The 3CR Chandra snapshot survey: extragalactic radio sources with 0.5$<z<$1.0
F. Massaro V. Missaglia C. Stuardi D. E. Harris R. P. Kraft A. Paggi E. Liuzzo G. R. Tremblay S. A. Baum C. P. O'Dea B. J. Wilkes J. Kuraszkiewicz W. R. Forman

 



1Dipartimento di Fisica, Università degli Studi di Torino, via Pietro Giuria 1, I-10125 Torino, Italy.
2Istituto Nazionale di Fisica Nucleare, Sezione di Torino, I-10125 Torino, Italy.
3INAF-Osservatorio Astrofisico di Torino, via Osservatorio 20, 10025 Pino Torinese, Italy
4Department of Physical Sciences, University of Napoli Federico II, via Cinthia 9, 80126 Napoli, Italy.
5Smithsonian Astrophysical Observatory, 60 Garden Street, Cambridge, MA 02138, USA.
6Istituto di Radioastronomia, INAF, via Gobetti 101, 40129, Bologna, Italy.
7Yale Center for Astronomy and Astrophysics, Physics Department, Yale University, PO Box 208120, New Haven, CT 06520-8120, USA.
8University of Manitoba,  Dept. of Physics and Astronomy, Winnipeg, MB R3T 2N2, Canada.
9Center for Imaging Science, Rochester Institute of Technology, 84 Lomb Memorial Dr., Rochester, NY 14623, USA.
10School of Physics & Astronomy, Rochester Institute of Technology, 84 Lomb Memorial Dr., Rochester, NY 14623, USA.
+Dan Harris passed away on December 6th, 2015. His career spanned much of the history of radio and X-ray astronomy. His passion, insight, and contributions will always be remembered. A significant fraction of this work is one of his last efforts.









 
This paper presents the analysis of  X-ray snapshot observations of a subsample of the extragalactic sources listed in the revised Third Cambridge radio catalog (3CR), previously lacking X-ray observations and thus observed during  Cycle 15. This data set extends the current  coverage of the 3CR extragalactic catalog up to redshift z=1.0. Our sample includes 22 sources consisting of one compact steep spectrum (CSS) source, three quasars (QSOs), and 18 FR II radio galaxies. As in our previous analyses, here we report the X-ray detections of radio cores and extended structures (i.e., knots, hotspots and lobes) for all sources in the selected sample. We measured their X-ray intensities in three energy ranges: soft (0.5–1 keV), medium (1–2 keV) and hard (2-7 keV) and we also performed standard X-ray spectral analysis for brighter nuclei. All radio nuclei in our sample have an X-ray counterpart. We also discovered X-ray emission associated with the eastern knot of 3CR 154, with radio hotspots in 3CR 41, 3CR 54 and 3CR 225B and with the southern lobe of 3CR 107. Extended X-ray radiation around the nuclei 3CR 293.1 and 3CR 323 on a scale of few tens kpc was also found. X-ray extended emission, potentially arising from the hot gas in the intergalactic medium and/or due to the high energy counterpart of lobes, is  detected for 3CR 93, 3CR 154, 3CR 292 and 3CR  323 over a few hundreds kpc-scale. Finally, this work also presents an update on the state-of-the-art of  and  observations for the entire 3CR sample.














§ INTRODUCTION


 Since the early 60's, the ensemble of extragalactic sources listed in the Third Cambridge radio catalog (3C) represents one of the most attractive samples to study the physics of radio-loud active galactic nuclei (AGNs). Originally created using radio observations performed at 159 MHz <cit.>, and subsequently at 178 MHz <cit.>, the 3C catalog went through two main revisions later in the 80's <cit.>. 

Since then, a vast suite of observations became available from the radio to optical wavelengths thus enriching the multifrequency database necessary to carry out broad band analyses. Radio images with arcsecond resolution for almost all 3CR extragalactic sources are already present in the NRAO Very Large Array (VLA) Archive Survey (NVAS)[http://archive.nrao.edu/nvas/] and in the MERLIN[http://www.jb.man.ac.uk/cgi-bin/merlin_retrieve.pl] archives <cit.>. At higher frequencies, Spitzer <cit.>, in particular for high redshift sources <cit.>, and Hubble Space Telescope observations cover more than 90% of the 3CR extragalactic catalog <cit.>. In addition near infrared observations are also available for a significant fraction of the 3CR objects <cit.>. Recently, the Herschel Space Observatory also observed several 3CR sources, mostly focusing on the higher redshift ones, <cit.>. Moreover, a dedicated spectroscopic campaign was carried out with the Telescopio Nazionale Galileo to provide a detailed optical classification and to study their nuclear emission <cit.>. All these observations make the extragalactic 3CR catalog an ideal sample to investigate AGN nuclear properties, extended radio structures, such as jet knots, hotspots and lobes, and/or their large-scale environments <cit.>.

