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Much-sought “Goldilocks” black hole identified through its belching

July 5, 2012
Courtesy of CSIRO
and World Science staff

Out­bursts of scald­ing gas have clinched the ident­ity of the first known “mid­dleweight” black hole, as­tro­no­mers say.

Be­fore it was found, sci­en­tists had good ev­i­dence only for “su­per­mas­sive” black holes—ones weigh­ing a mil­lion to a bil­lion Sun­s—and “stel­lar mass” ones, weigh­ing three to 30 Suns. “This is the first ob­ject that we’re really sure is an intermediate-mass black hole,” said Sean Far­rell, a post­doc­tor­al fel­low at the Uni­vers­ity of Syd­ney in Aus­tral­ia and a mem­ber of the re­search team on the proj­ect. 

An ar­row shows the lo­ca­tion of the claimed black hole HLX-1 in the gal­axy ESO 243-49. (Cred­it: NA­SA, ESA and S. Far­rell (U. Syd­ney))


The find­ings are re­ported in the July 5 is­sue of the jour­nal Sci­ence.

A black hole is an ob­ject so heavy and com­pact that its gra­vity over­whelms and pulls in an­y­thing that strays too close, even light rays. 

Re­search­ers have hoped to find a Gold­i­locks-like black hole of a weight in be­tween the gi­ants and the lil­li­pu­tian va­ri­eties. This in­ter­est came about in large part be­cause it’s not clear how the gi­ants form, but one pos­si­bil­ity is through merg­ers of many mid­dle­weight black holes. 

Work­ing out this prob­lem would help shed light on the ev­o­lu­tion of the uni­verse, be­cause the huge “su­per­mas­sive” black holes have a spe­cial struc­tur­al im­por­tance, sit­ting as they do at the cen­ters of most ga­lax­ies.

Since 2010, Far­rell and col­leagues have been stu­dy­ing the Goldi­locks-like black hole with the Com­pact Ar­ray ra­di­o tel­e­scope near Narrabri, New South Wales, of Aus­tral­ia’s Com­mon­wealth Sci­en­tif­ic and In­dus­t­ri­al Re­search Or­ga­nisa­t­ion. Called HLX-1, for hyper-luminous X-ray source 1, the object lies in a gal­axy called ESO 243-49, about 300 mil­lion light-years away. A light-year is the dis­tance light trav­els in a year.

“We don’t know for sure how su­per­mas­sive black holes form, but they might come from medium-size ones merg­ing. So find­ing ev­i­dence of these intermediate-mass black holes is ex­cit­ing,” said Ron Ek­ers of the or­gan­iz­a­tion, who stud­ies su­per­mas­sive black holes in the cen­tres of ga­lax­ies.

The ob­ject was dis­cov­ered by chance in 2009, astron­omers said, be­cause it was blast­ing out X-rays with un­usu­al gus­to. As gas from a star or gas cloud is be­ing sucked in­to a black hole, it is heat­ed to ex­treme tem­per­a­tures and shines in X-rays.

“A num­ber of oth­er bright X-ray sources have been put for­ward as pos­sibly be­ing mid­dle­weight black holes. But all of those sources could be ex­plained as re­sult­ing from low­er mass black holes,” Far­rell said. “Only this one can’t. It is ten times brighter than any of those oth­er can­di­dates. We are sure this is an intermediate-mass black hole—the very first.”

The X-rays and the burp-like out­flows of gas are con­nect­ed, Far­rell added.

“From stu­dy­ing oth­er black holes we know that suck­ing in the gas cre­ates X-rays, but there’s then a sort of re­flux, with the re­gion around the black hole shoot­ing out jets of high-energy par­t­i­cles that hit gas around the black hole and gen­er­ate ra­di­o waves,” he ex­plained. “So what we tend to see is the X-ray emis­sion and then, a day or two or even a few days lat­er, the source flar­ing up in ra­di­o waves.” By look­ing at the X-ray out­put, the re­search­ers said they cor­rectly pre­dicted, twice, when the ob­ject should al­so be bright­en­ing in ra­di­o waves.


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Outbursts of scalding gas have clinched the identity of the first known “middleweight” black hole, astronomers say. Before it was found, scientists had good evidence only for “supermassive” black holes—ones weighing a million to a billion Suns—and “stellar mass” ones, weighing three to 30 Suns. “This is the first object that we’re really sure is an intermediate-mass black hole,” said Sean Farrell, a postdoctoral fellow at the University of Sydney in Australia and a member of the research team on the project. The findings were reported July 5 in the research journal Science. A black hole is an object so heavy and compact that its gravity overwhelms and pulls in anything that strays too close, even light rays. Researchers have hoped to find a Goldilocks-like black hole of a weight in between the giants and the lilliputian varieties. This interest came about in large part because it’s not clear how the giants form, but one possibility is through mergers of many middleweight black holes. Working out this problem would help shed light on the evolution of the universe, because the huge “supermassive” black holes have a special structural importance, sitting as they do at the centers of most galaxies. Since 2010, Farrell and colleagues have been studying the Goldilocks-like black hole with Compact Array radio telescope near Narrabri, New South Wales, of Australia’s Commonwealth Scientific and Industrial Research Organisation. Called HLX-1, for hyper-luminous X-ray source 1, the black hole lies in a galaxy called ESO 243-49, about 300 million light-years away. “We don’t know for sure how supermassive black holes form, but they might come from medium-size ones merging. So finding evidence of these intermediate-mass black holes is exciting,” said Ron Ekers of the organization, who studies supermassive black holes in the centres of galaxies. The object was discovered by chance in 2009, because it was blasting out X-rays with unusual gusto. As gas from a star or gas cloud is being sucked into a black hole, it is heated to extreme temperatures and shines in X-rays. “A number of other bright X-ray sources have been put forward as possibly being middleweight black holes. But all of those sources could be explained as resulting from lower mass black holes,” Farrell said. “Only this one can’t. It is ten times brighter than any of those other candidates. We are sure this is an intermediate-mass black hole—the very first.” The X-rays and the burp-like outflows of gas are connected, Farrell added. “From studying other black holes we know that sucking in the gas creates X-rays, but there’s then a sort of reflux, with the region around the black hole shooting out jets of high-energy particles that hit gas around the black hole and generate radio waves,” he explained. “So what we tend to see is the X-ray emission and then, a day or two or even a few days later, the source flaring up in radio waves.” By looking at the X-ray output, the researchers said they correctly predicted, twice, when the object should also be brightening in radio waves.