Clumpy winds and wind accretion in high mass X-ray binaries

To-scale sketch of the high mass X-ray binary Vela X-1. The B-type companion star is shown in blue, the orbit of the neutron star in black. Red marks orbital phases discussed in Grinberg et al. 2017. The background shows the variable large-scale structures that are formed through the interactions of the neutron star with the stellar wind of the mass donor based on simulations in Manousakis (2011). [Credit: Grinberg et al. 2017]

Sketch of clumpy wind structure in a high mass X-ray binary. The overdense regions (clumps) smoothly move with the wind in the radial direction and induces variable absorption along the line of sight towards the compact object (solid line). [Credit: Grinberg et al. 2017]

High resolution observations of the Silicon lines in the spectrum of the HMXB Cyg X-1 at varying level of absorption from minimal (non-dip) to strongest (strong dip). As the dip deepens, lines from lower charged ions appear and become stronger. This behavior can be explained by structured clumps, with less ionized inner parts and more ionized outer parts. [Credit: Hirsch et al. 2019]

We are made of stardust—or, at least in significant parts, of material processed in stars. Hot, massive giant stars can drive the chemical evolution of galaxies and trigger and quench star formation through their strong winds and their final demise as supernovae. The winds of such stars are fast, reaching velocities up to a few thousands kilometres per second, and strong, carrying away up to 10^-5 solar masses per year. Such winds can obviously strongly influence the evolution of the star and its interaction with the environment. Yet, optical and X-ray measurements of the wind mass loss disagree and can only be reconciled if the winds are highly structured, with colder, dense clumps embedded in a tenuous hot gas. This agrees with theoretical predictions that winds in massive stars should be highly unstable to perturbations in their velocity. However, measuring the clumpy structure of such winds is challenging. In (quasi-)single stars wind properties are inferred for the whole clump ensemble and no measurements of individual clumps or clump groups are possible.

Luckily, nature provides us with perfect laboratories to study clumpy winds: high mass X-ray binaries (HMXBs), systems where compact objects accretes matter from the wind of an giant stellar companion of O/B type. HMXBs are an important steps in the evolution of massive stars binaries and possible progenitors of compact object merger events that have been recently also detected as sources of gravitational waves with LIGO/VIRGO.

Historically, HMXBs have been mostly used to study accretion and ejection processes close to black holes and neutron stars. But HMXBs are also unique labs to study winds: The radiation created close to the compact object effectively X-rays the wind that passes through the line of sight towards the observer. Employing absorption resolved approaches we can thus shed light on the wind structure, specially the elusive layers within a few stellar radii from the the O/B star, including possible distortion of the wind by the presence of the compact object and intrinsic clump structure.