CSIRO’s Australia Telescope Compact Array Has Discovered A Middleweight Black Hole.

CSIRO's Australia Telescope Compact Array, which was used to study the black hole HLX-1. Credit: D. Smyth

Its name is HLX-1 and this “Hyper-Luminous X-Ray Source” is the first of its type to be positively identified.We thought we had black holes all figured out.

Thanks to observations done with an Aussie CSIRO radio telescope, this “middleweight” black hole was caught in the act of expelling outbursts of super-heated gas in a galaxy cataloged as ESO 243-49 about 300 million light years distant. Up until now, astronomers only had good evidence for supermassive black holes and stellar mass black holes – a range from a million to billion times the mass of our Sun – down to about three to thirty times. We knew there were big ones and we knew there were small ones. Now we have proof there’s a middle class in space! 

“This is the first object that we’re really sure is an intermediate-mass black hole,” said Dr. Sean Farrell, an ARC Postdoctoral Fellow at the University of Sydney and a member of the research team, which included astronomers from France, Australia, the UK and the USA.

CSIRO’s Dr. Ron Ekers, who studies supermassive black holes in the centers of galaxies, said “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.”

First Light

HLX-1 was first ubiquitously detected in 2009 as a very bright X-ray source.. a source point fueled by a star or cloud of gas being pulled into a black hole. As it neared its death, the fueling source heated to extreme temperatures – emitting a huge amount of 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,” Dr. 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.”

Since 2010 the researchers have been studying the black hole with CSIRO’s Compact Array radio telescope near Narrabri, New South Wales. “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,” said Dr. Farrell. “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.”

Cross Checking

However, confirmation is everything when it comes to astronomy and researchers had to do their homework. By studying the source’s X-ray emissions, they were able to predict two times when it should be brightening in the radio band of the electromagnetic spectrum – and they were correct both times. According to Dr. Farrell’s computations, there should be a companion star which travels in an eccentric orbit around HLX-1.

When it brushes by, the black hole pulls gas away from its partner and causes an X-ray flare. The brightness of both the X-ray and radio flares determines the mass and, in this case, it has an upper limit of about 90,000 times the mass of the Sun. However, Dr. Farrell cautions this is a conservative estimate and (for a variety of reasons) a more realistic size of about 20,000 solar mass is more likely.

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

Why have we found only this one confirmed intermediate-mass black hole? “There maybe lots of others out there that are not currently feeding, and so are not detectable, or are feeding at a very low rate, so they don’t stand out as intermediate-mass black holes,” Dr. Ekers said. Dr. Farrell also explains that HLX-1 could very well have been the central black hole of a low-mass dwarf galaxy consumed by the larger galaxy, ESO 243-49. What’s more, there’s hints of star-forming areas in HLX-1 which supports the “plunging dwarf” theory.

Is the work over yet? Not quite. The CSIRO team is still studying HLX-1, looking for more signs of disturbance around the area of this middleweight champ. They are hoping to find anomalies, such as gas streams. which would further solidify their findings. “Jets should also be produced by intermediate mass black holes, although evidence for this third class of black hole has until recently been weak.” says the research team. “The detection of transient radio emission at the location of the intermediate mass black hole candidate ESO 243-49 HLX-1, is consistent with a discrete jet ejection event. These observations also allow us to refine the mass estimate of the black hole.”

Original Story Source: CSIRO News Release.