Radio waves detected from intermediate black holes

They don’t broadcast top pop songs but black holes do emit radio waves as they accumulate matter. That is well known, but now the first radio emissions have been detected from an intermediate mass black hole by an international team of astronomers based in Australia, Britain, France and the United States.

The researchers discovered the first intermediate mass black hole, called HLX-1, in 2009 and published their findings in the journal Nature.

Their latest paper, published in Science this month, examines radio emissions from HLX-1, which are the first radio waves observed from an intermediate mass black hole. The findings allowed the scientists to refine the estimated size of the black hole.

Astronomers note that 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.

They have classified black holes into ‘stellar mass black holes’, 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 the sun, so it has been dubbed an ‘intermediate mass black hole’. It is located in a galaxy called ESO 243-49, about 300 million light years away from Earth.

Using NASA’s Swift satellite and the Australia Telescope Compact Array, the team examined radio emissions during two state transitions of the black hole, in 2010 and 2011.

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

“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 Sean Farrell, the team leader from the University of Sydney’s school of physics.

“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 researchers made observations concerning two state changes in HLX-1 – one in 2010 which they observed in September and December, and another in August 2011.

From studying other black holes, astronomers know that when black holes suck in gas it creates X-rays but there is 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 X-ray emission are first seen and then, a day or two or even a few days later, the source flares up in radio waves.

As HLX-1 changed state, the astronomers measured variable radio emission coming from the black hole that was consistent with a transient jet ejection event. From the radio emissions released they could also calculate its approximate size and have refined the estimate of how big the black hole is to between around 9x103 times the size of the sun and 9x104 times – further proof that HLX-1 is indeed sized as an intermediate mass black hole.