Accretion mode of the Ultra-Luminous X-ray source M82 X-2

TL;DR

This paper investigates the accretion mode of the ULX M82 X-2, proposing that it is likely a neutron star accreting from a Be-star disc, based on observed spin and orbital period relations.

Contribution

It demonstrates that standard wind accretion models do not fit M82 X-2, but an extended Be-type HMXB model can explain its properties.

Findings

Spherical wind accretion is inconsistent with M82 X-2.

Be-type HMXB disc wind accretion explains observed spin-orbit relation.

Roche lobe overflow is a possible accretion mode under certain conditions.

Abstract

Periodic pulsations have been found in emission from the ultra-luminous X-ray source (ULX) M82 X-2, strongly suggesting that the emitter is a rotating neutron star rather than a black hole. However, the radiation mechanisms and accretion mode involved have not yet been clearly established. In this paper, we examine the applicability to this object of standard accretion modes for high mass X-ray binaries (HMXBs). We find that spherical wind accretion, which drives OB-type HMXBs, cannot apply here but that there is a natural explanation in terms of an extension of the picture for standard Be-type HMXBs. We show that a neutron star with a moderately strong magnetic field, accreting from a disc-shaped wind emitted by a Be-companion, could be compatible with the observed relation between spin and orbital period. A Roche lobe overflow picture is also possible under certain conditions.