However, although most of the 3CR extragalactic radio sources were observed thanks to extensive X-ray campaigns carried out with ,  and  <cit.>, until Cycle 9 the  archive covered only up to ∼60% of the 3CR extragalactic sample <cit.>, while the others, such as , covered less than 1/3 of the entire catalog. Thus we started our  snapshot survey to ensure that all 3CR extragalactic sources have at least an exploratory X-ray observation, with an angular resolution similar to those at lower energies, available to the astronomical community. Adopting a step-wise strategy, we requested observations in narrow redshift, z, ranges resulting in modest proposals each cycle to minimize the impact on the  schedule. To date all the 3CR sources with z<1 have at least a snapshot observation (i.e., less than 20ksec total exposure time) available in the  archive <cit.> and several of them inspired follow up X-ray observations on interesting objects <cit.>.

Here, we present the analysis of the  snapshot observations approved during Cycle 15 including all 3CR radio sources lying between z=0.5 and z=1 that were previously unobserved by . 

The paper is organized as follows. The update of the ongoing  campaign of the 3CR sources is described in  <ref> together with some details on the current sample. A brief overview of the data reduction procedures are given in  <ref> while results are described in  <ref>.  Then, in  <ref> we present our summary and conclusions. Finally, in the Appendix, we show X-ray images with radio contours overlaid for all the sources in the current sample ( <ref>) together with the updated summary of the  observations for the entire of 3CR extragalactic catalog ( <ref>).

Unless otherwise stated we adopt cgs units for numerical results and we also assume a flat cosmology with H_0=69.6 km s^-1 Mpc^-1, Ω_M=0.286 and Ω_Λ=0.714 <cit.>. Spectral indices, α, are defined by flux density, S_ν∝ν^-α.







§ STATE-OF-THE-ART OF THE 3CR EXTRAGALACTIC CHANDRA SNAPSHOT SURVEY














The revised 3C extragalactic catalog includes 298 sources <cit.>. We have already analyzed and published all the data collected to date for the  observations carried out in Cycles 9, 12 and 13 for a total of 75 sources <cit.> and an additional 140 objects, listed in the  archive, were also presented adopting the same data reduction procedures <cit.>. Several subsets of the 3CR sample have been also observed by other groups <cit.>. Table <ref> summarizes the references for the  observations of the 3CR extragalactic sources already analyzed and published as part of this compilation.

In our previous archival analyses we excluded 7 sources, namely 3CR 66A <cit.>, 3CR 71 <cit.>, 3CR 84 <cit.>, 3CR 186 <cit.>, 3CR 231 <cit.>, 3CR 317 <cit.> and 3CR 348 <cit.>, since each of these source has accumulated an exposure time of greater than 80 ks, and has been discussed extensively in the literature. In addition, we also did not re-analyze: 3CR 236, 3CR 326, 3CR 386 since the PI of these observations is currently carrying out the analysis (M. Birkinshaw, priv. comm.).

The  archive now includes all the 3CR extragalactic sources up to z=0.5 (i.e., 150 sources), with the only exceptions being those objects for which spectroscopic observations, performed after the last revision <cit.>, reported different redshift estimates, namely: 3CR 27, at z=0.184, 3CR 69 at z=0.458 <cit.> and 3CR 93 at z=0.357, as confirmed by Ho & MinJin (2009). The present analysis extends the   database up to z=1.0 including 22 more targets. During these Cycle 15 3CR snapshot observations, we also observed 3CR 142.1 and 3CR 277 for which the redshift reported in the literature <cit.> updates the earlier estimates from Spinrad et al. (1985). These two sources, together with 3CR 93, belong to the sample of 22 targets analyzed in the present work.  snapshot observations of 3CR 27 and 3CR 69 were proposed and obtained in subsequent observing cycles (Stuardi et al. 2017 in prep.).

Twenty-five of the 298 3CR extragalactic sources are still unidentified, lacking an optical counterpart and/or an optical spectroscopic observation necessary to unveil their nature. We recently observed 21 of these 25 targets with  snapshot observations discovering X-ray counterparts for eleven of them, but even using optical data available thanks to the instruments on board of the  satellite, we could not discern and/or confirm their extragalactic nature <cit.>.

A summary table on the state-of-the-art of the  observations for all the 3CR extragalactic sources, including detections of extended components: jet knots, hotspots, lobes and X-ray emission from the hot intergalactic medium present in galaxy groups/clusters, is reported in Appendix  <ref>. 

 










§ DATA REDUCTION AND DATA ANALYSIS


Data reduction and analysis procedures adopted for all the  observations presented here were extensively described in our previous papers, thus only a brief overview is reported in the following. We adopted the same procedures since our final aim is to create a uniform database for the entire 3CR extragalactic survey once all the sources listed therein will be observed by . 

We followed the standard procedure described in the  Interactive Analysis of Observations (CIAO) threads[http://cxc.harvard.edu/ciao/guides/index.html],  to perform our data reduction and we used CIAO v4.7 with the  Calibration Database (CALDB) version 4.6.2.







