SRG/eROSITA discovery of eRASSU J050213.8-674620, a candidate Be+WD X-ray binary system in the LMC

ATel #13789; F. Haberl, C. Maitra, J. Greiner, S. Carpano (all MPE), J. Wilms, I. Kreykenbohm, P. Weber, O. Koenig (all ECAP/FAU), A. Udalski (Univ. of Warsaw)
on 7 Jun 2020; 22:22 UT
Credential Certification: Frank Haberl (fwh@mpe.mpg.de)

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In the course of the first all-sky survey (eRASS1), the eROSITA instrument on board the Russian/German Spektrum-Roentgen-Gamma (SRG) mission has nearly finished scanning the Large Magellanic Cloud (LMC). A source with supersoft X-ray spectrum was detected at the following position (after preliminary astrometrical corrections):

RA (J2000) = 05:02:13.85, DEC (J2000) = -67:46:20.8

with a 1.0" statistical 1σ uncertainty. As likely optical counterpart we identify the emission-line star AL 55 (Howarth 2013, A&A 555, A141) with magnitudes and colours consistent with an early-type star.

eROSITA scanned the source from 2020-05-21 04:56 UTC (MJD 58990.20) to 2020-05-27 12:56 (MJD 58996.54) accumulating a total exposure of 1504 s. The average background-subtracted, vignetting and PSF corrected count rate in the 0.2-0.6 keV (0.2-5.0) band was 0.29 ± 0.02 (0.30 ± 0.02) cts s^-1. The spectrum can be described by an absorbed black-body model with a temperature kT = 43 eV (37-50 eV) and an absorption column density of 8×10²⁰ (5-12 ×10²⁰) H cm^-2. The source flux was ~4.8×10^-13 erg cm^-2 s^-1 in the 0.2-2.0 keV band, corresponding to an absorption-corrected luminosity of 1.8×10³⁶ erg s^-1, assuming a distance of 50 kpc. All spectral parameters are based on preliminary calibration and 90% confidence ranges are given.

The source is not listed in any X-ray catalogue, but was covered by two pointed ROSAT PSPC observations (500063p, 500060p) within 50' off-axis angle in 1991 and 1992, unfortunately near the window support structure. The most stringent 3σ upper limit of 2×10^-3 cts s^-1 (0.2-2 keV) from the 1991 observation corresponds to a factor of about 25 lower flux than expected from the eROSITA spectrum, assuming the best-fit model parameters from above. Like for SWIFT J004427.3-734801 (ATel #13709), the association of the supersoft X-ray source with an early-type star suggests a Be plus White Dwarf (Be+WD) X-ray binary. From the normalisation of the best-fit black-body model a radius for the emission region of ~3500 km is derived, consistent with the surface of a white dwarf.

During the observing period of OGLE-IV, AL 55 was located in the gap between CCD chips, while during OGLE-III it was located in two independent fields (OGLE ids lmc124.1.4920 and lmc125.4.33202), enhancing the cadence of the observations. The combined OGLE-III I-band light curve shows typical variability seen from Be/X-ray binaries with brightness changes within a band of ±0.025 magnitudes on top of a larger long-term trend. The average brightness over the OGLE-III epoch (2002-2009) was 13.56 mag.