The 3CR Chandra Snapshot Survey: Extragalactic Radio Sources with Redshifts between 1 and 1.5

Radio images were retrieved from the NVAS¹⁴ and the DRAGN¹⁵ web pages.

For the peculiar case of 3CR 297 (see Section 3), we also computed the radio spectral index map, using archival observations at 4.85 and 8.4 GHz. The radio spectral index, α_R, is a diagnostic tool to obtain useful information on the particle energy distribution of electrons emitting via synchrotron emission. Spatial variations in spectral index across the radio structure were used to distinguish different regions of 3CR 297, since they could indicate differences in the underlying physical processes occurring in cores and in jets: radio emission arising from compact cores typically has a flat radio spectrum (i.e., α_R ≲ 0.5), while that from extended structures is characterized by a steeper spectrum (see, e.g., Pauliny-Toth & Kellermann 1968).

Archival VLA data analyzed are from project IDs AE0059 (4.85 GHz) and AV0164 (8.4 GHz) and were obtained on 1988 October 30 and 1990 May 11, respectively. In both observing runs, the VLA configuration was A. We performed postcorrelation processing and imaging with the Common Astronomy Software Applications (CASA).

We first calibrated the images in amplitude and phase with the primary calibrators 1127–145 (4.85 GHz) and 1328+307 (8.4 GHz). We set the uv range for the computation of gain solutions in the latter case to a maximum of 400 kλ, following the specification on the VLA fringe calibrators.¹⁶ For the 4.85 GHz data, we flagged the antenna VA20 in the RR correlation in the bin time 14:28:55.0 to remove the bad response and achieve better gain solutions. Then, a self-calibration procedure was iteratively applied to improve the signal-to-noise ratio, using the CASA task gaincal + applycal. We obtained the model image through the clean task using the weighting parameter set on natural first, and uvtaper at the end, to model also the extended emission. During this procedure, we flagged additional deviating data (at 8.4 GHz the antenna VA22 for time less than 06:42:00 and VA13 between 07:00:0 and 07:01:15).

We computed the two final continuum maps with the same beam (098, 088, −35°, task clean using uvtaper), and we regridded both to the same cell size (task imregrid) to properly compute the spectral index image for pixels with values >3σ with the relation ${\alpha }_{R}=-\mathrm{log}({S}_{1}/{S}_{2})/\mathrm{log}({\nu }_{1}/{\nu }_{2})$, with ν₁ = 4.85 GHz, ν₂ = 8.4 GHz, and S_1,2 being the respective flux at the two frequencies (task immath). The resulting average rms of the 3CR 297 radio map was 0.4 ( 0.15) mJy beam⁻¹, and the peak flux was 0.510 (0.299) Jy at 4.85 (8.4) GHz. Typical uncertainties for flux density measurements in 3CR 297 radio maps are ∼10% for 4.85 GHz and ∼3% for 8.4 GHz.

The 3CR source sample observed during Cycle 17 is listed in Table 1, together with the basic parameters of each source (e.g., radio and optical classification, celestial coordinates, redshift, visual magnitude, and radio flux). Sources were observed for a nominal exposure time of 12 ks, but actual live times are given in Table 2. All Chandra observations were performed with the ACIS-S back-illuminated chip in VERY FAINT mode to provide high sensitivity and a low background level. The four chips turned on were I2, I3, S2, and S3 with the nominal aim point centered on S3.