 §.§ X-ray photometry

We initially generated level 2 event files using the acis_process_events task and filtering for grades 0,2,3,4,6.  We checked the absence of high background intervals inspecting lightcurves extracted for each data set, but this never occurred. We aligned the X-ray position of each core with that of the radio to perform the astrometric registration <cit.>. The source 3CR 292 has been observed twice with , obsID 16065 and obsID 17488, with exposure times of ∼4ksec and ∼8ksec, respectively. In this case a merged event file was created using the CIAO routine reproject_obs, thus reprojecting event files to the reference coordinates of the deeper observation (i.e., obsID 17488). 

Then we created flux maps in the X-ray energy ranges: 0.5 – 1 keV (soft), 1 – 2 keV (medium), 2 – 7 keV (hard), taking into account exposure time and effective area. In our procedure, as previously done, we used monochromatic exposure maps set to the nominal energies of 0.75, 1.4, and 4 keV for the soft, medium and hard band, respectively. All flux maps were converted from units of counts/sec/cm^2 to cgs units by multiplying each event by the nominal energy of each band. However, we made the necessary correction to recover the observed erg/cm^2/s, when performing X-ray photometry <cit.>. 

We measured observed fluxes for all the X-ray detected nuclei and extended components. This was done choosing a region of size and shape appropriate to the observed X-ray emission and matching the radio structure. We also chose two background regions, having the same shape and size so as to avoid X-ray emission from other parts of the source.  The flux for each energy band and region was measured using funtools[http://www.cfa.harvard.edu/∼john/funtools] as in our previous analyses. Uncertainties are computed assuming Poisson statistics (i.e., square root of the number-of-counts) in the source and background regions. X-ray fluxes, not corrected for the Galactic absorption, measured for the cores are reported in Table <ref> while those for detected jet knots, hotspots and lobes are given in Table <ref>. The name of each component (e.g., knot or hotspot) is a combination of one letter indicating the orientation of the radio structure and one number indicating distance from the core in arcseconds.

Since the  native pixel size for the ACIS instrument is 0.492, the data are undersampled, thus to recover the resolution we regridded our images to 1/2, 1/4, or 1/8 of the native ACIS pixel size. This was dictated by the angular size of each radio source and by the number of counts in each source components. For sources of large angular extent 1/2 or no regridding was adopted <cit.>.

Finally, we performed a comparison between radio and X-ray images at similar angular resolution to verify if extended structures in radio sources, such as jet knots, hotspots and lobes, have an X-ray counterpart. To achieve this goal, we used radio images retrieved from publicly available websites as that of the National Radio Astronomy Observatory VLA Archive Survey (NVAS) [http://archive.nrao.edu/nvas/], NASA Extragalactic Database (NED) [http://ned.ipac.caltech.edu/], the (DRAGN)[http://www.jb.man.ac.uk/atlas/] website as well as personal websites of our colleagues[http://3crr.extragalactic.info][http://www.slac.stanford.edu/ teddy/vla3cr/vla3cr.html]. Image parameters for each radio observation used are given in the figure captions of  <ref>.







 §.§ X-ray spectral analysis

We performed spectral analysis for the X-ray counterparts of radio cores of those sources having more than 400 counts, to determine their X-ray spectral indices α_X, the presence or absence of significant intrinsic absorption, and the role played by mild pileup in artificially hardening the spectrum. 

The spectral data were extracted from a 2 aperture, as for photometric measurements, using the CIAO routine specextract, thereby automating the creation of count-weighted response matrices. Background spectra were extracted in nearby circular regions of radius 10 not containing obvious sources. The source spectra were then filtered in energy between 0.5-7 keV and binned to allow a minimum number of 30 counts per bin to ensure the use of the Gaussian statistic. We used the Sherpa[http://cxc.harvard.edu/sherpahttp://cxc.harvard.edu/sherpa] modeling and fitting package to fit our spectra. For each source we adopted two models: (1) a redshifted power-law with Galactic and intrinsic photoelectric absorption components, and (2) the same model with an additional pileup component using the jdpileup Chandra CCD pileup model developed by Davis (2001). 

Prior to fitting, the Galactic hydrogen column density and the source redshift were fixed at the measured values for each source. When considering the first (1) fitting model, the two main variable parameters - namely the intrinsic absorption (N_H(z)) and X-ray photon index Γ - were allowed to vary in a first pass fit, but subsequently stepped through a range of possible physical values to explore the parameter space, determine 68% confidence intervals, and quantify the degree to which N_H(z) and Γ are degenerate. 

We have also explored the possible effect of pileup on our sources adding a jdpileup[http://cxc.harvard.edu/sherpa/ahelp/jdpileup.htmlhttp://cxc.harvard.edu/sherpa/ahelp/jdpileup.html] component to our baseline model (2). We left the parameters of the jdpileup model fixed to their default values, with the exception of alpha (the probability of a good grade when two photons pile together) and f (the fraction of flux falling into the pileup region). In no case were we able to constrain the value of alpha since it was usually degenerate with f and/or the intrinsic absorption, so we decided to freeze it to its default value of 1. The value of f was left free to vary between 0.85 and 1, and was constrained in two cases. The results of spectral fitting are given in Table <ref>. 











