A COMPREHENSIVE X-RAY AND MULTIWAVELENGTH STUDY OF THE COLLIDING GALAXY PAIR NGC 2207/IC 2163

Galaxies in collision are known to host intense star-formation activity. Presumably this is due to dynamical shocks that are induced by the supersonic relative speeds of the galaxies in comparison to the thermal speeds of the stars and gas clouds within the galaxies. These shocks, in turn, trigger the collapse of molecular clouds, leading to the formation of star clusters with a wide spectrum of stellar masses, including O and B stars (Struck 1997; Struck et al. 2005; Bonnell et al. 2006). Furthermore, as is well known empirically, many of these O and B stars will naturally be found in binary systems.

Accompanying these star-formation events in galaxy collisions are the production of numerous different classes of high-energy astrophysical objects, such as core-collapse supernovae (Hamuy et al. 2000; Konishi et al. 2011), high-mass X-ray binaries (HMXBs; David et al. 1992; Grimm et al. 2003; Swartz et al. 2004; Liu et al. 2006), and gamma-ray burst sources (Bloom et al. 2002). In the case of HMXBs and gamma-ray burst sources, binary stars are an intrinsic part of the evolution of these objects (see, e.g., Bhattacharya & van den Heuvel 1991; Woosley & Heger 2012), and they may also be relevant to the evolution of many supernovae (see, e.g., Podsiadlowski et al. 1993), both core collapse and thermonuclear events. In general, the more massive star in the binary evolves first and may lose its envelope via mass transfer or ejection from the system, and this is followed by the collapse of the core, which produces either a black hole or a neutron star. When that collapsed star accretes matter from the original secondary in the system, either via stellar wind accretion or Roche-lobe overflow, a massive X-ray binary is formed.

Among the many HMXBs that are found in collisional galaxies, a small fraction (∼10%, according to Mineo et al. 2012a) are so-called "ultraluminous X-ray sources" (ULXs). These consist of off-nuclear sources with L_X > 10³⁹ erg s⁻¹, a luminosity that corresponds to the Eddington limit for an accreting 10 M_☉ black hole and is taken as a fiducial reference point. It is not known exactly what mass of black holes may power ULXs, but both super-Eddington accretion onto stellar-mass black holes (see, e.g., Done & Gierliński 2003; Madhusudhan et al. 2008; Gladstone et al. 2009) and sub-Eddington accretion onto intermediate-mass black holes (IMBHs; with masses in the range of 10³–10⁴ M_☉; Colbert & Mushotzky 1999; Farrell et al. 2009) have been discussed and are plausible⁸ (see Feng & Soria 2011 for a review).

On average, ∼30% of the ULXs hosted by star-forming galaxies have L_X > 4–5 × 10³⁹ erg s⁻¹, and ∼10% are very luminous (L_X > 10⁴⁰ erg s⁻¹). Many of the high-luminosity ULXs are hosted by colliding galaxies and become X-ray bright ≈10–20 Myr after the end of star formation (Swartz et al. 2004, 2009; Walton et al. 2011).

The donor stars for most ULXs, i.e., for ULXs hosted by star-forming galaxies, are massive young stars and may be either blue supergiants (Roberts et al. 2001; Liu et al. 2002; Smith et al. 2012), Wolf-Rayet stars (Liu et al. 2013), or red supergiants (Copperwheat et al. 2005; Patruno & Zampieri 2008; Heida et al. 2014).

There are now numerous collisional galaxies that are known to host a substantial number of ULXs (see, e.g., Smith et al. 2012). These include the famous Antennae galaxies (Whitmore & Schweizer 1995; Zezas & Fabbiano 2002), the spectacular Cartwheel galaxy with its prominent spoke-like features (Higdon 1995; Gao et al. 2003; Wolter et al. 2006), the "cigar" galaxy M82 (Kaaret et al. 2004), and the more recently studied NGC 2207/IC 2163 (Mineo et al. 2013).

There is a well-established correlation between the total star-formation rate (SFR) in a galaxy and the total number of luminous X-ray sources harbored by that galaxy (see, e.g., Grimm et al. 2003; Smith et al. 2005; Mapelli et al. 2010; Swartz et al. 2011; Mineo et al. 2012a). There are numerous different techniques for determining the SFRs, which include using separately, or in combination, UV continuum, Hα recombination lines, forbidden lines ([O ii]), far-infrared (FIR) continuum, and thermal radio emission (see, e.g., Kennicutt 1998; Kennicutt & Evans 2012 for detailed discussions). It has been suggested by a number of researchers that an appropriate linear combination of the GALEX UV and Spitzer FIR bands may be an especially robust indicator of SFR in nearby and starburst galaxies (e.g., Bell 2003; Hirashita et al. 2003; Iglesias-Páramo et al. 2004, 2006; Hao et al. 2011; Kennicutt & Evans 2012). In particular, Leroy et al. (2008) proposed a formulation for computing the SFR based on a specific linear combination of GALEX far-ultraviolet (FUV) (centered at 1575 Å) and Spitzer 24 μm fluxes to enable the creation of spatially resolved (at the few arcseconds level) images of SFR per unit area (see specifically their Equations (D10) and (D11)).

In our previous work on this subject (Mineo et al. 2013), we developed a new approach toward investigating the correlation between the number and luminosity densities of luminous X-ray binaries and the local SFR in the regions immediately surrounding the X-ray sources, using the Leroy et al. (2008) prescription. This novel technique enables us to probe these correlations on a galaxy-by-galaxy basis. In this approach, we quantitatively compare the location of the luminous X-ray sources imaged with Chandra with the spatial structures in the SFR images. Furthermore, as has been suggested by Calzetti et al. (2005, 2007), Kennicutt & Evans (2012), M. Krumholz (2012, private communication), and S. Rappaport et al. (2014, in preparation), the UV fluxes detected by GALEX tend to indicate older regions of star formation (∼10–50 Myr), after the embedded dust has been mostly cleared, in contrast with the 24 μm Spitzer images that highlight more recent star formation (i.e., ∼5–10 Myr) where the regions are still enshrouded by dust. These latter regions may harbor more ULXs at the upper end of the luminosity function (i.e., with L_X ≳ 10⁴⁰ erg s⁻¹). This kind of correlation analysis, done on the local level, enables some of the theoretical ideas concerning the formation and evolution of very massive binaries to be constrained.

Recent results by Luangtip et al. (2014) show that in a sample of 17 nearby (<60 Mpc) luminous infrared galaxies (LIRGs) with SFRs >7 M_☉ yr⁻¹ and low foreground Galactic column densities (N_H ≲ 5 × 10²⁰ cm⁻²) there is a large deficit (a factor of ∼10) in the number of ULXs detected per unit SFR when compared to the detection rate in nearby, normal star-forming galaxies. The study is based on Chandra observations with sufficiently sensitive imaging to permit the detection of all ULXs present in the galaxy. The authors suggest that it is likely that the high column of gas and dust in these galaxies, which fuels the high SFR, also acts to obscure many ULXs from our view.

Based on a sample of Arp interacting galaxies, Smith et al. (2012) found a deficiency of ULXs in the most infrared-luminous galaxies, in agreement with the results mentioned above. They conclude that, although the active galactic nuclei (AGNs) may contribute to powering the far-infrared, the ULXs in these galaxies may be highly obscured and therefore not detected by Chandra.

On the other hand, Basu-Zych et al. (2013a, 2013b) show that the total X-ray luminosity output per unit SFR in distant star-forming galaxies weakly evolves with redshift. They suggest that the L_X/SFR evolution is driven by metallicity (see also Mapelli et al. 2010; Prestwich et al. 2013; Brorby et al. 2014, for similar conclusions on nearby galaxies) and show that the dust extinction (L_IR/L_UV) has insignificant effects on the observed values of L_X/SFR. These results seem consistent with that found by Mineo et al. (2012a), who investigated the correlation of L_IR/L_NUV with L_X − SFR for HMXBs. They found no correlation, although the average L_X/SFR seems to be decreasing with increasing values of L_IR/L_NUV (see their Figure 11(d)).

Our prior analysis of a 13 ks Chandra observation of NGC 2207/IC 2163 (Mineo et al. 2013), an impressive pair of spiral galaxies in the initial stages of collision, revealed a total of 21 ULXs within the D25 ellipse (de Vaucouleurs et al. 1991). Such a production efficiency of luminous X-ray sources per unit stellar mass is comparable with that of the Antennae pair of colliding galaxies (Zezas & Fabbiano 2002). Because NGC 2207/IC 2163 turned out be so rich in ULXs, based on an initially short 13 ks observation (Mineo et al. 2013; see also Kaufman et al. 2012), we were granted three further Chandra observations (for a collective additional exposure time of 50 ks).

The colliding galaxies NGC 2207/IC 2163 are estimated to be at a distance of 39.6 ± 5.5 Mpc (Arnett 1982), which is based on measurements of type Ia supernovae (see NASA/IPAC Extragalactic Database, NED). Figure 1 shows a collection of images of NGC 2207/IC 2163 recorded with the Hubble Space Telescope (HST), GALEX, Spitzer, and Herschel, all to the same scale and orientation. The larger galaxy on the right is NGC 2207, and the smaller galaxy on the left is IC 2163. The collisional dynamics of such galaxy pairs in general, and of NGC 2207/IC 2163 in particular, have been well modeled with N-body codes (see, e.g., Struck et al. 2005, and references therein). Such simulations can, for example, indicate which of the currently observed features in these galaxies have been created or substantially modified by the collision. The consensus is that the major spiral arms of NGC 2207 existed prior to the collision and have not been substantially perturbed by the interaction. By contrast, the noteworthy "ocular" feature in IC 2163 (see, in particular, the Spitzer 8 μm image) was apparently produced in the encounter. These facts indicate that the collision between NGC 2207 and IC 2163 is likely in its initial phases, e.g., for perhaps only a single orbit of the galaxy pair. Given the timescale for this grazing encounter, the star formation that has been induced has likely been underway only for the past dynamical timescale, i.e., a few times 10⁸ yr. Mass estimates for NGC 2207 and IC 2163 (including dark matter), used in the Struck et al. (2005) simulations as well as those measured by Mineo et al. (2013), are quite comparable at 1.5 × 10¹¹and1.1 × 10¹¹ M_☉, respectively, for the two galaxies.

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In the present paper we reinvestigate the X-ray emission from this same galaxy pair with about five times the net Chandra exposure (63 ks versus13 ks) used for our previous study. NGC 2207/IC 2163 was observed with Chandra on three subsequent occasions for a total additional exposure of 50 ks. This deeper exposure enables more sensitive studies of the X-ray population (3.4 × 10³⁸ erg s⁻¹ versus 10³⁹ erg s⁻¹) and diffuse emission. The four different epochs also allow us to study the long-term variability of individual sources and that of the X-ray luminosity function (XLF). We also utilize multiwavelength data from the GALEX, Spitzer, Herschel, and Two Micron All Sky Survey (2MASS) archives. In Section 2 we describe the data products and the basic steps in the analysis. We discuss the 56 point sources (excluding the central AGN) detected in the combined exposures, including 28 ULXs, in Section 3. In this same section we present cumulative luminosity functions for the individual exposures as well as for the sum, we discuss the X-ray source variability, and we search for optical and infrared counterparts to the X-ray sources. In Section 4 we describe the construction of spatially resolved maps of SFR density and L_IR/L_NUV, along with the related multiwavelength data acquisition. In Section 5 we repeat our correlation study between the local star-formation rate in NGC 2207/IC 2163 and the number and X-ray luminosity of the ULXsthis time with improved significance. As a new feature of the analysis, in Section 6 we also compute the correlation between a local L_IR/L_NUV and the number and luminosity of the ULXs, with quantitative considerations about the age of the stellar population associated with bright X-ray binaries. The spectrum of the diffuse emission is presented in Section 7. A summary and our conclusions follow in Section 8.

2.1. Data Preparation

2.2. Source Photometry

2.3. Luminosities and Hardness Ratios

The net count rates measured in the 0.5–8.0 keV band for each X-ray point source (Section 2.2) were converted into fluxes, i.e., units of erg cm⁻² s⁻¹. The count-rate-to-flux conversion factor was obtained as follows. For each observation, we first extracted the combined spectrum of all point sources detected within the D25 ellipse that have a number of net counts ⩾10, in order to avoid undue contamination from diffuse emission and faint unresolved X-ray sources.¹¹ The task dmextract was used for this purpose, and the central source was excluded from the multisource region because it may be an AGN (Elmegreen et al. 2006; Kaufman et al. 2012). Using the sky2tdet tool, we obtained the weights maps that are needed to create the weighted ancillary response files (ARFs) with mkwarf. The weighted response matrix files (RMFs) were created using mkrmf. The background spectrum was similarly extracted from multiple large regions between, and far enough from, the point sources within the D25 ellipse on the same detector chip. Finally, we used the script combine_spectra to sum the four composite source spectra, the associated background spectra, and the source and background ARF and RMF instrument responses.

