Picking up signal from faraway galaxy, Israelis challenge black hole assumptions

Black holes can go months or years before gobbling up last of dead star’s remains, physicists say, after detecting pattern of radio emissions from aftermath of star’s destruction

Nathan Jeffay is The Times of Israel's health and science correspondent

Illustration of a black hole with an accretion disk around it. As a result of the accretion of material from a destroyed star, powerful jets are launched. (Elen11 via iStock by Getty Images)
Illustration of a black hole with an accretion disk around it. As a result of the accretion of material from a destroyed star, powerful jets are launched. (Elen11 via iStock by Getty Images)

Israeli researchers have picked up radio emissions from a faraway galaxy that they say may radically change the understanding of how black holes behave.

When stars get close to black holes, they are in danger of being torn apart. If they are, much of the material is expected to be consumed by the black hole, in a process that has been assumed to take place within days and last for several weeks.

But physicists from the Hebrew University of Jerusalem and Tel Aviv University are raising the possibility of an as-yet unexplained process that causes some black holes to go months or years before gobbling up the last remains of a destroyed star.

“There seems to be a physical process we don’t understand yet that causes a long delay between the star being destroyed and the black hole accreting its material,” Hebrew University physicist Assaf Horesh told The Times of Israel.

Illustration of A star torn apart by a black hole. The stellar debris are then accreted onto the black hole. (Elen11 via iStock by Getty Images)

The peer-reviewed journal Nature Astronomy just published the research by Horesh, Iair Arcavi of Tel-Aviv University, and NASA Swift space telescope director Brad Cenko. After observing optical radiation from the destruction of a star in a galaxy 700 million light years away in 2015, they started searching for radio emissions, expecting to detect them for a short period.

Following the destruction of a star, physicists believe it is common for some material to enter the black hole that caused its death. As part of this process, other material is forced away by jets caused by magnetic fields around the black hole. Given that it is thrown back at high velocity, this is thought to create shockwaves that generate radio emissions.

3D rendering of a black hole (Philipp Tur via iStock by Getty Images)

Scientists have experience of detecting this immediately after a star’s destruction becomes observable on Earth, but Horesh and his colleagues ended up breaking new ground, by detecting emissions over a four-year period.

“We saw optical radiation after the star was destroyed, and then no radio emissions for months,” said Horesh, a lead author of the new research. “We decided to give it what we thought would be one last shot. Suddenly, six months later there was radio emission, which faded away for a year.”

Diagram showing timescale of detection of radio emissions by Israeli researchers after the destruction of a star. (Courtesy of Dr Assaf Horesh, Dr. Iair Arcavi, et al)

“Then, four years after the initial observation, we found that once again there was a very strong signal.”

He said the research suggests that the period over which a black hole consumes material from a star and throws back other material — generating radio emissions in the process — is more drawn out “and somewhat more chaotic” than previously assumed. He added that his team is looking to expand its research by identifying more radio transmissions from various galaxies.

read more:
Never miss breaking news on Israel
Get notifications to stay updated
You're subscribed