A CHANDRA STUDY OF THE INTERSTELLAR METALLICITY IN THE LARGE MAGELLANIC CLOUD USING SUPERNOVA REMNANTS

0453-68.5 is a faint, aged (>17,000 years) SNR showing a clear limb-brightened shell that is dominated by X-ray emission from the swept-up ISM (McEntaffer et al. 2012). 0453-68.5 contains a PWN at its center solidifying its classification for a CC explosion (Gaensler et al. 2003; McEntaffer et al. 2012). We selected six regions from around the outer swept-up shell, as shown in Figure 1(a), to estimate abundances for O, Ne, Mg, and Fe. Our abundance measurements from these six sub-regions are all consistent within statistical uncertainties, and thus we summarize the average abundances in Table 2. We were unable to constrain the Si and S abundances in 0453-68.5 because of the spectrally soft nature of the X-ray emission (photon statistics are poor at $E\gt \;1.5$ keV). McEntaffer et al. (2012) studied several regions partially overlapping with the regions used in this work. Their measured O abundance (O $=\quad 0.17\pm 0.02$) is significantly lower compared to Russell & Dopita (1992) value ($O=0.263$), but is consistent with H98's measurement within uncertainties (O = 0.21 ± 0.05). Our measured value for the O abundance (O = 0.08 ± 0.01) is lower than that by H98. We note that McEntaffer et al. (2012) allowed only the O abundance to vary in their spectral model fits while fixing all other abundances at the LMC values by Russell & Dopita (1992). Thus, their O abundance measurement might have been affected by those fixed abundance values for other elements.

DEM L71 shows a double-shock morphology consisting of an outer swept-up ISM shell surrounding a central bright region of reverse-shock heated ejecta (Hughes et al. 2003). The composition of the metal-rich ejecta shows enhanced Si and Fe abundances, with a lack of O, classifying it as a Type Ia SNR (Hughes et al. 2003). We measured the elemental abundances for O, Ne, Mg, Si, and Fe based on spectra extracted from seven regions along the outer swept-up shell (Figure 1(b)). The measured elemental abundances were consistent among all these seven sub-regions, and the averaged abundance values are shown in Table 2. Our measured elemental abundances are generally in agreement with those found in the inner rim region of the swept-up shell by Hughes et al. (2003).

N23 is an irregular shaped SNR that appears to originate from a CC explosion of a massive star (Hughes et al. 2006). N23 shows a compact object near its center. This compact source is probably the stellar remnant from the explosion of a massive star, although its association with the SNR is still under debate (Hayato et al. 2006, Hughes et al. 2006). Hughes et al. (2006) found that most of the emission from N23 shows LMC-like elemental abundances, and found only one region (located near the center of the remnant, west of the compact object) with enhanced elemental abundances (∼2–3 × LMC abundances). We selected four regions from all around the outer boundary of the SNR (Figure 1(c), avoiding the metal-rich region found by Hughes et al. 2006) and measured the elemental abundances for O, Ne, Mg, Si, and Fe. The measured elemental abundances are consistent among the four sub-regions. We present the average values for our measured abundances in Table 2. Our measured ISM abundance values are in good agreement with those in the east rim estimated by Hughes et al. (2006).

0519-69.0 has been classified as a Type Ia SNR by Hughes et al. (1995). They found that 0519-69.0 is an O-poor SNR with Fe-rich ejecta throughout its central regions. 0519-69.0 shows a clumpy morphology with a clear inner ejecta nebula surrounded by a thin shell of swept-up ISM (Kosenko et al. 2010). We selected four thin red regions (dominated by spectrally soft X-ray emission) along the outermost boundary of the SNR (Figure 1(d)) to measure the elemental abundances for O, Ne, Mg, Si, and Fe. The measured elemental abundances are consistent among all four sub-regions. The averaged abundances are shown in Table 2. Abundance measurements from this outer swept-up shell of 0519-69.0 have not been previously reported in the literature.