Table 2. Completeness Luminosities and Global Source Properties

ObsID	LC	NX(> LC)	LX(> LC)	NAGN(> LC)	LAGN(> LC)	XLF slope γ
	(erg s−1)		(erg s−1)		(erg s−1)
(1)	(2)	(3)	(4)	(5)	(6)	(7)
11228	1.2 × 1039	20	6.4 × 1040	1.4	7.0 × 1039	$1.32^{+0.33}_{-0.29}$
14914	8.2 × 1038	27	7.1 × 1040	1.9	7.5 × 1039	$1.04^{+0.23}_{-0.21}$
14799	1.3 × 1039	23	6.9 × 1040	1.3	6.9 × 1039	$1.56^{+0.36}_{-0.31}$
14915	7.7 × 1038	26	5.2 × 1040	2.0	7.6 × 1039	$1.27^{+0.27}_{-0.24}$
Combined	3.4 × 1038	56	7.8 × 1040	3.6	8.4 × 1039	$^{\dagger }0.92^{+0.14}_{-0.13}$

Notes. All values are referred to the D25 ellipse of NGC 2207/IC2163. Column 1: Chandra identification numbers; Column 2: completeness luminosity at which >90% of point sources are detected in the 0.5–8 keV band; Column 3: number of point sources detected above L_C (the central AGN was excluded from this number); Column 4: cumulative 0.5–8 keV luminosity of compact sources above L_C; Column 5: expected number of background AGNs above the completeness luminosity; Column 6: expected equivalent luminosity of the background AGNs. See Section 2.4 for details; Column 7: best-fitting XLF slope of the simple power-law model (Equation (2)), in cumulative form. †: we fitted the combined XLF with a power-law model with an exponential cutoff (Equation (3)), obtaining a best-fitting slope α = 0.66 ± 0.04 (1.66 in differential form) and the exponential cutoff at L_o = (3.39 ± 0.28) × 10³⁹ erg s⁻¹ (see Section 3.2 for details).

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The final spectrum was binned in order to have a minimum of 20 counts per channel to apply χ² statistics. This background-subtracted co-added spectrum (see Section 7) was modeled as an absorbed power law using XSPEC v. 12.7.1b. The best-fit parameters for this model are N_H = (3.0 ± 0.3) × 10²¹ cm⁻² and Γ = 1.95 ± 0.08, with χ² = 78.4 for 76 degrees of freedom (i.e., reduced χ² = 1.03). This is in full agreement with the results of the population study by Swartz et al. (2004), who showed that the distribution of the power-law photon indexes for luminous compact X-ray sources in star-forming galaxies is centered on Γ = 1.97 ± 0.11.

Our best-fit absorbed power law was adopted to convert the net count rate of each detected point source into a flux (erg cm⁻² s⁻¹). The fluxes were then converted into luminosities (erg s⁻¹), assuming a distance of 39.6 Mpc (Arnett 1982).

Because we do not have sufficient statistics for X-ray spectral fitting of individual sources, to investigate the ULX spectral properties we used hardness ratios. The procedure described in Section 2.2 was applied to the reference source list to obtain count rates in both soft (S: 0.5–2 keV) and hard (H: 2–8 keV) bands. The respective source counts were used to calculate the X-ray hardness ratio as

$$\begin{equation} {\rm {HR}} = \frac{{H}-{S}}{{H}+{S}}\protect. \protect \end{equation} \tag{ 1 }$$

2.4. Completeness Analysis

3.1. Discrete Source Content

In Table 2 we list the number of compact sources detected with luminosities above L_C within the D25 region, with significance of at least 3σ, along with their integrated luminosity. The same table also includes the expected number and luminosity of background AGNs computed above an equivalent L_C within the D25 region. The contribution of background AGNs was estimated using the log N − log S function of Georgakakis et al. (2008). We converted their log N − log S function, defined over the 0.5–10 keV band, to apply in our somewhat more narrow band (0.5–8 keV). After obtaining the predicted total flux of background AGNs, S_AGN, above the completeness threshold flux, S_C, we computed the equivalent AGN luminosities as 4π D² S_AGN(> S_C), where D is the distance to the galaxy pair. From Table 2 it is evident that the contribution of background AGNs to the bright X-ray compact source population of NGC 2207/IC 2163 is within the 5%–7% range.

The properties of the X-ray point sources detected within the D25 ellipse are listed in Tables 7, 8, 9, 10, and 11. For all sources in each observation, we provide the Chandra positions, the net counts after background subtraction in several bands, the hardness ratio, the X-ray luminosity, and the X-ray flux. There were 74 sources detected in the composite image, 57 of which (∼77%) were above the completeness luminosity. One source in each observation is associated with the low-luminosity AGN near the center of NGC 2207 (Kaufman et al. 2012) and is indicated with a † symbol in the tables in the Appendix.

We indicate, with a ‡ symbol, the X-ray source associated with the elongated soft X-ray feature, called feature i by Elmegreen et al. (2000). This source has a spectrum compatible with the rest of the ULXs (see Figure 4) and could actually be a ULX embedded in the diffuse emission. Smith et al. (2014) also found that this source is quite extended, but with a low surface brightness, and that it has a soft X-ray spectrum as well as an X-ray flux similar to that measured in the present work.

The feature is located in the outer spiral arm of NGC 2207, ∼15 N-W from its center in the middle of a dusty starburst region. Other symbols (^⋆, ^◊, and *) are used to indicate X-ray sources that match (within ∼3'') the position of the supernovae (SNe) hosted by NGC 2207: SN 2003H, SN 2013ai, and SN 1999ec.

A Chandra X-ray image of NGC 2207/IC 2163 is shown in Figure 2. It was obtained by combining the four available observations (see Table 1), and it unveils the presence of soft, diffuse emission (discussed in Section 7) in addition to the bright X-ray compact source population. The signal from diffuse emission is weak and was apparently not visible in the single pointing (ObsID 11228) analyzed in our previous paper (Mineo et al. 2013). The detection of diffuse emission in the current work is also aided by the fact that we now use the CIAO task csmooth, while in the previous paper only a Gaussian smoothing kernel was applied.

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In Figure 3 we show the locations of the 57 point X-ray sources above the completeness threshold superposed on the HST image of NCC 2207/IC 2163. Not surprisingly, many of these sources lie along the prominent spiral arms of NGC 2207 (larger galaxy on the right), though the same does not seem to follow for the smaller galaxy IC 2163.

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In Figure 4, the net photon fluxes (photons cm⁻² s⁻¹) in the 0.5–8 keV band are plotted versus the hardness ratios defined in Equation (1) for all point sources detected above the completeness luminosity of each individual observation (see Table 2 for details). The sources are indicated with four different colors, depending on the observation in which they were detected. The figure shows that the bulk of ULXs in NGC 2207/IC 2163 have a soft spectrum (HR < 0). The hardness ratios measured for the central AGNs are well separated from the ULX population, having a harder (0.5 < HR < 1) spectrum and being bright.

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We also note that the spectrum of the X-ray source matching the position of the supernova SN 2013ai, detected in ObsID 14799, is harder than the spectrum of the bulk of the ULXs. Most type II SNe have X-ray spectra well described by a kT = 1 keV to kT = 10 keV mekal model, but a couple of type IIn SNe have been observed with rather hard X-ray spectra, e.g., a power law with photon index of −0.2 in SN 2005kd (Pooley et al. 2007) and a power law with photon index of 1.1 in SN 2001em (Pooley & Lewin 2004).

3.2. X-Ray Luminosity Function

After excluding the central AGN, we constructed the cumulative X-ray luminosity function of the X-ray point sources in NGC 2207/IC 2163 for each individual observation and for the combined observations (left and middle panels of Figure 5). The cumulative luminosity distribution of background AGNs (see Section 3.1 for details) is marked in the middle panel of Figure 5 by a dot-dashed (gray) curve. Figure 5 shows that combining the four Chandra observations allowed for a significant improvement in sensitivity: 3.4 × 10³⁸ erg s⁻¹ versus ∼10³⁹ erg s⁻¹ for the individual observations (see also column 2 in Table 2).

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The XLF was modeled with a simple power law. We fixed the cutoff at $L_{\rm {cut}} = 10^{41}\,\rm {erg}\,\rm {s}^{-1}$, i.e., a luminosity exceeding that of the brightest detected compact source ($L_{0.5\hbox{--}8\,\rm {keV}}=1.2\times 10^{40}\,\rm {erg}\,\rm {s}^{-1}$, detected in ObsID 11228):

$$\begin{equation} N(>L_{38})=\xi \times L_{38}^{-\gamma },\quad L\le 10^{41}\,\rm {erg}\,\rm {s}^{-1}. \end{equation} \tag{ 2 }$$

We fitted the cumulative XLFs using a maximum likelihood (ML) method. The best-fitting power-law slopes for individual observations are listed in column 7 of Table 2. They range between 1.04 and 1.56 (corresponding to 2.04 and 2.56 in differential form). The ML fit of the combined observations yielded a slope of $\gamma = 0.92^{+0.14}_{-0.13}$ (1.92 in differential form), which is indicated with a green line on the middle panel of Figure 5. This slope is steeper than the typical slope of ≈0.6 (1.58 ± 0.02 in differential form) found for the high-mass X-ray binary (HMXB) luminosity distributions below ${\sim }10^{40}\,\rm {erg}\,\rm {s}^{-1}$ (Grimm et al. 2003; Swartz et al. 2011; Mineo et al. 2012a). In our previous work, (Mineo et al. 2013), we found a similar result and speculated that this might have been related to the limited sensitivity of the single Chandra observation (ObsID 11228) that we analyzed. In fact, with a sensitivity of ∼10³⁹ erg s⁻¹, we could be observing only the high-luminosity roll-off of the power-law distribution that extends to lower luminosities with a slope similar to 1.6. In the present work we improved the sensitivity down to 3.4 × 10³⁸ erg s⁻¹ by combining four observations. We also fitted the combined XLF using a power-law model with an exponential cutoff:

$$\begin{equation} N(>L_{38}) = \xi \times L_{38}^{-\alpha }\,\exp (-L_{38}/L_{o}), \end{equation} \tag{ 3 }$$

which yielded a best-fitting slope of α = 0.66 ± 0.04 (1.66 in differential form) and the exponential cutoff at L_o = (3.39 ± 0.28) × 10³⁹ erg s⁻¹. The slope is now in full agreement with that of the average XLF for HMXBs, and the exponential cutoff somewhat confirms the speculation that we may be observing only the bright end roll-off of a more extended power-law distribution with slope 1.6.

We performed a Kolmogorov–Smirnov (KS) test to determine the goodness of fit. The test statistic D_KS, i.e., the maximum absolute value of the differences between the two distributions, is 0.31 and 0.27 for the power-law and exponentially cut off power-law models, respectively. The p values for the two-sided hypothesis are 5.5 × 10⁻⁴ and 3.8 × 10⁻³, respectively, i.e., smaller than the canonically assumed significance level of 0.05 for rejection. Formally the null hypothesis is rejected, but it still appears to be a good qualitative fit.

The observed power-law roll-off is due to the lack of sources brighter than ${\sim }10^{40}\,\rm {erg}\,\rm {s}^{-1}$, also observed in our previous work (Mineo et al. 2013). However, this could well be expected based on the star-formation rate of NGC 2207/IC 2163: the number predicted by the XLF from Mineo et al. (2012a) for bright HMXBs is marginally consistent (2  ±  1.4) with what we observe.

3.3. ULX Variability

To investigate the variability of the ULX population of the colliding galaxy pair NGC 2207/IC 2163, we started from the list of sources detected in the combined observation. At the position of each detection, we used the same tools and technique as described in Section 2.2 to measure the net number of counts and the net photon flux (photons cm⁻² s⁻¹) in each individual observation in the soft (0.5–2 keV), hard (2–8 keV), and full (0.5–8 keV) bands. The soft and hard source counts were used to calculate the X-ray hardness ratios as in Equation (1). The full band count rates were used to investigate the source variability, following Fridriksson et al. (2008), by means of the significance S_flux for long-term flux variability:

$$\begin{equation} S_{\rm {flux}} = {\max }_{i,j}\frac{|F_{i}-F{j}|}{\sqrt{\sigma ^{2}_{F_{i}}+\sigma ^{2}_{F_{j}}}}, \end{equation} \tag{ 4 }$$

where F_i and F_j are the net photon fluxes in the 0.5–8 keV band of the given source in the ith and jth Chandra observations, and $\sigma _{F_{i}}$ and $\sigma _{F_{j}}$ are the respective uncertainties. A source is variable if S_flux > 3, and it is a transient candidate if it is variable (S_flux > 3) and its measured flux is consistent with zero during at least one observation. For sources with zero counts (and therefore a null count rate F_{i, j} = 0), we used the 1σ upper limit for zero counts assuming Poisson statistics, based on the tables in Gehrels (1986), which is 1.84 counts. We converted to the photon flux uncertainty $\sigma _{F_{i,j}}$ by dividing the 1σ upper limit for zero counts by the value of the exposure map (cm² s counts photon⁻¹) at the position of the source.

