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June 16, 2011

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Scientists: black hole kills star, blasts beam at Earth

June 16, 2011
World Science staff

One of the big­gest, bright­est ex­plo­sions ev­er recorded comes from a huge black hole at the cen­ter of a dis­tant gal­axy, as­tro­no­mers say. The dark be­he­moth ap­par­ently tore up a star that wan­dered too close—con­vert­ing its en­er­gy in­to a pow­er­ful beam that we can see be­cause we’re in its path, ac­cord­ing to the sci­en­tists.

“This is truly dif­fer­ent from any ex­plo­sive event we have seen be­fore,” said Josh­ua Bloom, an as­tron­o­mer at the Un­ivers­ity of Cal­i­for­nia at Berke­ley.

What Uni­ver­si­ty of War­wick, U.K. re­search­ers think the star may have looked like at the start of its dis­rup­tion by a black hole. (Cred­it: U. of War­wick / Mark A. Gar­lick)

Black holes are ob­jects so heavy and com­pact that their gra­vity over­whelms and drags in an­ything that strays too close, in­clud­ing light. They aren’t di­rectly vis­i­ble, but of­ten the vi­o­lent ac­ti­vity sur­round­ing black holes gives off de­tect­a­ble light.

“The only ex­plana­t­ion that so far fits the size, in­tens­ity, time scale, and lev­el of fluctua­t­ion” meas­ured “is that a mas­sive black hole at the very cen­ter of that gal­axy has pulled in a large star and ripped it apart by tid­al dis­rup­tion,” said An­drew Levan of the Uni­vers­ity of War­wick, U.K., one of the re­search­ers. In oth­er words, it’s the same ef­fect by which the Moon’s gra­vity gently dis­torts our oceans, cre­at­ing the tides, ex­cept the black hole’s gravita­t­ional force would be around 100 tril­lion times stronger than the Moon’s. 

The spin­ning black hole then cre­at­ed two je­ts of en­er­gy, “one of which point­ed straight to Earth,” he added. 

“De­spite the pow­er of this the cat­a­clys­mic event, we still only hap­pen to see [it] be­cause our so­lar sys­tem hap­pened to be look­ing right down the bar­rel of this je­t,” said Levan. The event, dubbed Sw 1644+57, took place at the heart of a gal­axy that lies in the di­rection of the con­stella­t­ion Dra­co, he added. The re­search is pub­lished June 16 in the re­search jour­nal Sci­ence.

The beam had to cross 3.8 bil­lion light-years of space be­fore reach­ing us, re­search­ers es­ti­mate. A light-year is a un­it of dis­tance equal to how far light trav­els in a year.

“This burst pro­duced a tre­men­dous amount of en­er­gy over a fairly long pe­ri­od of time, and the event is still go­ing on more than 2½ months lat­er,” said Bloom. “That’s be­cause as the black hole rips the star apart, the mass swirls around like wa­ter go­ing down a drain, and this swirling pro­cess re­leases a lot of en­er­gy.”

What Uni­ver­si­ty of War­wick, U.K. re­search­ers think the af­ter­math of a large star be­ing con­sumed by a black hole at the cen­ter of a gal­axy 3.8 bil­lion light years dis­tant may have looked like. (Cred­it: U. of War­wick / Mark A. Gar­lick)

When NASA’s Swift Gam­ma Burst Mis­sion space­craft first de­tected the flash, as­tro­no­mers thought it was a gam­ma-ray burst from a col­laps­ing star. But on March 31 Bloom sent out an e­mail cir­cu­lar sug­gest­ing it was ac­tu­ally a high-en­er­gy je­t pro­duced as a star about the size of our Sun fell in­to a black hole a mil­lion times heav­i­er. Bloom and col­leagues pro­pose that some 10 per­cent of the in­falling star’s mass is turned in­to en­er­gy and ra­di­at­ed off as X-rays and gam­ma rays.

