Speeding the spinning top with wind-captured discs and clumpy wind accretion in Supergiant X-ray binaries
|Date||5 June 2019|
The historical detection of a gravitational wave signal from a double neutron star (NS) merger last year ushered in a new era for the study of these fascinating compact objects. With more double NS detections expected in the incoming years, a better understanding of the impact of binarity on stellar evolution is now necessary to trace back the evolution of these highly magnetized spinning tops. In high Mass X-ray Binaries (HMXB), the transient albeit decisive phase preceding double NS systems, a NS orbits a supergiant O/B star and captures part of its stellar wind. We aim at characterizing the mass and angular momentum transfer to the accreting NS.
Our numerical simulations of wind accretion onto the NS reveal a complex 3D geometry : depending on the efficiency of the wind launching, on the extent of the NS magnetosphere and on the cooling mechanism of the flow, a wind-captured disc can form around the accretor, with dramatic consequences on the spinning up/down of the NS. Because the spin of the NS retroactively controls accretion and outflows through magneto-centrifugal gating effects, a better understanding of the angular momentum loss and transfer in wind-fed HMXB is required to interpret the observed torques and evaluate their capacity to eventually lead to a NS merger within a Hubble time. In this talk, I will discuss the conditions of formation of a wind-captured disc and the impact of the inhomogeneities in the wind (or “clumps”) on the time variability of the X-ray flux in Supergiant X-ray binaries. I will also present how slow winds can provoke a significant enhancement of the mass transfer rate to the accretor, up to levels suitable for the super-Eddington accretion regime observed in Ultra-luminous X-ray sources.