Table 2.  Nuclear X-Ray Fluxes

3CR	Shifta	LivTimb	N2c	Ext. ratiod	f(Soft)e	f(Medium)e	f(Hard)e	f(Total)e	HRf	LXg
	('')	(ks)	(counts)		0.5–1 keV	1–2 keV	2–7 keV	0.5–7 keV		(1044 erg s−1)
27	–	11.91	61(8)	0.90(0.06)	–	2.3(1.0)	86(11)	88(11)	<0.94	0.09(0.01)
69	0.62	11.92	497(22)	0.94(0.01)	9(2)	74(5)	350(20)	433(21)	0.65(0.03)	3.4(0.2)
36	0.66	11.91	335(18)	0.94(0.01)	27(4)	65(5)	143(13)	235(15)	0.37(0.05)	24.3(1.6)
119	0.60	11.90	383(20)	0.97(0.01)	18(4)	64(5)	220(16)	303(17)	0.54(0.04)	17.2(0.9)
124	0.48	11.92	21(5)	0.49(0.09)	1.5(1.0)	0.5(0.5)	22(5)	24(5)	< 0.96	1.6(0.3)
173	1.10	12.40	124(11)	0.96(0.03)	11(3)	21(3)	43(7)	76(8)	0.34(0.09)	4.4(0.5)
194	0.36	11.91	125(11)	0.87(0.03)	0.8(0.8)	18(3)	89(10)	108(11)	0.67(0.05)	8.8(0.9)
208.1	0.73	11.91	228(15)	0.92(0.02)	25(4)	35(4)	102(11)	162(12)	0.48(0.06)	9.1(0.7)
220.2	0.44	11.92	684(26)	0.94(0.01)	78(7)	119(7)	273(18)	470(20)	0.39(0.04)	36.3(1.5)
222	0.68	11.91	3(2)	–	–	0.8(0.6)	1.5(1.5 )	2.3(1.6)	–	0.3(0.2)
230	–	12.35	25(5)	0.7(0.1)	–	2.4(1.0)	21(5)	24(5)	<0.80	3.5(0.7)
238	0.13	11.91	23(5)	0.5(0.1)	2.0(1.0)	3.5(1.2)	8(3)	14(3)	–	1.8(0.4)
255	0.73	11.92	5(2)	–	0.7(0.7)	1.0(0.6)	1.4(1.4)	3.1(1.7)	–	0.4(0.2)
297	0.54	12.34	17(4)	0.24(0.06)	1.3(0.9)	2.5(0.9)	8(3)	12(3)	–	1.5(0.4)
300.1	0.24	11.92	19(4)	0.42(0.09)	0.6(0.6)	0.8(0.6)	22(6)	23(6)	<0.93	1.8(0.5)
305.1	–	11.91	16(4)	0.7(0.2)	0.8(0.8)	–	20(5)	20(5)	<0.92*	1.5(0.4)

Notes. Fluxes are given in units of 10⁻¹⁵ erg cm⁻² s⁻¹. Values in parentheses are 1σ uncertainties.

^aAngular shift imposed on the X-ray image to align the nuclear X-ray position with that of the radio. The symbol "–" marks the sources without astrometric registration. ^bLivTim is the observation live time. ^cTotal number of counts within a circle of radius r = 2'' centered on the source position. The uncertainties given in parentheses are computed as $\sqrt{\mathrm{total} \mbox{-} \mathrm{number} \mbox{-} \mathrm{of} \mbox{-} \mathrm{counts}}$. ^d"Extent ratio": the ratio of the net counts (i.e., background subtracted) in the r = 2'' circle to the net counts in the r = 10'' circle. Values less than 0.5 indicate the presence of extended emission around the nuclear component of 3CR 124, 3CR 297, and 3CR 300.1. We did not report values of "extent ratio" for 3CR 222 and 3CR 255, having less than nine counts in the 2'' circle. A 1σ uncertainty is derived from the Poisson uncertainties on the number of counts in the circular region of radius 2'' and in the annular region between the radii 2'' and 10'', taking into account the covariance of the terms and the uncertainty on the number of background counts measured on the CCD. ^eFluxes extracted from the flux maps in the three energy bands (0.5–1, 1–2, 2–7 keV) and the total flux in the energy range 0.5–7 keV. The uncertainties are derived from the relative uncertainties on the number of counts in the source and background regions, added in quadrature. The symbol "–" indicates that no counts were detected in the photometric region in the respective energy range. ^fThe observed nuclear fluxes have been used to determine the hardness ratios (HR) with the relation (H − M)/(H + M), where H and M are the X-ray fluxes in the hard and medium bands, respectively. We computed the HRs for those sources having more than nine counts in the hard and medium bands, while we only estimated the upper bound of the HR value for those sources with nine or more counts in the hard band alone. The HR upper bound for the source 3CR 305.1, labeled with " * ," was computed with the soft X-ray flux S (i.e., (H − S)/(H + S)) due to the lack of counts detected in the medium band. The uncertainties have been derived from the X-ray flux uncertainties. ^gX-ray luminosity in the range 0.5–7 keV, computed using the values of D_L given in Table 1. A 1σ uncertainty is derived from the uncertainties on the flux.

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We performed the data reduction following the standard reduction procedure described in the Chandra Interactive Analysis of Observations (CIAO) threads,¹⁷ using CIAO version 4.9 and the Chandra Calibration Database (CALDB) version 4.7.3. Level 2 event files were generated using the acis_process_events task, and events were filtered for grades 0, 2, 3, 4, and 6 and corrected for bad pixels. Light curves for every data set were extracted and checked for high background intervals, but none were found.

Since the on-axis width of the point-spread function (PSF) of the Chandra telescope is smaller than the size of the ACIS pixels (0492), to recover the native angular resolution, it is necessary to avoid the undersampling. This is achieved by regridding all of the images to one-fourth of the native size to obtain a common pixel size of 0123.

