Author: Ryosuke Hirai, Monash University December 13, 2021

Artist's impression of CygnusX-1. Image source: Mark Myers, OzGrav-Swinburne University

The first evidence of the existence of a black hole was discovered in the 1960s, when strong X-rays were detected in a system called Cygnus X-1. In this system, a black hole is surrounded by a massive star blowing extremely strong wind, whose wind is more than 10 million times stronger than the wind from the sun. Part of the gas in this wind is attracted to the black hole by gravity, forming an "accretion disk", which emits the strong X-rays we observe. These systems with black holes and massive stars are called "massive X-ray binary star systems", which are very helpful for understanding the properties of black holes.

Nearly 60 years after the first discovery, only a few similar high-quality X-ray binary stars have been discovered. It is expected that more such black holes will exist, especially considering that many double black holes (the future state of massive X-ray binary stars) have been discovered through gravitational waves in the past few years. Many binary stars have also been discovered in our Milky Way galaxy, and it is expected that they will eventually become high-quality X-ray binary stars. So, we have seen many predecessors and descendants, but where are all the high-quality X-ray binary stars themselves hidden?

One explanation states that even if a black hole is surrounded by a massive star with strong winds, it does not always emit X-rays. In order to emit X-rays, the black hole needs to create an accretion disk, where the gas spins and heats up before falling in. To create an accretion disk, the falling gas needs "angular momentum" so that all gas particles can rotate in the same direction around the black hole. However, we find that it is usually difficult to have enough angular momentum to fall into a black hole in a high-quality X-ray binary star. This is because it is generally believed that the wind blows symmetrically, so the amount of gas flowing through the black hole clockwise and counterclockwise is almost the same. As a result, the gas can fall directly into the black hole without forming an accretion disk, so the black hole is almost invisible.

But if this is true, why do we see any X-ray binary stars? In our paper, we solved the equation of motion of the stellar wind, and we found that when the black hole is close enough to the star, the wind does not blow symmetrically. Due to tidal forces, the wind blows at a slower speed in the direction towards and away from the black hole. Because of this symmetry breaking in the wind, the gas can now have a large amount of angular momentum, enough to form an accretion disk around the black hole and glow in X-rays. The necessary conditions for this asymmetry are quite strict, so only a small part of black holes + massive binary stars can be observed.

The model in our study explains why there are only a few high-quality X-ray binaries detected, but this is only the first step in understanding asymmetric stellar winds. By further studying this model, we may be able to solve many other mysteries of massive X-ray binary stars. Written by Ryosuke Hirai, OzGrav postdoctoral fellow, Monash University

Reference: "Acretion Disk Formation Conditions and Observability of Accretion Wind X-ray Binary Stars", Authors: Ryosuke Hirai and Ilya Mandel, November 18, 2021, Australian Astronomical Society publication. DOI: 10.1017/pasa.2021.53

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