§ RESULTS









 §.§ General

We detected the X-ray counterpart of all radio cores in our sample and we measured their fluxes adopting a circular region of 2 centered on the radio position used for the astrometric registration. All the results of our X-ray photometry, i.e., nuclear X-ray fluxes in the three different bands (see  <ref>) together with their X-ray luminosities, are reported in Table <ref> while X-ray images for the whole selected sample are presented in  <ref>. In Table <ref>, we also report the values of the 'extended emission' parameter, computed as the ratio of the total number of counts in a circular region of radius r = 2 circle to that of a circle of radius r = 10 both centered on the radio position of each 3CR source (i.e., Ext. Ratio “Extent Ratio”).  This ratio is close to unity for unresolved (i.e., point-like) sources since the on-axis encircled energy for 2 is ≃0.97, and only a small increase is expected up to 10. Thus parameter values significantly less than 0.9 indicate the presence of extended emission around the nuclear component <cit.>. In our sample this situation clearly occurs for 3CR 107, 3CR 293.1 and 3CR 323. 

Four of the 22 sources./, namely: 3CR 93, 3CR 114, 3CR 154 and 3CR 288.1, show X-ray nuclei with more than 400 counts within a circular region of 2 radius. Thus, according to our previous works, we performed a basic X-ray spectral analysis for them. Their spectra are consistent with a mildly absorbed power-law with the absorption consistent with the Galactic value <cit.>. 3CR 93 and 3CR 154 show a fraction of pileup of 10%, however their fits do not improve significantly when including a jdpileup spectral component [http://cxc.harvard.edu/sherpa/ahelp/jdpileup.html]. Results of the spectral analysis are reported in Table <ref>, the statistical uncertainties quoted refer to the 68% confidence level.

We also discovered X-ray emission associated with three hotspots in three different sources; their X-ray fluxes are reported in Table <ref> together with detection significances, all above 3σ, computed assuming a Poisson distribution for the background. The X-ray counterpart of a radio knot in the 3CR 154 eastern lobe as well as that of the whole southern radio lobe of 3CR 107 was also found (see Table <ref> for details).

Three of 22 3CR sources in our sample lie within optically known galaxy cluster, namely: 3CR 34, 3CR 44 and 3CR 247 <cit.> and to search for possible X-ray emission due to the hot gas permeating the intergalactic medium around the radio structure, we adopted the following procedure for all our targets. We measured the total number of counts in a circular region with diameter equal to the total extent of the radio source and we subtracted those counts within circular regions of 2 corresponding to the radio nucleus and/or to background sources lying within the larger circle. Then assuming a Poisson distribution for the background events, we computed the probability of obtaining the measured value given the expected number of counts in the background. 

We detected an excess of X-ray photons, above 3σ significance, around the radio structures of 3CR 93, 3CR 154 and 3CR 323, on a scale of few hundreds kpc as shown in Fig. <ref>. Such X-ray extended emission could be due to the presence of hot gas filling their large-scale X-ray environment (see Fig. <ref>) but it could also be contaminated by the radiation arising from the lobes of these FR II radio galaxies which would be aligned with their large-scale radio structure. These scenarios are indistinguishable due to the low number of counts available, however we tend to favor the former (i.e. presence of hot gas in the intergalactic medium) since the peaks of the X-ray surface brightness do not appear coincident with those of the radio intensity (i.e., hotspots in the lobes). We also found a marginal detection (i.e. 2σ significance) of X-ray emission from the group/cluster around 3CR 34 (see also Fig. <ref>). For this source we do not claim that the extended X-ray emission could be associated with the radio lobes because it lies in a galaxy-rich large scale environment <cit.>. Then we discovered extended X-ray emission also for 3CR 292 on a hundreds of kpc scale (see Fig. <ref>) above 3σ significance. Here an excess of X-ray photons is also found associated with the northern hotspot/lobe but it is consistent with a 3σ fluctuation of the X-ray diffuse background (more details are given in the following section). A specific analysis to search for galaxy groups or cluster signatures surrounding the 3CR sources observed during the  snapshot survey is out of the scope of the paper and it will be presented in a forthcoming work.





Finally, we remark that X-ray images with radio contours overlaid for all the sources in the current sample are shown in the Appendix ( <ref>).






 §.§ Source details


3CR 34. This is an FR II radio galaxy at z=0.69 and optically classified as a high-excitation line radio galaxy <cit.>. It lies near the center of a compact cluster of galaxies <cit.> appearing as one of the reddest members and being surrounded by fainter companions. There is a double hotspot in the western lobe. Best et al. (1997) reported the detection of a strong jet-cloud interaction at 120 kpc distance from the core and HST images also show a narrow region of blue emission orientated along the radio axis and directed towards a radio hotspot. In the  observation analyzed here we only detected the X-ray core. We tested the presence of diffuse X-ray emission due to the intergalactic medium of the known galaxy cluster where 3C 34 lies according to the procedure previously described. We found an X-ray excess of about 3σ significance (see Fig. <ref>) where the number of counts in an annular region of radii 2and 30, centered on the radio core position, is 90, compared with 45.7 expected background counts in that region. We also estimated the luminosity of the X-ray extended emission, L_X, adopting an annular region of inner radius 2and outer radius 30, centered on the position of the 3CR 34 radio nucleus, to exclude the nuclear contribution of the central radio galaxy. Our estimate is L_X=(5.2±2.2)·10^43erg/s. We note that this L_X estimate, as in the following cases of 3CR 93, 3CR 154, 3CR 292 and 3CR 323, is an upper limit on the X-ray luminosity of the hot gas in the intergalactic medium since there could be a possible contamination due to the lobe X-ray emission.

