A Swift X-Ray View of the SMS4 Sample—X-Ray Properties of 31 Quasars and Radio Galaxies

Abstract

We present Neil Gehrels Swift Observatory (hereafter Swift) observations of 31 sources from the SMS4 catalog, a sample of 137 bright radio sources originally designed to extend the well-studied 3CRR radio sample to the Southern Hemisphere. All these sources had no Chandra or XMM-Newton observations: 24 of these were observed with Swift through a dedicated proposal in 2015, and data for the remaining seven were retrieved from the Swift archive. The reduction and analysis of data collected by the Swift X-ray Telescope (XRT) led to 20 detections in the 0.3–10 keV band. We provide details of the X-ray emission in this band for these 20 detections, as well as upper limits for the remaining 11 SMS4 sources. When statistics allowed, we investigated the extent of the X-ray emission and the hardness ratio, and we carried out a spectral analysis. We matched the 20 X-ray-detected sources with infrared (AllWISE, CatWISE2020) and optical (GSC 2.3.2, DES DR2) catalogs to establish associations with infrared and optical sources and compared our results with previously published counterparts in these bands. Requiring a detection in both the infrared and optical bands to establish a candidate counterpart for our X-ray detections, we obtain reliable counterparts for 18 sources, while the remaining two sources need further investigation to establish firm identifications. In the infrared, we confirm 12 previously established counterparts and provide six new candidates. In the optical, we find agreement with 13 previously established counterparts, while we provide an alternative candidate for five SMS4 sources. We find that ∼35% of all the SMS4 sources lie below the lower limit of 10.9 Jy for the flux density at 178 MHz established for the 3CRR sample, at variance with the values extrapolated using measurements at higher frequencies. Therefore, for future studies where flux-density-limited samples are needed, we encourage the use of the more recent G4Jy sample. We present the list of 56 SMS4 sources that in 2022 March remain to be observed in the X-rays with narrow-field instruments, to highlight interesting sources and organize further observational campaigns to achieve complete X-ray coverage for the whole SMS4 in the forthcoming years.

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

“…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.…”

“…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).…”

“…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).…”

Powerful Radio Sources in the Southern Sky. I. Optical Identifications

“…Then, radio spectra for the MS4 sources were compiled from the literature to estimate flux densities at 178 MHz, and the subset of SMS4 with S 178 > 9 Jy was extracted. Some sources listed in the SMS4 were also recently observed in the soft X-rays (Maselli et al 2022).…”