What happens when a star is swallowed up in a black hole?

A tidal disruption event (TDE for short) occurs when a star passes by a super-massive black hole and disintegrates due to the gravity of the black hole, i.e. its tidal forces. Two articles published in the journal Nature Astronomy illuminate the internal structure of such rare events that were observed a few years ago 700 million light-years away. One article led by Dr. Assaf Harash from the Hebrew University of Jerusalem deals with measuring radio waves emitted long after an appeal event, and another article on the possible detection of neutrino emissions related to another appeal event. Both studies use many observatories operating at a variety of wavelengths , Radio waves and X-rays, and even in the neutrino observatory, a particle that is not in the electromagnetic spectrum.This combined study allows the understanding of phenomena that could not be detected with the help of traditional astronomy, using only visible light.

Supermassive black holes probably exist at the center of most galaxies in the universe. The mass of such a black hole can be between a million and a billion solar masses, and the process of their formation is related to the formation of the galaxy that contains them. These are dark bodies that cannot be seen directly, but are sometimes surrounded by gas that falls rapidly around the event horizon, heats up, and emits energy at different wavelengths. When such a black hole swallows large amounts of gas, the galaxy appears “active”, meaning it emits a lot of energy that can be measured even millions of light-years away. This type of process is relatively continuous: radiation is emitted over the years, and although significant changes in the amount of radiation can be measured, in most cases an active galaxy remains active over time.

However, there are rare events where the center of a galaxy emits a large amount of radiation for a few weeks or months, after which the galaxy returns to normal. These events are rare and very bright, and are appropriate for the theoretical scenario where a star is torn to shreds by the difference in gravity created near the black hole. The radiation is emitted from the impact of the star’s gas on the gas surrounding the black hole, and from the heat emitted when the gas surrounds the black hole in small circles, a phenomenon known as adsorption disk. Sometimes a gas jet is also created that is ejected over the disc, and contains a material that is accelerated to high speeds. It is not yet known exactly which part of the radiation is emitted from which physical part of the gas in the vicinity of the black hole. Because these are events that occur in distant galaxies, they cannot be directly observed, and in order to better understand the physics of the process of appeal and absorption of such stars, radiation over time and at different wavelengths must be carefully measured.

The spectrum of the event, i.e. the intensity of the radiation at different wavelengths, can indicate the temperature of the gas emitting the radiation and its composition. The intensity of the radiation helps to understand how much energy is released when the star breaks down and is swallowed. The length of time it takes for radiation to clear up and disappear helps to understand the distances and velocities of the various gas particles. But even very accurate measurements in the visible light field are not enough to understand what is happening near the black hole. To better understand those rare phenomena it is necessary to increase the wavelength range used by astronomers, and even to use techniques that are not based on electromagnetic radiation.

The electromagnetic spectrum describes radiation of various types, consisting of detectors in an electric field and a magnetic field. This radiation fills the universe, and outside of astronomy itself its effects can be discerned in everyday life as well. At one end of the spectrum we find the longest waves, with the least energy, like the radio waves and microwave waves familiar in the media and in the kitchen. At medium wavelengths we find, among other things, infrared radiation, which is used by night vision devices, for example; The visible light that includes the colors of the rainbow, and the ultraviolet radiation that is already energetic enough to cause skin burns and health damage to those who have been exposed to it for a long time. At the end of the short wavelengths are the most energetic types of radiation, X-rays (X-rays) and gamma radiation (nuclear radiation) that penetrate through living tissues and can cause severe damage to living things.

Each of these types of radiation can be used by astronomers to observe different bodies or phenomena in the universe. In some cases such as X-ray radiation, the telescope must be placed above the atmosphere, which effectively absorbs X-rays, and use a space telescope or an airborne telescope. In other cases large telescopes are placed on a high mountain to collect a large amount of radiation, for example visible light.

In addition to electromagnetic radiation, in recent years astronomers have also used new information carriers that provide new knowledge about astronomical phenomena: gravitational waves, neutrino particles and cosmic radiation. The neutrinos are particularly intriguing because they do not yet know the origin of the high-energy neutrino particles that come from the universe. These particles are very elusive, and unlike electromagnetic radiation, their existence is difficult to measure. To detect a few neutrino particles a year, out of trillions passing through Earth every second, huge and complex detectors are needed, such as the IceCube, which consists of an array of sensors arranged in the shape of a square kilometer, deep inside the Antarctic ice sheet A unique star that detects neutrino particles from space.

The combination of different types of radiation with particles and gravitational waves has been given the overall name “multi-messenger astronomy”, and it has yielded surprising discoveries in various fields. The most important characteristic is the cooperation between observatories operating in different technologies and in a variety of places in the world, and even in space. In this case, the value of the combined observations exceeds the sum of the values ​​of all the observations individually.

The fact that the radio waves were emitted long after the visible light indicates that there are processes that release energy that come into action even after the star decays and disperses in the gravitational field of the black hole. One possibility is that gas orbiting the black hole continues to fall slowly, or that it takes time for the material splashed from the star to fall back into an adsorption disk. Existing models describing TDE events do not yet explain the late emission of radio radiation, and this discovery is an opportunity to improve and refine our understanding of TDE events. Apparently, it is worthwhile to follow TDE events using radio waves, even months and years after they are observed. In the future it may be possible to find more examples of such radio eruptions, which will help to understand exactly where the radiation comes from, and it may even be possible to detect TDE eruptions only by radio waves emitted months after the star has disintegrated.

In the second article, a large international team of researchers, including Dr. Assaf Harash and doctoral student Itai Sephardi from the Hebrew University and Prof. Avishai Gal-Yam from the Weizmann Institute of Science, reports the discovery of high-energy neutrino particles that apparently came from the 2019 TDE event. The presence of high-energy neutrinos, if indeed formed as part of the TDE, indicates the acceleration of high-energy particles.The most common explanation is that neutrinos can be formed in a jet of gas emitted near the black hole, while the material torn from the star falls in and produces strong radiation and magnetic fields. In its circular path into the black hole.If the jet contains protons that are accelerated to high energies they can collide with other protons or photons (light particles) and produce particles, which decompose in a series of processes leading to neutrino formation.Unlike visible light Of the black hole, through clouds of dense gas that block most types of radiation.They reveal to us the energy of the protons that are accelerated in the jet, and help to understand the structure of the various parts in the process of decomposition of the star and its absorption in the black hole. In addition, if the relationship between TDE events and neutrino emissions is proven, we can finally know where the energetic neutrino particles discovered in the ice cube come from, and perhaps we can conclude that TDE events are the source of high-energy cosmic rays, the origin of which is still unknown.

Thanks to the collaboration of researchers from different countries who operate a wide range of astronomical observatories, it is possible to detect and characterize phenomena such as the rupture of a super-massive black guy star. The crossover of information at different wavelengths and new information carriers that are not in the electromagnetic spectrum opens up a new and dynamic universe that could not be seen with the help of one type of radiation. Over time, more and more such combined observations will reveal the processes that produce the radiation generated during TDE and the physics of one of the most violent and energetic phenomena in the universe.

Guy Nir is a doctoral student at the Weizmann Institute of Science and a writer on the Davidson Institute for Science Education website