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

TL;DR

This study reports eROSITA's detection of unprecedented X-ray flares from the Be/X-ray binary A0538-66, revealing new insights into its plasma environment and accretion behavior far from periastron.

Contribution

First all-sky survey detection of flares from A0538-66 by eROSITA, showing neutron star accretion far from periastron and providing new system environment information.

Findings

eROSITA detected two flares at different orbital phases.

Flares had peak luminosities of 2-4 x 10^36 erg/s.

Optical monitoring revealed asymmetric orbital modulation peaks.

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 (SRG) satellite. eROSITA caught two flares within one orbital cycle at orbital phases $\phi = 0.29$ and $\phi = 0.93$ (where $\phi=0$ corresponds to periastron), with peak luminosities of $\sim 2-4 \times 10^{36}$ erg/s (0.2-10 keV) and durations of $42 \leq \Delta t_{\rm fl} \leq 5.7\times 10^4$ s. The flare observed at $\phi \approx 0.29$ 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.