When possible, we registered X-ray images, changing the appropriate keywords in the header of the event file so as to align the nuclear X-ray position with that of the radio. In most cases, the total angular shift (reported for each registered source in Table 2) was less than 1'', as occurred in all previous sources analyzed and collected in the XJET website¹⁸ (Massaro et al. 2011). The registration facilitates accurate searches for X-ray emission from within the extended radio structures by comparing radio and X-ray data with similar angular resolution. The distribution of the offsets in Table 2 is consistent with the level of uncertainty in the Chandra absolute astrometry (08 at 90% confidence, 14 at 99%).¹⁹ We have also confirmed by eye the identification of the X-ray, radio, and optical nuclei in all cases.

2.2.1. Flux Maps

2.2.2. X-Ray Spectral Analysis of the Stronger Nuclei

We performed X-ray spectral analysis for the nuclei containing 300 or more total counts to estimate their X-ray spectral indices, α_X. The 400-count threshold adopted in previous analysis (see, e.g., Massaro et al. 2018) was lowered for this higher redshift sample. We carried out the analysis on the level 2 event files using Xspec version 12.9 (Arnaud 1996) and CIAO 4.9 Sherpa version 1 (Freeman et al. 2001) software packages, with consistent results.

We extracted the spectral data from the same 2'' photometric aperture centered around the nuclei described above, using the ciao routine specextract, while the background spectra were extracted in nearby rectangular regions not containing obvious X-ray sources. Then, we binned the background-subtracted spectra to a 30 counts per bin minimum threshold to ensure the validity of the χ² statistics, and we selected the energy range 0.5–7 keV for the spectral fitting.

We fitted the photon fluxes of the four brightest sources with a simple multiplicative model, phabs×powerlaw in xspec syntax: a power law absorbed by the Galactic-equivalent hydrogen column density, N_H,Gal (Table 1, Kalberla et al. 2005). The normalization parameter and the X-ray spectral index, α_X, were allowed to vary. Results of the fit procedure are reported in Table 4.

Table 4.  Spectral Analysis of Bright Nuclei

3CR	N2a	αXb	χ2(dof)c
36	335(18)	0.7(0.1)	5.68(7)
69	492(22)	0.4(0.1)	21.38(13)
119	381(20)	0.5(0.1)	8.49(9)
220.2d	683(26)	0.60(0.07)	13.7(19)
220.2d	683(26)	${0.81}_{-0.25}^{+0.08}$	13.2(17)

Notes. These four nuclei have 300 or more total counts in the photometric aperture and are thus suitable for spectral analysis.

^aTotal number of counts within a circle of radius = 2''. The uncertainties given in parentheses are computed as $\sqrt{\mathrm{total} \mbox{-} \mathrm{number} \mbox{-} \mathrm{of} \mbox{-} \mathrm{counts}}$. ^bX-ray spectral index α_X with 1σ uncertainties derived from the fit in parentheses. ^cχ² of the fit procedure with degrees of freedom in parentheses. ^dFor the source 3CR 220.2, whose observation was found to be affected by a pileup fraction of ∼8.5%, we reported fit results obtained with the simple model of a power law with Galactic absorption (first row) and with the addition of the jdpileup model (second row).

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We also considered the possible presence of mild pileup in Chandra observations of the two nuclei with the highest number of counts in our sample: 3CR 69 and 3CR 220.2. Pileup occurs on X-ray CCDs for sources with high flux levels when two or more photons arrive within the same detector pixel within a single CCD frame integration time, and they are counted as a single photon of higher energy.

We first produced pileup maps with the CIAO pileup_map tool, which indicated a pileup fraction of ∼5% and ∼10% in the nuclei of 3CR 69 and 3CR 220.2, respectively. To constrain more precisely the pileup amount in these observations, we performed a spectral fitting with Sherpa, adding to the previous model the jdpileup model (Davis 2001). Following the Sherpa thread,²⁰ we fixed the parameters: grade zero probability, g₀ = 1; number of detection cells, n = 1; and maximum number of photons considered for pileup in a single frame, n_terms = 30. In addition, the frame time, ftime, and fractional exposure, fracexp, parameters were fixed to the values of the keywords EXPTIME and FRACEXPO of the event and ARF files. The probability of a good grade when two photons pile together, α, and the fraction of flux falling into the pileup region, f, were left free to vary during the fit.

The power-law spectral index we obtained for 3CR 69 is consistent with the 0.4 value reported in Table 4, obtained without the pileup model, and the pileup fraction as evaluated from the get_pileup_model Sherpa command is zero. Since the quality of the data collected for 3CR 36 and 3CR 119, observed to have lower flux levels, is not sufficient to allow a more detailed spectral analysis, we conclude that these sources are affected by a pileup fraction ≲5% and that the estimate of their spectral index is not compromised.

