学术文献库

We present Swift observations of 31 sources from the SMS4 catalog, a sample of 137 bright radio sources in 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, 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 the 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. 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. We present the list of 56 SMS4 sources that in 2022 March remain to be observed in the X-rays with narrow-field instruments.