eRASSt J100130.9-614021: A new bright X-ray source discovered by SRG/eROSITA

ATel #13844; A. Rau, C. Maitra (both MPE), L. Ducci (IAAT), J. Schmitt, (Univ. Hamburg), J. Wilms, I. Kreykenbohm, Ph. Weber (all ECAP/FAU), M. Salvato (MPE), G. Lamer, A. Schwope (both AIP)
on 1 Jul 2020; 13:50 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Arne Rau (arau@mpe.mpg.de)

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During the second all-sky survey (eRASS:2), the eROSITA instrument on board the Russian/German Spektrum-Roentgen-Gamma (SRG) mission discovered a new X-ray source, eRASSt J100130.9-614021, localized to

RA(J2000) = 10:01:30.93 (150.37887 deg)
Dec(J2000) = -61:40:21.5 (-61.67265 deg)

with an estimated uncertainty of 2.5" radius (incl. systematics).

The source was detected by the eROSITA Near-Real Time Analysis when it first entered the eROSITA Field of View on 2020 June 28th 05:15 UTC (MJD 59028.22). The source was scanned by eROSITA over the following ~40hr with a 4hr cadence and ~40s integration each. the lightcurve showed an initial rise to a count rate of ~3 cnt/s (0.2-10keV) and a subsequent slow decline to ~1cnt/s.

The X-ray spectrum with an exposure of ~392s can be described equally well by a blackbody model with kT=0.46+/-0.06 keV and by an absorbed power-law model with a slope of 2.5+/-0.5 and an N_H=(2.5+/-1.2)E21 cm^-2 (uncertainties are given at 90% confidence level). In the case of the power-law model, the required equivalent neutral Hydrogen column density is consistent with the Galactic foreground value (N_H=~3.4E21 cm^-2; HI4PI Collaboration, N. Ben Bekhti et al., 2016). N_H is unconstrained in the black body fit.

Assuming the black body model, the resulting 0.2-5 keV flux is ~2.9E-12 erg/cm^2/s. The location of the source was already observed by eROSITA as part of the first all-sky survey on 2019 December 23rd-26th. No source was detected during this time period with a 3-sigma upper limit on the 0.2-5keV flux of 2.9E-13 erg/cm^2/s.

The position of eRASSt J100130.9-614021 coincides with a bright optical/IR source listed as 5255954374373045888 in Gaia DR2 (https://www.cosmos.esa.int/gaia) and J100130.87-614020.9 in CatWISE2020 (W1_Vega=11.31+/-0.01 & W2_Vega=11.39+/-0.01; Eisenhardt, 2020, ApJS, 247, 69). The Gaia BP-RP color and effective temperature suggest the object to be a K-type star. The distance, as provided by Bailer-Jones et al. (2018, AJ, 156, 58) is ~1.1kpc. The corresponding X-ray luminosity of ~7E32 erg/s, or L_X/L_bol = 0.07, coupled with the observed long duration, is extremely unusual for K-type star. The light curve and fx/fopt agree well with an accreting compact object. The size of the emission region, as calculated from the distance, luminosity, and black body temperature, would be consistent with a hot spot on a white dwarf or neutron star.