Completing the 3CR Chandra Snapshot Survey: Extragalactic Radio Sources at High Redshift

One of the most valuable catalogs of radio-loud active galactic nuclei (AGNs) is the Third Cambridge (3C) catalog of radio sources, originally built at 159 MHz (Edge et al. 1959; Bennett 1962) and revised afterward at 178 MHz (see e.g., 3CR and 3CRR by Spinrad et al. 1985; Laing et al. 1983, respectively). The extragalactic fraction of the 3C catalog is a premiere sample of powerful radio galaxies and quasars spanning a wide range of radio powers and redshifts, from very nearby sources at z ∼ 0.001 up to more distant quasars lying at z ∼ 2.5, all radio selected at 178 MHz in the 3CR release (Spinrad et al. 1985). It lists both radio galaxies (RGs, i.e., narrow emission line objects as defined by Jackson & Browne 1990) and quasars (QSOs, i.e., broad emission line objects as defined by Jackson & Browne 1990), as well as edge-darkened (i.e., FR I) and edge-brightened (i.e., FR II) radio sources, according to the FR classification scheme (Fanaroff & Riley 1974), and, both high-excitation (HERG) and low excitation (LERG) radio sources, classified on the basis of their optical spectra. This last classification scheme was first introduced by Hine & Longair (1979) and has been used since then by several authors to classify radio sources (e.g., Laing et al. 1994; Buttiglione et al. 2010; Best & Heckman 2012; Baldi et al. 2019).

This catalog has been extensively studied with the goal of exploring feedback processes occurring between radio galaxies and their environments (see e.g., Fabian 2012; Kraft et al. 2012) as well as investigating their nuclear emission and the emission from X-ray counterparts of their extended radio structures (i.e., jet knots, hotspots, and lobes; see e.g., Croston et al. 2005; Ineson et al. 2013, 2015, 2017; Mingo et al. 2017). Thus, a vast suite of observations were performed in the last decades for the 3CR catalog in the radio (see e.g., Law-Green et al. 1995; Giovannini et al. 2005), infrared (see e.g., Baldi et al. 2010; Werner et al. 2012; Dicken et al. 2014; Ghaffari et al. 2017), optical (see e.g., Hiltner & Roeser 1991; Chiaberge et al. 2000; De Lucia et al. 2004; Buttiglione et al. 2009, 2011; Tremblay et al. 2009), X-ray (see e.g., Prieto 1996), and gamma (Torresi 2018) bands, using all major observing facilities, such as the Very Large Array (VLA), Spitzer and Herschel, the Hubble Space Telescope (HST), ROSAT, and Fermi satellites, to name a few examples.

Then to complete the X-ray coverage of the 3CR catalog, in 2007, we embarked on the 3CR Chandra Snapshot Survey, with the goal of obtaining snapshot observations (i.e., exposure time of the order of 20 ks) for the entire catalog. Before this project, only 150 extragalactic radio sources out of 298 listed in the 3CR catalog were already present in the Chandra archive (see e.g., Evans et al. 2006; Hardcastle et al. 2006). Thus in Chandra Cycle 9, we started observing all 3CR sources at z < 0.3 lacking X-ray observations. Of the targets, 29 were observed during Chandra Cycle 9 (Massaro et al. 2010) and the remaining 27 during Cycle 12 (Massaro et al. 2012), with nominal exposure times of 8 ks each. We then continued requesting radio sources at z > 0.3 (increasing the exposure time and taking into account the decrease in the low-energy effective area due to the buildup of contaminant on the ACIS-S filter) and observed 19 radio galaxies in Chandra Cycle 13 up to z = 0.5 (Massaro et al. 2013), 23 more targets in Cycle 15 (Massaro et al. 2018), and completed all 3CR observations up to z = 1.5 in Cycle 17 (16 more sources; Stuardi et al. 2018).

Here we present the last paper in our survey reporting Chandra observations of the remaining nine 3CR sources, completing observations of all identified 3CR sources up to z =  2.5—namely, 3C 239, 3C 249, 3C 257, 3C 280.1, 3C 322, 3C 326.1, 3C 418, 3C 454, and 3C 454.1. These are the highest-redshift (from 1.554 to 2.474) sources listed in the 3CR Chandra Snapshot Survey. Observations in the 3CR Chandra Snapshot Survey have inspired several X-ray follow-up observations of different targets of the survey (see e.g., Massaro et al. 2009; Hardcastle et al. 2010, 2012; Balmaverde et al. 2012; Orienti et al. 2012; Ricci et al. 2018; A. Paggi et al. 2020, in preparation). Furthermore, this survey led to several discoveries of X-ray counterparts of radio jet knots and hotspots (see e.g., Massaro et al. 2015) and of extended X-ray emission around these radio sources (Massaro et al. 2018; Stuardi et al. 2018; A. Jimenez-Gallardo et al. 2020, in preparation).

However, during our investigation, we also discovered that the 3CR catalog lists 25 targets that are still unidentified (Massaro et al. 2015; Maselli et al. 2016), lacking an association with an optical counterpart, and thus also being unclassified. There is a high chance that these 3CR unidentified sources are (i) high-redshift quasars; (ii) highly obscured radio-loud active galaxies, thus very different from the rest of the 3CR catalog, deserving a deeper analysis; or (iii) LERGs and thus lacking radiatively efficient AGN signatures in the optical and X-rays (see Hardcastle et al. 2009; Maselli et al. 2016). A Chandra campaign is currently ongoing to observe the first nine unidentified targets and results will be presented in a forthcoming paper (V. Missaglia et al. 2020, in preparation).

