A Swift X-Ray View of the SMS4 Sample. III. Deeper Insight into Previously Undetected Sources

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Abstract
We update the X-ray information given in A. Maselli et al. for five bright radio sources in the SMS4 catalog, thanks to additional observations with the X-Ray Telescope on board the Neil Gehrels Swift Observatory (hereafter Swift) carried out through 2026 February. X-ray emission from MRC B1754−597, previously based only on data from the eROSITA-DE DR1 catalogs, is now detected also by Swift, which yields a more precise positional uncertainty. Thanks to ∼7 ks of additional exposure with Swift, we are now able to detect MRC B1817−391, a source for which no X-ray counterpart is found in eROSITA-DE DR1. Another source that is not detected in the eROSITA-DE DR1 catalogs, and that was not observed by Swift earlier than 2025, is MRC B1827−360: with a ∼6 ks X-ray observation, the source is detected. Finally, based on ∼8 ks of additional exposure, we detect X-ray emission for MRC B2032−350, a source that lies out of the DR1 footprint. In contrast, additional exposure (∼2 ks) just gives an upper limit on the X-ray emission of MRC B1814−519. The analysis of the extent and the hardness ratio of the four detected X-ray sources suggests the presence of soft, diffuse X-ray emission as expected from galaxy group cores or hot galaxy coronae. Using the positional uncertainty of the X-ray detections to constrain the search for counterparts at lower energies, we provide a new infrared/optical counterpart for MRC B1817−391 and confirm the counterparts previously reported in the literature for the three remaining sources.

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