Powerful Radio Sources in the Southern Sky. II. A Swift X-Ray Perspective

The Third Cambridge Catalog (3C; Edge et al. 1959) and its revised versions (3CR and 3CRR; Bennett 1962; Laing et al. 1983, respectively) is widely considered the gold standard among catalogs of powerful radio sources. Since their releases, they have enabled core investigations into the nature of radio-loud active galactic nuclei (i.e., radio galaxies and quasars; Begelman et al. 1984; Urry 1995; Harvanek et al. 2001; Hardcastle & Croston 2020), their environments at all scales, and feedback processes occurring therein (McNamara & Nulsen 2007, 2012; Fabian 2012; Morganti 2017).

The 3C benefits from an extensive list of surveys, all carried out over six decades, searching for counterparts at all wavelengths from radio (Wills & Parker 1966; Law-Green et al. 1995; Hardcastle & Worrall 2000; Giovannini et al. 2005), to infrared (IR; Simpson et al. 1999; Baldi et al. 2010; Werner et al. 2012; Dicken et al. 2014), optical (Djorgovski et al. 1988; Hiltner & Roeser 1991; Longair et al. 1995; McCarthy et al. 1995, 1996, 1997; de Koff et al. 1996; Lehnert et al. 1999; Martel et al. 1999; Chiaberge et al. 2000; Buttiglione et al. 2009, 2010, 2011), and X-ray energies (Evans et al. 2006; Hardcastle et al. 2006; Balmaverde et al. 2012; Wilkes et al. 2013; Kuraszkiewicz et al. 2021). Many results were also achieved via follow-up observations performed at all frequencies as those obtained by (i) the Hubble Space Telescope (HST) snapshot survey of 3CR radio source counterparts ²⁵ (Madrid et al. 2006; Privon et al. 2008; Chiaberge et al. 2015; Hilbert et al. 2016); (ii) the 3CR Chandra snapshot survey (Massaro et al. 2010, 2012, 2013, 2018; Stuardi et al. 2018; Jimenez-Gallardo et al. 2020); and, more recently, (iii) the MUse RAdio Loud Emission line Snapshot survey (MURALES; Balmaverde et al. 2018, 2018, 2019, 2021, 2022).

However, despite its success, the 3C suffers from an artificial limitation since it is restricted to the Northern Hemisphere. About 2/3 of 3C sources are not visible from the Southern Hemisphere, thus having limited access to modern astronomical facilities and instruments, such as the Multi Unit Spectroscopic Explorer (MUSE; Bacon et al. 2010) and the Enhanced Resolution Imager and Spectrograph (ERIS; Kenworthy et al. 2018) mounted at the Very Large Telescope (VLT), the Atacama Large Millimeter/submillimeter Array (ALMA), the MeerKAT radio telescope (see, e.g., Sejake et al. 2023), and the High Energy Stereoscopic System (HESS) at very high energies. In the near future, new facilities located in the Southern Hemisphere will include the Square Kilometre Array (SKA; McMullin et al. 2020), the Vera Rubin Observatory (a.k.a., Large Synoptic Survey Telescope—LSST; Ivezić et al. 2019), the Extremely Large Telescope ²⁶ (ELT), and the Cherenkov Telescope Array (CTA).

Several attempts were made to create the southern equivalent of the 3C. For instance, Best et al. (1999) used the Molonglo Reference Catalogue (MRC; Large et al. 1981) to select an equatorial sample of 178 radio sources with flux density above 5 Jy at 408 MHz, in the range of decl. between −30° and 10° and having Galactic latitudes ∣b∣ ≥ 10°. This sample is characterized by a high spectroscopic completeness and its footprint mitigates the 3C observability limitation. Burgess & Hunstead (2006a, 2006b) created the Molonglo Southern 4-Jy (MS4) sample, a sample of southern radio sources at 408 MHz with integrated flux densities S₄₀₈ > 4.0 Jy. However it was only until the Murchison Widefield Array (MWA; Tingay et al. 2013) became operational in Western Australia that the southern counterpart of the 3C at ∼178 MHz was finally created. We recently built a sample of 264 extragalactic radio sources extracted from the GaLactic and Extragalactic All-sky MWA 4-Jy (G4Jy) catalog (White et al. 2020a, 2020b), namely: the G4Jy-3CRE (Massaro et al. 2023, hereinafter Paper I), that is the southern equivalent to the 3CR in terms of its nominal flux limit threshold of 9 Jy (Bennett 1962; Spinrad et al. 1985).

In Paper I, we carried out a comparison between archival radio maps and optical images to search for G4Jy-3CRE host galaxies. This analysis was combined with an extensive literature search performed to identify those radio sources with a redshift, z, measurement. We found that 79% of the G4Jy-3CRE sources (i.e., 208 out of 264) have a clear optical counterpart to their radio cores. For 181 radio sources, the optical counterpart is also coincident with the mid-IR one reported in the G4Jy catalog (White et al. 2020a, 2020b). Using both the NASA Extragalactic Database (NED) ²⁷ and the SIMBAD Astronomical Database, ²⁸ we found spectroscopic z measurements for a total of 145 sources (out of 264), corresponding to ∼55% of the G4Jy-3CRE catalog. To be considered for this work, redshifts must have (i) a published figure of the optical spectrum or (ii) a description of such spectrum with emission and/or absorption lines clearly reported in a table and/or in the publication. These conservative criteria were already adopted in previous analyses and spectroscopic campaigns (see, e.g., Massaro et al. 2016; Peña-Herazo et al. 2020, 2022; Kosiba et al. 2023).

In this second paper of the series, we present a first X-ray perspective of the G4Jy-3CRE catalog based on targeted X-ray observations. By searching the archive of the X-Ray Telescope (XRT; Burrows et al. 2000, 2005) on board the Neil Gehrels Swift Observatory (Gehrels et al. 2004), we found that a total of 90 G4Jy-3CRE radio sources, out of 264, were already observed, with at least one observation having a nominal exposure time, T_exp, above 250 s. The main aims of the current X-ray analysis are to (i) use the position of X-ray counterparts, when detected, to verify and eventually refine the results of the previous optical analysis (see Paper I for more details); (ii) test which sources show extended X-ray emission that could be a signature of emission from the intracluster medium (ICM) for those harbored in galaxy clusters and groups, and (iii) obtain measurements of their X-ray count rate, useful to plan X-ray follow-up observations.

We also compared Swift X-ray images with radio maps at different frequencies. Similar to Paper I, we mainly used those radio maps available in the databases of the Very Large Array (VLA) Low-Frequency Sky Survey Redux ²⁹ (VLSSr; Cohen et al. 2007), the Tata Institute of Fundamental Research (TIFR) Giant Metrewave Radio Telescope (GMRT) Sky Survey (TGSS; Intema et al. 2017), the Sydney University Molonglo Sky Survey (SUMSS; Mauch et al. 2003), the National Radio Astronomy Observatory (NRAO) VLA Sky Survey (NVSS; Condon et al. 1998), and the VLA Sky Survey (VLASS; Lacy et al. 2020), corresponding to nominal frequencies of 74 MHz, 150 MHz, 843 MHz, 1.4 GHz, and 3 GHz, respectively, and the NRAO VLA Archive Survey (NVAS) ³⁰ databases. This analysis is only devoted to X-ray observations and it does not include any optical and ultraviolet investigation feasible thanks to the observations collected with the Ultra-violet Optical Telescope (UVOT) instrument (Roming et al. 2005) on board Swift. A dedicated paper on broadband photometry for the G4Jy-3CRE sample, that includes also UVOT data, is currently in preparation (A. García-Pérez et al. 2023, in preparation).

