SRG discovery of SRGA J043520.9+552226 = SRGE J043523.3+552234, an X-ray counterpart of optical transient ATLAS19bcxp

ATel #13571; I. Mereminskiy, P. Medvedev, A. Semena, M. Pavlinsky, S. Molkov, A. Lutovinov, R. Burenin, S. Sazonov (IKI), R. Sunyaev, M. Gilfanov (IKI, MPA)
on 20 Mar 2020; 19:01 UT
Credential Certification: Ilya Mereminskiy (i.a.mereminskiy@gmail.com)

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On March 18, during the ongoing all-sky survey, SRG/ART-XC discovered a bright X-ray source at RA=68.837, Dec=55.374 (J2000) with a 90% error radius of 30 arcsec. The X-ray flux is about 10^-11 erg/cm²/s in the 4-12 keV band, with no significant variability detected between four ART-XC consecutive passages separated by 4 hour intervals. The ART-XC localization region contains optical transient ATLAS19bcxp (AT 2019wey), which was discovered on December 7, 2019 and later independently detected by GAIA and classified as a hostless transient/possible supernova.

The X-ray source was also detected by SRG/eROSITA, which provided a more accurate localization: RA=68.8472, Dec=55.3760 with a 68% error circle radius of ~5 arcsec. These coordinates are just 0.8 arcsec off the position of ATLAS19bcxp, thus confirming their association. No previously known X-ray sources or other obvious optical counterparts were found within the eROSITA and ART-XC error boxes. The eROSITA spectrum can be equally well described by a power-law model with NH = (6.8±1)x10²¹ and Gamma = 2.25±0.35 and by a thermal bremsstrahlung model with NH = (5.5±0.7)x10²¹ and kT=3.2_-1^+1.8 keV. No emission lines were detected in the X-ray spectrum, with the apec model giving a 68% (one parameter) upper limit on the metal abundance of 0.2, assuming plasma in collisional equilibrium. The source's flux is 4.7x10^-12 and 1.3x10^-11 erg/cm²/s in the 0.3-2 and 2-8 keV energy bands, respectively.

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