N49B shows Mg- and Si-rich ejecta near its center (Park et al. 2003a). N49B shows bright circumferential filaments that are emission from dense shocked ISM. We selected six regions along the outermost boundary (Figure 1(e)). We measured the elemental abundances for O, Ne, Mg, Si, and Fe. Our measured elemental abundances are consistent among all six sub-regions, and the average abundance values are shown in Table 2. Our measured abundances are in agreement with those measured in the bright southern swept-up shell by Park et al. (2003a).

N132D is the brightest SNR in the LMC and belongs to the rare class of O-rich SNR. Optical observations have shown high-velocity O-rich ejecta, indicating a CC origin for this SNR (Morse et al. 1995). N132D interacts with CO clouds located all around the SNR supporting its CC origin (Banas et al. 1997; Sano et al. 2015). N132D shows a complicated filamentary morphology in X-rays with moderately enhanced metal-rich ejecta in its central regions (Borkowski et al. 2007). We measured the elemental abundances of O, Ne, Mg, Si, and Fe based on spectra extracted from eight sub-regions from the outermost boundary (Figure 1(f)) and bright central filaments of the shocked ISM. The measured abundances are consistent among these eight sub-regions, and we present average abundance values in Table 2. We note that in their ejecta spectral analysis Borkowski et al. (2007) assumed 0.4 solar abundances for the underlying emission component originating from the swept-up medium.

A soft Gamma-ray repeater (SGR 0526-66), a rare class of highly magnetized neutron stars, is projected within the boundary of N49 (Cline et al. 1982; Rothschild et al. 1994), suggesting a CC origin for N49, although the physical association between SGR 0526-66 and N49 is unclear (Gaensler et al. 2001; Klose et al. 2004; Badenes et al. 2009; Park et al. 2012). N49 shows some regions of Si- and S-rich ejecta (including an ejecta "bullet" in the southwest boundary (Park et al. 2012)). N49 is also interacting with clumpy molecular clouds (Vancura et al. 1992) in the eastern and southeastern regions giving rise to the complicated spectral nature of X-ray emission in that area (Park et al. 2003b). Avoiding these ejecta features and regions where the shock is interacting with clumpy molecular clouds, we selected two regions from the outermost boundary in the southern rim of the SNR (Figure 1(g)). Our measured abundances are consistent between the two selected regions, and are also in agreement with those measured by Park et al. (2012). We measured the elemental abundances for O, Ne, Mg, Si, and Fe, and the average abundance values are shown in Table 2.

N206 is a mixed-morphology SNR (Williams et al. 2005) with a bright eastern band surrounded by a diffuse swept-up ISM shell. N206 is suggested to be a CC SNR based on observed ejecta elemental abundance structure (Williams et al. 2005). We selected two regions from the diffuse ISM shell (Figure 1(h)) and measured the O, Ne, Mg, Si, and Fe abundances. The elemental abundances were consistent between these two sub-regions. The average abundance values are shown in Table 2. We note that our measured abundances are lower (by a factor of approximately two for O, Ne, Mg, and Fe) than those estimated by Williams et al. (2005) while our Si value is similar to that estimated by Williams et al. (2005). This discrepancy appears to be due to metal-rich ejecta contamination of the ISM abundance measurements in Williams et al. (2005) from an ejecta feature in the western part of their ISM shell region.

0534-69.9 and 0548-70.4 show a bright central metal-rich ejecta nebula surrounded by a limb-brightened swept-up ISM shell (Hendrick et al. 2003). Based on low O/Fe abundance ratios in the central ejecta regions of both SNRs Hendrick et al. (2003) classified them as Type Ia explosions. In these SNRs, the swept-up ISM emission can be clearly separated from the central ejecta emission. We extracted X-ray spectra from two outer boundary regions of 0534-69.9 (Figure 1(i)) to measure the elemental abundances for O, Ne, Mg, Si and Fe. We selected four regions in the outermost shell for 0548-70.4 (Figure 1(p)) and measured the elemental abundances for O, Ne, Mg, and Fe. The measured elemental abundances are consistent in all sub-regions in each SNR. The averaged elemental abundances for these SNRs are shown in Table 2. There are no previous estimates of metal abundances in the swept-up shell in these SNRs in the literature.