This paper is structured as follows: Section 2 is dedicated to a brief overview of the G4Jy-3CRE catalog and the selection criteria underlying the Swift-XRT archival search. Section 3 is devoted to a description of the data reduction and analysis procedures adopted here while Section 4 is dedicated to the results we achieved. A comparison with previous X-ray analyses is also presented in Section 5 while Section 6 illustrates the search for mid-IR and optical counterparts using X-ray images. Summary, conclusions, and future perspectives are given in Section 7. Finally, the Appendix reports all Swift X-ray images with radio contours overlaid, including a comparison between Chandra and Swift X-ray observations for four sources having unpublished Chandra data sets.

As in Paper I, we adopt cgs units for numerical results and we assume a flat cosmology with H₀ = 69.6 km s⁻¹ Mpc⁻¹, Ω_M = 0.286, and Ω_Λ = 0.714 (Bennett et al. 2014). Spectral indices, α, are defined by flux density, S_ν ∝ ν^−α . For optical images, we used those available in the archives of the Panoramic Survey Telescope & Rapid Response System (Pan-STARRS; Flewelling et al. 2020) and the Dark Energy Survey (DES; Abbott et al. 2018). These optical data were complemented by images available in the red filter of the Digital Sky Survey ³¹ (DSS), for those sources outside the Pan-STARRS and DES footprints. Pan-STARRS magnitudes are reported in the AB system (Oke 1974; Oke & Gunn 1983) while DES magnitudes are given in optical filters similar to those of Pan-STARRS and the Sloan Digital Sky Survey (SDSS; Ahn et al. 2012).

The G4Jy-3CRE sample contains a total of 264 extragalactic radio sources (Paper I). The parent sample of the G4Jy-3CRE catalog was the larger complete sample of extragalactic radio sources with flux densities above the threshold of 4 Jy at 151 MHz, namely the G4Jy catalog (White et al. 2020a, 2020b) based on the recent GaLactic and Extragalactic All-sky MWA survey ³² (GLEAM; Wayth et al. 2015; Hurley-Walker et al. 2017). It is worth noting that MWA is considered as the SKA precursor at low radio frequencies (Tingay et al. 2013).

All radio sources included in the G4Jy-3CRE catalog lie at decl. < −5°, also having Galactic latitudes ∣b∣ > 10°, and are located outside the original footprint of the 3CR catalog. G4Jy-3CRE sources have flux density measurements at 174 MHz, S₁₇₄ and at 181 MHz, S₁₈₁, integrated in the GLEAM subbands, above 8.13 Jy and 7.85 Jy, respectively, where these thresholds correspond to the 9 Jy limiting sensitivity, at ∼178 MHz, which is the nominal value adopted to create the 3CR. The flux density thresholds were computed assuming a power-law radio spectrum and adopting the spectral indices reported in the G4Jy catalog. Thus the G4Jy-3CRE is equivalent, in terms of radio flux density selection, to the northern 3CR extragalactic sample (Bennett 1962; Spinrad et al. 1985). We highlight that relatively bright sources at decl. < +30° and Galactic latitudes ∣b∣ > 10°, including the Orion Nebula, were all masked in the GLEAM extragalactic catalog and are not listed in the G4Jy sample (see White et al. 2020a, 2020b, and references therein for a list of masked sources and additional details).

Here, we cross-matched the G4Jy-3CRE catalog with the Swift archive, adopting a search radius of 10', and then we selected only sources having at least one XRT observation with nominal T_exp longer than 250 s. Our search was restricted only to X-ray observations performed in Photon Counting mode (Hill et al. 2004) to ensure that an X-ray image can be obtained and compared with those at other frequencies. Adopting these criteria, we found a total of 90 radio sources with Swift-XRT data available in the archive. We excluded G4Jy 1038 (aka 3C 279), one of the most famous blazars (see, e.g., Lynds et al. 1965; Stocke et al. 1998) because its Swift-XRT observations are already extensively discussed in the literature (see, e.g., Collmar et al. 2010; Hayashida et al. 2015; Larionov et al. 2020).

We processed a total of 624 individual observations for 89 radio sources belonging to the G4Jy-3CRE catalog. All these observations were acquired between 2005 May and 2022 December. Given that G4Jy 1748 and G4Jy 1749 lie in the same Swift-XRT field, with an ∼4' angular separation, we processed only the nine archival observations centered on the former radio source.

The distribution of the integrated T_exp for the 89 radio sources is shown in Figure 1. We remark that even when the integrated T_exp appears relatively large (i.e., above ∼10 ks), it is the result of merging several, relatively short observations. Since the integrated T_exp for a large fraction of the sources (i.e., ∼90%) is smaller than ∼25 ks, this prevents us from performing a uniform spectral analysis. Thus, we focused on searching for X-ray counterparts and refining the mid-IR and optical analyses.

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During our archival search, we also found four radio sources, namely, G4Jy 171, G4Jy 260, G4Jy 411, and G4Jy 1613 with unpublished Chandra X-ray observations. In these cases, we also reduced these Chandra data and we compared them with the results obtained from the Swift-XRT images. A brief overview of the Chandra data reduction procedure adopted here, together with a comparison between X-ray images collected by the two satellites, is reported in the Appendix.

3.1. X-Ray Source Detection

We adopted the same data reduction procedures described in previous Swift-XRT analyses (see, e.g., Massaro et al. 2008a, 2008b; Paggi et al. 2013; Marchesini et al. 2019, 2020), providing results in agreement with those of the Swift-XRT X-Ray point source catalogs (D'Elia et al. 2013; Evans et al. 2014, 2020). Therefore, we report here only basic details about the data reduction while additional information can be obtained from the references previously listed. The entire X-ray analysis performed here and all X-ray images shown in the following sections are restricted to the 0.5–10 keV energy range, unless stated otherwise.

Raw Swift-XRT data were downloaded from the archive ³³ and reduced to obtain clean event files using the xrtpipeline task, which is part of the Swift X-Ray Telescope Data Analysis Software (XRTDAS; Capalbi et al. 2005), distributed within the HEAsoft package (version 6.30.1). Event files were calibrated and cleaned with standard filtering criteria using the xrtpipeline task, combined with the latest calibration files available in the High Energy Astrophysics Science Archive Research Center (HEASARC) calibration database (CALDB) version (v. 20220907). ³⁴ In particular, using xselect, we excluded time intervals (i) with count rates higher than 40 photons per second and (ii) having the CCD temperature exceeding −50 °C in regions located at the edges of the XRT camera (D'Elia et al. 2013). Clean event files for the same sources were merged using the xselect task, while corresponding exposure maps were merged with the XIMAGE software. ³⁵

We created images for all merged event files and a detection run was then performed using the sliding cell DETECT (i.e., det) algorithm in XIMAGE (Giommi et al. 1992). We chose a threshold of the signal-to-noise ratio (S/N) equal to 3 to claim an X-ray detection and we adopted the following X-ray detection flags (XDFs) to characterize the detection and the identification:

• 1.  
  x (X-ray counterpart): all G4Jy-3CRE sources having a unique X-ray source located at the position of the radio source, within the X-ray positional uncertainty region, e.g., G4Jy 85 shown in left panel of Figure 2;
• 2.  
  e (extended X-ray source): radio sources for which the X-ray counterpart, detected in the Swift-XRT image, shows diffuse/extended X-ray emission, e.g., G4Jy 540 in the central panel of Figure 2;
• 3.  
  u (undetected X-ray counterpart): those merged event files for which the det algorithm did not report an X-ray detection above the chosen threshold at the location of the radio source (see, e.g., G4Jy 718 in the right panel of Figure 2). We also used this flag for four sources for which their X-ray emission is contaminated by a nearby extended object, thus making it challenging to claim a detection (see the following sections for more details).

