LSXPS discovery of the transient Swift J150636.7-411910 coincident with the galaxy ESO 328-19

ATel #16239; Sergio Campana (INAF-OA Brera), Phil Evans & Kim Page (University of Leicester)
on 12 Sep 2023; 13:55 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Sergio Campana (sergio.campana@brera.inaf.it)

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The Living Swift XRT Point Source Catalogue experiment (Evans et al. 2023, MNRAS 518 174) detected a new transient in the field of GRB230911C.
The transient, named Swift J150636.7-411910, has coordinates:
RA(J2000): 15h 06m 36.76s
DEC(J2000): -41^o 19' 10.8"
with an error radius of 6.1".
The transient position is consistent with the nearby S0 galaxy ESO 328-19, at z~0.01554 (2.94" separation).
As announced (GCN 34633), the UVOT produces elongated images due to a problem under investigation with one of the gyroscopes. Swift J150636.7-411910 is ~7' from the observation target (GRB 230911C) and is therefore outside the actual UVOT field of view (~4' radius).
The transient was discovered together with the GRB on 2023-09-11 at 10:10:26.
GRB230911C has been observed again during the same day, and Swift J150636.7-411910 as well.
The count rate rose with respect to the first observation by a factor of ~2.
The peak count rate (0.3-10 keV) is now 1.2 (+0.4,-0.3)x10^-2 ct s^-1.
Taking 5.7 ks of data, we obtain 36 source counts. Fixing the absorbing column density to the Galactic value of (9.8x10²⁰ cm^-2) we can constrain the power law photon index to 2.0+-0.5 (90% c.l.). The average 0.3-10 keV unabsorbed flux is 3.2x10^-13 erg s^-1 cm^-2, corresponding to a luminosity of 1.9x10⁴¹ erg s^-1 (assuming a distance of 70 Mpc).
A fit with a blackbody works as well: the blackbody temperature is kT_BB=0.45+-0.17 keV and the flux is a factor 1.7 lower. The equivalent radius at 70 Mpc is R_BB~5x10⁸ cm. The relatively hot temperature and small radius are not typical of a tidal disruption event.
The LSXPS transients page is:
https://www.swift.ac.uk/LSXPS/transients/2784