eROSITA detection of flares from the Be/X-ray binary A0538-66

¹ Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
e-mail: ducci@astro.uni-tuebingen.de
² ISDC Data Center for Astrophysics, Université de Genève, 16 chemin d’Écogia, 1290 Versoix, Switzerland
³ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica, Via A. Corti 12, 20133 Milano, Italy
⁴ Max-Planck-Institut für extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
⁵ INAF - Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (LC), Italy
⁶ Universität Erlangen/Nürnberg, Dr.-Remeis-Sternwarte, Sternwartstraße 7, 96049 Bamberg, Germany
⁷ Astronomical Observatory, University of Warsaw Al. Ujazdowskie 4 00-478 Warszawa, Poland

Received: 23 March 2021
Accepted: 28 May 2021

Abstract

In 2018, XMM-Newton observed the awakening in X-rays of the Be/X-ray binary (Be/XRB) A0538-66. It showed bright and fast flares close to periastron with properties that had never been observed in other Be/XRBs before. We report the results from the observations of A0538-66 collected during the first all-sky survey of eROSITA, an X-ray telescope (0.2-10 keV) on board the Spektrum-Roentgen-Gamma satellite. eROSITA caught two flares within one orbital cycle at orbital phases φ = 0.29 and φ = 0.93 (where φ = 0 corresponds to periastron), with peak luminosities of ~2-4 × 10³⁶ erg s^-1 (0.2-10 keV) and durations of 42 ≤ Δt_fl ≤ 5.7 × 10⁴ s. The flare observed at φ ≈ 0.29 shows that the neutron star can accrete considerably far from periastron, although it is expected to be outside of the circumstellar disk, thus providing important new information about the plasma environment surrounding the binary system. We also report the results from the photometric monitoring of A0538-66 carried out with the REM, OGLE, and MACHO telescopes from January 1993 until March 2020. We found that the two sharp peaks that characterize the orbital modulation in the optical occur asymmetrically in the orbit, relative to the position of the donor star.