eRASSt J160827.6-605822: SRG/eROSITA detection of an extreme X-ray brightening of a candidate RS CVn system

ATel #14046; Iuliia Grotova, Arne Rau (both MPE), Jonathan Knies, Ralf Ballhausen (both ECAP), Jan Robrade, Juergen Schmitt (both University of Hamburg), Ingo Kreykenbohm, Philipp Weber, Joern Wilms (both ECAP)
on 28 Sep 2020; 09:16 UT
Credential Certification: Arne Rau (arau@mpe.mpg.de)

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In the course of the second all-sky survey (eRASS2), the eROSITA instrument aboard the Russian/German Spektrum-Roentgen-Gamma (SRG) mission discovered a new, soft X-ray source, designated as eRASSt J160827.6-605822, and located at:

RA(J2000)= 16:08:27.66 (242.11523 deg)
DEC(J2000)= -60:58:22.3 (-60.97286 deg)

with an estimated positional uncertainty of 4“ radius.

The source was discovered by the eROSITA Near-Real Time Analysis on 2020 Sep 13 10:31 UTC (MJD 59105.44) and detected also in the following five visits (cadence of 4hrs) before leaving area covered during the all-sky scan. The light curve is consistent with being flat with a count rate of ~0.5 cnts/s in the 0.2-5 keV band.

The X-ray spectrum can be represented using the APEC model (Smith 2001), which describes emission from the collisionally ionized plasma. With the abundance parameter fixed at Solar values, we find kT= 1.5(+0.76, -0.3) keV and an equivalent neutral hydrogen column density of N_H<10^21 cm^-2. Alternatively, the spectrum can be described with an absorbed power law of slope 2.8+/-0.6 with NH consistent with the Galactic foreground value (N_H=1.9e21cm^-2, HI4PI Collaboration, N. Ben Bekhti et al., 2016). Statistical errors are given at 90% confidence level.

Assuming the thermal plasma model, the 0.2-5keV flux is 4.6e-13 erg/s/cm^2. This is over 5 times larger than the 3-sigma upper flux limit of 8e-14 erg/s/cm^2 derived from the non-detection during the first eROSITA all-sky survey on 2020 March 14-15. Upper limits from archival ROSAT and XMM observations computed using the Upper Limit Server (http://xmmuls.esac.esa.int/hiligt/) are less constraining.

The position of eRASSt J160828-605822 coincides with an optical/IR point source with a Gaia r'-band magnitude of 13.7 at distance of ~2kpc (DR2: source id 5832870672429289728). It is also listed as J160827.85-605823.5 in the AllWISE Source Catalog with W1= 11.81+/-0.02 mag and W2=11.80+/-0.02 mag. The source is also included in the ASAS-SN Variable Stars Database (https://asas-sn.osu.edu/variables) with an optical period of ~1.2 days.

The apparent short optical period, combined with the large X-ray luminosity and thermal X-ray spectrum with a temperature of 10^7 K are consistent with an active star or stellar system, possibly a distant RS Canum Venaticorum binary system, however, the computed X-ray luminosity of ~2x10^32 erg/s, constant over >20hrs, is unusually large for this class of objects.

Follow-up observations, in particular optical spectroscopy in chromospheric emission lines, is strongly encouraged to help verifying the nature of this source.