As in previous data papers, this work is organized as follows. A brief overview of the data reduction procedures, uniformly applied to all previous analyses, is given in Section 2 while results are described in Section 3. X-ray images of the nine 3CR radio galaxies are shown Section 3, with radio contours overlaid. Then, Section 4 is devoted to our summary, conclusions, and a brief overview of future perspectives.

Unless otherwise stated, we adopted cgs units for numerical results and assumed the same flat cosmology with H₀ = 69.6 km s⁻¹ Mpc⁻¹, Ω_M = 0.286, and Ω_Λ = 0.714 (Bennett et al. 2014), previously adopted in earlier papers in this series. Spectral indices, α, are defined by the flux density, S_ν ∝ ν^−α. Average background level of the X-ray images is ∼0.02 photons pixel⁻¹.

Here we present an overview of the data reduction and analysis procedures performed on Chandra observations. To create a uniform Chandra database of 3CR sources, procedures performed are the same as those in our previous papers and, therefore, details about them can be found in Massaro et al. (2010, 2011, 2012, 2013, 2015, 2018) and Stuardi et al. (2018). Details for each observation are reported on Table 1.

Data reduction was done following the standard procedure described in the Chandra Interactive Analysis of Observations (CIAO; Fruscione et al. 2006) threads.¹⁵ In particular, we used CIAO v4.10 with the Chandra Calibration Database (CALDB) version 4.8.4.1. Level 2 event files were created using the CIAO task chandra_repro. Astrometric registration was carried out by aligning the radio core position, when detected, with its X-ray position (following the same procedure described in Massaro et al. 2011). In our case, only three sources—namely, 3C 280.1, 3C 418 and 3C 454—were registered since radio nuclei could not be detected using radio maps available for the remaining sources. For sources registered, the total shift was of the order of 05 or less, which corresponds to ∼4 kpc at redshifts of 1.5–2.5, consistent with previous campaign papers.

Flux maps were created by taking into account the exposure time and the effective area. We obtained maps in the X-ray energy ranges: 0.5–1 keV (soft), 1–2 keV (medium), and 2–7 keV (hard) by using monochromatic exposure maps set to the nominal energies of 0.75, 1.4, and 4 keV for the soft, medium, and hard bands, respectively. All flux maps were converted from units of counts s⁻¹ cm⁻² to cgs units by multiplying each event by the nominal energy of each band, assuming that every event in the same band has the same energy (see Massaro et al. 2015).

We defined the position of X-ray nuclei and lobes using radio maps. However, when the core was not detected in radio, the position of the X-ray nucleus was defined as the emission peak in the 5–7 keV band (in contrast to the 0.5–7 keV band used for images and for the rest of the analysis) since we expect nuclei to have hard spectra in the X-ray band. The positions found are consistent (within 2'') with those of the host galaxy in the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS¹⁶ ; Chambers et al. 2016) and those reported in Spinrad et al. (1985). Then, observed X-ray fluxes were measured using circular regions of 2'' radii centered on the X-ray nuclei and, for lobes, circular regions containing the radio emission at more than three times the root mean square (rms) level of the background in the radio maps and centered on the peak of the radio emission. Background regions were chosen as circular regions at least as big as the radio sources and located on the same charge-coupled device (CCD) chip, far enough from the radio galaxy (i.e., at least a few tens of arcsec) to avoid the smearing of the point-spread function (PSF) on CCD borders and contamination from the source. The average background level is ∼0.02 photons pixel⁻¹.

Table 1.  Source List of the Chandra AO20 Snapshot Survey

3CR	Class	R.A. (J2000)	Decl. (J2000)	z	kpc scale	DL	log(${N}_{{\rm{H}},\mathrm{Gal}}$)	mv	S178	Chandra	Obs. Date
name		(hh mm ss)	(dd mm ss)		(kpc arcsec−1)	(Mpc)	(cm−2)		(Jy)	Obs. ID	yyyy mm dd
239	RG–FR II–HERG	10 11 45.284	+46 28 18.79	1.781	8.598	13715.2	19.84	22.5	13.2	21395	2019 Feb 20
249	QSO–LDQ	11 02 03.774	-01 16 16.67	1.554	8.612	11587.3	20.60	⋯	16.9	21396	2019 Feb 16
257	RG	11 23 09.391	+05 30 18.50	2.474	8.245	20524.7	20.47	⋯	9.7	21397	2019 Feb 13
280.1	QSO–LDQ	13 00 33.364	+40 09 07.28	1.667	8.615	12639.0	20.21	19.4	9.2	21398	2019 Jul 29
322	RG–FR II	15 35 01.269	+55 36 52.33	1.681	8.614	12770.4	20.11	23.0	10.1	21399	2019 Feb 25
326.1	RG	15 56 10.170	+20 04 20.73	1.825	8.586	14134.4	20.51	R	8.2	21400	2019 May 20
418	QSO	20 38 37.042	+51 19 12.43	1.686	8.613	12817.4	21.72	20.0	13.1	21401	2018 Sep 3
454	QSO–CSS	22 51 34.722	+18 48 40.02	1.757	8.603	13487.5	20.75	18.5	11.6	21403	2019 Aug 19
454.1	RG–FR II–CSS	22 50 33.067	+71 29 18.02	1.841	8.582	14287.3	21.45	⋯	9.8	21402	2019 Mar 16