In total, 12 sources out of 57 (∼20% including the AGN at the center of NGC 2207) show significant long-term variability (S_flux > 3). Of these, seven are transient candidates (source No. 51, 31, 37, 74, 22, 46, 66). We list all of the variable sources and transient candidates in Table 6, along with their count rates in the 0.5–8 keV band, the hardness ratios (computed with Equation (1)), and the significance, S_flux, of the variability. The long-term light curves of these sources are shown in Figure 6. The ratio between maximum and minimum count rate ranges between 2 and 3.8 for variable sources, and between ∼34 and ∼72 for transient candidates. One of the variable sources (41 in Table 6) is the central AGN in NGC 2207. Another one (22 in Table 6), is associated (011 separation) with the supernova SN 2013ai. The source was detected only during ObsID 14799, where it happens to have a rather hard X-ray spectrum (HR ∼0.7 ± 0.2) and 0.5–8 keV luminosity $L_{\rm {X}}=(6.3\pm 1.3)\times 10^{39}\,\rm {erg}\,\rm {s}^{-1}$ (see Table 9).

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Correlated X-ray spectral and luminosity changes have been observed in a number of ULXs (e.g., Kubota et al. 2001; Pintore et al. 2014). Figure 4 shows no significant long-term variability in the hardness ratio of the ULXs in NGC 2207/IC 2163. It also shows that there are no other sources located close to the AGN in the HR > 0.5 area of the diagram, suggesting there are probably not many background AGNs among the sources shown here, in line with our other estimates (see Section 3.1).

We looked more carefully for possible spectral changes in the 12 variable sources mentioned above, and we found none. The lack of such changes is common when the flux variability is only a factor of a few (Fridriksson et al. 2008; Soria et al. 2009; Webb et al. 2014).

3.4. HST and Spitzer Counterparts

Using only the data from Chandra ObsID 11228, along with Spitzer 24 μm and GALEX FUV images, Mineo et al. (2013) investigated, for the first time, the spatial and luminosity distributions of ULXs as a function of the local SFRs within a galaxy. They found that the relation between the total number of ULXs and the integrated SFR of the host galaxy (Mapelli et al. 2010; Mineo et al. 2012a; Smith et al. 2012) is also valid on subgalactic scales, i.e., a local N_X–SFR relation. Because of the small number of X-ray sources (21 ULXs) detected in ObsID 11228, Mineo et al. (2013) were not able to study the local L_X–SFR relation in a statistically meaningful manner. Using the same SFR density image constructed by Mineo et al. (2013) (see their Section 4.1 for details) and following exactly the same technique (described in their Section 5), we now revise the spatially resolved N_X–SFR and L_X–SFR relations for ULXs in NGC 2207/IC 2163 using the combined data from all available Chandra observations (Table 1). In addition, adopting the same technique, we explore the effects of age and dust extinction on the bright XRB population in NGC 2207/IC 2163 on subgalactic scales. To characterize the dust extinction or age effects, we use the ratio of 8–1000 μm luminosity (L_IR) to observed (i.e., uncorrected for attenuation effects) NUV (2267 Å) luminosity (L_NUV). We note that the SFR density map was constructed using GALEX FUV (1516 Å) and Spitzer MIPS 24 μm, following the calibration of Leroy et al. (2008). However, the FUV image of NGC 2207/IC 2163 has poorer statistics compared with the NUV image, which makes it less suitable for a pixel-by-pixel analysis; this is why we use the NUV image to construct the L_IR/L_NUV map.

For the basic IR data, we used a MIPS 24 μm Large Field image from the "post Basic Calibrated Data" products provided by the Spitzer Space Telescope Data archive.¹² These are images calibrated in $\rm {MJy}\, \rm {sr}^{-1}$, suitable for photometric measurements. We measured the 24 μm background in a region away from the galaxy and subtracted it from the image. The total net counts were then converted from units of $\rm {MJy}\, \rm {sr}^{-1}$ into Jy using a conversion factor C_24 μm = 1.41 × 10⁻⁴. The monochromatic fluxes (Jy) at 24 μm were then converted into spectral luminosities ($\rm {erg}\,\rm {s}^{-1}\,\rm {Hz}^{-1}$). The total IR luminosity (8–1000 μm) was estimated using the relations from Bavouzet et al. (2008): $L_{\rm {IR}} (L_{\odot }) =6856\times (\nu L_{\nu }/L_{\odot })_{24\,\mu \rm {m, rest}}^{0.71}$.

We based the ultraviolet analysis on GALEX NUV and FUV background-subtracted intensity map data that are publicly available in the archive of the GR6/GR7 Data Release.¹³ These are images calibrated in units of counts per pixel per second, corrected for the relative response, with the sky background subtracted. We converted the net counts to flux using the conversion factors¹⁴ between GALEX count rate (countss⁻¹) and flux ($\rm {erg}\,\rm {cm}^{-2}\,\rm {s}^{-1}\,\mathring{\rm{A}}^{-1}$): C_NUV = 2.06 × 10⁻¹⁶ and C_FUV = 1.40 × 10⁻¹⁵. The fluxes were then converted into a broadband NUV luminosity by taking the product λF_λ and multiplying by 4πD².

Using the routine HASTROM from the NASA IDL Astronomy User's Library,¹⁵ we oversampled the 24 μm image (245 pixels⁻¹) so as to match the better angular resolution (15 pixels⁻¹) of the GALEX images. This routine interpolates without adding spatial frequency information beyond the intrinsic resolution of the original image. The resulting maps, displayed in Figures 7 and 9, have the same spatial resolution and pixel coordinates as the GALEX images but are limited by the Spitzer resolution. Figures 1, 3, and 9 show that IC 2163 is dustier than NGC 2207.

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With the SFR density image (Section 4 and Figure 7 in this paper and Section 4.1 of Mineo et al. 2013), we now improve the spatially resolved N_X–SFR and L_X–SFR relations for ULXs in NGC 2207/IC 2163 (Mineo et al. 2013), using the combined data from all available Chandra observations (Table 1). We defined a grid of uniformly spaced logarithmic bins of SFR density based on the pixel values in the SFR image (see Figure 7). Based on the SFR density value at the position of a given X-ray source, we counted the number of sources brighter than $10^{39}\,\rm {erg}\,\rm {s}^{-1}$ and their collective luminosity for each bin of SFR density. We counted the number of pixels in each bin of SFR density (over the D25 region) and thereby computed the corresponding integrated area. We use this area to normalize the background AGN log N − log S function, which we take from Georgakakis et al. (2008), and thereby calculate the predicted number of background AGNs, $N_{\rm {AGN}}(L> 10^{39}\,\rm {erg}\,\rm {s}^{-1})$, and their equivalent luminosity, $L_{\rm {AGN}}(>10^{39}\,\rm {erg}\,\rm {s}^{-1})$, above the same luminosity threshold. The respective values for the bright X-ray sources and the AGNs were then subtracted to yield $N_{\rm {X}}(L> 10^{39}\,\rm {erg}\,\rm {s}^{-1})$ and $L_{\rm {X}}(> 10^{39}\,\rm {erg}\,\rm {s}^{-1})$, respectively. Dividing these values by the area underlying each bin of SFR density, we obtained the surface density for X-ray point sources (source number kpc⁻²) and luminosity density (erg s⁻¹ kpc⁻²) as a function of SFR density.

These quantities are plotted against the SFR surface density in Figure 8. Table 3 describes the data used to prepare Figure 8. Because of ULX variability (Section 3.3), we now detect 28 ULXs, seven of which were not visible in ObsID 11228 alone. This ∼30% increase in ULX number and a better average value for L_X allow for an improvement in both the N_X–SFR and L_X–SFR relations, which is more appreciable in the latter case. Pixels with SFR density less than $6\times 10^{-4}\,M_{\odot }\,\rm {yr}^{-1}\,\rm {kpc}^{-2}$ were not used because they are dominated by background noise (see Section 5 in Mineo et al. 2013). Two ULXs were located in these regions of low SFR density, so they are not included in our plot. In Figure 8, we also plot the average N_X–SFR and L_X–SFR relations for ULXs obtained for large samples of nearby star-forming galaxies by Mineo et al. (2012a, their Equations (20) and (22) as solid lines in Figure 8), after having converted the SFR estimate for the different initial mass functions (IMFs) used in the present work (IMF as in Calzetti et al. (2007), i.e., with a slope −1.3 for the 0.1–0.5 M_☉ mass range and −2.3 for 0.5–120 M_☉; see Section 4.2 and Table 2 in Mineo et al. 2013). Similarly, we compare our results with the N_ULX–SFR relation from Mapelli et al. (2010). The dashed line in the top panel of Figure 8 shows their Equation (6), which is slightly nonlinear. We also plot the N_ULX–SFR relation from Smith et al. (2012) as a dotted line, after having applied all of the necessary conversions to make it compatible with the units used in Figure 8. The latter relation is almost identical to the scaling from Mineo et al. (2012a).

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Table 3. Table of the Data Used to Prepare Figure 8

ΣSFR	NX	LX	NAGN	LAGN	Area
($M_{\odot }\,\rm {yr}^{-1}\,\rm {kpc}^{-2}$)		($10^{38}\,\rm {erg}\,\rm {s}^{-1}$)		($10^{38}\,\rm {erg}\,\rm {s}^{-1}$)	(kpc2)
(1)	(2)	(3)	(4)	(5)	(6)
0.0013	1	10	0.13	6.7	100
0.0027	1	41	0.25	13	190
0.0054	3	130	0.26	13	200
0.011	8	180	0.26	13	195
0.022	7	200	0.19	9.7	145
0.045	6	160	0.07	3.6	55

Note. Column 1: value of Σ_SFR at the bin center; Column 2: number of ULXs per bin of Σ_SFR; Column 3: luminosity of ULXs per bin of Σ_SFR; Column 4: expected number of background AGNs per bin of Σ_SFR; Column 5: expected luminosity of background AGNs per bin of Σ_SFR; Column 6: area underlying each bin of Σ_SFR in kpc².

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Figure 8 confirms that the multiple-galaxy-averaged relation between the total number of X-ray point sources and the integrated SFR of the host galaxy also holds on subgalactic scales.

Mineo et al. (2012a) investigated the correlation of L_IR/L_NUV with L_X/SFR for HMXBs and found virtually no correlation (the Spearman's rank correlation coefficient is r_S = −0.35, corresponding to a probability of P = 7% for the null hypothesis). On the other hand, although within large error bars, the ratio of multiple-galaxy average L_X to SFR suggests a decreasing trend with increasing values of L_IR/L_NUV (see their Figure 11(d)). The ratio L_IR/L_NUV is affected by both dust extinction and age. We investigated the possible effects of dust extinction and age on the bright XRB population in NGC 2207/IC 2163 on subgalactic scales.

With the spatially resolved image of L_IR/L_NUV (Section 4, Figure 9), we studied the number of X-ray point sources above the completeness luminosity and their luminosities as a function of the local L_IR/L_NUV. We applied the same pixel-by-pixel analysis to the L_IR/L_NUV image that we utilized to study the local correlation between numbers and luminosities of X-ray sources and the local SFR density, which is described in Sections 4 and 5. We thereby obtained the densities N_X/kpc² and L_X/kpc² for the luminous X-ray sources and plotted these against the value of L_IR/L_NUV in Figure 10 (upper and lower panel, respectively). Table 4 describes the data used to prepare Figure 10. The figure shows a modest increase of N_X/kpc² and L_X/kpc² with L_IR/L_NUV at small L_IR/L_NUV values, up to L_IR/L_NUV ∼ 1. This is followed by a decrease of the N_X/kpc² and L_X/kpc² values when L_IR/L_NUV increases. Figure 10 shows these quantities plotted against seven discrete bins in L_IR/L_NUV. In order to take into account possible binning effects on the statistical significance of the observed number and luminosity density trends with L_IR/L_NUV, we also analyzed plots with five bins in L_IR/L_NUV. The observed trend for N_X/kpc² has a higher statistical significance (∼3.3σ and ∼2.8σ for the central bins in the case of five bins of L_IR/L_NUV versus seven bins, respectively) than that for L_X/kpc² (∼1.3σ and ∼2σ for the central bins in the case of five bins and seven bins, respectively). The significance was estimated by comparing the data points and their uncertainties with the best-fit average values: N_X/kpc² = (1.5 ± 0.3) × 10⁻², $L_{\rm {X}}/(10^{38}\,\rm {erg}\,\rm {s}^{-1})/\rm {kpc}^{2}=(1.5\pm 0.3)\times 10^{-1}$ (seven-bin plot). We view all of this as tentative evidence for a smoothly peaked dependence of N_X/kpc² and L_X/kpc² on L_IR/L_NUV.