“Here, you have a black hole sit­ting qui­es­cent­ly, not gob­bling up mat­ter, and all of a sud­den some­thing sets it off,” Bloom said. “This could hap­pen in our own gal­axy, where a black hole sits at the cen­ter liv­ing in qui­es­cence, and oc­ca­sion­ally bur­bles or hic­cups as it swal­lows a lit­tle bit of gas. From a dis­tance, it would ap­pear dor­mant, un­til a star ran­domly wan­ders too close and is shred­ded.”

Prob­a­ble tid­al dis­rup­tions of a star by a mas­sive black hole have pre­vi­ously been seen at low­er en­er­gies, but nev­er one that pro­duces gam­ma rays, the most en­er­get­ic form of light, Bloom said. Such ran­dom events, es­pe­cially look­ing down the je­t’s bar­rel, hap­pen “prob­ably once in 100 mil­lion years in any giv­en gal­axy,” he con­tin­ued. “I would be sur­prised if we saw anoth­er one of these an­ywhere in the sky in the next decade.” The gam­ma-ray emis­sions probably be­gan March 24 or 25, he added; “we think this event was de­tected around the time it was as bright as it will ev­er be.”

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One of the biggest, brightest explosions ever recorded comes from a huge black hole at the center of a distant galaxy, astronomers say. The dark behemoth apparently tore up a star that wandered too close—converting its energy into a powerful beam that we can see because we’re in its path, according to the scientists. “This is truly different from any explosive event we have seen before,” said Joshua Bloom, an astronomer at the University of California at Berkeley. Black holes are objects so heavy and compact that their gravity overwhelms and drags in anything that strays too close, including light. They aren’t directly visible, but often the violent activity surrounding black holes gives off energetic emissions detectable by telescopes. “The only explanation that so far fits the size, intensity, time scale, and level of fluctuation” measured “is that a massive black hole at the very center of that galaxy has pulled in a large star and ripped it apart by tidal disruption,” said Andrew Levan of the University of Warwick, U.K., one of the researchers. In other words, it’s the same effect by which the Moon’s gravity gently distorts our oceans, creating the tides, except in the black hole’s gravitational force would be around 100 trillion times stronger than the Moon’s. The spinning black hole then created two jets of energy, “one of which pointed straight to Earth,” he added. “Despite the power of this the cataclysmic event, we still only happen to see [it] because our solar system happened to be looking right down the barrel of this jet,” said Levan. The event, dubbed Sw 1644+57, took place at the heart of a galaxy that lies in the direction of the constellation Draco, he added. The research is published June 16 in the research Journal Science. The beam had to cross 3.8 billion light-years of space before reaching us, researchers estimate. A light-year is a unit of distance equal to how far light travels in a year. “This burst produced a tremendous amount of energy over a fairly long period of time, and the event is still going on more than two and a half months later,” said Bloom. “That’s because as the black hole rips the star apart, the mass swirls around like water going down a drain, and this swirling process releases a lot of energy.” When the Swift Gamma Burst Mission spacecraft first detected the flash in the direction of the constellation Draco, astronomers thought it was a gamma-ray burst from a collapsing star. But on March 31 Bloom sent out an email circular suggesting it was actually a high-energy jet produced as a star about the size of our Sun fell into a black hole a million times heavier. Bloom and colleagues propose that some 10 percent of the infalling star’s mass is turned into energy and radiated off as X-rays and gamma rays. “Here, you have a black hole sitting quiescently, not gobbling up matter, and all of a sudden something sets it off,” Bloom said. “This could happen in our own galaxy, where a black hole sits at the center living in quiescence, and occasionally burbles or hiccups as it swallows a little bit of gas. From a distance, it would appear dormant, until a star randomly wanders too close and is shredded.” Probable tidal disruptions of a star by a massive black hole have previously been seen at lower energies, but never one that produces gamma rays, the most energetic form of light, Bloom added. Such random events, especially looking down the jet’s barrel, happen “probably once in 100 million years in any given galaxy,” said Bloom. “I would be surprised if we saw another one of these anywhere in the sky in the next decade.” The gamma-ray emissions probably began March 24 or 25, he added; “we think this event was detected around the time it was as bright as it will ever be.”