Earlier this year, astronomers found what seemed to be a pair of supermassive black holes heading towards a collision so strong it would generate a surge of gravitational waves through space-time. Now, in a study in the journal Nature, astronomers at Columbia University provide additional evidence that a pair of closely orbiting black holes is causing rhythmic flashes of light from quasar PG 1302-102. Based on calculations of the pair’s mass — together, and relative to each other — the researchers predict a collision 100,000 years from now, a long time for humans but a blink of an eye for a star or black hole. Spiraling together 3.5 billion light-years away, deep in the Virgo constellation, the pair is separated by a mere light-week. By contrast, the closest previously confirmed black hole pair is separated by 20 light-years.
“This is the closest we’ve come to observing two black holes on their way to a massive collision,” said the study’s senior author, Zoltan Haiman, an astronomer at Columbia. “Watching this process reach its culmination can tell us whether black holes and galaxies grow at the same rate, and ultimately test a fundamental property of space-time: its ability to carry vibrations called gravitational waves, produced in the last, most violent, stage of the merger.”
At the center of most giant galaxies, including our Milky Way, lies a supermassive black hole so dense that not even light can escape. Over time, black holes grow bigger — millions to billions times more massive than the Sun — by consuming stars, galaxies, and even other black holes. A supermassive black hole about to consume another can be detected by the mysterious flickering of a quasar, the beacon of light produced by black holes as they consume gas and dust swirling around them. Normally, quasars brighten and dim randomly, but when two black holes are about to unite, the quasar appears to flicker at regular intervals, like a light bulb on a timer.
Recently, a team led by Matthew Graham, a computational astronomer at the California Institute of Technology, designed an algorithm to identify repeating light signals from 247,000 quasars monitored by telescopes in Arizona and Australia. Of the 20 pairs of black hole candidates discovered, they focused on the most compelling bright quasar — PG 1302-102. In a January study in Nature, they showed that PG 1302-102 seemed to brighten by 14 percent every five years, indicating the pair was less than a tenth of a light-year apart.
Intrigued, Haiman and his colleagues wondered if they could build a theoretical model to explain the repeating signal. If the black holes were as close as predicted, one had to be circling a much larger counterpart at nearly a tenth of the speed of light, they hypothesized. At that speed, the smaller black hole would appear to brighten as it approached Earth’s line of sight under the relativistic Doppler beaming effect. If correct, they predicted they would find a five-year cycle in the quasar’s ultraviolet emissions — only two-and-a-half times more variable in its intensity. Analyzing UV observations collected by NASA’s Hubble and GALEX space telescopes, they found exactly that.
Previous explanations for the repeating signal include a warp in the debris discs orbiting the black holes, a wobble in the axis of one black hole, and a lopsided debris disc formed as one black hole draws material off the other — all creating the impression of a periodic flicker from Earth.
The new study introduces a novel method for studying the convergence of black holes, according to researchers. By determining the combined and relative mass of PG 1302-102's black holes, they have narrowed down the expected collision time for the pair to a range of 20,000 to 350,000 years from now, with the most likely estimate being around 100,000 years. (Previously, Graham's team had predicted a collision time of 10,000 to several million years from now, with a best estimate of 250,000 years). Study co-author David Schiminovich, an astronomer at Columbia, mentioned, "We are now able to quantify the rates at which black holes merge and form larger black holes, and this knowledge helps us in the search for more pairs of black holes." The increasing number of discoveries of binary black hole systems has raised hopes among astronomers that a collision may be observed within the next decade. Graham and his team recently reported 90 additional candidates, while astronomers at Columbia are expecting to announce their own findings soon based on data collected at California's Palomar Observatory. With a growing list of black holes under observation, the likelihood of witnessing a collision and detecting the gravitational waves predicted by Einstein's general theory of relativity, though not yet observed, is increasing. The lead author of the study, Daniel D'Orazio, a graduate student at Columbia, stated, "The detection of gravitational waves allows us to explore the mysteries of gravity and test Einstein's theory in the extreme environment of black holes. Achieving this goal is a major aspiration in our field."
No comments:
Post a Comment