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Table 4. Table of the Data Used to Prepare Figure 10

LIR/LNUV	NX	LX	NAGN	LAGN	Area
		($10^{38}\,\rm {erg}\,\rm {s}^{-1}$)		($10^{38}\,\rm {erg}\,\rm {s}^{-1}$)	(kpc2)
(1)	(2)	(3)	(4)	(5)	(6)
0.16	4	59	1.3	30	455
0.32	12	230	1.5	35	525
0.67	16	200	1.3	31	465
1.4	17	200	0.9	21	315
2.9	4	50	0.28	6.7	100
6.1	1	4	0.11	2.7	40
13	1	19	0.039	0.91	15

Note. Column 1: value of L_IR/L_NUV at the bin center; Column 2: number of X-ray binaries above the completeness luminosity per bin of L_IR/L_NUV; Column 3: luminosity of X-ray binaries per bin of L_IR/L_NUV; Column 4: expected number of background AGNs per bin of L_IR/L_NUV; Column 5: expected luminosity of background AGNs per bin of L_IR/L_NUV; Column 6: area underlying each bin of L_IR/L_NUV in kpc².

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The L_IR/L_NUV ratio may also be seen as a rough indicator of the timescale of the star-formation event. The UV emission originating from the photospheres of O and B stars is absorbed and re-emitted in the IR band by dust grains heated by the embedded young ionizing stars. The IR should trace somewhat younger star formation (<10 Myr, Calzetti et al. 2007), while UV traces older star formation (30–100 Myr, Calzetti et al. 2005); see also Kennicutt & Evans (2012). This might suggest, as a rough timescale, that at L_IR/L_NUV < 1, the age may be ≳ 10 Myr, and therefore the trend in Figure 10 might be tentatively ascribed to an age effect on the bright XRB formation and evolution. The UV dominates where the dust absorption is very low. In Milky Waytype galaxies, this means that the molecular clouds have dissipated or at least the stars have moved out of the clouds. To summarize: (1) high L_IR could mean such high dust obscuration that we fail to detect some X-ray sources. (2) High L_NUV could mean an older stellar population that has become free of dust. (3) The L_IR/L_NUV ratio could indicate an age that might, importantly, affect how many ULXs have already evolved away. Additionally, there are further complications due to the chemical composition, which we do not address in the present work.

Interpreting the trend observed in Figure 10 as an effect of age or star-formation timescale, we see that the number of bright XRBs and their luminosity peak at ≈10 Myr, i.e., around the epoch where young stars are escaping from the dust clouds that enshroud them. We note that these qualitative conclusions are in agreement with the results found by Swartz et al. (2009) for a large sample of ULXs detected in 58 nearby galaxies. They found that the most luminous ULXs (or equivalently, the most common phases of very high mass transfer) are biased toward early B-type donors with an initial mass of ≈10–15 M_☉ and an age ∼10–20 Myr, perhaps at the stage where the B star expands to become a blue supergiant. Similarly, an age of ≈10–20 Myr was inferred for the stars around NGC 4559 X-1 by Soria et al. (2005) and ≈20 Myr for those around NGC 1313 X-2 by Grisé et al. (2008).

However, this interpretation remains mostly tentative. Not only do the data points in Figure 10 have rather large error bars, but there is also some uncertainty introduced as a result of the pixel interpolation applied to obtain the L_IR/L_NUV map (see Section 4). Moreover, the 24 μm emission may depend on dust geometry, and the conversion from 24 μm to total IR luminosity has a very large uncertainty. The L_IR/L_NUV is not a perfect age indicator, but it should be at least statistically meaningful.

The L_IR/L_NUV ratio of galaxies also depends upon metallicity, with lower metallicity systems having less dust and therefore lower L_IR/L_NUV (see, e.g., Kunth & Oumlstlin 2000; Johnson et al. 2007; Basu-Zych et al. 2013b); therefore, the range L_IR/L_NUV < 1 may also show the low metallicity regions of NGC 2207/IC 2163. Based on the emission line analysis of H ii regions in interacting galaxies, Rupke et al. (2010) found that the metallicity in NGC 2207/IC 2163 is in the range 12 + log ([O/H]) ≈ 8.8–9.2, and they show that IC 2163 hosts regions with, on average, higher metallicity (12 + log ([O/H]) ≳ 9) than those in NGC 2207 (12 + log ([O/H]) ≲ 9). On the other hand, a full range of ≈0.4 dex may not be enough in terms of the implied dust-to-gas ratio, suggesting that in the case of NGC 2207/IC 2163 the effects of metallicity may not be important. We may conclude that either bright XRBs have not yet formed in the dustier galaxy (IC 2163) or they are hidden by high extinction (in agreement with the results of Smith et al. 2012; Luangtip et al. 2014).

Star-forming galaxies are known to emit significant amounts of X-rays at ∼sub-kiloelectron volt temperatures (typically in the range of 0.2–0.3 keV, sometimes with evidence of a second thermal component at ∼0.7 keV) because of hot ionized gas. The luminosity of the diffuse thermal X-ray emission correlates with the SFR of the host galaxy. The gas is thought to be in a state of outflow, driven by the collective effects of supernovae and winds from massive stars (Chevalier & Clegg 1985; Strickland et al. 2000, 2004; Grimes et al. 2005; Tüllmann et al. 2006; Owen & Warwick 2009; Yukita et al. 2012; Mineo et al. 2012b; Li & Wang 2013).

7.1. Isolating the Hot Interstellar Medium in NGC 2207/IC 2163

Recently, Mineo et al. (2012b) isolated the contribution of hot, diffuse interstellar medium (ISM) in a sample of 21 local, star-forming galaxies. They took special care with various systematic effects and controlled the contamination by "spillover" counts from bright resolved compact sources of the diffuse emission. Here we use the same procedures to isolate the truly diffuse emission that is due to the hot ISM from contaminating components and to obtain its luminosity.

We first searched for the optimal size of the regions to be used to remove the point-source counts from the image and to minimize the contamination by "spillover" counts. We used the same procedure described in Section 2 to search for point-like sources in the soft (0.5–2 keV), hard (2–8 keV), and total (0.5–8 keV) energy bands. For each source, we used the information about the shape of the PSF at the source position to determine the radius of the circular region containing 90% of the source counts, i.e., R_90% PSF. From the source lists obtained in each energy band, we created a set of source regions having radii ranging from 0.5 R_90% PSF to 3.5 R_90% PSF with a step of 0.1. A corresponding set of diffuse emission images was created for each observation of NGC 2207/IC 2163, adopting the following method. We removed the source regions from the image, and, using the CIAO task dmfilth (POISSON method), we filled in the holes left by the source removal with pixel values interpolated from the surrounding background regions. The background region for this interpolation purpose was defined as a circle with radius three times the radius of the source region. We ensured that the chosen background annuli did not contain neighboring point sources. For each background region listed in the input file, we subtracted all of the overlapping neighboring point-source regions and merged them into a single source-removal region. For each of the resulting images, we estimated the count rate within the D25 ellipse using the CIAO task dmextract and plotted it against the radius of the removed-source region. This plot showed a sharp decrease of the count rate at small source radii ⩽R_90% PSF, followed by a flattening of the curve at R >1.5–2 R_90% PSF. We found that on average the difference between excluding source regions with R = 1.5 R_90% PSF and R = 2 R_90% PSF is only ∼4% of the background-subtracted soft band count rate. Based on this analysis, we adopted a source region radius of R = 1.5 R_90% PSF to minimize the contamination of diffuse emission by point-source counts without compromising the statistics for the diffuse emission itself. The diffuse emission spectrum was extracted in each individual observation from the D25 region, after having removed the circular regions with R = 1.5 R_90% PSF for all detected point sources.

The background spectrum was extracted in each observation from a region defined by the whole Chandra chip outside 1.3 × D25, to avoid contamination from true diffuse emission in the outskirts of the D25 region and to retain good count statistics for the background spectrum itself. This takes into account both the instrumental and cosmic X-ray background.

The co-added spectrum of diffuse X-ray emission from all four observations in the 0.5–8 keV band is shown in Figure 11, along with the composite point-source spectrum that was obtained in Section 2.3.

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7.2. Spectral Analysis

7.3. X-Ray Luminosity of the Hot ISM

We have presented a comprehensive study of the total X-ray emission from the colliding galaxy pair NGC 2207/IC 2163.

We repeated our correlation study between the local SFR in NGC 2207/IC 2163 and the number and X-ray luminosity of the ULXs with improved significance with respect to our pilot study (Mineo et al. 2013), which was based on only one (ObsID 11228) of the four observations utilized in the present work. Because of ULX variability (Section 3.3), we now detect 28 ULXs, 7 of which were not visible previously. This ∼30% increase in ULX number allows for an improvement in both the spatially resolved N_X–SFR and L_X–SFR relations, which is most evident in the latter case. We confirm that the global relation between the number of X-ray point sources and the integrated SFR of the host galaxy also holds on local scales within a given galaxy. Thanks to the improved statistics utilized in the present analysis, we can now show that the relation between X-ray luminosity and local SFR is also in general agreement with the multiple-galaxy-wide average relation between the cumulative luminosity of ULXs and the integrated SFR.

We investigated the long-term flux and spectral variability of the ULX population and found that 12 sources out of 57 (∼20% including the AGN at the center of NGC 2207) show significant long-term variability (S_flux > 3; see Section 3.3). Of these, seven are transient source candidates. The ratio between maximum and minimum count rate ranges between 2 and 3.8 for the variable sources, but between ∼34 and ∼72 for the transient source candidates. One of the variable sources (41 in Tables 6 and 11) is the central AGN in NGC 2207. One of the transient sources (22 in Tables 6 and 11) is associated (011 separation) with the supernova SN 2013ai. We find no evidence for X-ray spectral changes in connection with the flux variability of the ULXs. All of the ULXs (except the one associated with SN 2013ai) have a soft spectrum during all four observations. Such behavior has been observed when the flux variability ranges over only a factor of a few.

We cross-checked the coordinates of 17 optical "superstar clusters" (mass 1–20 × 10⁴ M_☉) identified with HST as well as the coordinates of 225 Spitzer 8 micron clumps with the positions of the 74 X-ray sources detected within the D25 ellipse in the co-added Chandra image. Within a 15 tolerance limit, we found that only one young SSC is coincident with a bright XRB ($L_{\rm {X}} < 10^{39}\,\rm {erg}\,\rm {s}^{-1}$); by contrast, we found a statistically significant set of approximately one-third of our X-ray sources that align with Spitzer 8 μm-detected young star complexes, and one-half of the matching sources are ULXs. Among the matches there are two SNe, SN 1999ec and SN 2013ai, and the extended X-ray source at the location of the dusty starburst region called "feature i.

We report that our X-ray source 18 corresponds to source "X1" in Kaufman et al. (2012), the nature of which was still uncertain. The latter authors interpreted their source X1 as a possible radio SN, an SNR, or a background quasar. The significant variability and spectral hardness of our X-ray source 18 are incompatible with an AGN, and, according to its luminosity, $L_{\rm {X}} > 10^{39}\,\rm {erg}\,\rm {s}^{-1}$ in all Chandra pointings, we conclude that the source is a ULX.

We constructed the X-ray luminosity function of the bright X-ray point sources in each individual observation and found that the best-fitting slopes are in agreement with each other within the uncertainties (Table 2). This suggests that the X-ray variability exhibited by about 20% of the detected sources does not significantly influence their XLFs. We also obtained the XLF from the four combined Chandra observations. The best-fitting model for the average XLF is a power law with slope α = 0.66 ± 0.04 (1.66 in differential form) and an exponential cutoff at L_o = (3.39 ± 0.28) × 10³⁹ erg s⁻¹. The slope is in full agreement with that of the average XLF for HMXBs, and the exponential cutoff in our model may correspond to the roll-off of the bright end of a more extended power-law distribution with slope 1.6.

We studied the possible effects of dust extinction and age on a bright XRB population in NGC 2207/IC 2163 on subgalactic scales. We applied the same technique as that used to obtain the image of the SFR density. To characterize the dust extinction within the galaxy pair, we used the ratio of 8–1000 μm luminosity (L_IR) to observed (i.e., uncorrected for attenuation effects) NUV luminosity (L_NUV at 2267 Å). We found that the number and luminosity of bright XRBs show a trend with the local L_IR/L_NUV ratio. In particular, we observe a peak in the N_X and L_X distributions at L_IR/L_NUV ∼ 1, which is more significant for N_X. The peak may be tentatively interpreted as an effect of the different star-formation timescales traced by the IR and NUV proxies, and we speculate that at L_IR/L_NUV ≈ 1 the age of the underlying stellar population may be around 10 Myr. That these qualitative conclusions are in agreement with more quantitative previous results suggests that the most luminous ULXs are biased toward donor stars having an age of ∼10–20 Myr (Soria et al. 2005; Grisé et al. 2008; Swartz et al. 2009). However, this interpretation still remains somewhat tentative.

We disentangled and compared the X-ray spectra of the diffuse emission with the population of bright XRBs hosted by NGC 2207/IC 2163. The hot ISM has a temperature $kT = 0.28^{+0.05}_{-0.04}$ keV, assuming solar metal abundances, and dominates the overall X-ray output of NGC 2207/IC 2163 at E ≲ 1 keV. Unresolved accreting compact objects most likely dominate the diffuse X-ray emission at E ≳ 1 keV.