Note. Col. (1): the 3CR name. Col. (2): the "Class" column specifies whether the source is an RG or QSO and the source classification according to the radio morphology (only FR II, in our case for radio galaxies; Fanaroff & Riley 1974; or lobe-dominated quasar, LDQ), the optical spectroscopic designation (HERG), and whether the source is classified as a compact steep spectrum (CSS; see also Perryman et al. 1984; Hes et al. 1996; Grimes et al. 2004 for more details). These classifications were listed when available in the literature. Col. (3)–(4): the celestial positions listed are those of the nuclei: R.A. and decl. (equinox J2000, see Section 2 for details). We reported here the X-ray peak in the 5–7 keV band, which corresponds to the X-ray nucleus position, for sources for which the radio core was not clearly detected, which were all except 3C 280.1, 3C 418, and 3C 454. Col. (5): redshift z. We verified in the literature (e.g., NASA/IPAC Extragalactic Database (NED) and/or SIMBAD databases) if new z values were reported after the release of the 3CR catalog. The only sources with the original Spinrad et al. (1985) are 3C 239, 3C 418, and 3C 454; redshifts of 3C 257, 3C 322, 3C 326.1, and 3C 454.1 were obtained from Hewitt & Burbidge (1991). Lastly, redshifts of 3C 249 and 3C 280.1 were obtained from Best et al. (2003) and Hewett & Wild (2010), respectively. Col. (6): the angular to linear scale factor in arcseconds. Cosmological parameters used to compute it are reported in Section 1. Col. (7): the luminosity distance in Mpc. Cosmological parameters used to compute it are reported in Section 1. Col. (8): galactic neutral hydrogen column densities ${N}_{{\rm{H}},\mathrm{Gal}}$ along the line of sight (Kalberla et al. 2005). Col. (8): the optical magnitude in the V band listed in the 3CR catalog (Spinrad et al. 1985). Col. (9): S₁₇₈ is the flux density at 178 MHz (Spinrad et al. 1985). Col. (10): the Chandra observation ID. Col. (11): the date when the Chandra observation was performed.

Download table as:  ASCIITypeset image

Fluxes were measured in each energy band and region using funtools¹⁷ and assuming a flat energy response in each band as in our previous analyses (see Massaro et al. 2015). Uncertainties were computed assuming Poisson statistics (i.e., square root of the number of photons) in the source and background regions. X-ray fluxes were not corrected for the Galactic absorption, as in our previous analyses (see e.g., Massaro et al. 2015), since we would need to assume a given photon index. X-ray luminosities were not corrected for neutral hydrogen column densities (either galactic or intrinsic), as in our previous analyses. Results for nuclei are reported in Table 2 while lobes fluxes are given in Table 3. We indicate here all diffuse emission detected in the 0.5–7 keV energy range consistent/associated with extended radio structures as X-ray counterparts of lobes, since at these high-redshift radio maps available do not have sufficient resolution to distinguish/identify hotspots. The lobes are labeled as a combination of one letter indicating their orientation and one number indicating their distance from the X-ray nuclei in arcseconds.

Table 2.  X-Ray Emission from Nuclei

3CR	Net	Detection	${F}_{0.5\mbox{--}1\mathrm{keV}}$ i	${F}_{1\mbox{--}2\mathrm{keV}}$ i	${F}_{2\mbox{--}7\mathrm{keV}}$ i	${F}_{0.5\mbox{--}7\mathrm{keV}}$ i	log(${N}_{{\rm{H}}}(z)$)	LX	${L}_{{\rm{X}},\mathrm{obs}}$
Name	Counts	Significance	(cgs)	(cgs)	(cgs)	(cgs)	(cm−2)	(1044 erg s−1)	(1044 erg s−1)
239	7.5 (2.7)	>5	⋯	0.6 (0.5)	4.4 (1.9)	5.0 (2.0)	22.863–23.845	0.283–0.940	0.189-0.300
249	11.5 (3.4)	>5	1.01 (1.08)	0.6 (0.4)	4.8 (1.8)	6.4 (2.1)	19.89–22.38	0.234–0.304	0.222–0.233
257	29.5 (5.4)	>5	⋯	3.18 (1.02)	15.5 (3.6)	18.7 (3.8)	23.412–23.813	3.192–6.087	1.791–2.528
280.1ii	719.6 (26.8)	>5	91.4 (11.8)	123.0 (6.5)	251.7 (14.5)	466.2 (19.8)	20.01–21.507	17.675–17.914	16.607–17.482
322	3.6 (1.9)	4	⋯	⋯	3.6 (1.9)	3.6 (1.9)	19.991–24.001	0.090–0.839	0.088–0.207
326.1iii	1.6 (1.3)	⋯	⋯	⋯	1.8 (1.4)	1.8 (1.4)	24.4–24.404	0.800–1.040	0.110–0.144
418iv	1513.6 (38.9)	>5	22.4 (4.7)	166.0 (7.2)	784.8 (25.4)	973.3 (26.8)	22.871–23.023	69.445–73.719	39.229–39.418
454ii	438.7 (20.9)	>5	50.7 (8.8)	70.8 (4.9)	155.2 (11.1)	276.7 (15.0)	19.902–22.338	12.269–13.608	10.728–11.388
454.1	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯	⋯