In the case of 3CR 220.2, the pileup fraction obtained from the spectral fitting with Sherpa is 8.5%, comparable with that evaluated from the pileup map, and the spectral index obtained with the pileup model is reported in Table 4. Although the two values are consistent within the 1σ range, we note that the pileup effect tends to be more severe in this source, hardening the spectrum. The spectral fit performed with the pileup model yields estimates of the flux in the soft, medium, hard, and total bands of 1.3, 1.6, 3.7, and 6.6 × 10⁻¹³ erg cm⁻² s⁻¹, respectively. The total source luminosity is 5.1 × 10⁴⁵ erg s⁻¹.

In the current sample of 16 X-ray-detected sources, three are classified as compact steep spectrum (CSS) radio sources: 3CR 119, 3CR 173, and 3CR 305.1 (O'Dea 1998). Among the other sources, three are quasars, nine are FR II radio galaxies, and one is an FR I. Radio classifications reported in Table 1 are based on a literature search, with the exception of the radio galaxy 3CR 255, for which we propose an FR II classification.

We detected X-ray emission for all of the nuclei in the sample with a level of significance higher than 3σ. All results of the X-ray photometry (i.e., nuclear X-ray fluxes in the three energy bands, together with their X-ray HRs and luminosities) are reported in Table 2 (see also Section 3.3 for more details on each source). X-ray images are presented in Appendix A with contours of the radio structures overlaid.

We detected the high-energy counterpart of two hotspots: one in the quasar 3CR 220.2 and one in 3CR 238 (see Figure 1). We discovered also X-ray emission associated with a lobe in 3CR 124 (Figure 2) and with a jet knot in the quasar 3CR 297 (Figure 3). Detection significances, fluxes, and luminosities of the components are reported in Table 3.

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Results of the X-ray spectral analysis performed for the brightest nuclei (3CR 36, 3CR 69, 3CR 119, and 3CR 220.2) are reported in Table 4: all spectra are consistent with a power-law model with Galactic absorption.

In Table 2 we also listed the total number of counts within a circular region of radius 2'', N₂, centered on the source radio nucleus. We used the radio position given in Table 1 for those sources for which the X-ray image was not registered, while for 3CR 297 we used the radio core position suggested from the flattening of the radio spectrum (see Section 3.3). As for the previous analyses of 3CR sources observed, we computed the "extent ratio," dividing N₂ by the number of counts in a region of radius 10'', N₁₀, centered on the same position, after the subtraction of background counts measured on the CCD from both values (i.e., Ext. ratio shown in Table 2). We used the "extent ratio" as a diagnostic tool for the presence of extended X-ray emission around the source region (e.g., Massaro et al. 2009a, 2010), though it does not provide information on whether the emission is related to any extended radio structure or not. We expect values close to unity for an unresolved (i.e., point-like) source since the on-axis encircled energy for r = 2'' is ≈0.97, so there is only a small increase between r = 2'' and r = 10'' for an unresolved source. We found that three of the 16 sources in our sample show values of "extent ratio" less than 0.5, namely 3CR 124, 3CR 297, and 3CR 300.1, indicating the possible presence of diffuse X-ray emission. For 3CR 300.1, the indication of the presence of extended X-ray emission, given by the low value of "extent ratio," is not reliable because its value is affected by the presence of an X-ray point source at ∼7'' distance from its nucleus.

All sources showing extended X-ray emission are at z > 1.0, so the combination of their radio projected size, close to the limit of Chandra resolution, and the short exposures of snapshot observations does not always allow us to distinguish between X-ray extended emission arising from the hot intergalactic medium (IGM) in their large-scale environment and the X-ray radiation related to their large-scale radio structure (as for example IC/CMB in their radio lobes). We interpret the diffuse X-ray emission as due to the high-energy counterpart of the extended radio structure when the peaks of X-ray surface brightness appear coincident with those of the radio intensity. Otherwise we conclude that it could be due to the presence of thermal emitting gas in the IGM on a scale of several tens to hundreds of kiloparsecs. Radio images in the megahertz band, together with X-ray spectroscopic observations, are necessary to clearly disentangle the two above scenarios (Croston et al. 2005). An example is the radio galaxy 3CR 124 (see Figure 2) with an "extent ratio" of 0.49. In this source, we detected both extended X-ray emission surrounding the radio source, on the scale of tens of kiloparsecs, and X-ray diffuse emission coincident with the external lobe, asymmetrically located with respect to the main radio axis (detection significance and fluxes of the lobe are all reported in Table 3). Both emission regions contribute to the low value of "extent ratio."

We detected X-ray emission with a brightness peak to the west side of the quasar 3CR 297 (see Figure 3). Since the radio core position of this source is unclear, radio and X-ray images are registered with the surface brightness peak position of the knot k-w 4 (see Table 3). The presence of diffuse X-ray emission in the westward direction is not related to any radio structure, and we did not find an obvious optical counterpart in HST images, so we interpreted it as X-ray radiation arising from the IGM. A detailed description of this peculiar source is given in Section 3.3.

