SRG/eROSITA detection of the bright, transient X-ray flare SRGt J123822.3-253206

ATel #13416; J. Wilms, I. Kreykenbohm, P. Weber, S. Falkner, T. Dauser, J. Knies, O. Koenig (all ECAP/FAU), A. Malyali, A. Rau, A. Merloni, D. Bogensberger, H. Brunner, J. Buchner, S. Carpano, M. Freyberg, F. Haberl, C. Maitra, M. Salvato (all MPE Garching), V. Doroshenko, L. Ducci, L. Ji (all IAAT Tuebingen), J. H.M. M. Schmitt (Hamburg), A. Schwope (AIP)
on 22 Jan 2020; 16:27 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Arne Rau (arau@mpe.mpg.de)

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On 31 December 2019 at 6:42 UTC (MJD 58848.280), the eROSITA instrument onboard the Russian/German Spektrum-Roentgen-Gamma (SRG) mission detected a bright X-ray flare localized to

RA(J2000) = 12:38:22.2
Dec(J2000) = -25:32:06

with an estimated positional uncertainty of 10" radius.

The source has been given the tentative designation SRGt J123822.3-253206.

The flare was found by the eROSITA Near Real Time Analysis (NRTA) during a 40s scan of the eROSITA all-sky survey over the position. The event was so bright that it caused pile-up in the eROSITA CCDs, allowing only to derive a lower limit on the count rate of 89+/-3 count/s in the 0.2-10keV band. This corresponds to a lower limit on the peak flux of ~2e-10 erg/cm^2/s.

The source was tentatively detected again in the next eROSITA scan over the field 14.4ks later. By that time the count rate had decreased to 0.37+/-0.23 count/s (~8e-13 erg/cm^2/s). There was no detection in the three subsequent scans nor in in three scans prior the flare.

The origin of the event is currently unknown. Short X-ray flares are typically either originating from stellar flares or neutron star-related X-ray bursts. A stellar flare origin appears unlikely: For typical bright stellar flare luminosities of 1e31 erg/s, the source would have to be within 20pc and thus likely known. There is no GAIA counterpart in the error circle, which excludes nearby star as a source of the burst.

We note that also an X-ray burst is difficult to reconcile without an optical counterpart. Three optically faint (~21mag) sources were found within the X-ray error circle in the VHS survey and in PanSTARRS DR2, with one also being detected by NEOWISE (J123822.64-253210.9) with W1=15.8mag, and W1-W2~0mag. While either of those sources may be associated with the detected X-ray flare, their observed optical/NIR brightness is inconsistent with the expectations for a 20pc M star.

The transient was also independently discovered by the hard X-ray instrument ART-XC on SRG. (See the accompanying ATel by Semena et al. for further information.)

About eROSITA: The SRG/eROSITA All-sky survey started on 2019 December 8th and for the next four years will cover the entire sky every 6 months. Bright transients will be discovered by an automatized Near Real Time Analysis and reported in future ATels.