SRGt J123822.3-253206: Short, bright and hard X-ray transient detected by SRG/ART-XC during the all-sky survey

ATel #13415; A. Semena, I. Mereminskiy, A. Lutovinov, S. Molkov, M. Pavlinsky (IKI RAS) on behalf of ART-XC collaboration
on 22 Jan 2020; 16:25 UT
Credential Certification: Ilya Mereminskiy (i.a.mereminskiy@gmail.com)

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During the ongoing survey observations the ART-XC telescope on board the SRG observatory detected a short and intense transient event. In the 26 second transit over the source position (middle of transit is at 2019-12-31 06:43:02 or 58848.27988 MJD) we found a bright source with the count rate of ~3 counts/s in the 4-20 keV band. The transient was also independently detected in the SRG/eROSITA data (see the accompanying ATel#13416 by J. Wilms). We chose to use same name for this transient, SRGt J123822.3-253206, with coordinates as determined by eROSITA, due to its better positional accuracy.
We estimated the source flux to be 7x10^-11 ergs/cm2/s in the 4-10 keV band and 6x10^-11 ergs/cm²/s in the 10-16 keV band. The source was below detection limits in scans before and after this event (separated by 4 hours in time).
We independently determined the position of this transient as RA, Dec = 189.5912, -25.5332 (J2000) with the 15" uncertainty (68% confidence interval), which is agrees well with eROSITA data. There are no known X-ray sources located within 2' radius around the source position. We also checked Swift/BAT and Fermi/GBM for subthreshold triggers and found none. We have requested Swift/XRT observation of the source in order to constrain its late-time properties, the observation is to be performed in next several days.
The source nature is still unclear and we consider different origins of the observed event, both galactic and extragalactic, although given the absence of bright optical sources in the localization region (Pan-STARRS DR1 shows only two sources with z-magnitude <20) we can preliminary exclude possibility that this transient is a flare on nearby star.
We strongly encourage follow-up observations in other wavebands to solve this puzzling picture.
ART-XC is an X-ray telescope on board the SRG observatory. The telescope is designed and built by IKI (Russian Academy of Sciences) and RFNC, with the participation from MSFC/NASA.