XMM-Newton observations of the peculiar Be X-ray binary A0538-66

TL;DR

This paper reanalyzes XMM-Newton observations of the peculiar Be X-ray binary A0538-66, revealing distinct spectral states and accretion regimes, and discusses their implications for understanding high-mass X-ray binaries.

Contribution

It provides a detailed timing and spectral analysis of A0538-66's active state, distinguishing accretion and propeller regimes and comparing them with other X-ray binaries.

Findings

Identification of direct accretion and propeller states not correlated with orbital phase.

Spectral components differ between accretion and propeller regimes.

Presence of a 6.4-keV emission line during accretion state.

Abstract

A0538-66 is a neutron star/Be X-ray binary located in the Large Magellanic Cloud and, since its discovery in the seventies, it showed a peculiar behavior which makes it a unique object in the high-mass X-ray binaries scene: the extremely eccentric orbit (e=0.72), the short spin period of the neutron star (P=69 ms), the episodes of super-Eddington accretion. These characteristics contribute to a remarkable bursting activity that lasts from minutes to hours and increases the flux by a factor 10^3-10^4. In 2018, A0538-66 was observed by XMM-Newton in a particularly active state, characterized by a forest of short bursts lasting 0.7-50 seconds each. In this contribution we present a reanalysis of these observations. The timing analysis allowed us to distinguish between the epochs of direct accretion and propeller state, that do not correlate with the orbital position of the neutron star. The spectral analysis revealed that during the accretion regime three components (a soft one, a hard one, and a ~6.4-keV emission line) equally contribute to the overall emission, while the propeller regime is characterized by a single soft component. We discuss these findings in the context of spherical and disk accretion regimes, highlighting the similarities and the differences with other X-ray binary systems.