Notes. Col. (1): 3CR name. Col. (2): background-subtracted number of photons within a circle of radius of r = 2'' in the 0.5–7 keV band. The number between parenthesis corresponds to the uncertainty in the number of photons computed assuming Poisson statistics. Col. (3): detection significance of the nucleus in the 0.5–7 keV band, assuming the background has a Poisson distribution with a mean scaled to a 2'' radius circle. Col. (4): measured X-ray flux between 0.5 and 1 keV. Col. (5): measured X-ray flux between 1 and 2 keV. Col. (6): measured X-ray flux between 2 and 7 keV. Col. (7): measured X-ray flux between 0.5 and 7 keV. Col. (8): the range of intrinsic column densities needed to obtain the X-ray fluxes reported assuming a model comprising Galactic absorption (fixed to the values reported in 1), a power law with a slope fixed to 1.8, and intrinsic absorption with column density ${N}_{{\rm{H}},\mathrm{int}}$ at the source redshift z. Col. (9): the range of X-ray luminosities in the 0.5–7 keV band without taking into account intrinsic or Galactic absorption. Col. (10): the range of observed X-ray luminosities in the 0.5–7 keV band evaluated from the model with Galactic and intrinsic absorption.

ⁱFluxes are given in units of 10⁻¹⁵ erg cm⁻² s⁻¹ and 1σ uncertainties are given in parenthesis. The uncertainties on the flux measurements are computed as described in Section 2. Fluxes were not corrected for Galactic absorption and were obtained by assuming a flat energy response in each band. ⁱⁱFor 3CR 280.1 and 3CR 454 the number of photons allowed us to extract their spectra and we obtained acceptable fits (i.e., reduced χ² ≃ 0.9) adopting an absorbed power-law model, with only Galactic column density. We found the best fit value of the photon index being 1.54 (0.09) and 1.59 (0.07) with an integrated flux equal to 3.19 × 10⁻¹³ erg cm⁻² s⁻¹ and 5.51 × 10⁻¹³ erg cm⁻² s⁻¹ for 3CR 280.1 and 3CR 454, respectively. Then, assuming a fixed value of the photon index of 1.8, we also estimated the value of the intrinsic absorption (i.e., ${N}_{{\rm{H}},\mathrm{int}}$) consistent with those obtained with the photometric analysis (i.e., hardness ratios) within 1σ being 1.84(0.07) × 10⁻²² and 0.97(0.04) × 10⁻²² for 3CR 280.1 and 3CR 454, respectively. ⁱⁱⁱFor 3CR 326.1 we report the upper limit of the intrinsic column density since the nucleus was not detected. ^ivFor 3CR 418 we also carried out a spectral analysis and fitting with an absorbed power-law model, both including the Galactic and the intrinsic absorption, with a fixed photon index of 1.8. We obtained an estimate of the pileup fraction of 0.13 (0.03) and a value of ${N}_{{\rm{H}},\mathrm{int}}$ equal to $4.52\times {10}^{-22}$ cm⁻² that is consistent with what expected from the source count rate using Portable, Interactive Multi-Mission Simulator (PIMMS) v4.10 (https://cxc.harvard.edu/toolkit/pimms.jsp) and that of the pileup_map task. All of this consistent with the photometric analysis.

Download table as:  ASCIITypeset image

Table 3.  X-Ray Emission from Radio Extended Structures (i.e., Lobes)

3CR	Component	Size	Net	Detection	${F}_{0.5\mbox{--}1\mathrm{keV}}$ a	${F}_{1\mbox{--}2\mathrm{keV}}$ a	${F}_{2\mbox{--}7\mathrm{keV}}$ a	${F}_{0.5\mbox{--}7\mathrm{keV}}$ a	LX
Name		(arcsec)	Counts	Significance	(cgs)	(cgs)	(cgs)	(cgs)	(1044 erg s−1)
239	e4	3	7.9 (2.8)	5	2.1 (1.7)	1.1 (0.6)	1.5 (1.1)	4.7 (2.1)	1.0 (0.5)
249	n11	4	4.1 (2.0)	3	0.7 (1.0)	0.5 (0.5)	1.8 (1.8)	3.0 (2.1)	0.5 (0.3)
249	s12	2.5	9.3 (3.0)	>5	2.0 (1.5)	1.9 (0.7)	0.4 (0.8)	4.2 (1.8)	0.7 (0.3)
257	w6	2	8.5 (2.9)	>5	⋯	0.9 (0.6)	3.9 (1.7)	4.9 (1.8)	2.5 (0.9)
280.1	w12	4.5	7.8 (2.8)	4	⋯	1.4 (0.8)	4.6 (2.5)	6.1 (2.6)	1.2 (0.5)
280.1	e7	5	26.0 (5.1)	>5	3.6 (2.3)	2.3 (0.9)	15.5 (4.1)	21.4 (4.8)	4.1 (0.9)
322	n14	3.5	7.7 (2.8)	5	0.9 (1.1)	1.4 (0.7)	1.9 (1.5)	4.2 (2.0)	0.8 (0.4)
326.1	e3	2.5	4.3 (2.1)	4	2.3 (1.7)	0.2 (0.2)	1.3 (1.2)	3.8 (2.1)	0.9 (0.5)
326.1	w3	2	7.6 (2.8)	>5	2.8 (2.0)	1.4 (0.7)	0.7 (1.0)	4.9 (2.3)	1.2 (0.5)

Notes. Col. (1): 3CR name. Col. (2): component name is a combination of one letter indicating the orientation of the radio structure and one number indicating distance from the nucleus in arcseconds. Col. (3): radius of the circle containing the three rms noise contours of the radio emission. Col. (4): background-subtracted number of photons within the photometric circle in the 0.5–7 keV band. The number between parenthesis corresponds to the uncertainty in the number of photons computed assuming Poisson statistics. Col. (5): detection significance of the lobes in the 0.5–7 keV band, assuming the background has a Poisson distribution with a mean scaled to a 2'' radius circle. Col. (6): measured X-ray flux between 0.5 and 1 keV. Col. (7): measured X-ray flux between 1 and 2 keV. Col. (8): measured X-ray flux between 2 and 7 keV. Col. (9): measured X-ray flux between 0.5 and 7 keV. Col. (10): X-ray luminosity in the range 0.5–7 keV with the 1σ uncertainties given in parenthesis.

