eRASSU J050810.4-660653: SRG/eROSITA discovery of a new Be/X-ray binary in the LMC
ATel #13609; F. Haberl, C. Maitra, S. Carpano (all MPE), L. Ducci, V. Doroshenko (all IAAT, Tuebingen), O. Koenig (ECAP/FAU), D. A.H. Buckley (SAAO), I. M. Monageng (SAAO/UCT), A. Udalski (Univ. of Warsaw)
on 2 Apr 2020; 15:29 UT
Credential Certification: Frank Haberl (fwh@mpe.mpg.de)
Subjects: Optical, X-ray, Binary, Neutron Star, Variables
During the beginning of the first all-sky survey (eRASS1), the eROSITA instrument on board the Russian/German Spektrum-Roentgen-Gamma (SRG) mission discovered a new hard X-ray source in the Large Magellanic Cloud (LMC). The X-ray source position at
RA (J2000) = 05:08:10.4, DEC (J2000) = -66:06:53
with an estimated uncertainty of 5.1" radius (dominated by systematics) is consistent with that of an early-type star (U-B=-0.939 and B-V=-0.067) in the MCPS catalogue (Zaritsky et al. 2004, AJ, 128, 1606). The association of the hard X-ray source with an early-type star makes it the first discovery of a new high mass X-ray binary by eROSITA in the LMC.
This conclusion is confirmed by optical spectroscopy of eRASSU J050810.4-660653 which was undertaken on 2020-03-21 using the Robert Stobie Spectrograph on the Southern African Large Telescope (SALT) under the SALT transient followup program. The PG0900 VPH grating was used, which covered the spectral region 3920-7000 Å at a resolution of 6.2 Å. A single 900 s exposure was obtained, starting at 18:17:52 UTC. The spectrum was dominated by a single-peaked H-alpha emission line. The measured line properties are: E.W. = -10.42 Å ± 0.50 Å and FWHM = 13.27 Å ± 0.10 Å, confirming a Be/X-ray binary nature of eRASSU J050810.4-660653.
eROSITA scanned the source from 2019-12-08 at UTC 13:00 hours (MJD 58825.54) to 2019-12-15 22:46 hours (58832.95) accumulating a total exposure of 1.6 ks. The 0.2-8.0 keV spectrum can be described by an absorbed power law with a photon index of ~1.2 and a column density of ~2.1×1021 H cm-2. The source flux was ~3.4×10-12 erg cm-2 s-1 in the 0.2-10.0 keV band, corresponding to an absorption-corrected luminosity of 1.2×1036 erg s-1, assuming a distance of 50 kpc. All spectral parameters are based on preliminary calibration. The eROSITA 0.2-10.0 keV light curve (background-subtracted, vignetting and PSF corrected, seven telescopes combined) shows a long-term increase over the seven days of eROSITA scans from 0.24 to 1.14 cts s-1 (daily averages).
From a recent X-ray observation using the Neil Gehrels Swift observatory on 2020-03-29 and 2020-04-01 (MJD 58937.11 - 58937.59 and 58940.43 - 58940.50, total exposure of about 1520 s) the source was detected with an XRT count rate of 2.5 ± 0.5 ×10-2 cts s-1 in the 0.2-10.0 keV band. Assuming the spectral parameters from above, this corresponds to an observed flux of 1.6 ± 0.3 ×10-12 erg cm-2 s-1 and a luminosity of 4.8 ± 0.9 ×1035 erg s-1 (0.2-10.0 keV). The flux measured by Swift/XRT is lower by a factor of about two than during the eROSITA observations.
The OGLE I-band light curve of the optical counterpart (lmc512.20.12) shows strong variability with brightness changes initially between ~14.6 and ~13.7 mag, but decreasing amplitude with time. Depending on the analysed epoch, timescales of ~333, 379 and 400 days are found, likely related to super-orbital variations of the Be disk (e.g. Rajoelimanana et al. 2011, MNRAS, 413, 1600).
