The first black holes were discovered when astronomers detected intense X-rays from a system called Cygnus X-1. Massive stars in this system blow strong winds so that some of the gas in them is attracted to the black hole. This results in the formation of a cumulus disk, which emits the strong X-rays we observed.
This system of black holes and superstars is called a “high-mass X-ray binary.” This binary has proven to be very useful in understanding the nature of black holes.
Since the system’s discovery, astronomers have found only a few similar high-mass X-ray binaries.
Where are all the same high-mass X-ray binaries hiding?
One possible explanation is that a Black holeOrbiting a massive star – doesn’t always emit x-rays. To emit X-rays, it is necessary to make an accretion disk. To form an accretion disk, the incoming gas requires angular momentum so that all gas molecules can orbit the black hole in the same direction.
Although it is very difficult for gas to have sufficient angular momentum to fall into a high-mass black hole X-ray binary. This is because the wind usually blows symmetrically. Thus, there is an equal distribution of gas flowing through the black hole clockwise and counterclockwise. As a result, gas can fall directly into the black hole without forming an accretion disk, making the black hole almost invisible.
But if this is true, why do we see X-ray binaries at all?
In a new study, Osgraph scientists solved the kinematic equation for star wind. They found that the stellar wind does not blow symmetrically when the black hole is close enough to the star.
Instead, wind slowly blows toward and away from the black hole due to tidal forces. This breaking of wind symmetry causes the gas to have a large angular momentum. This angular momentum is high enough to form an accretion disk around the black hole and glow in X-rays.
The scientist noted, “The conditions for this asymmetry are rather tight, so only a small fraction of massive black holes + binaries will be observed.”
OzGrav Postdoc Ryosuke Hirai, Universitas Monash, He saidAnd The model in our study explains why only a small number of high-mass X-ray binaries were detected, but this is only the first step in understanding asymmetric stellar winds. By investigating this model further, we may be able to solve many other mysteries of high-mass X-ray binaries. “
Journal reference:
- Hirai, Ryosuke, Mandel, Elijah. Conditions for the formation of the accretion disk and possible observations of winds collecting X-ray binaries. DOI: 10.1017/pass .2021.53
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