^aFluxes are given in units of 10⁻¹⁵ erg cm⁻² s⁻¹ and 1σ uncertainties are given in parenthesis. The uncertainties on the flux measurements were computed as described in Section 2. Fluxes were not corrected for Galactic absorption and were obtained by assuming a flat energy response in each band.

Download table as:  ASCIITypeset image

The Chandra native pixel size for the ACIS instrument is 0492, while the half-energy radius is 0418, so the data are under-sampled.¹⁸ To recover the resolution, images were regridded to half, one-fourth, or one-eighth of the native ACIS pixel size, according to the angular sizes of radio sources and to their number of counts (see also Massaro et al. 2012, 2013 for more details).

As in previous analyses, a comparison between radio and X-ray images was carried out. This allowed us to verify the presence of X-ray counterparts of radio structures and to identify those sources that present diffuse X-ray emission in the soft band that is extended beyond the position of the radio lobes. This diffuse emission could be either due to inverse Compton (IC) scattering of seed photons arising from the cosmic microwave background (CMB) or X-ray emission arising from intracluster medium (ICM) in galaxy clusters or a combination of both processes (see e.g., Croston et al. 2005). We expect this diffuse X-ray emission to peak in the soft band, regardless of its origin. To obtain the detection significance of the diffuse X-ray emission, we assumed Poisson statistics as for nuclei and lobes, using also the same background regions. Emission regions were chosen as circles centered in the X-ray nuclei and including all the radio emission above a three rms level and excluding the regions chosen for the nuclei and the lobes.

Radio images were obtained from the VLA archive website¹⁹ as well as one of our colleague's personal website²⁰ and the Tata Institute of Fundamental Research (TIFR) Giant Metrewave Radio Telescope (GMRT) Sky Survey (TGSS²¹ ). Image parameters for each radio observation used are given in the figure captions of Section 3.

An additional difference with respect to previous survey papers is the lack of spectral analysis. Since all sources in this sample are at high redshift, using only snapshot observations meant that the total number of photons received from each source was too small to carry out this kind of analysis.

3.1. General

We detected X-ray emission in the full band (0.5–7 keV) above a 3σ significance level for all the nuclei in our sample with the only exception of 3C 326.1 and 3C 454.1. Background-subtracted number of photons within a circle of r = 2'' as well as the nuclear fluxes in the soft (0.5–1 keV), medium (1–2 keV), hard (2–7 keV), and full (0.5–7 keV) bands and the X-ray luminosity for each source are shown in Table 2. In contrast with previous analyses (see e.g., Massaro et al. 2012), we do not report the extent ratio of the sources (i.e., the ratio of photons in the 2'' circle and those in an annulus between 2'' and 10'') since nearly all sources in our sample are enclosed within 10'' from their X-ray nucleus.

We detected nine lobes above a 3σ confidence level in six of the sources in our sample. Sizes, the background-subtracted number of photons, X-ray fluxes, and X-ray luminosities of the X-ray counterparts of radio lobes are listed in Table 3.

X-ray detection of nuclei and lobes was carried out in the 0.5–7 keV band as in previous analyses for the 3CR Chandra Snapshot Survey; although, we used the 0.5–3 keV band to detect the diffuse X-ray emission, since we expect this emission to peak in the soft band, regardless of its origin. This diffuse emission was estimated excluding the regions selected for nuclei and lobes.

Out of the seven nuclei detected in X-rays in our sample, three are affected by pileup (see Davis 2001 for details about pileup)—namely 3C 280.1 (5%), 3C 418 (10%), and 3C 454 (3%). Therefore, X-ray fluxes obtained for these sources are underestimated.

Zoom In Zoom Out Reset image size

Out of the nine sources in our sample, we only detected diffuse X-ray emission in the soft band along the radio axis and beyond the lobe radio emission observed at frequencies in the order of GHz in 3C 249, with a 2σ confidence level. This diffuse emission is shown in the left panel of Figure 2, where the X-ray emission in the 0.5–3 keV band with 150 MHz TGSS radio contours overlaid is shown. Additionally, only two out of the nine selected targets—namely 3C 249 and 3C 322—presented extended morphology in TGSS (see left panels of Figures 2 and 5), while the rest are unresolved.

Zoom In Zoom Out Reset image size

As carried out for the analyses of 3CR sources at low redshifts (i.e., below 0.3) as well as in works such as the ones by Hardcastle et al. (2006, 2009), Mingo et al. (2014, 2016), Panessa et al. (2016), Ursini et al. (2018), and Butler et al. (2019) on similar samples, we performed spectral simulations with sherpa, using a model comprising Galactic absorption (fixed to the values reported in Table 1 for each source), a power law with a slope fixed to 1.8, and intrinsic absorption with column density ${N}_{{\rm{H}},\mathrm{int}}$ at the source redshift z. We computed the ranges of the hardness rations (HRs) as the ratio between the fluxes in the soft (0.5–3 keV) and the hard band (3–7 keV) and, from those, we derived the range of intrinsic absorbing column density ${N}_{{\rm{H}},\mathrm{int}}$ as reported in Table 2 for all our detected nuclei, as well as the ranges of X-ray luminosities in the 0.5–7 keV band without taking into account any absorption (L_X) and from the model with Galactic and intrinsic absorptions (${L}_{{\rm{X}},\mathrm{obs}}$).

