The discovery of the first black hole was made when astronomers detected intense X-rays from a system called Cygnus X-1. The massive star in this system blows the mighty wind that some part of gas in it is gravitationally attracted towards the black hole. This results in forming an accretion disc, which emits the strong X-rays that we observe.
Such systems with a black hole and a massive star are called ‘high-mass X-ray binaries. These binaries have proven very helpful in understanding the nature of black holes.
Since the discovery of the system, astronomers have detected only a handful of similar high-mass X-ray binaries.
Where are all the high-mass X-ray binaries themselves hiding?
One possible explanation states that a black hole– orbited by a massive star- does not always emit x-rays. For x-ray emission, it needs to create an accretion disc. For the accretion disc formation, the falling gas needs angular momentum so that all the gas particles can rotate around the black hole in the same direction.
Although, it is quite difficult for gas to have enough angular momentum to fall onto the black hole in high-mass X-ray binaries. This is because the wind is usually considered to be blowing symmetrically. Hence, there is an even distribution of gas flowing past the black hole clockwise and counter-clockwise. As a result, the gas can fall into the black hole directly without creating an accretion disc, so the black hole is almost invisible.
But if this is true, why do we see any X-ray binaries at all?
In a new study, Ozgrav scientists solved the equations of motion for stellar winds. They found that the stellar wind does not blow symmetrically when the black hole is close enough to the star.
Instead, the wind blows slowly towards and away from the black hole due to the tidal forces. This break in symmetry in the wind causes the gas to have a large angular momentum. This angular momentum is high enough to form an accretion disc around the black hole and shine in X-rays.
Scientists noted, “The necessary conditions for this asymmetry are rather strict, so only a small fraction of black hole + massive star binaries will be able to be observed.”
OzGrav Postdoc Ryosuke Hirai, Monash University, said, “The model in our study explains why there are only a small number of detected high-mass X-ray binaries, but this is only the first step in understanding asymmetric stellar winds. By investigating this model further, we might be able to solve many other mysteries of high-mass X-ray binaries.”
- Hirai, Ryosuke, Mandel, Ilya. Conditions for accretion disc formation and observability of wind-accreting X-ray binaries. DOI: 10.1017/pasa.2021.53