The encounters between black holes and stars are not rare occurrences on a cosmic scale, and when such an event occurs, the powerful gravitational tidal forces of the black hole are responsible for the emission of an impressive jet of matter, while the star finds itself inexorably dislocated. For the first time, astrophysicists have been able to observe the formation and evolution of such a jet.
The phenomenon occurred within a pair of colliding galaxies, called Arp 299 and located about 150 million light-years away from Earth, in the constellation Ursa Major. It is composed of two irregular barred galaxies named IC 694 and NGC 3690. Arp 299 is a region of star-formation bursts (region with a fast rate of star formation) and up to now 8 supernovas have already been detected. As in most galaxies, IC 694 and NGC 3690 have a central supermassive black hole.
Using radio telescopes and infrared telescopes, an international team of astrophysicists was able to observe a star twice as massive as the Sun passing near one of the supermassive black holes with a mass of 20 million solar masses. At this distance, the star was hit hard by the gravitational tide of the black hole, and was dislocated under these extremely violent forces. One such process is called the tidal disruption event (TDE). The results of the observation were published in the journal Science.
A TDE appears when a star passes below the Roche limit (distance under which an object is dislocated by the tidal forces of the gravitational body that it orbits) from a black hole and approaches the horizon sufficiently to undergo a deformation and tearing due to tidal forces of the black hole. Since 1976, astrophysicists Juan Frank and Martin F. Rees theorized on such events. However, these phenomena have never been directly observed and never with great precision. “Never before have we been able to directly observe the formation and evolution of a jet coming from one of these events,” explains Miguel Perez-Torres, astrophysicist at the Institute of Astrophysics of Andalusia (Spain). ).
During a TDE, the star is first deformed and then disintegrated; it then forms an accretion disk around the equator of the black hole before falling progressively beyond its horizon of events. During the fall of the star material, the gravitational and friction forces produce a considerable increase in the temperature of the star, causing the periphery of the black hole to shine dramatically. Part of the matter of the star is expelled to the poles of the black hole via magnetic field lines and ejected in the form of a stream of relativistic particles.
The observation of this phenomenon, dubbed “Arp 299-B AT1”, began on January 30, 2005, when the Canary Islands-based William Herschel telescope observed an infrared burst from the center of Arp 299. On July 17, In the same year, the Very Long Baseline Array (VLBA) – an American network of radio telescopes using very long base interferometry – detected another event of the same type. However, the telescopes, operating in the visible spectrum, could not locate these phenomena, implying that Arp 299 is surrounded by dust blocking the visible light.
“Over time, the new object remained bright in the radio and infrared domains, but not in the visible range. The most likely explanation is that concentrations of interstellar gas and dust on the periphery of the galactic center have absorbed X-rays and visible light to re-emit them in the form of infrared, “explains Seppo Mattila, astrophysicist at the University of Turku. (Finland).
During the last ten years, astrophysicists have continued to observe the phenomenon through several telescopes, including VLBA and European radio telescopes, to better understand it. These observations eventually showed the presence of a unidirectional emission of matter, from the edge of the black hole, moving about 1/4 of the speed of light and thus confirming its relativistic jet nature.
Scientists believe that TDEs are not rare events, but that many could be hidden by dust clouds on the periphery of galactic centers hosting supermassive black holes, such as Sgr A * in the center of the Milky Way. Arp 299-B AT1 is a fabulous opportunity to learn more about the formation and evolution of relativistic jets as well as the growth of black holes. “Because of the dust that absorbs visible light, this particular TDE may actually be just the tip