Electromagnetic emission from black hole mergers in a giant disk of gas
Researchers provide a first plausible clue for environments of black hole mergers by explaining their possible associations of electromagnetic emission. Their suggested solution, now published in Astrophysical journal, opens a novel window for understanding the universe through multi-messenger astronomy.
In 2015, gravitational waves from merging black holes, which is predicted by general relativity, are detected at the first time. Currently, black hole mergers, weekly discovered by gravitational wave observations, are one of the most attracting phenomena for expanding the understanding of the universe.
On the other hand, it has not been understood how these merging black holes got bound and merged. Their progenitor stars might be bound from their birth, collapse to become black hole binaries, and merge with each other. Or they might merge due to dynamical interactions in star clusters. This is an important question to uncover the evolution of the universe by gravitational wave observations. In this study, researchers provide an important clue for this mystery.
Recently, as a black hole merger with most characteristic properties, the gravitational wave event, named GW190521, has been reported. In this event, in addition to the significantly higher mass of merging black holes compared to that expected in theoretical predictions, an optical flare is likely associated with the merger. Since such characteristic properties are difficult to be explained by mergers in usual environments, plausible models are intensely discussed in the astronomical community.
As environments where such properties could be explained, the research group focused on central regions of galaxies. In the regions, supermassive black holes with masses more than million solar mass stay, and they are often surrounded by giant disks of gas. In these gaseous disks, many smaller black holes are predicted to be embedded. By interacting with gas and other objects, these black holes come close to each other, form binaries, and merge with each other. Furthermore, due to repeated mergers expected in deep potential well of the supermassive black hole, they can become massive enough to explain the high mass reported in GW190521. In the environments, light may be also produced due to accretion of gas onto black holes. However, the properties of the optical emission found with GW190521 and the reason that emission is produced only after black hole mergers have not explained well, and hence, the association of the optical emission was regarded as coincidence.
To explain the optical association, the research group focused on emission from strong shocks emerged due to the collision between the jet produced by the merged remnant and gas in the giant disk. In the environments, circum-black hole disks are highly magnetized, which produce strong jets due to the Blandford-Znajek process. At mergers, the directions of black hole spins are randomized, which also reorients the directions of jets. After merger, the jet can collide with unshocked gas and strong shocks emerge, in which strong emission is produced from heated and accelerated electrons. This is the process that optical emission is associated with black hole mergers (Fig 1). The researchers calculated the delay time of the optical emission from the merger, the duration, and the luminosity of the optical emission, and found that all the properties of the observed light with GW190521 can be well explained by the model by considering merger at 10 light year from the central supermassive black hole.
Furthermore, the research group focused on the other event, in which short-duration gamma-ray emission is possibly associated with the first gravitational wave event, named GW150914. This association was also regarded as coincidence mainly due to the difficulty of theoretical explanations. By applying the model developed above to this association, the delay time of gamma-ray emission from the merger, the duration, and the spectral energy distribution can be well explained by mergers in a giant gas disk. Thus, the research group successfully explained several possible electromagnetic associations with significantly different properties by the unified model. As a smoking gun signature of the scenario, the research group predicted that flares can be simultaneously observed in infrared and X-ray bands. If the scenario is confirmed, these systems will be highly useful to improve our understanding of the evolution of compact objects in AGN disks, the structure of AGN disks, plasma physics, and the expansion history of the universe by helping to constrain the Hubble constant, in addition to unveiling the origin of black hole mergers. Thus, this work most likely opened a new window for the understanding of the universe.
Paper information:
【Journal】The Astrophysical Journal
【Title】Observable Signature of Merging Stellar-mass Black Holes in Active Galactic Nuclei
【Author】Tagawa, Hiromichi; Kimura, Shigeo S; Haiman, Zoltán; Perna, Rosalba; Bartos, Imre
【DOI】10.3847/1538-4357/acc4bb
【URL】https://iopscience.iop.org/article/10.3847/1538-4357/acc4bb