None of the 16 sources in our sample has been reported in the literature as being a member of a galaxy cluster or group, although we found that, based on the literature search reported in Section 3.3, 3CR 220.2, 3CR 255, and 3CR 300.1 could lie in large-scale (i.e., 100 kpc) galaxy-rich environments. For these sources, we searched for possible X-ray emission due to the hot gas of the intracluster medium around their radio structures (see also Massaro et al. 2018). We measured the total number of photons in a circular region containing the entire extent of the radio source, and we subtracted the X-ray counts within circular regions of 2'', corresponding to the radio nucleus, as well as that of point-like sources (e.g., hotspots or background/foreground objects) lying within the same region. Assuming a Poisson distribution for the background events, we computed the probability of obtaining the measured value. We did not find any X-ray excess above 3σ significance on an environment with a scale of a few hundreds of kiloparsecs.

3CR 27  is a classical FR II radio galaxy at z = 0.184. From an optical perspective, it is an elliptical high-excitation radio galaxy (HERG), as confirmed by the spectrum used to update its optical identification after the last revision of the 3CR catalog (Hiltner & Röser 1991). It is located in a region of high optical obscuration at low Galactic latitude (i.e., b = 55). With the current radio map at 1.4 GHz, it was not possible to locate the radio core. However, its X-ray nucleus was probably detected with a total of 63 counts in a region of radius ∼2'' centered on the celestial coordinates reported in Table 1 (Hiltner & Röser 1991) and located between the two lobes of the radio galaxy (see Figure 4). The SW hotspot has a flux density ∼4 times greater than the northern one at 1.4 GHz. Salter & Haslam (1980) reported the detection of a possible overdensity of sources in the radio observations at 408 MHz and at 2.7 GHz over an area of 31 × 13 surrounding 3CR 27, without any information on its redshift. However, given the z estimate available in the literature, this corresponds to a linear size of ∼35 Mpc, making the possible detection of a galaxy-rich environment around 3CR 27 unlikely. In addition, extended X-ray emission arising from the IGM on a scale of hundreds of kiloparsecs was not detected in our Chandra snapshot observation.

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3CR 36  is classified as an FR II HERG at z = 1.301 (Hewitt & Burbidge 1991; Jackson & Rawlings 1997). In the 8.4 GHz map, its angular extension is ∼9'', corresponding to a projected size of less than 80 kpc, with a dominant knotty jet in the north. In the HST optical image, 3CR 36 appears as a compact galaxy, and only a central nuclear component is detected (McCarthy et al. 1997). We detected the nucleus also in our Chandra snapshot survey (see Figure 5), and the total 335 X-ray photons observed in the nuclear region allowed us to perform a spectral analysis, which yielded a spectral index α_X = 0.7 (see Table 4).

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3CR 69  is an FR II radio galaxy, optically classified as HERG, at z = 0.458 (Hiltner & Röser 1991). Since the source lies in a heavily obscured region at Galactic latitude b = −09, its optical identification was made only after the last revision of the 3CR catalog. The radio image at 8.4 GHz shows two extended lobes and a fainter nucleus that was the only X-ray detection (see Figure 6). The spectral analysis of the nucleus yielded a spectral index α_X = 0.4, as reported in Table 4, and the pileup fraction estimate is ∼5%.

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3CR 119  is a CSS radio source (O'Dea 1998), and its updated redshift is z = 1.023 (Eracleous & Halpern 1994). The source, which in our VLA 8.4 GHz map is a compact 05 structure (∼5 kpc at the source redshift), shows a very complex radio morphology on the 1 kpc scale observed with the VLBI at 5 GHz: the nucleus was identified with a weak, compact (<0003) component with a flat spectrum, while the extended structure has a spiral-like form with two knots along the jet (Fanti et al. 1986). This distorted morphology was interpreted as the result of the source interaction with dense clouds in its environment, and this idea was supported by the presence of a large gradient of the rotation measure (i.e., 2300 rad m⁻² mas⁻¹ in the 8.4 GHz band) and strong depolarization between 8.4 and 5 GHz (Nan et al. 1999; Mantovani et al. 2010). The X-ray snapshot observation revealed point-like emission on a scale of ∼2'', corresponding to ∼15 kpc at the source redshift, shown in Figure 7. The spectral analysis of its nucleus yielded a value of α_X = 0.5 (see also Table 4).

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3CR 124  is an FR II radio galaxy at z = 1.083 (Hewitt & Burbidge 1991). Optically, it is classified as a HERG (Jackson & Rawlings 1997). The 8.4 GHz image shows a small-scale, compact double-lobe structure, with a fainter central core and angular extension of 25, corresponding to about 20 kpc. However, 3CR 124 also shows additional, more extended, asymmetric radio emission misaligned by ∼45° measured in the W direction from the line connecting the other two, separated by ∼6'' (see Figure 8), suggestive of a relic lobe (McCarthy 1989; Dunlop & Peacock 1993). The optical galaxy observed with HST is more extended than the radio source; it has a highly curved morphology and shows an emission-line region that is very closely aligned with the radio one (McCarthy et al. 1997; Privon et al. 2008). The two radio components aligned in the NS direction can be interpreted as the two sides of an outflow, because a velocity gradient aligned with the radio axis was measured in the form of extended [O III] λ5007 emission regions (Shih et al. 2013).