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View figure set (90 panels)

Tarball of figure set images

For all sources that are undetected in X-rays with the det algorithm (labeled u in Table 1), we carried out a second detection run using the SOurce STAtistic (sosta) tool available in XIMAGE. The sosta algorithm uses a local background to determine the significance of a source detection and to estimate its count rate while detections performed using the det procedure are based on the global background. Thus, the former algorithm could be more accurate for observations with relatively low integrated T_exp. To perform this detection run we were prompted to select the radio source position to compute the background, source counts, count rate, and detection significance. Radio sources having a candidate X-ray counterpart detected using sosta have all an S/N below 3, computed using the det task, and all the numbers of photons are reported without uncertainties in Table 1 to indicate that they correspond to a 3σ upper limit. The sosta algorithm does not provide the X-ray source position and thus it is not reported in Table 1.

Table 1. Results for All G4Jy-3CRE Sources with Archival Swift-XRT Observations

G4Jy	R.A.(r)	Decl.(r)	R.A.(x)	Decl.(x)	n90(nb,90)	Ctr	S/N	Texp	Nobs	XDF	z	NH,Gal
name	(J2000)	(J2000)	(J2000)	(J2000)	(ph)	(10−2 ph s−1)		(s)				(1 × 1020 cm−2)
20	00:10:30.55	−44:22:57.00	⋯	⋯	24(20)	0.2(–)	⋯	5932	6	u	⋯	1.12
27	00:16:02.68	−63:10:07.21	⋯	⋯	25(36)	0.1(0.05)	1.8	4072	6	u	⋯	1.52
77	00:41:29.21	−09:22:40.01	⋯	⋯	0(109)	⋯	⋯	6722	6	u	⋯	2.69
78	00:42:09.19	−44:14:01.68	00:42:08.5	−44:13:57.2	47(58)	0.48(0.11)	4.2	6038	6	x	0.346 (a)	2.61
85	00:46:16.85	−42:07:42.89	00:46:17.4	−42:07:55.7	103(26)	1.04(0.14)	7.4	7128	5	x	0.0526 (b)	1.13
86	00:47:33.66	−25:17:07.37	00:47:33.2	−25:17:32.8	4474(118)	13.5(0.23)	59.6	32,008	13	e	0.00081 (c)	2.79
93	00:52:14.88	−43:06:29.18	00:52:14.6	−43:06:27.3	44(30)	0.45(0.11)	4.1	5273	5	x	⋯	1.17
120	01:05:20.91	−45:05:28.00	⋯	⋯	14(10)	0.51(–)	⋯	2267	3	u	⋯	1.64
162	01:30:27.96	−26:09:58.86	⋯	⋯	2(5)	1.54(–)	⋯	995	1	u	2.34665 (d)	1.31
168	01:31:36.53	−07:03:57.20	⋯	⋯	2(4)	2.46(–)	⋯	1033	1	u	⋯	3.5
171	01:34:00.69	−36:29:01.79	01:33:57.5	−36:29:37.1	165(39)	0.86(0.1)	8.7	11,744	2	x	0.02977 (e)	1.72
213	02:00:12.15	−30:53:26.48	02:00:12.3	−30:53:28.7	31(4)	2.93(0.57)	5.1	1221	1	x	0.6768 (f)	1.54
217	02:02:14.04	−76:20:06.29	02:02:12.8	−76:19:56.7	124(12)	7.53(0.74)	10.1	1750	3	x	0.38939 (g)	5.51
247	02:19:02.81	−36:26:12.23	02:19:03.0	−36:26:05.7	50(16)	5.4(0.9)	6.0	930	1	x	0.488815 (h)	1.81
249	02:20:08.18	−70:22:27.80	02:20:08.2	−70:22:29.9	111(33)	2.56(0.28)	9.1	4606	8	x	⋯	6.06
257	02:23:57.55	−70:59:46.55	⋯	⋯	12(72)	0.4(–)	⋯	3603	5	u	⋯	6.17
260	02:25:02.61	−23:12:47.59	02:25:02.8	−23:12:47.7	111(77)	13.4(0.31)	43.7	20,139	7	x	0.232241 (d)	1.59
290	02:43:44.50	−51:12:33.59	⋯	⋯	1(1)	0.57(–)	⋯	311	1	u	⋯	2.0
293	02:46:56.26	−55:41:22.74	⋯	⋯	17(39)	0.28(–)	⋯	4315	7	u	⋯	2.59
304	02:52:46.31	−71:04:36.19	⋯	⋯	17(19)	0.1(0.06)	1.6	2888	2	u	0.56288 (a)	4.24
373	03:38:46.45	−35:22:46.26	⋯	⋯	0(202)	0.0(–)	⋯	34,042	15	u	0.1126 (i)	1.42
392	03:52:31.88	−07:11:04.03	03:52:30.6	−07:11:05.2	93(43)	0.95(0.14)	6.9	6599	5	x	0.96354 (j)	4.87
404	04:00:16.51	−16:10:13.58	⋯	⋯	12(3)	2.34(–)	1.4	1033	1	u	0.584 (d)	1.97
411	04:05:33.94	−13:08:14.39	04:05:34.1	−13:08:12.3	3510(134)	9.68(0.19)	50.0	37,306	15	x	0.57055 (j)	4.11
415	04:07:48.44	−12:11:33.40	04:07:48.4	−12:11:34.0	1375(23)	34.4(1.0)	33.4	4398	4	x	0.5731 (k)	3.54
416	04:08:20.25	−65:45:10.66	⋯	⋯	35(11)	0.68(–)	⋯	1996	1	u	0.962 (l)	3.68
427	04:12:48.02	−56:00:48.82	⋯	⋯	33(37)	0.29(–)	⋯	4448	3	u	⋯	0.9
446	04:20:56.43	−62:23:36.94	⋯	⋯	15(49)	0.13(0.08)	1.7	4002	5	u	⋯	1.82
464	04:29:40.22	−36:30:56.02	04:29:40.0	−36:30:50.7	58(16)	0.99(0.21)	4.8	3678	2	x	⋯	1.34
492	04:44:37.45	−28:09:48.33	04:44:37.6	−28:09:53.7	1661(70)	11.5(0.3)	38.9	15,533	3	x	0.147 (a)	2.46
506	04:55:14.31	−30:06:47.20	04:55:13.9	−30:06:50.4	38(23)	0.41(0.12)	3.4	4533	2	x	⋯	1.06
510	04:56:09.84	−21:59:15.98	04:56:08.9	−21:59:11.5	489(18)	10.8(0.51)	21.0	4982	1	x	0.533 (m)	2.77
518	05:06:43.75	−61:09:41.51	05:06:43.8	−61:09:41.1	45,86(771)	4.97(0.09)	53.6	96,348	82	x	1.093 (n)	1.95
540	05:22:57.90	−36:27:29.67	05:22:57.9	−36:27:29.3	44,910(376)	56.6(0.27)	210.9	88,761	50	e	0.056546 (o)	3.24
563	05:36:13.67	−49:44:23.10	05:36:13.9	−49:44:20.3	35(30)	0.48(0.11)	4.2	5120	6	x	⋯	3.01
580	05:49:23.49	−40:51:13.04	⋯	⋯	43(47)	0.11(0.06)	2.0	7479	7	u	⋯	3.57
613	06:26:47.21	−54:32:46.20	06:26:49.8	−54:32:35.9	447(41)	0.54(0.11)	4.9	6629	2	e	0.051976 (p)	7.83
614	06:27:06.94	−35:29:12.93	06:27:06.7	−35:29:16.2	7613(578)	24.7(0.33)	73.9	33736	14	x	0.054855 (q)	6.32