DEM L238 is characterized by a bright central emission surrounded by limb-brightened swept-up ISM emission to the north, east, and west. The bright central emission shows an enhanced Fe abundance, suggesting that DEM L238 is a Type Ia SNR (Borkowski et al. 2006). We selected two regions from the outer shell to measure the elemental abundances for O, Ne, Mg, and Fe (Figure 1(j)). The measured elemental abundances from both regions are consistent with each other. The average elemental abundances are shown in Table 2. There are no previous measurements of the abundances in the swept-up shell in DEM L238.

N63A is the second brightest remnant in the LMC and is embedded in an H ii region with its position coincident with the OB association NGC 2030 (Warren et al. 2003). This suggests a massive Population I progenitor for N63A (Chu & Kennicutt 1988). N63A shows many interesting morphological features such as loops and plumes at the outermost boundary of the SNR along with many filamentary features throughout the SNR. The loop and plume features appear to show LMC-like abundances (Warren et al. 2003). However, we exclude these regions from our ISM study since their origin (shocked ISM vs. ejecta) is still unclear (Warren et al. 2003). Warren et al. (2003) showed that the column density toward N63A varies across the SNR. We selected 17 regions in N63A from all around the outermost boundary (Figure 1(k)). These regions show similar column densities to one another (as shown in Warren et al. 2003). Initially, we fitted these 17 sub-regional spectra with the column density varied. The measured column densities are all consistent within statistical uncertainties, and thus we repeated our spectral model fits for all of these sub-regions with ${N}_{{\rm{H}},\mathrm{LMC}}$ fixed at the mean value (${N}_{{\rm{H}},\mathrm{LMC}}$ = 1.1 × 10²¹ cm⁻²). We measured the elemental abundances for O, Ne, Mg, Si, and Fe (Table 2). Our elemental abundances are in good agreement with those found by Warren et al. (2003).

The Honeycomb nebula is a peculiar SNR. Its name is due to its optical morphology consisting of over 10 loops (with sizes of ∼2–3 pc) created by an SN blast wave interacting with sheets of dense but porous interstellar gas (Chu et al. 1995). Honeycomb's overall morphology is unlike most SNRs who show a coherent shell-like structure. We probed Honeycomb for possible metal-rich ejecta and found no evidence of enhanced elemental abundances. We identified an X-ray point source located within the southwestern limb as a CXOXASSIST source (X053543.01-691817.5). We found that the X-ray spectrum of this point source can be equally fitted by several different models because of the poor photon statistics (240 counts in the 0.5–7 keV band), e.g., a power law (photon index ${\rm{\Gamma }}\;\sim$ 1.9, ${N}_{{\rm{H}},\mathrm{LMC}}\;\sim$ 7 × 10²⁰ cm⁻² with ${\chi }_{\nu }^{2}$ = 0.7), a blackbody (kT ∼ 0.6 keV, ${N}_{{\rm{H}},\mathrm{LMC}}\;\sim$ 2 × 10²⁰ cm⁻² with ${\chi }_{\nu }^{2}$ = 1.4), and a plane shock (kT ∼ 8 keV, ${N}_{{\rm{H}},\mathrm{LMC}}\quad \sim$ 5 × 10²⁰ cm⁻² with ${\chi }_{\nu }^{2}$ = 1.1). We note that the best-fit electron temperature for the plane shock model is extremely high and it is unlikely adequate for this point source. This suggests either a power-law or blackbody model would be more appropriate to describe the observed X-ray spectrum of this source. The best-fit column for the blackbody model (${N}_{{\rm{H}},\mathrm{LMC}}$ ∼ 2 × 10²⁰ cm⁻²) favors this source's physical association with the Honeycomb nebula, while the column implied for the best-fit power-law model (${N}_{{\rm{H}},\mathrm{LMC}}$ ∼ 7 × 10²⁰ cm⁻²) is considerably larger than that for the Honeycomb nebula (${N}_{{\rm{H}},\mathrm{LMC}}$ ∼ 2 × 10²⁰ cm⁻²). However, the uncertainties on the column measurements for this point source in these models are large due to poor photon count statistics. Deeper observations are required to reveal the true nature of this X-ray point source. We extracted two spectra from Honeycomb to measure the ISM abundances (Figure 1(l)). One region from the entire northeastern limb and one region from the entire southwestern limb (excluding the X-ray point source). We measured the elemental abundances for O, Ne, Mg, and Fe (Table 2). There are no previous ISM abundance measurements for this SNR in the literature.