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The value of "extent ratio" computed for this source, 0.49 ± 0.09 (see Table 2), suggests the presence of diffuse emission around the nuclear region of the source. Examining the image more closely, we detect 22 photons in the 2''–10'' radial region, where we expect five background photons giving a detection of extended emission of more than 6σ. In this region, the total luminosity is (1.1 ± 0.2) × 10⁴⁴ erg s⁻¹.

We found an excess of X-ray photons with respect to the local background with more than 3σ level of significance in a region of radius 25 (see Table 3). This is spatially coincident with the lobe/relic radio emission appearing in the NE direction (see Figure 2). The local background was estimated at the same angular separation of the radio lobe/relic, thus using an annulus of 5'' inner radius and 10'' outer radius. However, the low number of X-ray photons does not allow us to distinguish if such emission is due to a fluctuation of the diffuse X-ray radiation that appears surrounding the nucleus also in the SE direction or due to emission arising from particles accelerated in the NE radio structure.

We could favor the latter scenario because (1) the excess of X-ray photons, spatially coincident with the NE radio structure, is ∼2 times larger than the number of counts in the SE direction, at the same angular separation and measured over the same area, and since (2) it lies at a larger distance in the NE direction than the rest of the diffuse X-ray emission in the southern side.

Nuclear X-ray emission was also detected by Chandra (see Figure 8).

3CR 173  is a CSS at z = 1.035, optically classified as a HERG (Jackson & Rawlings 1997). A double structure is visible in the radio image at 4.9 GHz, with the two peaks of radio surface brightness separated by only ∼2'', corresponding to ∼15 kpc at the source redshift (see Figure 9). The core is the dominant SW component, characterized by a flattening of the integrated radio spectrum near 38 MHz (Tyul'bashev & Chernikov 2000), and it is the only X-ray detection in this source.

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3CR 194, at z = 1.184 (Strom et al. 1990), was optically classified as a HERG (Jackson & Rawlings 1997), and it is an FR II radio galaxy with a double hotspot in the NW lobe (see Figure 10). A high-sensitivity VLA image at 8.4 GHz showed that the two radio lobes are asymmetrically located with respect to the radio core, and the northwestern lobe does not extend along the radio source axis, but is almost perpendicular to it (Fernini 2014). Weak radio emission in the EW direction was also observed at 5 GHz, giving an irregular quadrupolar shape and suggesting a movement of the source in the ambient medium in a westward direction (Strom et al. 1990). The optical galaxy in HST images is elongated NS and is composed of two components separated by 2'' (Djorgovski et al. 1988; McCarthy et al. 1997). 3CR 194 displays rotation measures (in high-resolution VLA observations at 1.4, 4.9, and 8.4 GHz) in excess of 1000 rad m⁻² with an enhancement coincident with the NW hotspot (Taylor et al. 1992): the excess was interpreted as being due to the compression of external gas and magnetic field by the terminal shock that created the hotspot. This suggests the presence of high-density, hot gas surrounding the galaxy, however lacking an X-ray detection. With 124 counts, the X-ray emission of the nuclear region of this galaxy was clearly observed by Chandra.

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3CR 208.1  is a quasar (QSO). The optical counterpart was first identified with an N-type galaxy at z = 1.02 (Spinrad et al. 1985). Gravitational amplification due to a foreground source ∼3'' SE at z = 0.159 was found to brighten the QSO by several tenths of a magnitude and to increase its radio flux by a factor of 1.5 (Le Févre & Hammer 1990). The optical image of 3CR 208.1 has a compact nucleus and a secondary component 06 NE (McCarthy et al. 1997), while the radio image at 14.9 GHz shows a lobe 5'' NW with a curved jet. The only feature detected by Chandra is the radio core (see Figure 11).

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3CR 220.2  is a QSO at z = 1.157 (Spinrad et al. 1985). In the radio image at 4.9 GHz, both lobes, separated by ∼60 kpc, show a hotspot in the outer region. The peak of the radio surface brightness in the southern hotspot is coincident with a feature visible in the HST optical image, but also with the peak of the X-ray emission detected and designated as h-s5 in Table 3. Two X-ray photons are detected in the radio NE lobe/hotspot region. However, given the presence of X-ray photons at similar angular separation from the radio core with no radio counterparts, we cannot exclude that those in the northern lobe/hotspot are fluctuations of the local background. The X-ray nuclear emission is consistent with an unresolved point source, despite the presence of an elongation of the X-ray emission along the radio axis visible in Figures 1 and 12.