The co-added spectrum of resolved X-ray point sources is well described by an absorbed power-law with index Γ = 1.95 ± 0.08, consistent with the centroid of the power-law photon index distribution for luminous X-ray compact sources in star-forming galaxies, Γ = 1.97 ± 0.11 (Swartz et al. 2004).

The 0.5–2 keV X-ray luminosity of the thermal plasma, based on the full-band fitting and corrected for both Galactic and local absorption, is $L_{\rm {0.5\hbox{--}2 keV}}^{\rm {mekal, int}}=7.9\times 10^{40}\,\rm {erg}\,\rm {s}^{-1}$, which is a factor of ∼2.3 larger than the average thermal luminosity produced per unit SFR in local star-forming galaxies and corresponds to ∼100% of the collective luminosity of all point sources detected above the completeness limit for the combined observations ($7.8\times 10^{40}\,\rm {erg} \,\rm {s}^{-1}$). Such a result is about 10% larger than what is typical in local star-forming galaxies, but it is within the dispersion of values. After having subtracted the estimated contribution of background AGNs, the total X-ray output of NGC 2207/IC 2163 is 1.5 × 10⁴¹ erg s⁻¹. The corresponding total, integrated SFR is 23.7 M_☉ yr⁻¹.

We thank the anonymous referee for helpful suggestions that greatly improved this paper. We acknowledge support from the NASA Astrophysics Data Analysis Program (ADAP) grant NNH13CH56C and by NASA Chandra grant GO3-14092A. We acknowledge Steven Willner, Luca Cortese, Bret Lehmer, and Antara Basu-Zych for insightful discussions on dust extinction, star formation, and their relation with the starburst age. We are grateful to Michele Kaufman and Debra Elmegreen, who kindly supplied us with the coordinates of the 17 superstar clusters that they had identified in the HST image, for further discussions. We made use of Chandra archival data and software provided by the Chandra X-ray Center (CXC) in the application package CIAO. We also utilized the software tool SAOImage DS9, developed by the Smithsonian Astrophysical Observatory. The FUV, 3.6 μm, and 24 μm images were taken from the GALEX and Spitzer archives, respectively. The Spitzer Space Telescope is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. GALEX is a NASA Small Explorer, launched in 2003 April. We also made use of data products from the Two Micron All Sky Survey (2MASS), which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and the National Science Foundation. Helpful information was found in the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration.

In this Appendix, we present the lists of the detected source properties for each of the four individual Chandra observations of NGC 2206/IC 2163 (Tables 7–10), as well as a similar table that combines the results from all four observations (Table 11). The tables include the source coordinates, the detected 0.5--8 keV counts, as well as the counts in two separate bands (0.5–2 keV and 2–8 keV), the hardness ratio, and both the X-ray luminosity and flux. The sources are numbered in the tables, but note that the numbering is different for each of the four individual observations as well as for the summed observations, due to the different sources that were detected in each. Only the source numbering for the combined image is used for reference in the main body of the paper and to label sources in Figures 3 and 6.

Table 7. NGC 2207/IC2163: X-Ray Source Properties for ObsID 11228

Source	αJ2000	δJ2000	0.5–8 keV	Signif	0.5–2 keV	2–8 keV	HR	LX	FX
	(deg)	(deg)	(counts)	(σ)	(counts)	(counts)		($10^{38}\,\rm {erg}\,\rm {s}^{-1}$)	($10^{-14}\,\rm {erg}\,\rm {cm}^{-2}\,\rm {s}^{-1}$)
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
1‡	94.06612	−21.3673	5.8 ± 4.5	3.4	3.9 ± 4.1	1.8 ± 3	−0.37 ± 0.84	7.6 ± 5.9	0.40 ± 0.32
2	94.06617	−21.3757	92 ± 11	38	63 ± 9.5	29 ± 6.9	−0.36 ± 0.12	120 ± 15	6.45 ± 0.79
3	94.06938	−21.3743	22 ± 6.2	8.5	20 ± 5.9	2.1 ± 3	−0.81 ± 0.25	30 ± 8.5	1.59 ± 0.45
4	94.07004	−21.3726	6.5 ± 4	2.8	3.3 ± 3.2	3.2 ± 3.3	−0.02 ± 0.71	8.9 ± 5.5	0.48 ± 0.29
5	94.07046	−21.3527	28 ± 6.8	13	21 ± 6	7.4 ± 4.2	−0.48 ± 0.24	38 ± 9.1	2.02 ± 0.48
6	94.07048	−21.3758	20 ± 6	8.6	9.7 ± 4.5	11 ± 4.7	0.05 ± 0.32	28 ± 8.3	1.50 ± 0.44
7	94.07176	−21.3806	41 ± 8	16	32 ± 7.2	9 ± 4.6	−0.56 ± 0.19	54 ± 11	2.87 ± 0.57
8	94.07191	−21.3599	18 ± 5.7	7.8	11 ± 4.8	6.5 ± 4	−0.26 ± 0.35	23 ± 7.5	1.24 ± 0.40
9	94.07487	−21.3679	33 ± 7.2	14	27 ± 6.6	6.4 ± 3.9	−0.62 ± 0.21	46 ± 9.9	2.44 ± 0.53
10	94.07500	−21.3647	14 ± 5.3	8.3	8 ± 4.4	6.1 ± 4	−0.13 ± 0.41	20 ± 7.4	1.04 ± 0.39
11	94.07521	−21.3686	6.5 ± 4.1	3.3	5.4 ± 3.9	1.1 ± 2.7	−0.65 ± 0.70	9.1 ± 5.7	0.48 ± 0.30
12	94.07536	−21.3707	5.1 ± 3.7	2.3	4.2 ± 3.5	0.94 ± 2.5	−0.63 ± 0.84	7.7 ± 5.5	0.41 ± 0.29
13	94.07818	−21.3743	25 ± 6.4	11	16 ± 5.4	8.6 ± 4.3	−0.30 ± 0.28	33 ± 8.5	1.74 ± 0.46
14	94.08054	−21.3641	13 ± 5	5.9	11 ± 4.7	2.2 ± 2.9	−0.66 ± 0.39	17 ± 6.6	0.91 ± 0.35
15	94.08430	−21.3851	18 ± 5.7	7.5	13 ± 5	5.4 ± 3.7	−0.41 ± 0.33	25 ± 7.6	1.31 ± 0.41
16	94.08525	−21.3740	5.8 ± 3.9	2.5	1.7 ± 2.9	4.1 ± 3.5	0.40 ± 0.79	7.8 ± 5.3	0.42 ± 0.28
17	94.08544	−21.3697	27 ± 6.7	11	20 ± 6	6.6 ± 4.1	−0.51 ± 0.25	36 ± 9	1.92 ± 0.48
18	94.08567	−21.3720	7.3 ± 4.1	2.9	4.2 ± 3.5	3.1 ± 3.2	−0.15 ± 0.64	9.8 ± 5.5	0.52 ± 0.30
19	94.08604	−21.3736	5.1 ± 3.8	2.1	5.6 ± 3.8	0 ± 2.1	−1.00 ± 0.74	6.9 ± 5.2	0.37 ± 0.28
20†	94.09177	−21.3727	70 ± 10	24	8.4 ± 4.8	61 ± 9.4	0.76 ± 0.12	95 ± 14	5.05 ± 0.74
21	94.09430	−21.3608	24 ± 6.6	11	17 ± 5.7	7 ± 4.3	−0.43 ± 0.28	33 ± 8.9	1.77 ± 0.48
22	94.09828	−21.3707	7.7 ± 4.3	3.3	5.7 ± 3.9	2 ± 2.9	−0.47 ± 0.62	11 ± 5.9	0.57 ± 0.32
23	94.10083	−21.3866	7.6 ± 4.1	3.4	7.6 ± 4.1	0 ± 2	−1.00 ± 0.53	13 ± 7	0.69 ± 0.37
24	94.10100	−21.3627	9.7 ± 4.6	3.9	5.2 ± 3.8	4.5 ± 3.6	−0.08 ± 0.54	13 ± 6.4	0.71 ± 0.34
25	94.10377	−21.3641	6.4 ± 3.9	2.6	4.2 ± 3.5	2.2 ± 2.9	−0.32 ± 0.71	8.9 ± 5.5	0.47 ± 0.29
26	94.10472	−21.3749	13 ± 5.3	5.9	7.3 ± 4.3	6.2 ± 4.1	−0.08 ± 0.44	19 ± 7.5	1.02 ± 0.40
27	94.10630	−21.3704	6.4 ± 4	2.6	0 ± 2.1	6.7 ± 4	1.00 ± 0.63	9.3 ± 5.8	0.49 ± 0.31
28	94.10824	−21.3771	14 ± 5.1	5.2	6.5 ± 3.9	7.4 ± 4.1	0.07 ± 0.41	22 ± 8	1.17 ± 0.43
29	94.11017	−21.3701	7.8 ± 4.3	5.2	6.8 ± 4.2	0.95 ± 2.6	−0.75 ± 0.59	11 ± 6.3	0.60 ± 0.34
30	94.11180	−21.3697	13 ± 5	5.2	12 ± 4.8	1.1 ± 2.5	−0.83 ± 0.37	22 ± 8.5	1.17 ± 0.45
31	94.11419	−21.3712	5.4 ± 3.7	2	4.3 ± 3.5	1.1 ± 2.5	−0.60 ± 0.79	9.1 ± 6.2	0.49 ± 0.33
32	94.11818	−21.3609	15 ± 5.3	7.2	11 ± 4.7	4.4 ± 3.5	−0.42 ± 0.37	24 ± 8.5	1.30 ± 0.46
33	94.12197	−21.3680	5.5 ± 3.8	2	5.5 ± 3.8	0 ± 2.1	−1.00 ± 0.75	8.1 ± 5.5	0.43 ± 0.29
34	94.12488	−21.3790	15 ± 5.5	8.3	11 ± 4.9	4.1 ± 3.5	−0.45 ± 0.38	21 ± 7.9	1.14 ± 0.42

Notes. Column 1: Source number; Column 2: right ascension (RA); Column 3: declination (Dec); Column 4: net counts in broad (0.5–8 keV) band. The uncertainty expressed here takes into account the fluctuations in the source as well as in the background; Column 5: Broadband source detection significance from wavdetect. This computes how unlikely it is for the background in the customized PSF region to fluctuate to yield the detected number of counts. Note that the PSF region is optimized differently in wavdetect than in the calculation of column (4) and is typically larger than in the latter; Columns 6 and 7: Net counts in soft (0.5–2 keV) and hard (2–8 keV) bands, respectively. Uncertainties in net counts are quoted to 1σ; Column 8: hardness ratio computed with Equation (1). Uncertainties were obtained by applying error propagation to the uncertainties in the net counts; Column 9: X-ray luminosity in the 0.5–8 keV band; Column 10: X-ray flux in the 0.5–8 keV band.† Central active galactic nucleus.‡ Extended soft X-ray source.

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Table 8. NGC 2207/IC2163: X-Ray Source Properties for ObsID 14914

