Hole lotta noise: Israeli scientists hear radio flares from black hole

Radio flares emitted by stars destroyed by black holes are, themselves, nothing new, and scientists have known about these before. But this only happens immediately, and the emission of radio flares long after the star's destruction is unheard of. 

But the team of researchers – led by Dr. Assaf Horesh from HUJI's Racah Institute of Physics and NASA Swift space telescope director Prof. Brad Cenko and Dr. Iair Arcavi from Tel Aviv University – has managed to find exactly that, discovering flares emitted months or even years later.

“According to existing theories of how these events occur, if no radio emission has been discovered in the immediate wake of the disruption, there is no expectation that one should occur later on,” Horesh explained in a statement. 

“However, we decided to conduct one last radio observation six months after the star was destroyed, and surprisingly we discovered bright radio emission. Once we discovered this delayed radio flare, we continued collecting data over a year, during which the radio emission faded away. Moreover, we found a second delayed flare, four years after the initial stellar disruption discovery. This is the first discovery of such delayed radio flares from such events, when a star is disrupted by a black hole.”

It is unclear what caused these delayed radio flares, as they defy existing theories of how black holes and these radio flares work. As a result of this discovery, entirely new models would need to be developed to better understand this phenomenon. However, the scientists are fully aware that this may not have been a rare occurrence – and in fact, could be common and simply never detected before.

One possible theory the team put forward was that a large amount of debris was eventually pulled into the hole long after the star was destroyed. However, the mechanism remains unclear. 

“What led to the delay and what is the exact physical process responsible for such late-time emission are still open questions,” Horesh said. 

“In light of this discovery, we are actively searching for more such delayed radio flares in other tidal disruption events.”

The findings of this study were published in the academic journal Nature Astronomy.

This study comes after black hole research by scientists from Technion-Israel Institute of Technology, which managed to measure the temperature of Hawking radiation emitted from black holes by studying a man-made sonic black hole.

This discovery lined up with predictions regarding its strength and temperature, which itself is dependent on the black hole itself, but saw sudden rapid growth later in the hole's lifespan. This, the researchers had hypothesized, was due to the formation of an inner horizon within the black hole, where the sound waves were no longer trapped.

This was never part of famed physicist Stephen Hawking's original theories regarding black holes – which itself made him the namesake for Hawking radiation – making it completely unprecedented, showing just how much there still is to learn about these mysterious celestial bodies. 

And like Horesh, the Technion scientist, Dr. Jeff Steinhauer, is determined to go beyond original analyses and theories to better understand how black holes work.

The findings of this study were published in Nature Physics.