618	06:35:45.05	−75:16:15.31	06:35:46.3	−75:16:15.8	9052(282)	16.7(0.21)	80.5	62,843	67	x	0.653 (r)	7.23
619	06:36:31.30	−20:34:38.00	06:36:32.2	−20:34:53.5	194(193)	0.34(0.05)	7.5	25,274	10	x	0.055198 (s)	20.6
644	07:09:21.90	−36:02:10.26	07:09:14.2	−36:01:22.6	1260(53)	6.83(0.23)	29.5	20,319	5	x	0.11 (t)	16.3
651	07:17:06.38	−36:21:52.95	07:17:08.9	−36:22:08.0	156(21)	0.98(0.26)	3.8	2061	2	e	0.031358 (h)	19.1
672	07:43:32.63	−67:26:28.61	07:43:31.8	−67:26:24.4	199(9)	8.61(0.68)	12.7	2868	2	x	1.512 (u)	8.95
718	08:37:14.08	−83:44:40.49	⋯	⋯	42(25)	0.33(–)	⋯	4473	4	u	⋯	10.8
721	08:39:50.80	−12:14:26.56	08:39:50.5	−12:14:34.4	8752(69)	33.4(0.42)	79.3	27,184	7	x	0.19787 (v)	5.43
723	08:41:26.19	−75:40:31.12	08:41:26.6	−75:40:25.6	101(17)	7.18(0.77)	9.4	1414	1	x	0.521 (w)	7.52
835	10:18:09.23	−31:44:15.50	10:18:08.9	−31:44:12.8	70(22)	1.68(0.24)	7.0	3786	3	x	1.346 (x)	5.1
836	10:19:43.84	−42:24:50.34	⋯	⋯	26(37)	0.37(–)	1.2	5403	7	u	⋯	7.91
854	10:33:13.07	−34:18:43.99	⋯	⋯	20(14)	0.67(–)	1.6	3360	5	u	⋯	5.5
876	10:51:30.35	−09:18:13.19	10:51:29.7	−09:18:09.3	1028(53)	11.5(0.38)	30.5	9523	10	x	0.345296 (k)	2.96
917	11:25:54.63	−35:23:22.66	⋯	⋯	11(4)	4.04(–)	0.8	451	1	u	0.033753 (y)	5.97
927	11:34:23.58	−17:27:51.70	11:34:23.8	−17:27:49.4	137(73)	0.59(0.08)	7.2	14,221	14	x	1.618 (d)	2.82
933	11:39:10.69	−13:50:42.90	11:39:10.5	−13:50:40.9	530(34)	7.11(0.36)	20.0	8066	1	x	0.556458 (a)	3.27
950	11:45:30.94	−48:36:10.40	11:45:30.9	−48:36:10.9	161(46)	2.9(0.3)	9.8	5203	7	x	⋯	10.4
1034	12:54:37.02	−12:33:29.47	12:54:35.4	−12:34:00.6	24(13)	1.21(0.4)	3.0	990	3	x	0.015464 (z)	3.07
1071	13:30:07.18	−21:42:04.50	13:30:07.0	−21:41:56.9	63(4)	5.58(0.77)	7.2	1279	3	x	0.528 (α)	7.1
1079	13:34:16.65	−10:09:23.54	13:34:18.6	−10:09:26.0	75(36)	0.26(0.08)	3.4	10,533	1	x	⋯	2.34
1080	13:36:38.91	−33:58:03.45	13:36:38.8	−33:57:57.6	294(78)	2.75(0.21)	12.8	7562	10	x	0.0125 (β)	3.96
1135	14:16:33.65	−36:40:47.02	14:16:33.1	−36:40:51.4	167(39)	2.05(0.19)	10.8	7108	5	x	0.0747 (γ)	4.0
1136	14:16:41.12	−21:46:18.01	14:16:40.8	−21:46:12.1	165(73)	0.37(0.05)	7.0	20,186	10	x	1.116 (δ)	7.74
1145	14:19:49.70	−19:28:32.22	14:19:49.6	−19:28:22.9	1500(40)	17.4(0.52)	33.8	8934	3	x	0.1195 ()	7.19
1148	14:20:04.09	−49:35:47.49	14:20:03.0	−49:35:40.3	360(31)	1.91(0.26)	7.5	4285	3	e	0.09142 (γ)	13.9
1158	14:24:31.90	−49:13:48.13	14:24:31.9	−49:13:54.5	65(35)	0.77(0.16)	4.8	4977	8	x	0.662 (ζ)	14.5
1192	14:48:29.05	−47:01:39.90	14:48:28.3	−47:01:45.3	33(18)	0.41(0.14)	3.1	3600	4	x	⋯	9.33
1203	14:54:28.53	−36:39:57.32	⋯	⋯	86(74)	0.3(–)	1.7	8808	9	u	⋯	5.31
1225	15:10:53.55	−05:43:07.10	15:10:53.4	−05:43:07.4	314(108)	1.87(0.13)	14.4	16,150	7	x	1.191 (η)	5.96
1279	15:48:59.13	−32:17:04.14	⋯	⋯	51(29)	0.51(–)	0.5	2896	1	u	0.1082 (γ)	6.9
1411	17:25:26.13	−80:04:46.04	⋯	⋯	3.0(3)	1.66(–)	⋯	817	2	u	⋯	6.25
1423	17:37:37.14	−56:33:50.71	17:37:35.7	−56:34:04.3	80(64)	2.82(0.38)	7.5	2482	4	x	0.09846 (a)	6.64
1432	17:42:01.59	−60:55:22.39	17:42:01.2	−60:55:12.6	1707(96)	18.1(0.52)	35.0	10,350	5	x	⋯	6.2
1472	18:19:34.76	−63:45:48.27	18:19:34.7	−63:45:47.7	312(34)	4.99(0.33)	15.3	6070	7	x	0.06412 (θ)	6.06
1477	18:22:16.23	−63:59:16.76	18:22:16.3	−63:59:17.1	27(16)	1.58(0.37)	4.2	1542	2	x	⋯	6.79
1498	18:37:42.05	−43:35:32.77	⋯	⋯	5.0(2)	5.55(–)	⋯	263	1	u	⋯	6.5
1518	19:15:47.88	−26:52:57.84	19:15:48.8	−26:52:51.9	189(37)	1.3(0.21)	6.0	4017	6	e	0.226 (d)	7.66
1605	20:10:28.97	−56:26:28.75	⋯	⋯	0.0(11)	⋯	⋯	837	2	u	⋯	4.48
1613	20:18:05.16	−55:40:28.39	20:18:01.0	−55:39:29.0	392(64)	1.85(0.12)	15.5	17,280	4	x	0.060629 (y)	4.77
1635	20:33:16.60	−22:53:20.90	20:33:16.6	−22:53:17.0	594(39)	7.43(0.35)	21.0	8939	4	x	0.131491 (r)	4.01
1640	20:35:47.60	−34:54:02.67	⋯	⋯	6.0(25)	1.46(–)	⋯	827	2	u	⋯	2.58
1664	20:56:04.34	−19:56:35.41	20:56:04.2	−19:56:30.6	88(19)	1.39(0.31)	4.5	1968	8	e	0.15662 (λ)	5.09
1671	21:01:38.97	−28:01:45.58	⋯	⋯	33(13)	0.52(0.29)	1.8	1346	2	u	0.039444 (y)	7.31
1708	21:37:44.45	−14:32:54.22	21:37:45.3	−14:32:53.9	2251(98)	20.7(0.51)	40.9	12,461	17	x	0.20047 (μ)	4.15
1717	21:43:33.18	−43:12:47.79	21:43:33.2	−43:12:46.1	77(26)	1.03(0.19)	5.5	4363	8	x	⋯	1.48
1723	21:47:23.97	−81:32:07.79	⋯	⋯	2.0(4)	3.46(–)	⋯	333	1	u	⋯	8.3
1748	21:57:06.78	−69:41:21.66	21:57:05.8	−69:41:22.0	5176(99)	32.4(0.46)	70.4	18,150	9	e	0.0281 (ν)	2.49
1749	21:57:47.60	−69:41:53.59	21:57:48.3	−69:41:53.3	264(99)	0.41(0.06)	6.9	18,150	9	x	⋯	2.5
1757	22:06:10.33	−18:35:39.01	⋯	⋯	23(18)	0.67(–)	1.3	3641	4	u	0.6185 (ξ)	2.3
1822	23:21:02.01	−16:23:05.21	23:21:01.8	−16:23:03.6	33(21)	0.61(0.15)	4.0	3337	2	x	1.414 (d)	1.69
1840	23:34:26.63	−41:25:26.02	⋯	⋯	8.0(12)	1.26(–)	1.3	1700	4	u	0.907 (x)	1.5
1863	23:59:03.71	−60:55:13.55	23:59:04.1	−60:54:59.8	236(31)	2.39(0.22)	11.0	7118	1	e	0.0959 (y)	1.4