Source	αJ2000	δJ2000	0.5–8 keV	Signif	0.5–2 keV	2–8 keV	HR	LX	FX
	(deg)	(deg)	(counts)	(σ)	(counts)	(counts)		($10^{38}\,\rm {erg}\,\rm {s}^{-1}$)	($10^{-14}\,\rm {erg}\,\rm {cm}^{-2}\,\rm {s}^{-1}$)
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
1	94.05535	−21.3669	7.8 ± 4.3	3.2	7.8 ± 4.3	0 ± 2.1	−1.00 ± 0.54	7 ± 3.9	0.37 ± 0.21
2‡	94.06608	−21.3677	18 ± 6.1	8.3	12 ± 5.2	6.3 ± 4	−0.30 ± 0.35	16 ± 5.3	0.85 ± 0.28
3	94.06618	−21.3757	130 ± 13	45	89 ± 11	46 ± 8.2	−0.32 ± 0.10	120 ± 12	6.26 ± 0.62
4	94.06938	−21.3743	38 ± 7.7	15	28 ± 6.8	9.6 ± 4.5	−0.49 ± 0.20	33 ± 6.7	1.76 ± 0.36
5	94.07044	−21.3759	16 ± 5.5	6.4	8.3 ± 4.4	7.4 ± 4.2	−0.06 ± 0.38	14 ± 4.8	0.73 ± 0.25
6	94.07048	−21.3527	46 ± 8.4	19	33 ± 7.3	13 ± 5.1	−0.44 ± 0.18	40 ± 7.3	2.14 ± 0.39
7	94.07074	−21.3809	8.3 ± 4.4	3.5	5.3 ± 3.8	3.1 ± 3.3	−0.27 ± 0.60	7.3 ± 3.9	0.39 ± 0.21
8	94.07178	−21.3806	68 ± 9.9	26	42 ± 8	26 ± 6.6	−0.23 ± 0.15	59 ± 8.6	3.15 ± 0.46
9	94.07187	−21.3599	14 ± 5.2	6.1	9.9 ± 4.6	4.4 ± 3.5	−0.38 ± 0.39	12 ± 4.5	0.66 ± 0.24
10	94.07493	−21.3678	26 ± 6.7	10	17 ± 5.6	8.7 ± 4.5	−0.32 ± 0.28	22 ± 5.8	1.18 ± 0.31
11	94.07506	−21.3647	19 ± 5.8	8.6	9.6 ± 4.5	9.5 ± 4.5	−0.01 ± 0.33	17 ± 5	0.88 ± 0.27
12	94.07529	−21.3688	9.6 ± 4.5	4.1	6.4 ± 3.9	3.3 ± 3.2	−0.32 ± 0.52	8.3 ± 3.9	0.44 ± 0.21
13	94.07817	−21.3743	30 ± 7	13	19 ± 5.8	11 ± 4.7	−0.29 ± 0.24	26 ± 6	1.38 ± 0.32
14	94.08011	−21.3827	7.5 ± 4.1	3.4	0 ± 2	7.6 ± 4.1	1.00 ± 0.54	6.8 ± 3.7	0.36 ± 0.20
15	94.08054	−21.3642	9.4 ± 4.6	4	8.4 ± 4.4	0.96 ± 2.6	−0.80 ± 0.51	8.1 ± 4	0.43 ± 0.21
16	94.08268	−21.3629	8.7 ± 4.4	3.6	4.4 ± 3.5	4.3 ± 3.5	−0.02 ± 0.57	7.9 ± 4	0.42 ± 0.21
17	94.08540	−21.3697	23 ± 6.3	9.8	12 ± 4.8	12 ± 4.8	0.00 ± 0.30	21 ± 5.6	1.10 ± 0.30
18	94.08613	−21.3737	14 ± 5.2	5.2	13 ± 5.1	1.1 ± 2.6	−0.84 ± 0.35	12 ± 4.5	0.63 ± 0.24
19	94.08674	−21.3808	39 ± 7.7	17	26 ± 6.5	13 ± 5	−0.33 ± 0.20	33 ± 6.6	1.77 ± 0.35
20	94.08898	−21.3738	9.3 ± 4.6	3.6	6.2 ± 4	3.2 ± 3.2	−0.32 ± 0.54	8 ± 3.9	0.43 ± 0.21
21†	94.09175	−21.3727	86 ± 11	30	19 ± 6.4	68 ± 9.9	0.56 ± 0.13	74 ± 9.7	3.94 ± 0.52
22	94.09426	−21.3609	31 ± 7.1	13	22 ± 6.2	8.7 ± 4.4	−0.43 ± 0.24	26 ± 6.1	1.41 ± 0.32
23	94.09786	−21.3719	39 ± 7.7	15	22 ± 6.1	17 ± 5.5	−0.12 ± 0.21	34 ± 6.6	1.80 ± 0.35
24	94.09819	−21.3708	22 ± 6.3	8	12 ± 5	9.8 ± 4.7	−0.10 ± 0.31	19 ± 5.4	0.99 ± 0.29
25	94.09921	−21.3710	6.7 ± 4.1	3	6.9 ± 4.1	0 ± 2	−1.00 ± 0.59	5.8 ± 3.6	0.31 ± 0.19
26	94.10011	−21.3697	7.5 ± 4.2	3.2	2 ± 3	5.6 ± 3.8	0.48 ± 0.64	6.5 ± 3.7	0.35 ± 0.19
27	94.10093	−21.3627	18 ± 5.7	7.3	14 ± 5.1	4.2 ± 3.5	−0.53 ± 0.33	15 ± 4.9	0.82 ± 0.26
28	94.10493	−21.3749	13 ± 5.1	5.5	5.4 ± 3.8	7.5 ± 4.2	0.16 ± 0.44	11 ± 4.4	0.59 ± 0.23
29⋆	94.10608	−21.3733	10 ± 4.7	4.4	7.2 ± 4.1	3.3 ± 3.2	−0.38 ± 0.49	9 ± 4	0.48 ± 0.21
30	94.10819	−21.3771	15 ± 5.5	7.1	11 ± 4.8	4.4 ± 3.6	−0.42 ± 0.38	13 ± 4.7	0.70 ± 0.25
31	94.10869	−21.3750	7.4 ± 4.1	3.2	3 ± 3.2	4.4 ± 3.5	0.18 ± 0.64	6.4 ± 3.6	0.34 ± 0.19
32	94.10971	−21.3702	12 ± 5	6.1	8.3 ± 4.3	4.2 ± 3.5	−0.33 ± 0.44	11 ± 4.3	0.58 ± 0.23
33	94.11052	−21.3701	33 ± 7.2	13	19 ± 5.8	14 ± 5.1	−0.16 ± 0.23	29 ± 6.3	1.53 ± 0.33
34	94.11171	−21.3698	33 ± 7.4	13	19 ± 5.9	15 ± 5.3	−0.13 ± 0.24	29 ± 6.4	1.54 ± 0.34
35	94.11250	−21.3632	23 ± 6.2	11	15 ± 5.3	7.5 ± 4.2	−0.34 ± 0.29	20 ± 5.4	1.05 ± 0.29
36	94.11316	−21.3785	4.9 ± 3.8	2.3	1.6 ± 2.9	3.3 ± 3.2	0.34 ± 0.92	4.4 ± 3.3	0.23 ± 0.18
37	94.11416	−21.3713	6.3 ± 4	2.7	5.2 ± 3.7	1.1 ± 2.6	−0.65 ± 0.70	5.5 ± 3.5	0.30 ± 0.19
38	94.11693	−21.3798	6.5 ± 4	2.3	5.5 ± 3.8	0.96 ± 2.6	−0.71 ± 0.70	6.5 ± 4	0.35 ± 0.21
39	94.11809	−21.3609	34 ± 7.3	14	20 ± 5.9	14 ± 5.2	−0.17 ± 0.23	29 ± 6.4	1.56 ± 0.34
40	94.12501	−21.3791	28 ± 6.9	15	25 ± 6.6	3.2 ± 3.2	−0.78 ± 0.21	28 ± 6.8	1.50 ± 0.36

Notes. Column 1: source number; Column 2: right ascension (RA); Column 3: declination (Dec); Column 4: net counts in broad (0.5–8 keV) band. The uncertainty expressed here takes into account the fluctuations in the source as well as in the background; Column 5: broadband source detection significance from wavdetect. This computes how unlikely it is for the background in the customized PSF region to fluctuate to yield the detected number of counts. Note that the PSF region is optimized differently in wavdetect than in the calculation of column (4) and is typically larger than in the latter; Columns 6 and 7: net counts in soft (0.5–2 keV) and hard (2–8 keV) bands, respectively. Uncertainties in net counts are quoted to 1 σ; Column 8: hardness ratio computed with Equation (1). Uncertainties were obtained by applying error propagation to the uncertainties in the net counts; Column 9: X-ray luminosity in the 0.5–8 keV band; Column 10: X-ray flux in the 0.5–8 keV band.† Central active galactic nucleus.‡ Extended soft X-ray source.^⋆ SN 2003H (31 match).

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Table 9. NGC 2207/IC2163: X-Ray Source Properties for ObsID 14799

Source	αJ2000	δJ2000	0.5–8 keV	Signif	0.5–2 keV	2–8 keV	HR	LX	FX
	(deg)	(deg)	(counts)	(σ)	(counts)	(counts)		($10^{38}\,\rm {erg}\,\rm {s}^{-1}$)	($10^{-14}\,\rm {erg}\,\rm {cm}^{-2}\,\rm {s}^{-1}$)
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
1	94.05536	−21.3669	6.5 ± 4	3	4.4 ± 3.5	2.1 ± 3	−0.36 ± 0.71	11 ± 6.9	0.60 ± 0.37
2	94.06615	−21.3757	31 ± 7	14	22 ± 6.1	8.8 ± 4.4	−0.43 ± 0.23	54 ± 12	2.87 ± 0.66
3‡	94.06619	−21.3676	8 ± 4.4	3.7	4.7 ± 3.8	3.3 ± 3.2	−0.17 ± 0.62	15 ± 8.3	0.81 ± 0.44
4	94.06937	−21.3742	12 ± 4.9	5.1	5.5 ± 3.7	6.2 ± 4	0.06 ± 0.47	20 ± 8.5	1.08 ± 0.45
5	94.07055	−21.3527	33 ± 7.3	15	24 ± 6.4	8.7 ± 4.4	−0.47 ± 0.22	59 ± 13	3.12 ± 0.69
6	94.07061	−21.3598	5.1 ± 3.8	2.4	4.4 ± 3.5	0.66 ± 2.6	−0.74 ± 0.91	8.8 ± 6.5	0.47 ± 0.35
7	94.07176	−21.3806	53 ± 8.8	22	31 ± 7.1	22 ± 6.1	−0.18 ± 0.18	92 ± 15	4.89 ± 0.82
8	94.07309	−21.3786	8.2 ± 4.4	3.9	7.4 ± 4.2	0.82 ± 2.6	−0.80 ± 0.57	14 ± 7.6	0.76 ± 0.40
9	94.07486	−21.3679	13 ± 5	5.7	9.6 ± 4.5	3.3 ± 3.2	−0.49 ± 0.41	23 ± 9	1.24 ± 0.48
10	94.07502	−21.3647	12 ± 4.8	5.4	7.3 ± 4.1	4.2 ± 3.5	−0.27 ± 0.46	20 ± 8.4	1.06 ± 0.45
11	94.07530	−21.3686	11 ± 4.7	4.9	7.6 ± 4.1	3.3 ± 3.2	−0.40 ± 0.47	20 ± 8.5	1.05 ± 0.45
12	94.07536	−21.3708	12 ± 4.9	5.9	5.4 ± 3.8	6.6 ± 4	0.09 ± 0.46	21 ± 8.5	1.10 ± 0.45
13⋆	94.07653	−21.3758	36 ± 7.7	18	5.4 ± 3.8	31 ± 7.2	0.70 ± 0.19	63 ± 13	3.34 ± 0.71
14	94.07819	−21.3743	22 ± 6.2	11	12 ± 4.8	11 ± 4.7	−0.04 ± 0.30	39 ± 11	2.05 ± 0.57
15	94.08047	−21.3642	9.9 ± 4.7	5.3	8.7 ± 4.5	1.1 ± 2.6	−0.77 ± 0.48	17 ± 8.1	0.91 ± 0.43
16	94.08548	−21.3697	8.8 ± 4.4	4.4	6.7 ± 4	2.1 ± 3	−0.52 ± 0.56	16 ± 8	0.85 ± 0.43
17	94.08617	−21.3776	4.2 ± 3.5	2	1.1 ± 2.5	3.1 ± 3.2	0.48 ± 0.98	7.3 ± 6	0.39 ± 0.32
18	94.08675	−21.3809	13 ± 5.2	6.5	5.6 ± 3.8	7.8 ± 4.3	0.16 ± 0.43	23 ± 8.9	1.24 ± 0.48
19	94.08907	−21.3738	4.2 ± 3.5	1.9	2 ± 2.9	2.2 ± 2.9	0.03 ± 0.98	7.2 ± 6	0.39 ± 0.32
20†	94.09177	−21.3728	39 ± 8	16	8.9 ± 4.6	30 ± 7.1	0.55 ± 0.20	70 ± 14	3.74 ± 0.76
21	94.09791	−21.3719	28 ± 6.8	11	15 ± 5.4	12 ± 4.9	−0.12 ± 0.26	50 ± 12	2.67 ± 0.65
22	94.10066	−21.3865	12 ± 4.9	5.5	6.6 ± 3.9	5.5 ± 3.7	−0.09 ± 0.45	21 ± 8.4	1.11 ± 0.45
23	94.10100	−21.3627	6.4 ± 4	2.6	4.3 ± 3.5	2.1 ± 3	−0.35 ± 0.72	11 ± 7	0.60 ± 0.37
24	94.10381	−21.3639	4 ± 3.6	1.7	0 ± 2.1	4.5 ± 3.6	1.00 ± 0.94	7 ± 6.3	0.37 ± 0.34
25	94.10484	−21.3748	8.9 ± 4.6	5.5	6.1 ± 4	2.9 ± 3.3	−0.35 ± 0.57	17 ± 8.8	0.93 ± 0.47
26	94.10820	−21.3771	7.7 ± 4.3	3.4	5.5 ± 3.9	2.1 ± 3	−0.44 ± 0.63	13 ± 7.4	0.71 ± 0.40
27	94.11050	−21.3702	15 ± 5.4	6.7	9.2 ± 4.5	6.2 ± 3.9	−0.19 ± 0.38	27 ± 9.4	1.44 ± 0.50
28	94.11194	−21.3698	10 ± 4.7	5.3	6.2 ± 3.9	4.2 ± 3.5	−0.19 ± 0.50	18 ± 8.1	0.97 ± 0.43
29	94.11255	−21.3631	5.4 ± 3.7	2.4	3.3 ± 3.2	2.2 ± 2.9	−0.20 ± 0.80	9.5 ± 6.5	0.51 ± 0.35
30	94.11814	−21.3609	8.8 ± 4.4	3.7	5.5 ± 3.7	3.3 ± 3.2	−0.25 ± 0.56	15 ± 7.7	0.83 ± 0.41
31	94.12123	−21.3808	3.9 ± 3.6	2.3	4.2 ± 3.6	0 ± 2.1	−1.00 ± 1.01	6.7 ± 6.2	0.36 ± 0.33
32	94.12492	−21.3788	15 ± 5.6	9.5	9.1 ± 4.7	6.3 ± 4.1	−0.18 ± 0.40	28 ± 10	1.52 ± 0.55

Notes. Column 1: source number; Column 2: right ascension (RA); Column 3: declination (Dec); Column 4: net counts in broad (0.5–8 keV) band. The uncertainty expressed here takes into account the fluctuations in the source as well as in the background; Column 5: broadband source detection significance from wavdetect. This computes how unlikely it is for the background in the customized PSF region to fluctuate to yield the detected number of counts. Note that the PSF region is optimized differently in wavdetect than in the calculation of column (4) and is typically larger than in the latter; Columns 6 and 7: net counts in soft (0.5–2 keV) and hard (2–8 keV) bands, respectively. Uncertainties in net counts are quoted to 1 σ; Column 8: hardness ratio computed with Equation (1). Uncertainties were obtained by applying error propagation to the uncertainties in the net counts; Column 9: X-ray luminosity in the 0.5–8 keV band; Column 10: X-ray flux in the 0.5–8 keV band.† Central active galactic nucleus. ‡ Extended soft X-ray source. ^⋆ SN 2013ai (023 match).

