SRG discovery of SRGA J204318.2+443815 = SRGE J204319.0+443820 - possible Galactic X-ray transient

ATel #14206; I. Mereminskiy(1), P. Medvedev(1), A. Lutovinov(1), M. Gilfanov(1, 2), A. Semena(1), R. Sunyaev(1, 2), S. Molkov(1), V. Arefiev(1) (1: IKI RAS, Moscow, Russia; 2: MPA, Garching, Germany)
on 23 Nov 2020; 22:23 UT
Credential Certification: Ilya Mereminskiy (i.a.mereminskiy@gmail.com)

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On Nov 20, 2020 during the second ongoing survey, the Mikhail Pavlinsky ART-XC telescope on board the SRG observatory discovered a bright X-ray source at RA= 310.8259, Dec=44.6374 (J2000) with a 90% error radius of 25". The source was registered with the X-ray flux of about 1.7 mCrab (~2.4x10^-11 ergs cm^-2 s^-1) in the 4-12 keV energy band. Note that no sources were detected at that position in the first survey in May 2020.

We used the SRG/eRosita data to obtain an enhanced position of the source and to perform a spectral analysis. The soft X-ray source was detected in both surveys at the position of RA= 310.8293, Dec=44.6390 (J2000, 95% confidence radius is 4"). The source spectrum obtained with eRosita in the second survey indicates a high level of absorption. Approximating data with a simple tbabs*powerlaw model we estimated the absorption column density to be about 10²³ cm^-2 and the photon index of 1.75. The observed flux in the 0.5-10 keV band is 1.8(-0.8/+0.3)x10^-11 ergs cm^-2 s^-1. No prominent variability is seen in the source light curve.

Interestingly, while the source flux in the 0.5-10 band is increased by a factor of four in comparison with the first eRosita survey, its soft flux (0.5-2 keV) seems to be down by half. This could indicate variations in the intrinsic absorption or a drastic change of the spectral shape.

The eRosita position is formally consistent with the optical transient ZTF18abjpmzf identified with a distant (approx. 5.5 kpc, Bailer-Jones+, 2018, n.b. a large proper motion) star, although this association is tentative. Given the low Galactic latitude of the transient (b<1.5 degree) we could assume that the transient is a Galactic object.

Multiwavelength observations are urged to uncover the nature of this transient.

ART-XC and eRosita are X-ray telescopes on board the SRG observatory which was launched from the Baikonur cosmodrome on 13 July 2019 by a Proton rocket. ART-XC has been renamed 'Mikhail Pavlinsky ART-XC telescope', after the tragic loss of its former PI, Dr. Mikhail Pavlinsky, who passed away in July 2020.