Note. Column (1): the G4Jy name of the radio source, also adopted in the G4Jy-3CRE catalog; columns (2, 3): the R.A. and the decl. (Equinox J2000) of the brightness-weighted radio centroid as reported in the G4Jy catalog, respectively; columns (4, 5): the R.A. and the decl. (Equinox J2000) measured from the distribution of the X-ray photons for radio sources with XDF = x or XDF = e, respectively; column (6): the number of photons measured in the 0.5–10 keV energy range within a circular region enclosing 90% of the Swift-XRT PSF, together with the average number of photons expected in the background for the same area, as reported in parenthesis; column (7): the X-ray count rate measured in photons s^–1 with the 1σ uncertainty in parenthesis as obtained using the det algorithm. For those sources having not statistical uncertainties reported, thus indicated with a dashed line, the count rate corresponds to a 3σ upper limit obtained using the sosta algorithm; column (8): the value of the S/N for all radio sources with an X-ray counterpart detected using the det algorithm. X-ray counterparts, at S/N < 3 correspond to those detected when running the sosta algorithm and in these cases no X-ray coordinates were computed; column (9): the integrated T_exp; col. (10): the total number of sources reduced and analyzed; column (11): the XDF assigned in our analysis ("x" labels radio sources with a detected X-ray point-like counterpart, "e" stands for radio sources with extended X-ray emission around their radio cores, and "u" for G4Jy-3CRE with radio cores undetected in the X-ray band); column (12): the redshift z obtained from the literature analysis presented in Paper I, a question mark indicates sources with uncertain z estimates; and column (13): the Galactic column density N_H,Gal (HI4PI Collaboration et al. 2016).

References. (a) Tadhunter et al. (1993); (b) Whiteoak (1972); (c) Springob et al. (2005); (d) Best et al. (1999); (e) Burbidge & Burbidge (1972); (f) Jones et al. (2004); (g) Jauncey et al. (1978); (h) Jones et al. (2009); (i) Carter & Malin (1983); (j) Lynds (1967); (k) Kinman & Burbidge (1967); (l) Labiano et al. (2007); (m) Wright et al. (1979); (n) Wright et al. (1977); (o) Sbarufatti et al. (2006); (p) Cava et al. (2009); (q) Quintana & Ramirez (1995); (r) Hunstead et al. (1978); (s) Storchi-Bergmann et al. (1996); (t) Parisi et al. (2014); (u) Bergeron & Kunth (1984); (v) Ho & Kim (2009); (w) Browne & Savage 1977); (x) di Serego Alighieri et al. (1994); (y) Tritton (1972); (z) Borne & Hoessel (1984); (α) Burbidge & Kinman (1966); (β) Younis et al. (1985); (γ) Simpson et al. (1993); (δ) Best et al. (2000); () Burbidge (1967); (ζ) Marshall et al. (2011); (η) Peterson & Bolton (1972); (θ) Thompson et al. (1990); (λ) Stickel & Kuehr (1994); (μ) Baldwin (1975); (ν) Marenbach & Appenzeller (1982); (ξ) Morton & Tritton (1982). (See also Paper I for more details.)

A machine-readable version of the table is available.

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In all merged event files, where we detected an X-ray counterpart to a G4Jy-3CRE radio source, we measured (i) the coordinates of the X-ray centroid (J2000 equinox) using the xrtcentroid task, when detected using the det algorithm; (ii) the number of photons n₉₀, within a circular region of radius equal to 120207 (i.e., 51 pixels) centered on the X-ray coordinates enclosing 90% of the Swift-XRT PSF; (iii) the expected number of background photons for the same area n_b,90; and (iii) the count rate, in photons s^–1, and the S/N of the X-ray detection when obtained with the det algorithm. If no statistical uncertainty is reported on the measured count rate, this corresponds to a 3σ upper limit obtained with the sosta algorithm. All these parameters, together with the total number of X-ray observations processed per source, the integrated T_exp, and the value of the XDF, are reported in Table 1.

Finally, in the Appendix, we show X-ray images for all merged event files with radio contours overlaid and a magenta dashed circle of 120207 superimposed, as in Figure 2. The circular region is centered on either the location of the X-ray counterpart of each G4Jy-3CRE radio source, if detected by Swift-XRT, or on its radio coordinates, as reported in the G4Jy catalog, if not. The frequencies of the radio maps used to draw the radio contours and the contour parameters are reported in the Appendix (see Table 2 for more details).

