Mar 03, 2021
JERUSALEM WEATHER

Astrophysicists at the Hebrew University of Jerusalem (HUJI), Tel Aviv University (TAU) and NASA (US National Aeronautics and Space Administration) have seen the first evidence of delayed radio flares from a tidal disruption event caused by a “black hole.” 

According to NASA, a black hole is a place in space where gravity pulls so much that even light can’t escape; the gravity is so strong because matter has been squeezed into a tiny space. This can happen when a star is dying. Because no light can get out, one can’t see black holes because they are invisible. Space telescopes with special tools can help find black holes – special tools can see how stars that are very close to black holes act differently than other stars.

Black holes can be big or small. Scientists think the smallest black holes are as small as just one atom. These black holes are very tiny but have the mass of a large mountain. Another kind of black hole whose mass can be up to 20 times more than the mass of the sun is called “stellar.” There may be many, many stellar mass black holes in Earth” galaxy, the Milky Way.

The largest black holes are called “supermassive” and have masses that are more than a million suns together. Scientists have found proof that every large galaxy contains a supermassive black hole at its center. The supermassive black hole at the center of the Milky Way galaxy is Sagittarius A. It has a mass equal to about four million suns and would fit inside a very large ball that could hold a few million Earths.

Scientists believe that the smallest black holes formed when the universe was created. Stellar black holes are made when the center of a very big star falls in upon itself, or collapses. When this happens, it causes a supernova – an exploding star that blasts part of the star into space. Astronomers believe that supermassive black holes were made at the same time as the galaxy they are in.

The team of researchers led by Dr. Assaf Horesh from HUJI’s Racah Institute of Physics have discovered the first evidence of radio flares emitted only long after a star is destroyed by a black hole.  Published in the prestigious journal Nature Astronomy under the title “Delayed radio flares from a tidal disruption event,” the discovery relied upon ultra-powerful radio telescopes to study these catastrophic cosmic events in distant galaxies called Tidal Disruption Events (TDE).  While researchers had known that these events cause the release of radio flares, this latest discovery saw those flares being emitted months or even years after the stellar disruption. 

Also on the team were NASA Swift space telescope director Prof. Brad Cenko and Dr. Iair Arcavi from TAU.

“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,” said Horesh. “However, we decided to conduct one last radio observation six months after the star was destroyed and surprisingly, we discovered bright radio emissions. 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.”

 

Flares are believed to be caused by a huge velocity jet launched when the star is destroyed and sucked into the black hole or as a result of the outward explosion of debris from the explosion. The analysis of the delayed radio flares led the research team conclude that new models have to be developed to explain such a long delay of radio flare emission. In addition, it is possible that such delayed radio flares are a common phenomenon, but to find more of them, teams will need to remain focused on observations surrounding the affected areas long after the initial disruption. It is also possible that a substantial amount of the stellar debris is eventually pulled into the black hole, but only long after the star was destroyed.

 

“What led to the delay and what is the exact physical process responsible for such late-time emission are still open questions,” concluded Horesh. “In light of this discovery, we are actively searching for more such delayed radio flares in other tidal disruption events.”