New X-ray transient Swift J162315.0-175233: a possible counterpart to Fermi J1623-1752

ATel #14945; K. L. Page, P. A. Evans and J. P. Osborne (U. Leicester)
on 29 Sep 2021; 15:15 UT
Credential Certification: Kim Page (kpa@star.le.ac.uk)

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Following the announcement by Fermi-LAT of a new gamma-ray source Fermi J1623-1752 (Rani et al., ATel #14939), with an error region encompassing the recurrent nova U Sco, Swift performed a ~5 ks Target of Opportunity observation of the field of U Sco between 2021 September 28 20:40 UT and September 29 03:17 UT. No X-ray source is detected at the location of U Sco by the XRT, to a 3-sigma upper limit of 2.2x10^-3 count s^-1. The UVOT measures magnitudes of u = 18.95 +/- 0.25, uvw1 = 19.60 +/- 0.32, uvm2 = 19.81 +/- 0.18 and uvw2 = 19.36 +/- 0.30. Given that U Sco in outburst reaches a visual magnitude of ~7.5, these observations show that the nova is not currently erupting, a conclusion also reached by Sokolovsky et al. in ATel #14941.

There is, however, a previously uncatalogued X-ray source, at a position of RA, Dec(J2000) = 245.81275 deg, -17.87598 deg, which is equivalent to

RA(J2000) = 16h 23m 15.06s
Dec(J2000) = -17d 52m 33.5s
with an estimated 90% uncertainty radius of 2.9 arcsec.

This source has a 0.3-10 keV XRT count rate of 0.020 +/- 0.003 count s^-1, with a spectrum which can be approximated by a power-law of Gamma = 1.5 +0.7/-0.6, with an absorbing column consistent with the Galactic value of 2.32x10²¹ cm^-2 (Willingale et al. 2013). The 0.3-10 keV flux is 9.3 x 10^-13 erg cm^-2 s^-1 (observed; 1.1 x 10^-12 erg cm^-2 s^-1 unabsorbed).

Combining all the previous Swift-XRT observations (a total of > 300 ks), the earlier upper limit at this position is 1.85 x 10^-4 count s^-1.

This new source is not detected by the UVOT, to the following limits: u > 19.61, uvw1 > 20.02, m2 > 20.83 and uvw2 > 19.78.

This source was automatically detected as a likely transient by the forthcoming Live Swift-XRT Point Source Catalogue (LSXPS), which is currently undergoing pre-release testing, and is named Swift J162315.0-175233.

The position of Swift J162315.0-175233 is only 29 arcsec from the Fermi-LAT position given in ATel #14939, and therefore fully consistent with that localisation. This new X-ray source may be the counterpart of the new Fermi-LAT detection.