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Table 10. NGC 2207/IC2163: X-Ray Source Properties for ObsID 14915

Source	αJ2000	δJ2000	0.5–8 keV	Signif	0.5–2 keV	2–8 keV	HR	LX	FX
	(deg)	(deg)	(counts)	(σ)	(counts)	(counts)		($10^{38}\,\rm {erg}\,\rm {s}^{-1}$)	($10^{-14}\,\rm {erg}\,\rm {cm}^{-2}\,\rm {s}^{-1}$)
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
1	94.05543	−21.3669	5.6 ± 3.8	2.3	4.5 ± 3.6	1.1 ± 2.6	−0.60 ± 0.79	4.9 ± 3.3	0.26 ± 0.18
2‡	94.06613	−21.3675	20 ± 6.3	11	10 ± 5	9.8 ± 4.6	−0.01 ± 0.34	17 ± 5.4	0.91 ± 0.29
3	94.06623	−21.3757	39 ± 7.9	16	27 ± 6.7	12 ± 5	−0.38 ± 0.21	33 ± 6.7	1.78 ± 0.36
4	94.06792	−21.3725	12 ± 5	4.5	7.7 ± 4.2	4.2 ± 3.6	−0.29 ± 0.46	10 ± 4.3	0.54 ± 0.23
5	94.06939	−21.3743	23 ± 6.3	8	13 ± 5.1	10 ± 4.6	−0.11 ± 0.30	19 ± 5.4	1.03 ± 0.29
6	94.06973	−21.3766	9.3 ± 4.6	4	8.4 ± 4.4	0.95 ± 2.6	−0.80 ± 0.51	8 ± 3.9	0.42 ± 0.21
7	94.07043	−21.3757	6.2 ± 4	2.7	3 ± 3.3	3.2 ± 3.3	0.02 ± 0.74	5.3 ± 3.4	0.28 ± 0.18
8	94.07057	−21.3527	13 ± 5.2	6	3.2 ± 3.3	10 ± 4.6	0.52 ± 0.41	23 ± 9	1.24 ± 0.48
9	94.07180	−21.3807	41 ± 8	17	29 ± 6.9	12 ± 4.9	−0.43 ± 0.20	35 ± 6.9	1.89 ± 0.37
10	94.07292	−21.3790	5.8 ± 4	2.6	3.7 ± 3.5	2.1 ± 2.9	−0.29 ± 0.79	5 ± 3.4	0.26 ± 0.18
11	94.07498	−21.3678	15 ± 5.3	6.5	11 ± 4.7	4.2 ± 3.5	−0.44 ± 0.38	13 ± 4.5	0.69 ± 0.24
12	94.07507	−21.3646	23 ± 6.4	10	16 ± 5.6	6.7 ± 4.1	−0.41 ± 0.29	20 ± 5.5	1.06 ± 0.29
13	94.07534	−21.3686	14 ± 5.2	6.1	10 ± 4.7	3.1 ± 3.2	−0.54 ± 0.40	12 ± 4.4	0.62 ± 0.24
14	94.07819	−21.3743	54 ± 8.9	23	37 ± 7.5	17 ± 5.6	−0.37 ± 0.17	46 ± 7.6	2.47 ± 0.41
15	94.08049	−21.3642	14 ± 5.3	7.2	10 ± 4.7	4.3 ± 3.5	−0.40 ± 0.39	12 ± 4.5	0.66 ± 0.24
16	94.08225	−21.3784	6.3 ± 4.1	3.2	1.8 ± 3	4.5 ± 3.6	0.42 ± 0.75	5.4 ± 3.5	0.29 ± 0.19
17	94.08265	−21.3628	11 ± 4.8	5.7	8.8 ± 4.5	1.7 ± 2.9	−0.67 ± 0.48	9 ± 4.2	0.48 ± 0.22
18	94.08532	−21.3740	13 ± 5.1	5	3.2 ± 3.3	9.7 ± 4.6	0.50 ± 0.42	11 ± 4.4	0.59 ± 0.24
19	94.08548	−21.3697	12 ± 5	5.3	8.9 ± 4.5	3.2 ± 3.3	−0.46 ± 0.45	10 ± 4.3	0.55 ± 0.23
20	94.08675	−21.3808	25 ± 6.4	11	21 ± 5.9	4.1 ± 3.5	−0.67 ± 0.25	21 ± 5.5	1.13 ± 0.29
21	94.08708	−21.3711	8.6 ± 4.5	4.2	5.5 ± 3.8	3.1 ± 3.3	−0.28 ± 0.59	7.4 ± 3.8	0.39 ± 0.20
22†	94.09180	−21.3727	160 ± 15	51	13 ± 5.6	150 ± 14	0.83 ± 0.07	140 ± 13	7.31 ± 0.68
23	94.09431	−21.3608	8.7 ± 4.4	4.7	6.4 ± 4	2.2 ± 3	−0.49 ± 0.56	7.5 ± 3.9	0.40 ± 0.21
24	94.09820	−21.3709	22 ± 6.2	9.1	14 ± 5.1	8.1 ± 4.3	−0.25 ± 0.30	19 ± 5.3	1.00 ± 0.28
25	94.10011	−21.3696	7.6 ± 4.2	3.1	3.1 ± 3.3	4.5 ± 3.6	0.18 ± 0.65	6.6 ± 3.7	0.35 ± 0.20
26	94.10064	−21.3818	7.1 ± 4.1	3.4	2.8 ± 3.2	4.2 ± 3.5	0.19 ± 0.67	6.5 ± 3.8	0.35 ± 0.20
27	94.10071	−21.3865	22 ± 6.2	9.7	20 ± 6	2.2 ± 3	−0.80 ± 0.25	19 ± 5.4	1.02 ± 0.29
28	94.10097	−21.3627	10 ± 4.7	4.4	9.1 ± 4.5	1.1 ± 2.5	−0.79 ± 0.45	8.9 ± 4.1	0.47 ± 0.22
29	94.10385	−21.3640	15 ± 5.2	6.4	9.6 ± 4.5	5.3 ± 3.7	−0.29 ± 0.39	13 ± 4.6	0.69 ± 0.24
30	94.10485	−21.3748	25 ± 6.5	10	14 ± 5.2	11 ± 4.8	−0.12 ± 0.28	21 ± 5.6	1.14 ± 0.30
31	94.10568	−21.3770	5.4 ± 3.7	2.2	5.4 ± 3.7	0 ± 2	−1.00 ± 0.75	4.9 ± 3.3	0.26 ± 0.18
32	94.10635	−21.3703	7.3 ± 4.1	3.5	0.8 ± 2.5	6.5 ± 3.9	0.78 ± 0.63	6.3 ± 3.5	0.34 ± 0.19
33	94.10823	−21.3770	6.2 ± 4.1	2.9	4.1 ± 3.6	2.1 ± 3	−0.32 ± 0.75	5.6 ± 3.6	0.30 ± 0.19
34	94.11048	−21.3701	70 ± 9.9	29	45 ± 8.2	25 ± 6.4	−0.29 ± 0.15	60 ± 8.6	3.20 ± 0.46
35	94.11177	−21.3698	35 ± 7.6	14	25 ± 6.6	10 ± 4.6	−0.43 ± 0.22	30 ± 6.5	1.60 ± 0.35
36	94.11363	−21.3738	4.2 ± 3.6	2.1	3.1 ± 3.3	1.1 ± 2.6	−0.47 ± 1.00	3.8 ± 3.2	0.20 ± 0.17
37	94.11431	−21.3713	9.1 ± 4.5	4	7.3 ± 4.1	1.8 ± 2.9	−0.61 ± 0.55	7.8 ± 3.9	0.42 ± 0.21
38	94.11816	−21.3609	23 ± 6.2	9.5	18 ± 5.6	5.2 ± 3.8	−0.54 ± 0.28	20 ± 5.4	1.06 ± 0.29
39	94.12033	−21.3768	7 ± 4.2	3.2	6.2 ± 4	0.83 ± 2.6	−0.76 ± 0.66	6.1 ± 3.6	0.32 ± 0.19
40	94.12192	−21.3680	19 ± 5.8	8.7	18 ± 5.7	0.95 ± 2.5	−0.90 ± 0.25	17 ± 5	0.88 ± 0.27
41	94.12480	−21.3786	6.5 ± 4	2.5	4.3 ± 3.5	2.2 ± 2.9	−0.32 ± 0.70	5.7 ± 3.5	0.30 ± 0.19

Notes. Column 1: source number; Column 2: right ascension (RA); Column 3: declination (Dec); Column 4: net counts in broad (0.5–8 keV) band. The uncertainty expressed here takes into account the fluctuations in the source as well as in the background; Column 5: broadband source detection significance from wavdetect. This computes how unlikely it is for the background in the customized PSF region to fluctuate to yield the detected number of counts. Note that the PSF region is optimized differently in wavdetect than in the calculation of column (4) and is typically larger than in the latter; Columns 6 and 7: net counts in soft (0.5–2 keV) and hard (2–8 keV) bands, respectively. Uncertainties in net counts are quoted to 1 σ; Column 8: hardness ratio computed with Equation (1). Uncertainties were obtained by applying error propagation to the uncertainties in the net counts; Column 9: X-ray luminosity in the 0.5–8 keV band; Column 10: X-ray flux in the 0.5–8 keV band. † Central active galactic nucleus. ‡ Extended soft X-ray source.

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Table 11. NGC 2207/IC2163: X-Ray Source Properties for the Combined Image