N157B contains one of the most energetic pulsars known (PSR J053747.39-691020.2). This pulsar is surrounded by a bright non-thermal X-ray nebula, which likely represents a toroidal pulsar wind terminal shock observed edge-on (Chen et al. 2006). While N157B shows metal-rich ejecta through its interior close to the pulsar, diffuse swept-up ISM emission extends significantly to the north (Chen et al. 2006). We used one region from the northeast to measure the elemental abundances of O, Ne, Mg, and Fe (Figure 1(m)). Our measured elemental abundances are shown in Table 2. There are no previous abundance measurements for the swept-up shell in this SNR.

0540-69.3 has been identified as one of a handful of O-rich SNR in the LMC, and it contains a bright pulsar and PWN, which have been throughly studied at many different wavelengths (e.g., Middleditch & Pennypacker 1985; Manchester et al. 1993a, 1993b; Gotthelf & Wang 2000; Kaaret et al. 2001; Petre et al. 2007). These O-rich ejecta and central pulsar solidify its CC origin. 0540-69.3 contains non-thermal emission in the form of "arcs" in the eastern and western boundaries of the SNR, which are suggestive of efficient cosmic-ray acceleration in the SNR shock (Park et al. 2010). Park et al. (2010) detected candidate metal-rich ejecta regions in the southern part of the SNR. We extracted spectra from two regions in the western boundary of the SNR avoiding these PWN, non-thermal arcs, and candidate ejecta regions (Figure 1(n)). We measured the elemental abundances for O, Ne, Mg, Si, and Fe. Our measured elemental abundances are consistent between both regions and the average values are shown in Table 2. Hwang et al. (2001) fixed elemental abundances at the LMC values by Russell & Dopita (1992) in their spectral model fit for their ambient medium region. However, their overall model fits are statistically poor (${\chi }_{\nu }^{2}=1.7\mbox{--}2.5$). They noted that their measurements might have been contaminated by metal-rich ejecta.

DEM L316B is a faint SNR that shows several central knots surrounded by diffuse shell-like emission. DEM L316B has been typed as a CC SNR based on the observed ejecta abundance ratios (Williams & Chu 2005). Avoiding metal-rich ejecta features throughout DEM L316B's center, we extracted the X-ray spectrum from a shell region surrounding the entire SNR (Figure 1(o)) to measure the ISM elemental abundances for O, Ne, Mg, Si, and Fe. Our swept-up shell region is similar to Region 12 in Williams & Chu (2005). We are unable to compare our elemental abundance values to those found by Williams & Chu (2005) since they did not exclusively provide the abundance values for Region 12. We note that Williams & Chu (2005) did find significantly under-abundant Fe (Fe $\sim \ 0.10\pm 0.03$) in some regions of DEM L316B, which is consistent with our measurement of the Fe abundances in this SNR.