3CR 41. This is an FR II radio galaxy with z=0.79, classified as a narrow emission line radio galaxy. In the radio image at 8.45 GHz there is no detection of a jet in the northern lobe while extended radio emission associated with the jet pointing towards the south-eastern hotspot is detected <cit.>. Both radio lobes are also visible even at 8.45 GHz. At higher energies, in our  snapshot observations, the southern hotspot is detected as well as the core of the radio galaxy.

3CR 44. This quasar, with a radio morphology similar to that of an FR II radio galaxy, is at z=0.66 being associated with a galaxy cluster visible in the optical image<cit.>. Kharb et al. (2008) reported a hint of a jet-like structure extending towards the southern hotspot in the 5GHz radio map while the HST image shows this radio galaxy to be either composed of two structures oriented north-south or, more likely, bisected by a dust lane running east-west <cit.>. We clearly detected the nuclear emission in the X-rays and we also found an excess of X-ray counts associated with the northern lobe but at <3σ significance.

3CR 54. This is an FR II radio galaxy at redshift z=0.8274. The lobe morphology on the southern side of the source is more extended than the northern one <cit.>. In the HST image, the galaxy has close companions and the source appears extended towards the southwest side but it is unclear if it is a bridge, a tidal tail, or a jet feature <cit.>. In the X-ray image the radio core is clearly detected together with the southern hotspot.

3CR 55. This is an FR II radio galaxy at z=0.735 and optically classified as a narrow emission line radio galaxy. The radio core is detected in the X-rays while the two hotspots are not.

3CR 93. This is a z=0.358 quasar with a lobe-dominated radio morphology <cit.>. In the optical image, available in the HST archive, 3CR 93 has a host galaxy with ∼3 diameter <cit.>. In the X-ray image the core is clearly detected with more than 1000 counts but there is no detection of hotspots. X-ray spectral analysis of the nuclear emission shows a power law spectrum with absorption consistent with the Galactic column density. Significant extended X-ray emission on hundreds of kpc scale was found around 3CR 93 with a significance above 3σ (see Fig. <ref>). The number of counts in an annular region of radii 2and 30, centered on the radio core position, is 114 while those expected for the same region in the background is 45.4. The X-ray luminosity of the extended emission, L_X, estimated adopting an annular region of inner radius 2and outer radius 30, centered on the location of 3CR 93, as previously done for 3CR 34, is L_X=(2.3±0.5)·10^43erg/s.

3CR 107. This FR II - HERG radio galaxy lies at z=0.785 <cit.>. In our  snapshot observation we clearly detected the nuclear emission as well as X-ray extended emission associated with the southern lobe (see Fig. <ref> and Table <ref>), as also indicated by the low value of the “ext ratio” in Table <ref>.


3CR 114. This FR II radio galaxy is at z=0.815 showing fairly weak emission lines in its optical spectrum that led to a LERG classification <cit.>. A faint compact nucleus with several clumps within the few arcseconds is observed at radio frequencies and a jet-like feature appears on the northern side in the 1.4GHz image <cit.> while single hotspots are detected both in the southern and in the northern lobes. Storm et al. (1990) claimed that this radio galaxy could lie in the core of a rich, distant cluster with some signatures of a merger on the basis of their optical observations. In the X-ray image the nucleus is detected but there is no evidence of hotspots and no signatures of X-ray emission from intracluster medium was found.

3CR 142.1. This double radio source (i.e., FR II morphology) at z=0.4061. In the radio there are two clear extended radio lobes with a radio bridge showing a constant spectral index <cit.>. In the X-ray image only the radio nucleus is detected.

3CR 154. This is a nearby lobe-dominated quasar <cit.> at z=0.58 <cit.> appearing as a point-like source in the HST optical image. In the  snapshot observation only the relatively bright radio core is detected, for which X-ray spectral analysis was performed. As in 3CR 34 and 3CR 93, 3CR 154 shows extended X-ray emission on kpc scale detected above 5σ level of confidence, measured adopting the same method previously described (see Fig. <ref>). We measured 243 X-ray photons in a annular region of radii 2and 40, centered on the location of the radio core, while 70.1 are expected in the background region. This X-ray extended emission has a luminosity L_X=(2.2±0.3)·10^44erg/s, measured using an annular region of inner radius 2and outer radius 40, centered on the location of radio nucleus of 3CR 154. In the eastern lobe we found an association between the peak of the radio and the X-ray intensities for a radio knot lying at 33 from the nucleus (see marked region in Fig. <ref>). Given this spatial coincidence, this excess of X-ray photons is probably due to the high energy counterpart of the radio extended structure. On the other hand, the southern knot in the same eastern lobe does not show a correspondence between the radio and the X-ray emissions (∼1.5 offset), indicating that this high energy emission could be linked to the hot gas in the intergalactic medium.


