Pulsing light signal yields clues about elusive black hole merger

The flashes of light detected from a quasar in deep space by astronomers are believed to be evidence of an ongoing merger between two supermassive black holes.

As two galaxies enter the final stages of merging, scientists have theorized that the galaxies' supermassive black holes will form a "binary" or two black holes in such close orbit they are gravitationally bound to one another.

In a new study, astronomers at the University of Maryland present direct evidence of a pulsing quasar, which might substantiate the existence of black hole binaries.

Suvi Gezari, assistant professor of astronomy at the University of Maryland, said that they might have observed two supermassive black holes in closer proximity than ever before.

The discovery could shed light on how often black holes get close enough to form a gravitationally bound binary and eventually merge together.

Black holes typically gobble up matter, which accelerates and heats up, emitting electromagnetic energy and creating some of the most luminous beacons in the sky called quasars. When two black holes orbit as a binary, they absorb matter cyclically, leading theorists to predict that the binary's quasar would respond by periodically brightening and dimming.

The researchers conducted a systematic search for so-called variable quasars using the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1) Medium Deep Survey. This Haleakala, Hawaii-based telescope imaged the same patch of sky once every three days and collected hundreds of data points for each object over four years.

In that data, the astronomers found quasar PSO J334.2028+01.4075, which has a very large black hole of almost 10 billion solar masses and emits a periodic optical signal that repeats every 542 days. The quasar's signal was unusual because the light curves of most quasars are arrhythmic.

To verify their finding, the research team performed rigorous calculations and simulations and examined additional data, including photometric data from the Catalina Real-Time Transient Survey and spectroscopic data from the FIRST Bright Quasar Survey.

The researchers plan to continue searching for new variable quasars. Beginning in 2023, their search could be aided by the Large Synoptic Survey Telescope, which was expected to survey a much larger area and could potentially pinpoint the locations of thousands of these merging supermassive black holes in the night sky.

The study is published in The Astrophysical Journal Letters.