A Swift X-Ray View of the SMS4 Sample. II. X-Ray Properties of 17 Bright Radio Sources

Based on a proposal to observe 18 bright radio sources from the SMS4 catalog with the Neil Gehrels Swift Observatory (hereafter Swift), we obtained X-ray observations of 17 targets (one target was not observed). Following up our first paper that discussed 31 sources (see Maselli et al.; 20 sources detected as point sources and one very extended source), we present results for this final sample of 17 radio sources that previously lacked dedicated, pointed narrow-field-of-view (FOV) X-ray observations. One of these 17 sources, undetected by Swift due to a very short exposure, was instead detected by eROSITA, and given in the Data Release 1 (DR1) Catalog. No 1eRASS source was found in the DR1 for the remaining source, unobserved by Swift. The new Swift observations led to 11 X-ray source detections in the 0.3–10 keV band and six upper limits. We investigated the extent of the X-ray emission and the hardness ratio, and when statistics allowed, we carried out a spectral analysis. The X-ray emission of eight sources is consistent with pointlike emission, while three sources show clear evidence of extent, each with peculiar properties. We used the X-ray determined positions and uncertainties of the 12 detected sources to establish associations with infrared and optical sources from the AllWISE and the GSC 2.4.2 catalogs. Requiring a detection in both the infrared and the optical bands to establish a candidate counterpart for our X-ray detections, we identify counterparts for all 12 sources. Following this X-ray-based approach to derive the position of the active nucleus, we are able to confirm the same IR counterparts previously proposed by White et al. for eight sources, and provide four new IR candidates. In the optical, we identify counterparts that match the candidates previously given by Burgess & Hunstead for all sources. We discuss the interesting structure of MRC B0344−345 and PKS B2148−555, two of the six extended X-ray sources that we detected in both our Swift campaigns, and suggest they are very promising for further X-ray and radio investigations. For the 38 SMS4 sources that lack pointed, narrow-FOV X-ray telescope observations, after our Swift campaigns, we list 18 likely counterparts from the eROSITA DR1 catalog.