Moreover, for three sources (namely, 3CR 280.1, 3CR 418, and 3CR 454), the number of detected photons for their nuclei allowed us to carry out detailed spectral analyses as we did for relatively bright 3CR sources at lower redshifts observed during our snapshot survey. All spectral fits in these cases were consistent with a simple power law with galactic absorption, where the photon index is consistent with that of typical AGNs (i.e., 1.8) within 3σ and, in agreement with the HR analysis, the column density from intrinsic absorption is negligible with respect to the Galactic one and/or is unconstrained. A summary of these results can be seen in Table 2.

3.2. Source Details

3C 239. This is an FR II radio galaxy at z = 1.781 (Laing et al. 1983) optically identified as an HERG (Jackson & Rawlings 1997). According to Hammer & Le Fevre (1990), this galaxy lies in a crowded field where there are two different populations of galaxies, one at the same redshift as 3C 239 and one in the foreground. They suggested that the first population belongs to a galaxy cluster surrounding 3C 239 that could be gravitationally lensed by the foreground population. The last statement was supported by Best et al. (1997) since they claim that a foreground galaxy within 5'' of the host of 3C 239, next to the eastern lobe, could cause the presence of a double hotspot and arc-like structures in that lobe. The HST images (Best et al. 1997) show a bright galaxy with two emission regions aligned with the radio axis and a string of emission to the southeast misaligned to the radio axis by ∼45°, as well as two faint tidal tails to the north and west of the galaxy, extending ∼40 kpc. These features could be the result of past galaxy collisions.

Both the nucleus and the eastern lobe of 3C 239 were detected in the X-rays (using the 0.5–7 keV band), using Poisson statistics (see Figure 1). We could not find an archival radio map in which the radio core was detected; therefore, we considered the position of the X-ray nucleus that of the peak emission in the 5–7 keV band, in which the nucleus was detected at a 3σ confidence level. However, we did not detect the presence of diffuse X-ray emission beyond the radio lobes that could indicate the presence of ICM from a galaxy cluster.

3C 249. This source is a lobe-dominated quasar at z = 1.554 (Law-Green et al. 1995). It has not been studied in detail in the literature and, as for the previous source, we could not find a radio map in which the radio core was detected. However, we obtained the position of the nucleus using hard X-rays (see the right panel of Figure 2), in which the nucleus was detected with a 2σ confidence level. We were able to detect the X-ray nucleus and both lobes in the 0.5–7 keV band. Additionally, diffuse X-ray emission was found beyond the position of the radio lobes in the 0.5–3 keV band (with a 2σ confidence level), as shown in the left panel of Figure 2.

3C 257. This is the highest-redshift (z = 2.474) radio galaxy in the 3C catalog. According to Hilbert et al. (2016), the infrared (IR) image shows a distorted morphology (∼2'', which is equivalent to 16.5 kpc at the source redshift), which implies that the source could be undergoing a merging event. Additionally, it shows some optical emission, which implies some ongoing star formation.

We defined the position of the X-ray nucleus using hard X-rays as in previous sources (in this band, the nucleus was detected with a 3σ confidence level). In the Chandra full band image, we detected the nucleus and the western lobe (see Figure 3). We did not detect diffuse X-ray emission in the soft band only.

Zoom In Zoom Out Reset image size

3C 280.1. This source is a lobe-dominated quasar at z = 1.667 (Laing et al. 1983; Véron-Cetty & Véron 2006). The 5 GHz radio map in Lonsdale et al. (1993) shows a strong jet.

In this case, we were able to define the core position using the radio emission (in this position, the X-ray nucleus was detected in the 5–7 keV band above a 5σ confidence level). We detected the nucleus and both lobes in the 0.5–7 keV band (see Figure 4). Furthermore, the eastern lobe of 3C 280.1, which corresponds to the strong jet detected by Lonsdale et al. (1993) at radio frequencies, is the brightest lobe in our sample. The X-ray nucleus of 3C 280.1 is affected by the 5% pileup. Lastly, we did not detect diffuse X-ray emission in the 0.5–3 keV band.

Zoom In Zoom Out Reset image size

3C 322. This is an FR II radio galaxy at z = 1.618 (Law-Green et al. 1995). In the IR, this source appears as an extended object (∼2'', which is equivalent to 17 kpc at the source redshift) with a core that is elongated in the northwest-southeast direction (Hilbert et al. 2016). According to Kotyla et al. (2016), it could be part of a galaxy cluster since there is an overdensity of galaxies on its field.

In the full X-ray band, we detected the nucleus (whose position we found using the peak emission in hard X-rays with a 4σ confidence level) and the north lobe (see the right panel of Figure 5). We found no diffuse emission in the 0.5–3 keV band that could indicate the presence of ICM from a galaxy cluster (see the left panel of Figure 5).