Table 2. Parameters of the Radio Contours Overlaid on the X-Ray Images

G4Jy	ν	lmin	Binning
Name	(MHz)	(Jy beam−1)	Factor
20	150	0.1	×4
27	843	0.3	×2
77	150	0.05	×2
78	150	0.3	×3
85	150	0.3	×3
93	150	0.3	×3
120	150	0.1	×3
162	150	0.1	×4
168	150	0.025	×4
171	150	0.05	×3
213	1400	0.4	×2
217	843	0.1	×3
247	150	0.1	×3
249	843	0.3	×2
257	843	0.2	×2
260	150	0.1	×4
290	150	1.6	×2
293	843	0.1	×2
843	843	0.3	×3
373	3000	0.002	×3
392	3000	0.01	×3
404	150	0.1	×4
411	150	0.1	×4
415	150	0.1	×4
416	843	1.6	×2
427	843	0.3	×3
446	843	0.6	×2
464	150	0.1	×4
492	3000	0.005	×2
506	3000	0.002	×2
510	150	0.1	×3
518	843	0.3	×2
540	150	0.1	×6
563	150	0.1	×4
580	150	0.1	×4
613	843	0.15	×2
614	150	0.1	×3
618	843	0.6	×2
619	150	0.1	×3
644	1400	0.005	×3
651	150	0.1	×2
672	843	0.1	×4
718	843	0.2	×2
721	150	0.1	×4
723	843	0.1	×3
835	150	0.1	×4
836	150	0.1	×4
854	150	0.1	×4
876	150	0.05	×4
917	3000	0.006	×2
927	150	0.1	×4
933	150	0.1	×4
950	150	0.1	×4
1034	3000	0.002	×2
1071	150	0.1	×4
1079	150	0.1	×4
1080	1400	0.04	×2
1135	150	0.1	×2
1136	150	0.1	×4
1145	150	0.2	×2
1148	150	0.2	×3
1158	150	0.1	×4
1192	150	0.1	×4
1203	1400	0.001	×4
1225	150	0.1	×4
1279	150	0.02	×3
1411	843	0.2	×2
1423	843	0.1	×4
1432	843	0.16	×2
1472	843	0.16	×2
1477	843	0.1	×3
1498	150	0.1	×3
1518	150	0.02	×4
1605	843	0.01	×2
1613	843	0.01	×3
1635	3000	0.001	×3
1640	150	1.6	×2
1664	150	0.1	×3
1671	150	0.1	×2
1708	150	0.1	×4
1717	150	0.3	×3
1723	843	0.1	×3
1748	843	0.1	×4
1757	150	0.1	×4
1822	150	0.1	×4
1840	150	0.16	×2
1863	843	0.1	×3

Note. Column (1): the G4Jy name of the radio source, also adopted in the G4Jy-3CRE catalog; column (2): frequency of the radio map used to draw contours in units of MHz; column (3): minimum level of the radio contours l_min; and column (4): binning factor for the radio contours.

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3.2. Extended X-Ray Emission

4.1. An X-Ray Overview of the Whole Sample

4.2. Extended X-Ray Emission around G4Jy 1518 and G4Jy 1664

We found clear signatures of extended X-ray emission around two radio sources in the Swift-XRT images: G4Jy 1518 and G4Jy 1664.

G4Jy 1518 belongs to the equatorial sample of Best et al. (1999) and it is classified as a radio galaxy lying at z = 0.226. It is also listed in the MRC of radio sources (MRC 1912-269; Large et al. 1981). From a radio perspective, G4Jy 1518 appears to be a "restarted" radio galaxy, as shown in the archival radio maps of Figure 3. The host galaxy identification is consistent with detection of the radio core, as recently shown using MeerKAT observations (Sejake et al. 2023).

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G4Jy 1664 is a radio galaxy at z = 0.15662 (aka 6dF J2056043-195635; Stickel & Kuehr 1994; Jones et al. 2009) with a classical FR II radio morphology, as shown in Figure 4. The redshift estimate was originally derived from stellar absorption features, clearly present in its optical spectrum. The radio core emission appears to be similar to giga-peaked radio sources (O'Dea et al. 1991).

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4.3. X-Ray Counterparts of Radio Lobes in G4Jy 1863

An intriguing case is G4Jy 1863, a narrow-line hard X-ray-selected giant radio galaxy (Cusumano et al. 2010; Oh et al. 2018; Bruni et al. 2020) at z = 0.0959 (Tritton 1972; Danziger & Goss 1983; Ramos Almeida et al. 2011) lying in the direction of the galaxy cluster A4067 (Abell 1958; Abell et al. 1989; Teague et al. 1990) and shown in Figure 5. G4Jy 1863 shows several signatures of a past merger in its optical image, for instance it has several irregular shells and two faint arcs (Ramos Almeida et al. 2011).

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In Figure 5 the red circles mark the positions of detected X-ray sources close to the radio structure of G4Jy 1863, and in particular those labeled as S1 and S2. These two point-like sources have clear optical and mid-IR counterparts, as shown in the central and right panels of the same figure. The proximity of these two X-ray sources prevented us from claiming the presence of extended X-ray emission according to the procedure previously described.

However, considering the number of photons associated with both the northern and the southern radio lobes, in circular regions of 20 pixel radius, in comparison with those expected in the background for the same area, we claim a detection above a 3σ level of confidence for the X-ray counterparts of its radio lobes. This could be due to inverse Compton scattering off seed photons arising from the cosmic microwave background (see, e.g., Hoyle 1965; Longair 1970; Harris & Grindlay 1979; Hardcastle et al. 2002; Croston et al. 2005; Harris & Krawczynski 2006; Worrall 2009).

The results obtained with our detection analysis are in agreement with those achievable when cross-matching radio positions, as well as mid-IR and optical ones (when available), with X-ray sources listed in the clean sample of the Second Swift-XRT Point Source Catalog ³⁷ (2SXPS; Evans et al. 2020). The only differences we found are summarized below:

• 1.  
  For four sources, namely G4Jy 27, G4Jy 446, G4Jy 854, and G4Jy 1757, all marked with XDF = u, there is a counterpart listed in the 2SXPS catalog found within the X-ray positional uncertainty at the 90% level of confidence and having an X-ray position consistent with their radio core. We also reported their X-ray detection based on the run carried out with the sosta algorithm having relatively low values of the S/N, below the threshold adopted for the det algorithm. This difference is based on the fact that the 2SXPS catalog used a lower threshold on the S/N to claim a detection. Moreover the 2SXPS catalog is built by reducing Swift-XRT observations with nominal T_exp lower than the threshold of 250 s adopted here.
• 2.  
  Two more sources, namely G4Jy 45 and G4Jy 1038 (aka 3C 279), have an X-ray counterpart listed in the 2SXPS catalog that we did not analyze here. G4Jy 45 has XRT observations beyond the threshold of 10' separation set to select sources and thus was automatically excluded when retrieving data sets from the Swift archive. G4Jy 1038, as previously stated, was not selected as it is extensively discussed in the literature.

We also compared our results with those recently presented by Maselli et al. (2022), who analyzed Swift observations of 31 radio sources listed in the SMS4 catalog (Burgess & Hunstead 2006a, 2006b), 22 in common with our sample. Our results are in agreement with those presented Maselli et al. (2022). For ten sources (G4Jy 93, G4Jy 249, G4Jy 506, G4Jy 563, G4Jy 672, G4Jy 723, G4Jy 950, G4Jy 1135, G4Jy 1192, and G4Jy 1432), we detect the same X-ray counterpart, while G4Jy 257, G4Jy 293, G4Jy 416, G4Jy 718, and G4Jy 836, all marked as XDF = u according to our analysis, have only upper limits on their X-ray count rates measured by Maselli et al. (2022). Moreover, as occurred when comparing our results with those of the 2SXPS catalog, for five sources, namely G4Jy 27, G4Jy 446, G4Jy 580, G4Jy 854, and G4Jy 1203, we detected an X-ray counterpart only by using the sosta algorithm, the same procedure adopted by Maselli et al. (2022).

Finally, the only difference between our analysis and that of the 2SXPS catalog and/or the literature (i.e., Maselli et al. 2022) is for G4Jy 20, which is marginally detected in both analyses but undetected in our investigation. The main reason is due to the threshold on T_exp chosen to retrieve Swift-XRT archival observations. Including all data sets with nominal T_exp above 50 s we also found a marginal detection when adopting the sosta algorithm, with S/N = 1.6 in agreement with previous analyses.