3CR 169.1. This classical FR II radio galaxy is at z=0.633. There is no detection of the jet and/or of the hotspots in the radio map at 8 GHz <cit.>. Harvanek et al. (1998) classified this source as a narrow emission line radio galaxy according to its optical spectrum. In the X-ray snapshot image the  nucleus is detected but there is no detection of the extended radio structure.

3CR 217. This is a FR II radio galaxy with narrow emission lines at z=0.898. In the radio image at 8 GHz there are no jet signatures, in either the eastern or the western lobe <cit.> and only the western lobe is detected. In the X-ray band the radio core is  detected but the  image is not registered because we were not able to locate precisely its position. Only a single photon is associated with the western hotspot being undetected in the X-rays.

3CR 225B. This is a FR II radio galaxy at z=0.582, optically classified as a narrow emission line galaxy. It does show only hotspots within its radio lobes <cit.>. In the X-ray there is a hint of diffuse emission near the nucleus but no detection for the two hotspots. We note that the nearby radio source 3CR 225A is also in the field of view of the  snapshot observation on a nearby CCD, but we did not detect any signature of X-ray emission arising from this radio object.

3CR 237.This is the only compact steep spectrum (CSS) radio source in our sample. It shows an FR II radio morphology and is at z=0.877. The radio source size is less than 2 (i.e., less than ∼15 kpc at z=0.877) and it has a clear X-ray counterpart.

3CR 247. This is an FR II radio galaxy at z=0.75, showing two hotspots on both the eastern and the western side, optically classified as a narrow emission line radio galaxy. The host galaxy associated with the radio source 3C247 lies in a very crowded field. In the HST optical image it appears as a symmetrical central galaxy with a close companion lying about 0.8 arcsec to the south. Two nearby galaxies also lie within the envelope of the infrared emission <cit.>. Radio spectral ageing analysis of this source has yielded an age of 3-5 million years, corresponding to a hotspot advance velocity of about 0.1c <cit.>. In the  X-ray image the radio core and the northern hotspot are detected.

3CR 272. A classical double FR II radio source at z=0.944, showing a relatively faint nuclear component with respect to the two hotspots at radio frequencies. Its optical spectrum shows a typical HERG spectrum with high ionization emission lines <cit.>. There is nothing to report in the  snapshot image other than the core detection.

3CR 277. This is a giant FR II radio galaxy at z=0.414 <cit.>, optically classified as a low excitation type radio galaxy (i.e., LERG). At 1.4 GHz the core and a jet-like structure are detected in the eastern lobe. Both radio lobes show double hotspots in high resolution radio images <cit.>. According to McCarthy et al. (1997) the source reveals a basic double morphology with the nuclear component slightly offset from the geometrical center. In the X-ray image the radio core is detected while while neither double hotspot has an X-ray counterpart.

3C 277.2. This FR II radio galaxy at z=0.767 is optically classified as a narrow line emission galaxy. It has a clear jet-like component in the southern lobe that is closer to the radio core but no signatures in the X-rays.

3CR 288.1. This is a lobe-dominated QSO lying at z=0.961 having a clear point-like optical counterpart <cit.>. In the X-ray image the nucleus is clearly detected and given the high number of photons with respect to the majority of the other targets, we also performed its spectral analysis finding its X-ray spectrum consistent with an absorbed power-law (see Table <ref>). Extended emission on kpc scale seems to be present around the core (see Table <ref>) but no evidence of a galaxy cluster or a group of galaxies is present in the optical images.

3CR 292. A narrow line FR II radio galaxy at redshift 0.713. Mullin et al. (2006) reported the detection of the two hotspots and the core in the radio map at 8.45 GHz but a part from the core none of these features is detected in the X-rays. Adopting the same procedure as in previous cases we found that 3CR 292 shows extended X-ray emission (more than 3σ detection significance) detected on kpc scale and potentially due to the presence of hot gas in the intergalactic medium (see Fig. <ref>). The number of X-ray photons measured in a annular region of radii 2and 75, centered on the radio core position, is 525 while those expected in the background, for the same area, is 364.3, where both X-ray counts have been measured using the merged event file. We estimated an X-ray luminosity for the extended emission of L_X=(2.5±0.7)·10^44erg/s, using the same annular region. Belsole et al. (2004) reported the X-ray detection of radio lobes in an  observation and as previously stated in the merged event file we clearly detected an excess of X-ray photons correspondent to the location of the northern hotspot. However, as shown in Fig. <ref>, in a circular region of 3.5 radius, centered on the northern hotspot, we measured 5 X-ray counts in the ∼4ksec  observation (i.e. obsID 16065) but only 1 in the deeper one (i.e. obsID 17488, ∼8ksec exposure time). Then, in the latter observation, the peak of the radio surface brightness is not coincident with the X-ray excess. Since this radio structure lies in a large scale environment permeated by X-ray emission from the hot intergalactic medium, we measured the level fo the X-ray background of the ∼4ksec observation within 75 from the radio core and excluding the nuclear X-ray emission. We found that, over the same area (i.e., circle of 3.5 radius), the expected number of counts is 1.5±1.2 (obsID 16065), thus this X-ray excess is consistent with a 3σ fluctuation of the diffuse X-ray background. A deeper investigation will be necessary to distinguish between X-ray emission associated with the hotspot or that of the intergalactic medium. Variability on monthly time scale has been excluded because the hotspot size, even if being relatively compact with respect to the lobe structure, covers a region of ∼25 kpc.