Source	αJ2000	δJ2000	0.5–8 keV	Signif	0.5–2 keV	2–8 keV	HR	LX	FX
	(deg)	(deg)	(counts)	(σ)	(counts)	(counts)		($10^{38}\,\rm {erg}\,\rm {s}^{-1}$)	($10^{-14}\,\rm {erg}\,\rm {cm}^{-2}\,\rm {s}^{-1}$)
(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)
1	94.05537	−21.3669	22 ± 6.3	8.8	18 ± 5.7	4 ± 3.6	−0.64 ± 0.29	6 ± 1.7	0.32 ± 0.09
2	94.06618	−21.3757	300 ± 19	82	200 ± 16	95 ± 11	−0.36 ± 0.06	81 ± 5.3	4.32 ± 0.28
3‡	94.06618	−21.3675	67 ± 10	19	43 ± 8.8	24 ± 6.4	−0.29 ± 0.15	19 ± 2.9	0.99 ± 0.15
4	94.06674	−21.3581	6.9 ± 4.4	3.3	6 ± 4	0.93 ± 3	−0.73 ± 0.76	2 ± 1.2	0.10 ± 0.07
5	94.06788	−21.3726	17 ± 6.1	5.4	11 ± 5.2	5.6 ± 4.1	−0.33 ± 0.39	4.6 ± 1.7	0.25 ± 0.09
6*	94.06856	−21.3692	11 ± 5.4	3.4	5.3 ± 4.6	5.2 ± 3.9	−0.01 ± 0.57	2.9 ± 1.5	0.15 ± 0.08
7	94.06937	−21.3743	95 ± 12	26	67 ± 10	28 ± 7	−0.41 ± 0.12	26 ± 3.2	1.40 ± 0.17
8	94.06969	−21.3766	14 ± 5.5	5	13 ± 5.2	1.3 ± 3.1	−0.81 ± 0.41	3.8 ± 1.5	0.20 ± 0.08
9	94.07010	−21.3726	18 ± 6.2	6.2	8.2 ± 4.7	10 ± 4.8	0.11 ± 0.37	5.1 ± 1.7	0.27 ± 0.09
10	94.07048	−21.3758	47 ± 8.7	14	26 ± 6.7	22 ± 6.3	−0.09 ± 0.19	13 ± 2.4	0.70 ± 0.13
11	94.07050	−21.3527	120 ± 13	45	80 ± 11	40 ± 8.1	−0.33 ± 0.11	41 ± 4.3	2.19 ± 0.23
12	94.07052	−21.3598	13 ± 5.3	5	8.5 ± 4.5	4.1 ± 3.9	−0.35 ± 0.47	3.5 ± 1.5	0.18 ± 0.08
13	94.07070	−21.3809	21 ± 6.5	7.3	10 ± 4.9	11 ± 5.1	0.02 ± 0.34	5.8 ± 1.8	0.31 ± 0.09
14	94.07177	−21.3807	210 ± 17	54	140 ± 14	74 ± 10	−0.30 ± 0.08	58 ± 4.6	3.09 ± 0.24
15	94.07189	−21.3599	41 ± 8.1	16	24 ± 6.5	17 ± 5.7	−0.17 ± 0.21	11 ± 2.2	0.61 ± 0.12
16	94.07299	−21.3724	7.9 ± 4.6	2.7	5.3 ± 4	2.6 ± 3.3	−0.35 ± 0.65	2.2 ± 1.3	0.12 ± 0.07
17	94.07301	−21.3787	23 ± 6.8	8.1	20 ± 6.3	3.3 ± 3.6	−0.72 ± 0.28	6.4 ± 1.9	0.34 ± 0.10
18	94.07493	−21.3678	88 ± 11	27	65 ± 9.8	24 ± 6.5	−0.47 ± 0.12	24 ± 3.1	1.31 ± 0.17
19	94.07506	−21.3647	75 ± 10	23	45 ± 8.4	30 ± 7	−0.19 ± 0.14	21 ± 2.9	1.11 ± 0.15
20	94.07530	−21.3687	41 ± 8.1	14	31 ± 7.2	10 ± 4.8	−0.49 ± 0.19	12 ± 2.3	0.61 ± 0.12
21	94.07536	−21.3708	20 ± 6.1	7.2	12 ± 5.1	7.9 ± 4.4	−0.21 ± 0.33	5.6 ± 1.7	0.30 ± 0.09
22◊	94.07649	−21.3758	41 ± 8.1	13	6.4 ± 4.2	35 ± 7.4	0.69 ± 0.18	11 ± 2.2	0.60 ± 0.12
23	94.07707	−21.3632	6.5 ± 4.2	2.8	3.9 ± 3.5	2.6 ± 3.3	−0.19 ± 0.75	1.8 ± 1.2	0.10 ± 0.06
24	94.07818	−21.3743	130 ± 13	42	85 ± 11	49 ± 8.6	−0.27 ± 0.10	37 ± 3.7	1.95 ± 0.20
25	94.08016	−21.3827	15 ± 5.5	6.4	0 ± 2.1	15 ± 5.5	1.00 ± 0.27	4.2 ± 1.5	0.22 ± 0.08
26	94.08054	−21.3642	49 ± 8.8	17	40 ± 8.1	8.7 ± 4.5	−0.64 ± 0.16	13 ± 2.4	0.71 ± 0.13
27	94.08208	−21.3620	14 ± 5.5	5.8	9.3 ± 4.6	4.8 ± 3.8	−0.32 ± 0.42	4 ± 1.5	0.21 ± 0.08
28	94.08222	−21.3784	15 ± 5.6	6.3	2.3 ± 3.4	13 ± 5.2	0.70 ± 0.38	4.3 ± 1.6	0.23 ± 0.08
29	94.08267	−21.3629	22 ± 6.4	9	16 ± 5.7	6.2 ± 4	−0.45 ± 0.29	6.2 ± 1.8	0.33 ± 0.10
30	94.08410	−21.3736	11 ± 4.9	3.5	5.9 ± 4	4.7 ± 3.8	−0.12 ± 0.52	2.9 ± 1.4	0.16 ± 0.07
31	94.08431	−21.3851	20 ± 6.2	8.4	13 ± 5.2	7.3 ± 4.3	−0.28 ± 0.33	5.6 ± 1.7	0.30 ± 0.09
32	94.08528	−21.3741	24 ± 6.7	7.4	9.7 ± 4.8	15 ± 5.4	0.21 ± 0.29	6.7 ± 1.8	0.36 ± 0.10
33	94.08544	−21.3697	71 ± 10	23	46 ± 8.4	25 ± 6.5	−0.31 ± 0.14	20 ± 2.8	1.05 ± 0.15
34	94.08568	−21.3719	11 ± 5.1	4	5.2 ± 4	5.8 ± 4	0.05 ± 0.52	3 ± 1.4	0.16 ± 0.07
35	94.08611	−21.3737	24 ± 6.5	7.1	23 ± 6.4	0.63 ± 2.6	−0.95 ± 0.21	6.5 ± 1.8	0.34 ± 0.09
36	94.08616	−21.3776	9.6 ± 4.8	3.4	4.7 ± 3.8	4.9 ± 3.8	0.03 ± 0.56	2.6 ± 1.3	0.14 ± 0.07
37	94.08675	−21.3808	77 ± 10	28	52 ± 8.7	25 ± 6.5	−0.34 ± 0.14	21 ± 2.8	1.13 ± 0.15
38	94.08706	−21.3712	21 ± 6.3	7.6	15 ± 5.6	6 ± 4	−0.43 ± 0.31	5.8 ± 1.8	0.31 ± 0.09
39	94.08906	−21.3738	18 ± 6	6.1	11 ± 4.9	7.5 ± 4.4	−0.19 ± 0.35	5 ± 1.6	0.27 ± 0.09
40	94.09023	−21.3654	8.9 ± 4.8	3.7	2.2 ± 3.3	6.7 ± 4.2	0.51 ± 0.60	2.5 ± 1.3	0.13 ± 0.07
41†	94.09177	−21.3727	350 ± 22	74	46 ± 9.4	300 ± 20	0.74 ± 0.05	97 ± 6	5.15 ± 0.32
42	94.09429	−21.3608	70 ± 10	24	50 ± 8.8	20 ± 6.1	−0.44 ± 0.14	19 ± 2.8	1.03 ± 0.15
43	94.09447	−21.3775	15 ± 5.5	5.7	10 ± 4.7	4.5 ± 3.7	−0.39 ± 0.41	4 ± 1.5	0.21 ± 0.08
44	94.09474	−21.3863	10 ± 4.8	4.5	5.9 ± 4	4.1 ± 3.5	−0.18 ± 0.53	2.8 ± 1.3	0.15 ± 0.07
45	94.09764	−21.3541	9.6 ± 4.6	4.1	3.3 ± 3.3	6.3 ± 4	0.30 ± 0.53	2.7 ± 1.3	0.14 ± 0.07
46	94.09788	−21.3719	69 ± 10	20	37 ± 7.7	32 ± 7.1	−0.07 ± 0.15	19 ± 2.8	1.02 ± 0.15
47	94.09820	−21.3708	51 ± 9	15	30 ± 7.3	21 ± 6.1	−0.19 ± 0.18	14 ± 2.5	0.76 ± 0.13
48	94.09843	−21.3665	15 ± 5.5	5.4	13 ± 5.2	1.5 ± 2.9	−0.80 ± 0.37	4.1 ± 1.5	0.22 ± 0.08
49	94.10012	−21.3697	17 ± 5.8	5.9	5.7 ± 4.1	11 ± 4.8	0.31 ± 0.38	4.7 ± 1.6	0.25 ± 0.09
50	94.10064	−21.3819	8.3 ± 4.6	3.6	3.2 ± 3.5	5.1 ± 3.7	0.23 ± 0.63	2.3 ± 1.3	0.12 ± 0.07
51	94.10071	−21.3865	41 ± 8	16	33 ± 7.3	7.3 ± 4.2	−0.64 ± 0.18	12 ± 2.3	0.62 ± 0.12
52	94.10097	−21.3627	44 ± 8.3	15	32 ± 7.3	12 ± 4.8	−0.47 ± 0.18	12 ± 2.3	0.65 ± 0.12
53	94.10377	−21.3641	26 ± 6.6	10	12 ± 5	14 ± 5.1	0.08 ± 0.28	7.1 ± 1.8	0.38 ± 0.10
54	94.10483	−21.3749	56 ± 9.4	19	34 ± 7.6	22 ± 6.4	−0.23 ± 0.17	16 ± 2.6	0.84 ± 0.14
55	94.10548	−21.3720	7.3 ± 4.3	3	5.4 ± 3.9	1.9 ± 2.9	−0.48 ± 0.65	2.1 ± 1.2	0.11 ± 0.06
56⋆	94.10608	−21.3733	24 ± 6.4	7.7	18 ± 5.8	5.3 ± 3.7	−0.55 ± 0.27	6.6 ± 1.8	0.35 ± 0.10
57	94.10637	−21.3704	20 ± 5.9	7	2.6 ± 3.2	17 ± 5.5	0.73 ± 0.30	5.5 ± 1.7	0.29 ± 0.09
58	94.10726	−21.3758	9 ± 4.7	3.6	4.5 ± 3.7	4.5 ± 3.7	−0.00 ± 0.59	2.6 ± 1.4	0.14 ± 0.07
59	94.10817	−21.3771	43 ± 8.1	15	27 ± 6.7	16 ± 5.4	−0.27 ± 0.20	12 ± 2.3	0.66 ± 0.12
60	94.10868	−21.3750	13 ± 5.1	4.9	7 ± 4.1	6.1 ± 3.9	−0.07 ± 0.43	3.9 ± 1.5	0.21 ± 0.08
61	94.10971	−21.3701	16 ± 5.8	6.9	12 ± 5.2	3.6 ± 3.7	−0.55 ± 0.38	4.5 ± 1.6	0.24 ± 0.09
62	94.11047	−21.3701	130 ± 13	40	83 ± 11	46 ± 8.3	−0.28 ± 0.10	36 ± 3.7	1.93 ± 0.20
63	94.11082	−21.3769	9.1 ± 4.7	4	7.4 ± 4.3	1.7 ± 2.9	−0.62 ± 0.55	2.7 ± 1.4	0.14 ± 0.07
64	94.11177	−21.3698	89 ± 11	27	60 ± 9.4	29 ± 6.8	−0.35 ± 0.12	26 ± 3.2	1.36 ± 0.17
65	94.11218	−21.3762	6.1 ± 4.1	2.4	0 ± 2.1	6.4 ± 4.1	1.00 ± 0.66	1.9 ± 1.2	0.10 ± 0.07
66	94.11249	−21.3632	31 ± 7.2	12	21 ± 6.2	9.8 ± 4.7	−0.37 ± 0.24	8.7 ± 2	0.46 ± 0.11
67	94.11423	−21.3713	22 ± 6.3	8	18 ± 5.8	4 ± 3.6	−0.64 ± 0.28	6.5 ± 1.8	0.35 ± 0.10
68	94.11599	−21.3825	9.5 ± 4.8	4	7.9 ± 4.4	1.6 ± 3	−0.65 ± 0.54	2.7 ± 1.4	0.14 ± 0.07
69	94.11693	−21.3759	8 ± 4.5	3.1	6 ± 4.2	2 ± 2.9	−0.49 ± 0.60	2.3 ± 1.3	0.12 ± 0.07
70	94.11814	−21.3609	79 ± 11	29	53 ± 8.8	26 ± 6.6	−0.35 ± 0.13	23 ± 3	1.21 ± 0.16
71	94.12022	−21.3768	9.8 ± 4.7	3.9	8 ± 4.4	1.8 ± 2.9	−0.63 ± 0.51	3.4 ± 1.6	0.18 ± 0.09
72	94.12192	−21.3680	30 ± 7	13	29 ± 6.8	0.95 ± 2.6	−0.94 ± 0.17	10 ± 2.3	0.54 ± 0.12
73	94.12476	−21.3786	39 ± 7.9	14	25 ± 6.6	14 ± 5.2	−0.28 ± 0.21	12 ± 2.4	0.64 ± 0.13
74	94.12505	−21.3792	46 ± 8.5	16	41 ± 8	5.2 ± 3.8	−0.77 ± 0.15	13 ± 2.5	0.71 ± 0.13

Notes. Column 1: source number; Column 2: right ascension (RA); Column 3: declination (Dec); Column 4: net counts in broad (0.5–8 keV) band. The uncertainty expressed here takes into account the fluctuations in the source as well as in the background; Column 5: broadband source detection significance from wavdetect. This computes how unlikely it is for the background in the customized PSF region to fluctuate to yield the detected number of counts. Note that the PSF region is optimized differently in wavdetect than in the calculation of column (4) and is typically larger than in the latter; Columns 6 and 7: net counts in soft (0.5–2 keV) and hard (2–8 keV) bands, respectively. Uncertainties in net counts are quoted to 1 σ; Column 8: hardness ratio computed with Equation (1). Uncertainties were obtained by applying error propagation to the uncertainties in the net counts; Column 9: X-ray luminosity in the 0.5–8 keV band; Column 10: X-ray flux in the 0.5–8 keV band. † Central active galactic nucleus. ‡ Extended soft X-ray source. ^⋆ SN 2003H (31 match). ^◊ SN 2013ai (012 match). * SN 1999ec (42 match).

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