eROSITA calibration and performance verification phase: High-mass X-ray binaries in the Magellanic Clouds

¹ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
e-mail: fwh@mpe.mpg.de
² Institut für Astronomie und Astrophysik, Sand 1, 72076 Tübingen, Germany
³ Dr. Karl-Remeis-Sternwarte und ECAP, Universität Erlangen-Nürnberg, Sternwartstraße 7, 96049 Bamberg, Germany
⁴ Astronomical Observatory, University of Warsaw, Al. Ujazdowskie 4, 00–478 Warszawa, Poland
⁵ Moscow State University, Sternberg Astronomical Institute, 119234 Moscow, Russia
⁶ Kazan Federal University, Department of Astronomy and Space Geodesy, 420008 Kazan, Russia

Received: 26 July 2021
Accepted: 25 August 2021

Abstract

Context. During its performance verification phase, the soft X-ray instrument eROSITA on board the Spektrum-Roentgen-Gamma (SRG) spacecraft observed large regions in the Magellanic Clouds in which almost 40 known high-mass X-ray binaries (HMXBs, including candidates) are located.

Aims. We looked for new HMXBs in the eROSITA data, searched for pulsations in HMXB candidates, and investigated the long-term behaviour of the full sample using archival X-ray and optical data.

Methods. For sufficiently bright sources, we performed a detailed spectral and temporal analysis of their eROSITA data. A source detection analysis of the eROSITA images in different energy bands provided count rates and upper limits for the remaining sources.

Results. We report the discovery of a new Be/X-ray binary in the Large Magellanic Cloud. The transient SRGEt J052829.5–690345 was detected with a 0.2–8.0 keV luminosity of~10³⁵ erg s⁻¹. It exhibits a hard X-ray spectrum, typical for this class of HMXBs. The OGLE I-band light curve of the V~15.7mag counterpart shows large variations up to 0.75 mag, which occur with a quasi-period of~511 days. The eROSITA observations of the Small Magellanic Cloud covered 16 Be/X-ray binary pulsars, 5 of which were bright enough for an accurate determination of their current pulse period. The pulse periods for SXP 726 and SXP 1323 measured from eROSITA data are~800 s and~1006 s, respectively, which is very different from their discovery periods. Including archival XMM-Newton observations, we update the spin-period history of the two long-period pulsars, which have shown nearly linear trends in their period evolution for more than 15 yr. The corresponding average spin-down rate for SXP 726 is 4.3 s yr⁻¹, while SXP 1323 has a spin-up rate of –23.2 s yr⁻¹. We discuss the spin evolution of the two pulsars in the framework of quasi-spherical accretion.