3CR 293.1. This faint FR II radio galaxy is at z=0.709. Its radio structures, i.e, the nucleus and both hotspots, are barely detected in the radio image available to us at 4.9 GHz and the comparison with the X-ray image indicates that the radio core is detected with possible extended X-ray emission around it (see Table <ref>).

3CR  323. This is a FR II radio galaxy at z=0.679. McCarthy et al. (1997) pointed out that the outer lobes are irregular and rather different in structure, the northern one undergoing a sharp bend to the east and ending in what may be a hotspot. In the X-ray we only detected the nucleus and there is also significant X-ray extended emission surrounding it both on scales of few tens and ∼300 kpc (above 5σ significance), potentially due to its intergalactic medium (see Fig. <ref>). There are 329 X-ray photons in a annular region of radii 2and 75, centered on the radio core position, while 269.3 background photons are expected in this region. In this case the number of X-ray counts within a circular region of 2 positioned on two point-like sources lying on the western side of 3CR 323 were also subtracted from the X-ray photons measured within the annular region. This does not affect the significance of the detection of the X-ray extended emission since only 8 and 20 photons were measured for the two nearby objects.  We estimated an X-ray luminosity of L_X=(4.7±1.6)·10^43erg/s for the extended emission surrounding 3CR 323, this was measured over an annular region of inner radius 2and outer radius 21, centered on the location of its radio core. A lower value of the outer radius for the annular region has been chosen to avoid contamination due to the presence of nearby foreground/background X-ray point-like sources.













§ SUMMARY AND CONCLUSIONS


This paper presents the analysis of  snapshot observations of a subsample of the extragalactic sources listed in the revised Third Cambridge radio catalog (3CR), previously lacking  observations and observed during Cycle 15. This data set extends the current  coverage of the 3CR extragalactic catalog up to redshift z=1.0. The 3CR extragalactic sample includes 22 sources listing one compact steep spectrum (CSS) source and three quasars (QSOs) while all the remaining sources are FR II radio galaxies. Nineteen targets lie at z in the range 0.5–1.0 plus 3C 93, 3CR 142.1 and 3CR 277. One additional target, 3CR 255A, lies in the  field of view of a nearby source (i.e., 3CR 255B) observed during these Cycle 15 observations, but its radio core is not detected in the X-rays.
 
The main aims of the 3CR  snapshot survey are: (i) to search for X-ray emission from jet knots, hotspots and lobes, (ii) to study the nuclear emission of their host galaxies and (iii) to investigate their environments at all scales, aiming to discover new galaxy groups/clusters via the X-ray emission of the intergalatic medium. 

In the present work, the basic source parameters for the newly acquired  data are presented. We created fluxmaps for all the X-ray snapshot observations and compared them with radio images to search for the high energy counterparts of extended radio structures (i.e., jet knots, hotspots, lobes). We measured their X-ray intensities in three energy ranges, namely soft, medium and hard band, for all radio cores and hotspots detected in the X-rays. Then, for the nuclei brighter than 400 X-ray photons, measured in a circular region of 2 radius in the 0.5–7 keV energy range, we also performed X-ray spectral analysis showing nuclear spectra all consistent with simple power-law model, with an eventual mild intrinsic absorption in a single case.

We found X-ray emission arising from three hotspots in 3CR 41, 3CR 54 and 3CR 225B. We also report the discovery of extended X-ray emission, on tens of kpc scale, around the radio nuclei of 3CR 107 and 3CR 293.1, for the former source due to the X-ray counterpart of the southern radio lobe. Three sources in our sample are members of, optically-known galaxy groups/clusters: 3CR 41, 3CR 44 and 3CR 247. In the last two cases we did not detect X-ray emission arising from the intergalactic medium while a marginal detection (i.e., 2σ) was found for 3CR 34. Moreover, we discovered extended X-ray emission on a scale of few hundreds of kpc around the radio structures of 3CR 93, 3CR 154, 3CR 292 and 3CR  323, all above 3σ significance. Then, for 3CR 154, we also detected the X-ray counterpart of a knot in the eastern radio lobe at 33 distance from the nucleus.