Zoom In Zoom Out Reset image size

3C 326.1. This source is an FR II radio galaxy at z = 1.825. McCarthy et al. (1987) found that the host galaxy is surrounded by a giant Lyα cloud of ∼100 kpc in diameter, which could imply that it is a young and/or star-forming galaxy. IR observations presented by Hilbert et al. (2016) show that the host is a dim galaxy, elongated in the east–west direction and surrounded by many small and irregular sources. Hilbert et al. (2016) also observed several clumps of ultraviolet (UV) emission due to, for example, star formation regions. Lastly, Kotyla et al. (2016) found an overdensity of sources in the field of 3C 326.1, suggesting that it could be part of a galaxy cluster.

We defined the position of the X-ray nucleus of this source using the 5–7 keV band emission, in which the nucleus was detected with a 2σ confidence level. We were not able to detect the X-ray nucleus above a 3σ level of confidence, using the full band X-ray emission. We detected both lobes (see Figure 6) with a confidence level of, at least, 4σ. As for 3C 322, we did not detect diffuse emission that could indicate the presence of ICM to corroborate Kotyla et al. (2016) results.

Zoom In Zoom Out Reset image size

3C 418. This is a QSO at z = 1.686 (Véron-Cetty & Véron 2006). According to Hilbert et al. (2016) in the IR this source can be seen as a bright target submerged in a dense field, due to its location at a low galactic latitude; while in the 4.86 GHz VLA map, the emission comes from the core and a small jet extending 6'' to the northwest.

Using the position of the radio core, we detected the X-ray nucleus in the 5–7 keV band above a 5σ confidence level. We also detected the nucleus in the full X-ray band and it was the brightest nucleus in the full X-ray band in our sample (see Figure 7). Due to the small angular size of the radio emission of 3C 418 (∼3''), we did not attempt to detect X-ray counterparts of radio lobes or diffuse X-ray emission in the soft band. The X-ray nucleus of 3C 418 is affected by a 10% pileup.

Zoom In Zoom Out Reset image size

3C 454. This source is a QSO at z = 1.757 (Véron-Cetty & Véron 2006) defined as a CSS source by O'Dea (1998). Using the 0.5–7 keV Chandra image, we detected the nucleus above a 3σ confidence level (see Figure 8). The position of this nucleus was given by the position of the radio core, in which the nucleus in the 5–7 keV was detected with a confidence level above 5σ. Radio lobes are not resolved in the radio map and, due to the small angular size of the radio emission of 3C 454 (∼15), we did not attempt to detect any extended X-ray counterparts of radio lobes or diffuse X-ray emission in the 0.5–3 keV. The X-ray nucleus of 3C 454 is affected by a 3% pileup.

Zoom In Zoom Out Reset image size

3C 454.1. This is a FR II radio galaxy at z = 1.841 classified as a HERG by Stickel & Kuehr (1996) and as a CSS by O'Dea (1998). Using optical observations, Chiaberge et al. (2015) claimed that this source recently underwent merger activity. Lastly, Kotyla et al. (2016) found an overdensity of sources in its field.

In the case of 3C 454.1, we were not able to identify the X-ray nucleus (see Figure 9). The position reported by Spinrad et al. (1985) matches the northern X-ray peak, however, this peak is most likely the X-ray counterpart of the northern radio lobe. On the other hand, the position of the X-ray peak in the 5–7 keV band matches the position of the southern radio lobe and, therefore we concluded that we could not detect the X-ray nucleus of 3C 454.1. We did not attempt to detect X-ray counterparts of radio lobes or diffuse X-ray emission in the soft band, due to the small angular size of the radio emission of 3C 454.1 (∼3'').

Zoom In Zoom Out Reset image size

We present the X-ray analyses carried out on the nine remaining identified 3C sources up to z = 2.5 (namely 3C 239, 3C 249, 3C 257, 3C 280.1, 3C 322, 3C 326.1, 3C 418, 3C 454, and 3C 454.1) observed by Chandra during Cycle 20 as part of the 3CR Chandra Snapshot Survey. This sample is particularly interesting since it covers the highest-redshift identified extragalactic sources in the 3CR catalog and, therefore, these are the highest-redshift sources observed so far during the 3CR Chandra Snapshot Survey. However, the high redshift prevents us from resolving hotspot emission from lobe emission and, therefore, we label all extended structures not being nuclei as lobes. All data presented in this paper, as well as all previous data from the 3CR Chandra Snapshot Survey, are publicly available in the Chandra archive.

In this paper, we present the main parameters of these newly observed sources. We followed the same data reduction and analysis procedures as in previous papers of the 3CR Chandra Snapshot Survey. We created flux maps in the soft (0.5–1 keV), medium (1–2 keV), and hard (2–7 keV) bands for detected nuclei and extended X-ray features (i.e., lobes) and reported their background-subtracted number of photons, detection significance, fluxes in each band, and X-ray luminosities.

We detected the X-ray nuclei using the 0.5–7 keV emission in all sources in our sample except for 3C 326.1 and 3C 454.1. Additionally, we detected X-ray counterparts of radio lobes in the same band in six of the nine sources in our sample—namely 3C 239, 3C 249, 3C 257, 3C 280.1, 3C 322, and 3C 326.1. In particular, we detected both lobes in 3C 249, 3C 280.1, and 3C 326.1. In the three sources left, the X-ray detected lobe was also the brightest in radio. Lastly, we found diffuse X-ray emission along the radio axis of 3C 249 in the 0.5–3 keV band.

This paper marks the completion of the Chandra Snapshot Survey of identified 3CR sources, carried out to guarantee the X-ray coverage of the entire 3CR extragalactic catalog with Chandra. Thanks to these last nine observations presented here, we completed the analysis of all identified sources listed in the 3CR.

