In 2019, astronomers noticed an uncommon gravitational chirp. Generally known as GW190521, it was the final scream of gravitational waves as a black gap of 66 photo voltaic plenty merged with a black gap of 85 photo voltaic plenty to develop into a 142 photo voltaic mass black gap. The info had been in step with all the opposite black gap mergers we’ve noticed. There was only one drawback: an 85 photo voltaic mass black gap shouldn’t exist.
All of the black gap mergers we’ve noticed contain stellar mass black holes. These kind when a large star explodes as a supernova and its core collapses to develop into a black gap. An previous star must be not less than ten instances the mass of the Solar to develop into a supernova, which may create a black gap of about 3 photo voltaic plenty. Bigger stars can create bigger black holes, up to a degree.
The primary era of stars within the cosmos had been possible a whole lot of photo voltaic plenty. For a star above 150 photo voltaic plenty or so, the ensuing supernova can be so highly effective that its core would bear what is called pair-instability. Gamma rays produced within the core can be so intense they decay into an electron-positron pair. The high-energy leptons would then rip aside the core earlier than gravity might collapse it. To beat the pair-instability, a progenitor star would want a mass of 300 Suns or extra. Which means that the mass vary of stellar black holes has a “pair-instability hole.” Black holes from 3 photo voltaic plenty to about 65 photo voltaic plenty would kind from common supernovae, and black holes above 130 photo voltaic plenty might kind from stellar collapse, however black holes between 65-130 photo voltaic plenty shouldn’t exist.
For GW190521, the 66 photo voltaic mass black gap is shut sufficient to the restrict that it possible fashioned from a single star. The 85 photo voltaic mass black gap, then again, is smack-dab in the midst of the forbidden vary. Some astronomers have argued that the bigger black gap might need fashioned from a hypothetical boson star generally known as a Proca star, but when that’s true, then GW190521 is the one proof that Proca stars exist. Extra possible, the 85 photo voltaic mass black gap fashioned from the merger of two smaller black holes, making GW190521 a staged merger. The issue with that concept is that black gap mergers are sometimes asymmetrical, in a means that the ensuing black gap is kicked out of its area of origin. A number of black gap mergers would solely happen underneath sure circumstances, which is the place a brand new examine in The Astrophysical Journal is available in.
The authors checked out how the mass, spin, and movement of a merging black gap pair decide the mass, spin, and recoil velocity of the ensuing black gap. By making a statistical distribution of outcomes, the staff might then work backwards. Given the mass, spin, and velocity of a “forbidden” black gap relative to its atmosphere, what had been the properties of its black gap ancestors? When the authors utilized this to the progenitors of GW190521, they discovered that the one potential ancestors would have given a comparatively massive recoil velocity. Which means that the merger will need to have occurred inside the area of an lively galactic nucleus, the place the gravitational properly can be robust sufficient to carry the system collectively.
This work has implications for what are generally known as intermediate mass black holes (IMBHs), which may have plenty of a whole lot or 1000’s of Suns. It has been thought that IMBHs kind inside globular clusters, but when the recoil velocities of black gap mergers are massive, this might be unlikely. As this examine exhibits, GW190521 couldn’t have occurred in a globular cluster.
Reference: Araújo-Álvarez, Carlos, et al. “Kicking Time Again in Black Gap Mergers: Ancestral Lots, Spins, Beginning Recoils, and Hierarchical-formation Viability of GW190521.” The Astrophysical Journal 977.2 (2024): 220.