Finally, we highlight that a table summarizing the state-of-the-art of the X-ray (i.e.,  and ) observations carried out to date is reported at the end of the present manuscript (see  <ref>).  detections are all based on both our current and previous analysis and represent an update with respect to previous works while those regarding , shown here for the first time, are only based on literature search.




We thank the anonymous referee for useful comments that led to improvements in the paper.

We are grateful to M. Hardcastle and C. C. Cheung for providing
several radio images of the 3CR sources while the remaining ones were
downloaded from the NVAS[http://archive.nrao.edu/nvas/] (NRAO
VLA Archive Survey), NED[http://ned.ipac.caltech.edu/] (Nasa
Extragalactic Database) and from the DRAGN
webpage[http://www.jb.man.ac.uk/atlas/].





This investigation is supported by the NASA grants GO1-12125A,
GO2-13115X, and GO4-15097X.  G.R.T. acknowledges support from the National Aeronautics and Space Administration (NASA) through Einstein Postdoctoral Fellowship Award Number PF-150128, issued by the Chandra X-ray Observatory Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060. This work was also supported by
contributions of European Union, Valle D'Aosta Region and the
Italian Minister for Work and Welfare. 

The National Radio Astronomy Observatory is operated by Associated Universities, Inc.,
under contract with the National Science Foundation.

This research has made use of data obtained from the High-Energy Astrophysics Science Archive
Research Center (HEASARC) provided by NASA's Goddard Space Flight Center; 

the SIMBAD database operated at CDS,
Strasbourg, France; the NASA/IPAC Extragalactic Database
(NED) operated by the Jet Propulsion Laboratory, California
Institute of Technology, under contract with the National Aeronautics and Space Administration.

TOPCAT[http://www.star.bris.ac.uk/∼mbt/topcat/] 
<cit.> for the preparation and manipulation of the tabular data and the images.

SAOImage DS9 were used extensively in this work
for the preparation and manipulation of the
images.  SAOImage DS9 was developed by the Smithsonian Astrophysical
Observatory. 

Facilities: VLA, MERLIN, CXO (ACIS)










 
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§ A: IMAGES OF THE SOURCES



For all the 3CR sources in our sample, radio morphologies are shown here 
as contours superposed on the re-gridded/smoothed X-ray event files.  
The full width half maximum (FWHM) of the Gaussian
smoothing function and the binning factor are reported in the figure captions.
X-ray event files were limited to the 0.5 to 7 keV band and rebinned
to change the pixel size with a binning factor 'f' (e.g. f=1/4
produces pixels 4 times smaller than the native ACIS pixel of 0.492).  
The labels on the color bar for each X-ray map are in units of
counts/pixel. We included in each caption also the radio
brightness of the lowest contour, the factor (usually 2 or 4) by which
each subsequent contour exceeds the previous one, the frequency of the
radio map, and the FWHM of the clean beam.
Figures appear so different from
each other mainly because of the wide range in angular size of the radio sources.































































§ B: THE STATUS OF THE  X-RAY 3CR OBSERVATIONS



Here we present the current status of the  and  observations for the entire 3CR catalog. While  X-ray observations have been uniformly re-analyzed, as reported in our previous investigations, all the  information provided here is based on a literature search <cit.>.

For all 3CR sources, we report their classification, labeling: radio galaxies (RG), according to the Fanaroff & Riley criterion <cit.>; quasars (i.e., QSRs);
Seyfert galaxies (Sy) and BL Lac objects (BL). We also indicate as “UNID” those sources which, lacking optical spectroscopy, remain unidentified. We include a column reporting the radio morphology for the radio galaxies (FR I vs FR II types) and indicating those objects that also show the radio structure of: (i) Compact Steep Spectrum (CSS) or X-shaped (XS) radio sources or (ii) have been classified in the literature as wide-angle tailed or narrow-angle tailed radio galaxies (WAT and NAT, resepctively). We also devoted a column to the optical classification of radio galaxies distinguishing them as HERG or LERG. The most updated value of the redshift z is also reported and we used a “cluster flag” to label sources that belong to a known galaxy group/cluster. We considered sources belonging to a galaxy-rich large scale environment those for which there is a known optical group/cluster reported in the literature and/or those for which there is an archival X-ray observation confirming the presence of hot gas in the intergalactic medium.

Regarding the X-ray analysis, we report X-ray detections of radio components adopting the following symbols: k= jet knot; h= hotspot; l= lobe and igm for sources that belong to a galaxy-rich large scale environment. No distinction has been made between sources lying in group or clusters of galaxies. We also adopt the symbol e for those radio objects that show extended X-ray emission of kpc scale as highlighted in our analyses using the “extent ratio” measurements. For  observations we only adopted l and igm symbols due to the lower angular resolution with respect to  that does not allow to see counterparts of jet knots and hotspots in the large fraction fo the 3CR sources.

Finally, the “X-ray flag” indicates if the source was already observed by  (c) and/or  (x). Sources marked with a ^* close to their 3CR name are those not re-analyzed in our previous studies (see  <ref> for more details). The table present in this work updates and thus supersedes those included in previous publications.