We also compared the results of the X-ray analysis with those obtained from the search for optical counterparts, and all results of this comparison are summarized as follows:

• 1.  
  For the radio source G4Jy 672, with an assigned mid-IR counterpart reported in Paper I, the X-ray counterpart lies within an angular separation of ∼2'' from the optical host galaxy, thus confirming its previous association.
• 2.  
  There are two more sources, namely G4Jy 249 and G4Jy 1432 (see Figure 6), for which available radio maps lack the angular resolution needed to locate their radio cores precisely and consequently their host galaxies in the optical band. However, using the Swift-XRT images it has been possible to determine the position of their optical counterparts using their X-ray emission. We are also able to confirm that the X-ray-detected sources are spatially coincident with their mid-IR counterparts associated in the G4Jy catalog.
• 3.  
  Despite the lack of a potential optical counterpart, the X-ray emission of G4Jy 1136 is spatially associated with its mid-IR counterpart (see also Figure 7) as previously stated in the G4Jy catalog (White et al. 2020a, 2020b). On the other hand, the comparison with mid-IR, optical, and X-ray data carried out for G4Jy 1192 revealed the location of its optical host galaxy, as shown in the right panel of Figure 8, in agreement with the mid-IR association described in Paper I.

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In this second paper of the series, we present a first overview of the X-ray archival observations for G4Jy-3CRE radio sources, mainly focusing on those observed with the XRT instrument on board the Neil Gehrels Swift Observatory.

The G4Jy-3CRE sample was recently extracted from the G4Jy catalog (White et al. 2020a, 2020b) to obtain a list of powerful radio sources, selected at low radio frequencies, equivalent, in terms of flux density, to the 3C catalog but having all sources located in the Southern Hemisphere (Paper I). The main advantage underlying the selection of this sample is the opportunity to study powerful radio sources and emission processes occurring in their large-scale environments with a catalog observable with modern Southern Hemisphere telescopes and instruments such as MUSE and ERIS at VLT and/or ALMA and, in the near future, also SKA, LSST, and ELT.

We retrieved, reduced, and analyzed all archival Swift-XRT data sets collected between 2005 May and 2022 December, having nominal T_exp larger than 250 s for a total of 624 observations processed for 89 sources. We found 30 radio sources with no X-ray counterpart detected above the chosen threshold of S/N = 3. We found nine radio sources with extended/diffuse X-ray emission, estimated according to the criterion based on the Swift-XRT PSF model. In particular, G4Jy 1518 and G4Jy 1664 show clear extended X-ray emission, previously unknown, suggesting that they could be hosted in galaxy clusters, while for G4Jy 1863 we claim the X-ray counterpart detection of its radio lobes. All remaining 50 radio sources have a detected X-ray point-like counterpart of their radio core. The main reason underlying the nondetection of X-ray counterparts for all radio sources analyzed here is due to their relatively short integrated T_exp.

Swift X-ray observations can provide first insights into the presence of diffuse/extended X-ray emission, used as the basis for future studies, and also to (i) confirm previous mid-IR and optical associations and to (ii) locate precisely the host galaxy position for those sources for which the angular resolution of the radio maps was not sufficient to reveal the position of the radio core.

We also obtained additional results that refine our previous radio-to-optical comparison (Paper I):

• 1.  
  For G4Jy 672, having no assigned mid-IR counterpart in the original G4Jy catalog, the X-ray counterpart lies within an angular separation of ∼2'' from the optical host galaxy, thus confirming its optical association since it is smaller than the typical Swift-XRT positional uncertainty for that T_exp (see, e.g., Evans et al. 2020). There is further confirmation of the host galaxy position via detection of the radio core in a recent MeerKAT image (Sejake et al. 2023).
• 2.  
  Our previous radio-to-optical comparison did not allow us to establish firmly the host galaxies of G4Jy 249 and G4Jy 1432 due to the relatively poor angular resolution of the radio maps used to locate their radio cores. However, using merged Swift-XRT images we determined precisely the locations of their optical counterparts from their X-ray emission.
• 3.  
  We also confirmed that the X-ray emission of G4Jy 1136 is spatially associated with its assigned mid-IR counterpart as listed in the original G4Jy catalog.
• 4.  
  The mid-IR, optical, and X-ray comparison carried out for G4Jy 1192 also revealed the location of its optical host galaxy.

Swift-XRT archival data sets, even if not homogeneous in terms of integrated T_exp that span a range between 5 and 96 ks, proved to be a powerful tool to refine the optical search for host galaxies of powerful radio sources and discover the presence of extended X-ray emission. In the cases of G4Jy 1518 and G4Jy 1664, having snapshot XRT observations of ∼4 and ∼2 ks integrated T_exp, it has been possible to reveal the presence of the surrounding ICM.

Swift-XRT observations are certainly paving the path to identify those radio sources deserving X-ray follow-up observations to perform detailed X-ray spectral analyses. This is crucial to complete optical spectroscopic observations necessary to (i) obtain the z measurements for the whole G4Jy-3CRE catalog and (ii) classify all radio sources listed therein from an optical perspective.

Additional X-ray data sets have been already requested and approved for the XMM-Newton satellite, as "filler programs" and as snapshot observations, to achieve a more complete high-energy overview of the G4Jy-3CRE catalog, while follow-up X-ray observations will be also requested in the near future to investigate deeply the large-scale environment of those radio sources harbored in galaxy clusters/groups.

We thank the anonymous referee for useful and valuable comments and suggestions that led to improvements in the paper. We wish to dedicate this paper to D. E. Harris and R. W. Hunstead, their insight, passion, and contributions to radio astronomy are an inspiration for most of us.

F.M. wishes to thank Dr. A. Moretti for their valuable discussions on the Swift-XRT PSF models and for all useful suggestions provided.

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." S.V.W. acknowledges financial assistance of the South African Radio Astronomy Observatory (SARAO; https://www.sarao.ac.za). W.F. and R.K. acknowledge support from the Smithsonian Institution and the Chandra High Resolution Camera Project through NASA contract NAS8-03060. W.F. also acknowledges support from NASA Grants 80NSSC19K0116, GO1-22132X, and GO9-20109X. This investigation is supported by the National Aeronautics and Space Administration (NASA) grants GO9-20083X, GO0-21110X and GO1-22087X. I.A., S.A.C. and V.R. are partially supported by grant PIP 1220200102169CO, Argentine Research Council (CONICET), and by the UNLP research project, G178 (2022-2025). A.G.-P. acknowledges support from the CONACyT program for their PhD studies. V.C. acknowledges support from the Fulbright — García Robles scholarship. This work was partially supported by CONACyT (Consejo Nacional de Ciencia y Tecnología) research grant 280789.

This research has made use of the NASA/IPAC Extragalactic Database (NED), which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France (Wenger et al. 2000).

