SRG/eROSITA detection of SWIFT J004427.3-734801 suggests a Be+WD X-ray binary system

ATel #13709; F. Haberl, C. Maitra, J. Greiner, A. Malyali, S. Carpano (all MPE), J. Wilms, I. Kreykenbohm, P. Weber (all ECAP/FAU), L. Ducci (IAAT, Tuebingen), A. Schwope, G. Lamer (all AIP), G. Vasilopoulos (Yale University)
on 5 May 2020; 18:48 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 started scanning the Small Magellanic Cloud (SMC). A source with supersoft X-ray spectrum was detected at a position consistent with that announced for SWIFT J004427.3-734801, a new Be/X-ray binary candidate (Coe et al. 2020, ATel #13626). The eROSITA source position (after small astrometrical corrections) was determined to

RA (J2000) = 00:44:27.86, DEC (J2000) = -73:48:02.4

which lies 1.1" from the proposed counterpart 2MASS J00442806-7348031, well within the 1.9" statistical uncertainty.

eROSITA scanned the source from 2020-04-29 at UTC 08:43 hours (MJD 58968.36) to 2020-05-01 00:43 hours (MJD 58970.03) accumulating a total exposure of 347 s. The average background-subtracted, vignetting and PSF corrected count rate in the 0.2-2.3 keV band was 0.22 ± 0.03 cts s^-1. The 0.2-3.0 keV spectrum can be described by an absorbed black-body model with a temperature kT = 76 eV (56-104 eV) and an absorption column density of 2.4×10²¹ (0.8-5.3 ×10²¹) H cm^-2. The source flux was ~2.5×10^-13 erg cm^-2 s^-1 in the 0.2-3.0 keV band, corresponding to an absorption-corrected luminosity of 2.2×10³⁶ erg s^-1, assuming a distance of 60 kpc. All spectral parameters are based on preliminary calibration and 90% confidence ranges are given.

The luminosity derived from the eROSITA spectrum is similar to that reported by Coe et al. A stringent 3-sigma upper limit of ~4.6×10^-15 erg cm^-2 s^-1 (0.2-2 keV) from an XMM-Newton pointed observation in April 2006 (http://xmmuls.esac.esa.int/upperlimitserver/ assuming a 60 eV black-body) demonstrates the high variability of SWIFT J004427.3-734801 of more than a factor of 50, which likely rules out an AGN nature of the source. The association of the supersoft X-ray source with an early-type star suggests a Be plus White Dwarf (Be+WD) X-ray binary, one of the very few found in the Magellanic Clouds so far (see e.g. Cracco et al. 2018, ApJ 862, 167). From the normalisation of the best-fit black-body model a radius for the emission region of 870 km is derived, which can be up to 8000 km due to the large uncertainty in absorption. This may suggest that we see X-ray emission from only a fraction of the WD surface.