SRG/eROSITA discovery of a strong outburst from the Be/X-ray binary RX J0529.8-6556 in the LMC
ATel #13828; F. Haberl (MPE), J. Wilms, A. Gokus, I. Kreykenbohm, P. Weber, O. Koenig (all ECAP/FAU), C. Maitra, S. Carpano (MPE), G. Vasilopoulos (Yale University)
on 23 Jun 2020; 14:12 UT
Credential Certification: Frank Haberl (fwh@mpe.mpg.de)
Subjects: Optical, X-ray, Binary, Neutron Star, Transient, Pulsar
Referred to by ATel #: 13830
In the course of the first all-sky survey (eRASS1), the eROSITA instrument on board the Russian/German Spektrum-Roentgen-Gamma (SRG) mission discovered a strong outburst from RX J0529.8-6556 in the Large Magellanic Cloud (LMC). RX J0529.8-6556 was discovered in ROSAT data (Haberl et al. 1997, A&A 318, 490) as Be/X-ray binary pulsar with a period of 69.5 s. During the ROSAT observation in Feb. 1993 the source was at a flux level of 3.8×10-12 erg cm-2 s-1 (0.1-2.4 keV) and in Oct. 2000 detected in a deep XMM-Newton observation (Haberl et al. 2003, A&A 406, 471) at 2.2×10-13 erg cm-2 s-1 (0.2-10 keV).
After an initial period of 20 days at the start of the survey when the source was off, eROSITA started scanning RX J0529.8-6556 again from 2020-06-06 00:58 UTC (MJD 59006.04) for about 30-40 s every 4 hrs and found the source in outburst. As of 2020-06-20 an exposure of 2584 s was accumulated. The average background-subtracted, vignetting and PSF corrected count rate in the 0.2-8.0 keV band was 6.43 ± 0.08 cts s-1. Quick-look data provided by the eROSITA near realtime analysis (NRTA) system indicate a flux decline over the last 2 days to about 2 cts s-1, which suggests a decay of the outburst. eROSITA will scan the source for another week and to monitor the outburst further follow-up observations are encouraged.
The average spectrum can be described by an absorbed power-law model with a photon index of 0.94 (±0.01) and an absorption column density of 2.8×1020 (1.9-3.8 ×1020) H cm-2. The average source flux was ~2.5×10-11 erg cm-2 s-1 in the 0.2-8.0 keV band, corresponding to an absorption-corrected luminosity of 7.7×1036 erg s-1, assuming a distance of 50 kpc. All spectral parameters are based on preliminary calibration and 90% confidence ranges are given. The eROSITA light curve shows variations by a factor of ~3, which can partly be attributed to variations within the pulses which are scanned at different pulse phase.
The OGLE I-band light curve of the optical counterpart (lmc512.20.12, see http://ogle.astrouw.edu.pl/ogle4/xrom/xrom.html ) between March 2010 and March 2020 shows strong outburst activity on top of a gradual long-term trend. About 20 outbursts are seen with brightness increases of more than 0.1 mag during individual outbursts. Most of the outbursts peak in phase with a ~155 day period, while several outbursts during the brightest phase of the light curve (~2013-2015) are out of phase by ~0.3. A Lomb-Scargle timing analysis of the first ~2.5 years of the light curve yields a significant signal at 151 days, while during the bright part a peak at 170 days was found in the periodogram. The analysis of the last 3.5 years yields several peaks at 211, 178 and 152 days, the latter consistent with the value from the first 2.5 years. Combining part 1 and 3 of the light curve yields a strong peak at 155 days. Since outbursts of Be/X-ray binaries occur at or near periastron passage of the neutron star (e.g. Reig 2011, Ap&SS 332, 1), we suggest an orbital period of 150-155 days for RX J0529.8-6556.
