First radio waves detected from intermediate mass black holes

9 July 2012

The black hole HLX-1, circled, is in the galaxy called ESO 243-49.
The black hole HLX-1, circled, is in the galaxy called ESO 243-49.

They don't broadcast top pop songs, but black holes do emit radio waves as they accumulate matter. The first radio emissions to be detected from an intermediate mass black hole have been published in the prestigious journal Science by an international team of astronomers, including Dr Emil Lenc and Dr Sean Farrell from the University of Sydney's School of Physics.

A team led by Dr Farrell, discovered the first intermediate mass black hole, called HLX-1, and published their findings in the journal Nature in 2009. This latest paper published in Science examines radio emissions from HLX-1, which are the first radio waves observed from an intermediate mass black hole and have allowed the scientists to refine the estimated size of this newly discovered black hole.

"Black holes are areas where the matter is so densely squeezed into a small space, that it makes gravity pull strongly enough to stop light from escaping," explained Dr Farrell.

"Astronomers have classified black holes into stellar mass black holes, which are up to tens of times the mass of our sun, and supermassive black holes, which are millions to billions of times the mass of our sun.

"HLX-1 lies in between these two sizes at around 20,000 times the mass of our sun, so we've called it an intermediate mass black hole," said Dr Farrell.

HLX-1 is located in a galaxy called ESO 243-49 about 300 million light years away from us.

Using the Australia Telescope Compact Array and NASA's Swift satellite, the team including scientists from France, the United Kingdom and USA examined radio emissions during two state transitions of the black hole HLX-1 in 2010 and 2011.

"Black holes change state from a low luminosity X-ray state to a high luminosity X-ray state, and back again. As they change state they release jets of superfast moving plasma which can be measured by the radio waves emitted," explained Dr Farrell.

"We made our observations of radio emissions from HLX-1 as it changed state, finding that it too - just like smaller stellar mass black holes and what we think happens with larger supermassive black holes over a longer time period - ejects jets of superfast moving plasma as it changes state," said Dr Farrell.

"It's the first evidence of a discrete jet ejection event from an intermediate mass black hole, showing that they produce radio flares like other black holes as they change state."

The team made observations over two state changes in HLX-1 - one in 2010 which they observed in September and December, and another in August 2011.

Dr Emil Lenc explained, "From studying other black holes, we know that when black holes suck in gas it 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."

"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 flares up in radio waves," said Dr Lenc.

As HLX-1 changed state, they measured variable radio emission coming from the black hole that was consistent with a transient jet ejection event.

"From the radio emissions released by our intermediate mass black hole HLX-1, we can also calculate its approximate size. So we've been able to refine our estimate of how big HLX-1 is to between around 9 x 103 times the size of our sun and 9 x 104 times the size of our sun," said Dr Farrell.

"It's further proof that HLX-1 is indeed sized as an intermediate mass black hole."

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