From providing a first glimpse into interesting sources that warranted deeper observations to providing data for more detailed studies, this survey has proven to be an extremely helpful tool in the study of radio sources (see e.g., Hardcastle et al. 2010, 2012; Balmaverde et al. 2012; Orienti et al. 2012; Ineson et al. 2013; Dasadia et al. 2016; Madrid et al. 2018). So far, during the 3CR Chandra Snapshot Survey, we have observed a total of 122 sources that had no previous observations in the Chandra archive. Using these observations, we have been able to detect X-ray counterparts of 119 radio cores.

According to the XJET database,²² thanks to the 3CR Chandra Snapshot Survey, we discovered eight more radio jet knots with an X-ray counterpart in seven different sources thus increasing their number by ∼10% (see Massaro et al. 2011 for latest results on the XJET project). In addition, we also doubled the number of sources that have at least one X-ray detected hotspot. Then, in all previous data papers of the 3CR Chandra Snapshot Survey, we reported the discovery of the X-ray counterpart of radio lobes for 11 3CR sources in addition to those listed in the literature. Additionally, X-ray emission arising from ICM in galaxy clusters was detected in 19 sources including those reported in the archival search (Massaro et al. 2015). Having such a large sample of X-ray counterparts of radio galaxies' components will allow us to study the origin of the X-ray emission from jets and hotspots, which is currently still uncertain, but believed to be nonthermal (see e.g., Harris & Krawczynski 2002, 2006; Worrall 2009), with the most likely scenarios being synchrotron emission or IC scattering.

We remark that the current summary about the X-ray detection of extended radio structures in 3CR sources does not include the following cases for which extensive analyses have been carried out in the literature: namely, 3CR 66A, 3CR 71 (a.k.a. NGC 1068), 3CR 84 (a.k.a Perseus A), 3CR 186, 3CR 231 (a.k.a. M82), 3CR 317 (a.k.a. Abell 2052), and 3CR 348 (a.k.a. Hercules A), as well as 3CR 236, 3CR 263, and 3CR 386 for which the multifrequency analysis is still ongoing (M. Birkinshaw 2020, private communication).

In Figure 10, we show the radio luminosity at 178 MHz obtained form Spinrad et al. (1985) versus the redshift for 3CR sources with Chandra observations, compared those we observed during the 3CR Chandra Snapshot Survey and those with previous archival observations. This comparison shows that at redshift z > 0.5, our snapshot observations pointed the faintest radio sources. In the left panel of Figure 11, we show the redshift distribution of the 3CR sources before and after our survey was completed, including all sources analyzed in this work, and not considering those still unidentified (i.e., lacking an optical counterpart and thus a z estimate). The comparison between these two z distributions highlights the impact of our 3CR Chandra Snapshot Survey on the number of sources at moderate redshift (i.e., above 1.2) as well as in the low redshift range (i.e., between 0.1 and 0.5), which were significantly increased with respect to those listed in the Chandra archive. Finally, the right panel of Figure 11 shows the distribution of the X-ray nucleus luminosities as measured in all data papers published to date (Massaro et al. 2010, 2012, 2013, 2015, 2018; Stuardi et al. 2018) and in this paper.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

To complete the 3CR Chandra Snapshot Survey, the next step will be to obtain observations of the 25 3C sources that remain unidentified. A Chandra campaign is currently ongoing to observe the first nine unidentified sources and results will be presented in a forthcoming paper. Additionally, we are planning another forthcoming paper presenting a statistical study on the whole 3CR Chandra Snapshot Survey in which we plan to include the spectral energy distribution modeling of these sources.

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 NVAS²³ (NRAO VLA Archive Survey), NED²⁴ (NASA Extragalactic Database), and from the DRAGN webpage.²⁵ We are also grateful to the anonymous referee for the constructive comments that helped improve this manuscript. This investigation is supported by the National Aeronautics and Space Administration (NASA) grants GO6-17081X and GO9-20083X. The Chandra X-ray Observatory Center is operated by the Smithsonian Astrophysical Observatory for and on behalf of NASA under contract NAS8-03060. This work is also supported by the "Departments of Excellence 2018-2022" Grant awarded by the Italian Ministry of Education, University and Research (MIUR) (L. 232/2016). This research has made use of resources provided by the Compagnia di San Paolo for the grant awarded on the BLENV project (S1618_L1_MASF_01) and by the Ministry of Education, Universities and Research for the grant MASF_FFABR_17_01. A.J. acknowledges the financial support (MASF_CONTR_FIN_18_01) from the Italian National Institute of Astrophysics under the agreement with the Instituto de Astrofisica de Canarias for the "Becas Internacionales para Licenciados y/o Graduados Convocatoria de 2017." F.M. acknowledges financial contribution from the agreement ASI-INAF No. 2017-14-H.0. A.P. acknowledges financial support from the Consorzio Interuniversitario per la Fisica Spaziale (CIFS) under the agreement related to the grant MASF_CONTR_FIN_18_02. C.S. acknowledges support from the ERC-StG DRANOEL, No. 714245. F.R. acknowledges support from FONDECYT Postdoctorado 3180506 and CONICYT project Basal AFB-170002. C.O. and S.B. acknowledge support from the Natural Sciences and Engineering Research Council (NSERC) of Canada. 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; and 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. TOPCAT²⁶ (Taylor 2005) was used 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 - Very Large Array, MERLIN - , CXO (ACIS) - , GMRT. -