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In IR and optical images, the source is aligned with a group of large spiral galaxies (Hilbert et al. 2016). Based on the photometric redshift estimations of the Sloan Digital Sky Survey (Albareti et al. 2017), these objects are probably foreground galaxies. However, we searched for extended X-ray emission on a scale of hundreds of kiloparsecs in the field of 3CR 220.2 and found no neat detection.

The nucleus of 3CR 220.2 is the brightest one within the sample (i.e., N₂ = 683), and it has a spectral index α_X = 0.6 (see Table 4). The pileup fraction obtained for this source is ∼8.5%, but the spectral index obtained using a pileup model (i.e., ${\alpha }_{X}={0.81}_{-0.25}^{+0.08}$) is consistent with 0.6.

3CR 222  is known to be the most distant FR I radio galaxy of the 3CR catalog to date, having z = 1.339 (Heckman et al. 1994), although there is no published spectrum in the literature. It was classified as an ultrasteep-spectrum radio source, since it has a radio spectral index α_R = 1.52 in the frequency range from 10 MHz to 1 GHz (Roland et al. 1982). 3CR 222 was described as a compact double radio source, dominated by the nuclear emission at 1.4 GHz and with steep-spectrum emission coming from a region separated by 12 to the south (Strom et al. 1990; Law-Green et al. 1995). The optical identification is the western member of an apparent faint quartet (Djorgovski et al. 1988). The X-ray nucleus is detected at 3σ level of significance with only three photons observed by Chandra in a circular region of radius 2'' (see Figure 13).

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3CR 230  is an FR II radio galaxy at z = 1.487 with a double hotspot at the end of the northern lobe (Hewitt & Burbidge 1991). It is optically classified as a HERG (Jackson & Rawlings 1997). The radio image at 8.4 GHz shows two knots along the southern jet: it was claimed that the radio core position is coincident with the northern one, at R.A. = 9^h51^m5890 and δ = −00°01'2787 (J2000.0; Steinbring 2011). However, a comparison with the HST (Hilbert et al. 2016) and WISE (Wright et al. 2010) images of the sources shows that the optical nucleus is 05 N of this position. Moreover, since the shift that should be applied to register the X-ray image with the radio core identified by Steinbring is ∼15, more than twice the average shift imposed on the other sources, we used the unregistered image (see Figure 14).

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By studying the relationship between source outflow and star formation in this galaxy, Steinbring noticed that the asymmetry of the 3CR 230 radio structure is mirrored in the optical surface brightness and in the strong turbulence in the direction of the shorter radio arm to the SW. The optical galaxy runs along a central spine SE to NW aligned with the radio lobes and ends in broader tails, while the observed linear structures due to star formation are possibly induced by the jet (Hilbert et al. 2016). 3CR 230 is identified as a possible cluster candidate according to the red sequence method (Kotyla et al. 2016). The nucleus is the only X-ray detection, although its position is uncertain.

3CR 238  is a HERG with a classical FR II morphology at z = 1.405 (Jackson & Rawlings 1997). It could be affected by gravitational lensing by the Abell cluster A949 with an estimated redshift z = 0.142 along its line of sight (Le Févre et al. 1988). However, the core of this galaxy cluster is outside the field of view of the Chandra observation. The optical observation made with the HST shows a compact nucleus and faint extensions in the NS direction (McCarthy et al. 1997). In the 8.4 GHz image, the southern jet shows two elongated knots separated by 1'' and 2'' from the nucleus, while the northern knot has a wider structure that is also detected in the X-ray (see Table 3 and Figure 1). The offset of ∼02 between the peak of X-ray and radio emission is within the Chandra position error (see, e.g., Massaro et al. 2011). We also checked the HST field, and we did not find any optical source that could be associated with this X-ray emission. The "extent ratio" value computed for this source, as reported in Table 2, is <0.9, and it indicates the presence of extended emission ascribed to the X-ray radiation associated with the hotspot (see Figure 15).

