Stars engulfed by a black hole release energy long after the process begins

The radio "awakens" - Dr. Asaf Haresh of the Hebrew University revealed a new and surprising phenomenon concerning the destruction of stars by black holes * The researcher and his team managed to reveal the release of flashes of radio radiation months and years after the star was first torn to pieces by the black hole. So far, no researcher has been able to prove that such radio radiation flashes exist, a relatively long time after the destruction of the star. The research even proves the power of the event and the partial understanding of the scientific world on the subject. The research was published in Nature Astronomy

Note: On the same day, another article by Dr. Haresh was also published that we reported on yesterday: For the first time, a neutrino particle has been detected that came to us from the vicinity of a supermassive black hole

In recent years, more and more bright flashes of visible (optical) light have been discovered in the centers of galaxies. The light flashes last several weeks to months. The popular opinion is that these flashes of light result from a particularly violent event, during which a star moves too close to a supermassive black hole located at the center of that galaxy, and is torn to pieces by the tidal forces exerted on it by the black hole. The star ceases to exist when the gas that made up the star until now turns into a stream of matter (gas), part of which remains in a circle around the black hole, and part of which flies away from the black hole at enormous speed. Events of this type are called Tidal Disruption Events, or TDEs for short.

Such events are of great importance in the field of black hole research. First, we now know that in most of the centers of galaxies today there is a supermassive black hole (for example, in our Milky Way galaxy there is a black hole with a mass of about four million solar masses, a discovery for which the Nobel Prize was awarded this year). In some galaxies the black hole is defined as active - there is a large reservoir of gas in the center of the galaxy in the shape of a disk, called an accretion disk, around the black hole and material from the disk is attached to the black hole. In this process, strong radiation is released and as a result, powerful jets are emitted from the vicinity of the black hole (but not directly from it) that are visible at different wavelengths and especially in radio radiation. There are also "dormant" black holes that do not have such a disk around them and do not emit unusually strong radiation from this area.

Star destruction events by the tidal forces of a black hole can also occur following the passage of a star in the vicinity of these (so-called dormant) black holes, and in fact the discovery of a temporary flash of radiation as a result of the tearing of the star by the dormant black hole indicates the existence of the same black hole at the center of that galaxy. This event, then, is another way to expose black holes. It also allows us to learn about the structure of the material in the very close vicinity of the black hole, even in distant galaxies, which is not possible in any other way today. Beyond that, these events allow us to study processes of material adsorption to the black hole. For example, we do not know how the accretion discs are formed. Even when the star is torn apart by the black hole, there is an open question as to what leads to the flash of light we see - is it caused by the rapid absorption of the star's material into the black hole? Some believe that the absorption of material from the star that has ceased to exist cannot occur immediately, and the stream of material torn from the star continues to circle around the black hole, and that shock waves created by the impact of one part of the stream on another part of it cause the flash of light we see.

The research group led by Dr. Assaf Haresh Rakah Institute of Physics at the Hebrew University, in collaboration with Swift Space Telescope Director Prof. Brad Senko, and Dr. Yair Hakabi from the Department of Astrophysics at Tel Aviv University, has recently been investigating events of this type through observations of the search for radio radiation capable of to reach Earth and announce the activity of the black hole. A new study by the group was published this week in the prestigious scientific journal nature astronomy. The researchers were able to distinguish radio radiation that was detected only in a few events of this type, and usually indicates the existence of material blown away immediately after a high-speed starburst event (or a powerful jet emitted from the vicinity of the black hole as a result of the absorption, if any, of material from the star destroyed by the hole's activity the black). However, in all the few cases in which radio radiation has been detected so far, in events of this type, it has been detected almost immediately after the destruction of the star, indicating material blown away at high speed from the vicinity of the black hole immediately after the event. This time, however, the researchers were surprised to discover radio flashes from the event that were emitted months (and then additionally years) after the time the star was destroyed.

The sensitive radio observations were made as part of a study of an event (known as ASASSN-15oi) of the destruction of a star by the tidal forces of a black hole, revealing for the first time the late (delayed) radio flashes emitted by such events. The event of the destruction of the star was revealed as a visible flash of light in the ASASSN project. As soon as the event was discovered, Dr. Haresh and his research colleagues made a radio observation of the event with the radio telescope known as the "Very Large Array", located in New Mexico in the USA (a telescope consisting of 27 radio antennas with a diameter of 25 meters each ).

In the first observation no radio radiation was detected despite the high sensitivity of the observation. The researchers repeated the observation twice more in the three months after the first observation, and no radio radiation was detected in these observations either. Amazingly, the insistence of the researchers led by Dr. Haresh to continue to carry out tests paid off. "The prevailing theory at this point is that if radio radiation has not been discovered, then it is not expected that radiation will be discovered in the future," says Dr. Haresh. "Nevertheless, we decided to go beyond the limits of the predictions of the known theories, and make another observation about six months after the time when the event was first discovered in visible light, and to our great surprise we discovered bright radio radiation. Immediately after the discovery, we collected additional and more detailed data using the radio telescope for about a year, during which the radio radiation faded. In addition, we discovered by analyzing data collected as part of a large sky survey (performed by the US National Radio Astronomy Observatory using the same radio telescope) in 2019 another (second) radio flash from the same point in the sky, about four years after the first detection of the event in visible light. This is the first time that flashes of radio radiation have been discovered that occur after a delay of months and years after the initial event in which a star was destroyed by a black hole."

The analysis of the findings revealed three main conclusions. The first, that none of the standard models of the destruction of a star by a black hole can explain the radio flashes that appear at such a long delay after the destruction of the star (including not the behavior of the radio radiation that developed differently from the way in which radio radiation develops in similar events, in which radio radiation was discovered in the past near the time of the destruction of the star). The second, that there is a possibility that the bursts of late radio radiation in these events are common phenomena and that it will be possible to discover more of these, and to investigate the newly discovered phenomenon if radio observations are made at later times than it has been customary to do so far. The third, which is possible because some of the material remaining from the destroyed star was attached to the black hole with high efficiency but not immediately, with a delay of several months.

"What caused this delay and what ultimately led to the absorption of the material into the black hole, these are questions that still remain open. If such a late absorption of material did occur, it is possible that a jet will be ejected at a high speed after the absorption, with the jet being responsible for the late radio flash that we discovered in the current study," explains Dr. Harsh, who also believes that his discovery is important for understanding how black holes grow (acquire mass) in the universe. , and how they develop and become super-massive. Adsorption of matter is one way for black holes to grow and discovering periods in which growth of the black hole occurs by delayed absorption (if this hypothesis is proven later) is another important step to understanding the entire process.

Also, Dr. Haresh and his research team believe that the discovery of delayed (delayed) radio bursts will allow not only to study the physics behind these events, but they can reveal and testify to the existence of additional black holes in the centers of galaxies. For example, a visible light flash will not always be detected when a star is destroyed by a black hole, but if several delayed radio radiation flashes are produced as a result, the latter will reveal the existence of the black hole involved in the event and allow it to be studied. "In light of these discoveries, we are preparing to perform additional radio observations of events of this type using radio telescopes around the world, in order to try and discover additional radio flashes of this type and get to the bottom of the phenomenon," notes Dr. Haresh.

for the scientific article

More of the topic in Hayadan:

• New light on black holes
• Are supermassive black holes formed by themselves and not as a result of collapsed stars?
• The disappearing black holes