This research has made use of the CIRADA cutout service at https://cutouts.cirada.ca, operated by the Canadian Initiative for Radio Astronomy Data Analysis (CIRADA). CIRADA is funded by a grant from the Canada Foundation for Innovation 2017 Innovation Fund (Project 35999), as well as by the Provinces of Ontario, British Columbia, Alberta, Manitoba and Quebec, in collaboration with the National Research Council of Canada, the US National Radio Astronomy Observatory and Australia's Commonwealth Scientific and Industrial Research Organisation. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Part of this work is based on the NVSS (NRAO VLA Sky Survey): The National Radio Astronomy Observatory is operated by Associated Universities, Inc., under contract with the National Science Foundation and on the VLA low-frequency Sky Survey (VLSS). We thank the staff of the GMRT that made these observations possible. GMRT is run by the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research. The Molonglo Observatory site manager, Duncan Campbell-Wilson, and the staff, Jeff Webb, Michael White, and John Barry, are responsible for the smooth operation of Molonglo Observatory Synthesis Telescope (MOST) and the day-to-day observing program of SUMSS. The SUMSS survey is dedicated to Michael Large whose expertise and vision made the project possible. The MOST is operated by the School of Physics with the support of the Australian Research Council and the Science Foundation for Physics within the University of Sydney. This scientific work makes use of the Murchison Radio-astronomy Observatory, operated by CSIRO. We acknowledge the Wajarri Yamatji people as the traditional owners of the Observatory site. Support for the MWA comes from the US National Science Foundation (grants AST-0457585, PHY-0835713, CAREER-0847753, and AST-0908884), the Australian Research Council (LIEF grants LE0775621 and LE0882938), the US Air Force Office of Scientific Research (grant FA9550-0510247), and the Centre for All-sky Astrophysics (an Australian Research Council Centre of Excellence funded by grant CE110001020). Support is also provided by the Smithsonian Astrophysical Observatory, the MIT School of Science, the Raman Research Institute, the Australian National University, and the Victoria University of Wellington (via grant MED-E1799 from the New Zealand Ministry of Economic Development and an IBM Shared University Research Grant). The Australian Federal government provides additional support via the Commonwealth Scientific and Industrial Research Organisation (CSIRO), National Collaborative Research Infrastructure Strategy, Education Investment Fund, and the Australia India Strategic Research Fund, and Astronomy Australia Limited, under contract to Curtin University. This work was supported by resources provided by the Pawsey Supercomputing Centre with funding from the Australian Government and the Government of Western Australia. We acknowledge the iVEC Petabyte Data Store, the Initiative in Innovative Computing, and the CUDA Center for Excellence sponsored by NVIDIA at Harvard University, and the International Centre for Radio Astronomy Research (ICRAR), a Joint Venture of Curtin University, and The University of Western Australia, funded by the Western Australian State government.

This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration.

This project used public archival data from the Dark Energy Survey (DES). Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, the Center for Cosmology and Astro-Particle Physics at the Ohio State University, the Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University, Financiadora de Estudos e Projetos, Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro, Conselho Nacional de Desenvolvimento Científico e Tecnológico and the Ministério da Ciência, Tecnologia e Inovação, the Deutsche Forschungsgemeinschaft, and the Collaborating Institutions in the Dark Energy Survey. The Collaborating Institutions are Argonne National Laboratory, the University of California at Santa Cruz, the University of Cambridge, Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas-Madrid, the University of Chicago, University College London, the DES-Brazil Consortium, the University of Edinburgh, the Eidgenössische Technische Hochschule (ETH) Zürich, Fermi National Accelerator Laboratory, the University of Illinois at Urbana-Champaign, the Institut de Ciències de l'Espai (IEEC/CSIC), the Institut de Física d'Altes Energies, Lawrence Berkeley National Laboratory, the Ludwig-Maximilians Universität München and the associated Excellence Cluster Universe, the University of Michigan, the National Optical Astronomy Observatory, the University of Nottingham, the Ohio State University, the OzDES Membership Consortium, the University of Pennsylvania, the University of Portsmouth, SLAC National Accelerator Laboratory, Stanford University, the University of Sussex, and Texas A&M University. Based in part on observations at Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. The Pan-STARRS1 Surveys (PS1) have been made possible through contributions of the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, Queen's University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation under grant No. AST-1238877, the University of Maryland, and Eotvos Lorand University (ELTE). Based on photographic data obtained using The UK Schmidt Telescope. The UK Schmidt Telescope was operated by the Royal Observatory Edinburgh, with funding from the UK Science and Engineering Research Council, until 1988 June, and thereafter by the Anglo-Australian Observatory. Original plate material is copyright (c) of the Royal Observatory Edinburgh and the Anglo-Australian Observatory. The plates were processed into the present compressed digital form with their permission. The Digitized Sky Survey was produced at the Space Telescope Science Institute under US Government grant NAG W-2166. We acknowledge the efforts of the staff of the Anglo-Australian Observatory, who have undertaken the observations and developed the 6dF instrument. This data set or service is made available by the Infrared Science Archive (IRSA) at IPAC, which is operated by the California Institute of Technology under contract with the National Aeronautics and Space Administration.

SAO Image DS9 development has been made possible by funding from the Chandra X-ray Science Center (CXC), the High Energy Astrophysics Science Archive Center (HEASARC), and the JWST Mission office at Space Telescope Science Institute (Joye & Mandel 2003). This research has made use of data obtained from the High-Energy Astrophysics Science Archive Research Center (HEASARC) provided by NASA Goddard Space Flight Center. We acknowledge the use of NASA's SkyView facility (http://skyview.gsfc.nasa.gov) located at NASA Goddard Space Flight Center. TOPCAT and STILTS astronomical software (Taylor 2005) were used for the preparation and manipulation of the tabular data and the images. The analysis is partially based on the OCCAM computing facility hosted by C3S (http://c3s.unito.it/) at UniTO (Aldinucci et al. 2017).

Four G4Jy-3CRE radio sources in the selected sample have X-ray observations in the Chandra archive that are still unpublished but publicly available. Thus we retrieved those data sets and we compare the Swift-XRT and Chandra event files. There are other G4Jy-3CRE radio sources observed with Chandra but these data sets were all already discussed in the literature (see, e.g., individual notes for the G4Jy-3CRE catalog in Appendix B of Paper I for more details).

Data reduction procedures adopted for Chandra observations are extensively described in our previous papers (see, e.g., Massaro et al. 2009a, 2009b, 2011; Jimenez-Gallardo et al. 2021; and references therein for more details). Here we followed the standard procedure described in the Chandra Interactive Analysis of Observations (CIAO) threads ³⁸ and we used CIAO v4.14 with the Chandra Calibration Database (CALDB) version 4.9.6. to generate level 2 event files. These were obtained using the acis_process_events task of Chandra textsubscriptre.pro script, and events were filtered for grades 0, 2, 3, 4, and 6. Light curves were also extracted and inspected for every data set to verify the absence of high-background intervals. Such time intervals were never found in the selected observations. The four radio sources analyzed here, namely G4Jy 171, G4Jy 260, G4Jy 411, and G4Jy 1613, were observed with the ACIS-S detector in VFAINT mode. In particular G4Jy 171 was observed twice (obs. ID numbers 16099 and 17577) but we only reduced the longer observation (25 ks nominal T_exp) carried out on 2015 February 2, while G4Jy 260 (obs. ID 21488, 2020 October 17, 10 ks), G4Jy 411 (obs. ID 23098, 2019 December 12, 10 ks) and G4Jy 1613 (obs. ID 23177, 2020 February 25, 10 ks) where all pointed one time with Chandra, the last two being selected as cool attitude targets. ³⁹

In Figure 9 we show a comparison between the Swift-XRT event file, as shown in Figure 2, with the Chandra level 2 event file restricted in the 0.5–7 keV band. We found that the Chandra X-ray image of G4Jy 260 clearly shows a readout streak (north–south direction) due to the presence of pileup (see, e.g., Davis 2001; McCollough & Rots 2005; and references therein) but we discovered that the radio hotspot, located on the western side of the nucleus, has an X-ray counterpart. Then we detected the X-ray counterpart of all radio cores for these four G4Jy-3CRE sources, thus confirming the Swift-XRT results. Finally, no clear signatures of extended X-ray emission were found in these Chandra data sets, with the only exception of a marginal detection of diffuse X-ray radiation around G4 Jy171.

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