A group of astronomers led by Tingting Liu of the University of Maryland has discovered a pulsing light signal that hints at a close pair of supermassive black holes.
Artist’s depiction of a black hole merger. Image credit: NASA / CXC / A. Hobart / Josh Barnes, University of Hawaii / John Hibbard, NRAO.
“We believe we have observed two supermassive black holes in closer proximity than ever before. This pair of black holes may be so close together that they are emitting gravitational waves, which were predicted by Einstein’s theory of general relativity,” explained Dr Suvi Gezari, also from the University of Maryland, who is a co-author of the paper published in the Astrophysical Journal Letters (arXiv.org preprint).
The cores of many glittering galaxies harbor supermassive black holes of millions, or even billions, of solar masses. What is more, these black holes and their galaxies appear to develop together. Theory predicts that as galaxies collide and merge, growing ever more massive, so too do their supermassive black holes.
Black holes by themselves are impossible to see, but their gravity can pull in surrounding gas to form an accretion disk. The spinning particles are accelerated to very fast speeds and release vast amounts of energy in the form of heat and powerful X-rays and gamma rays.
When this process happens to a supermassive black hole, the result is a quasar – an extremely luminous object that outshines all of the stars in its host galaxy.
When two black holes orbit as a binary, they absorb matter cyclically, leading scientists to predict that the binary’s quasar would respond by periodically brightening and dimming.
Dr Gezari, Liu and their colleagues used the Pan-STARRS1 Medium Deep Survey to search for so-called variable quasars. They found a quasar called PSO J334.2028+01.4075, which has a very large black hole of almost ten billion solar masses and emits a periodic optical signal that repeats every 542 days.
PSO J334.2028+01.4075’s signal was unusual because the light curves of most quasars are arrhythmic. To verify their finding, the scientists performed rigorous calculations and simulations and examined additional data.
“The discovery of a compact binary candidate supermassive black hole system like PSO J334.2028+01.4075, which appears to be at such close orbital separation, adds to our limited knowledge of the end stages of the merger between supermassive black holes,” said Mr Liu, who is the lead author on the study.
_____
Tingting Liu et al. 2015. A Periodically Varying Luminous Quasar at z = 2 from the Pan-STARRS1 Medium Deep Survey: A Candidate Supermassive Black Hole Binary in the Gravitational Wave-driven Regime. ApJ 803, L16; doi: 10.1088/2041-8205/803/2/L16