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3CR 255  is a z = 1.355 radio galaxy with two radio components separated by 10 kpc (Giraud 1990). It is optically classified as a HERG (Jackson & Rawlings 1997). The 8.4 GHz map indicates a weak signal at the center of the host galaxy, as well as a brighter, compact radio source to the SE (see Figure 16). The northern radio component is located at the position of the optical galaxy, but more data are necessary to verify if it is compact and if it has a flat radio spectrum, as expected for the core of radio galaxies. In the optical band, the source appears elongated on a 1'' scale, and an overdensity of objects with optical magnitude in the R band between 22.5 and 24 was found in a 62'' × 62'' area around the target (Giraud 1991). This suggests the presence of a group or cluster of galaxies on the scale of ∼500 kpc. An IR HST image with ∼01 resolution shows several extended sources in the vicinity of 3CR 255 (Hilbert et al. 2016). In the NASA/IPAC Extragalactic Database, these objects are all cataloged as infrared sources with no redshift estimation. All of them have a clear optical counterpart, suggesting that active star formation is ongoing (Hilbert et al. 2016). The possible suggestion of the presence of a small group of galaxies in the field of 3CR 255 comes also from the red sequence method (Kotyla et al. 2016). However, we did not detect extended X-ray emission in the hundreds of kiloparsecs environment of this source. 3CR 255 lies entirely in a region of 2'' radius in which Chandra observed only five X-ray photons.

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3CR 297  is a quasar with a peculiar radio morphology, lying at z = 1.406 (Spinrad et al. 1985). The emission observed in the 8.4 and 4.85 GHz maps, computed in Section 2.1, is elongated and curved with a knotty enhancement at the end of the northern jet (see Figure 3). In both radio images, the emission to the south spreads over a much wider area, with a stronger concentration of signal centered on a double elongated source approximately 8 kpc (i.e., ∼1'') in projected distance from the host galaxy. The optical image shows two components separated by 03 (McCarthy et al. 1997). Recent HST IR and optical images suggest the presence of ongoing merger activity in the area (Hilbert et al. 2016).

X-ray emission surrounding the radio jet is extended, and there are 33 X-ray photons coincident with the position of the peak of the radio intensity of the jet knot in the NW (see Figure 17). Its detection, X-ray fluxes, and luminosity are reported in Table 3. The precise location of the 3CR 297 nucleus in the radio maps available to us was uncertain, so we decided to perform the astrometric registration using the radio position of the jet knot (i.e., R.A. = 14^h17^m23905, δ = −04^d00^m4642, J2000.0), assuming that the peak of X-ray surface brightness coincides with the radio peak. We then estimated the radio core position, indicated by the flattening of the radio power-law spectrum, using the spectral index map computed in Section 2.1. As shown in Figure 3, the minimum value of spectral index, α_R = 0.68, is reached at R.A. = 14^h17^m24095, δ = −04^d00^m4906 (J2000.0), almost coincident with the optical host galaxy. We used also the HST observation to compare the morphology of the source in the radio, X-ray, and optical bands.

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X-ray diffuse emission was observed in a 2'' × 4'' region west of the source with six X-ray counts and a total luminosity of (6 ± 2) × 10⁴³ erg s⁻¹: since no optical counterpart was found in the HST image, this emission could be due to hot gas in the large-scale environment.

3CR 300.1  is a classical FR II HERG at z = 1.159 (Djorgovski et al. 1988), with two lobes separated by ∼10'' (i.e., ∼80 kpc at the source redshift) and a central core fainter than the lobes in the 14.9 GHz map (Jackson & Rawlings 1997). The radio core is not detected in the 8.4 GHz radio image. In HST IR images, the galaxy appears as an extended irregular source with diffuse emission that suggests the presence of active star-formation regions (Hilbert et al. 2016).

Kotyla et al. (2016) reported a possible presence of a source overdensity surrounding 3CR 300.1 and a possible cluster/group detection using the "red sequence" method that does not appear clearly evident to us, but we checked for the presence of diffuse X-ray emission on the tens of kiloparsecs scale in our Chandra snapshot observation. At a projected distance of ∼7'' in the SW direction, we detected an X-ray point source that has an IR counterpart in an HST image (see Figure 18). The value of "extent ratio" computed in Table 2 indicates the presence of extended emission around its nucleus, but we did not find a relevant detection in the annular region surrounding the radio source, if we exclude the background/foreground source.

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The two radio lobes are detected with more than 2σ level of significance using a local background. This was estimated in an annulus of 3'' inner radius and 8'' outer radius.

3CR 305.1  at z = 1.132 (Spinrad et al. 1985) is cataloged as a CSS (O'Dea 1998), and optically it is the only low-excitation radio galaxy (LERG) in our sample. The optical galaxy has a complex morphology suggestive of ongoing merger activity (Hilbert et al. 2016): the optical emission can be seen over more than 2'' with a curved S-shaped structure in the inner 1'' radius and a close agreement between optical and radio orientation (McCarthy et al. 1997). Radio images at 8.4 GHz show the northern jet with more elongated emission and a southern circular lobe. In the 15 GHz radio map, the northern jet shows three knots. The southernmost has a radio spectral index of 0.6 and is highly polarized (60%), so it is unlikely to be the core (van Breugel et al. 1992). No nuclear emission is detectable in the radio maps available to us, ranging from 1.5 to 22.5 GHz. Thus the registration was not performed. This source is detected with 16 X-ray photons in our Chandra image (see Figure 19).

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