eRASSU J052914.9-662446: SRG/eROSITA discovery of a second Be/X-ray binary in the LMC

ATel #13610; C. Maitra, F. Haberl, S. Carpano (all MPE), O. Koenig (ECAP/FAU), V. Doroshenko, L. Ducci (all IAAT, Tuebingen), D. A.H. Buckley (SAAO), I. M. Monageng (SAAO/UCT), A. Udalski (Univ. of Warsaw)
on 2 Apr 2020; 15:38 UT
Credential Certification: Frank Haberl (fwh@mpe.mpg.de)

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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) = 05:29:14.9, DEC (J2000) = -66:24:46

with an estimated uncertainty of 5.1" radius (dominated by systematics) is consistent with that of an early-type star (U-B=-0.71 and B-V=-0.21) 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 second discovery of a new high mass X-ray binary by eROSITA in the LMC (after eRASSU J050810.4-660653; Haberl et al. 2020, ATel #13609).

This conclusion is confirmed by optical spectroscopy of eRASSU J052914.9-662446 which was undertaken on 2020-03-20 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 1200 s exposure was obtained, starting at 18:33:47 UTC. The spectrum was dominated by a double-peaked H-alpha emission line. The measured line properties are: E.W. = -8.32 Å ± 0.37 Å and FWHM = 16.72 Å ± 0.11 Å, confirming a Be/X-ray binary nature of eRASSU J052914.9-662446.

eROSITA scanned the source from 2019-12-08 at UTC 16:01 hours (MJD 58825.67) to 2019-12-22 23:34 hours (MJD 58839.98) accumulating a total exposure of 2.9 ks. The 0.2-8.0 keV spectrum can be described by a power law with a photon index of ~0.6 and little source-intrinsic absorption. Therefore, the column density was fixed at the Galactic foreground value of 6.0×10²⁰ H cm^-2. The source flux was ~1.53×10^-12 erg cm^-2 s^-1 in the 0.2-10.0 keV band, corresponding to an absorption-corrected luminosity of 4.7×10³⁵ 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 variations on a time scale of days over the two weeks of eROSITA scans between 0.08 and 0.27 cts s^-1 (1.2 day averages).

From recent X-ray observations using the Neil Gehrels Swift observatory on 2020-03-30 between 13:40 and 23:28 UTC (MJD 58938.57 - 58938.98, 2322 s exposure) the source was detected with an XRT count rate of 2.22 ± 0.41 ×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 2.0×10^-12 erg cm^-2 s^-1 and a luminosity of 6.0×10³⁵ erg s^-1 (0.2-10.0 keV), which is at a similar brightness level as during the eROSITA observations.

The OGLE I-band light curve of the optical counterpart (lmc519.13.19023) shows strong variability with brightness changes between ~14.7 and ~14.3 mag around a mean of 14.5 mag. A signature for a coherent periodicity is found at ~151 days which could indicate the orbital period of the binary system.