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Before news of the powerful early October gamma-ray burst got out, there was another “show” which featured a black hole. This time, however, it is not a question of the emission of a powerful energy beam linked to the birth of this kind of very particular objects, but rather of an event that has lasted so long that its origin is to be sought from 2018.
About two years ago in fact a black hole665 million light-years away from Earth, had destroyed one star due to its strong gravitational pull. Nothing particularly strange from this point of view considering that phenomena of this type were already known. What was the peculiarity of the event is that two years later (in 2022), the same black hole it kept re-emitting parts of that same star when it usually ends in a short time.
A black hole ejected parts of a star after two years
The merit of the observation goes to the Harvard & Smithsonian Center for Astrophysics which, thanks to the scientists and the observation campaign of tidal destruction eventshad the intuition to go and re-observe the same part of the sky in search of new information (then leading to the realization of a study). L’expulsion of parts of the star it happened at very high speeds, up to half that of light, allowing it to become very noticeable due to the emissions you generate.
The event took the name of AT2018hyz and it has to do with the “spaghetti making” and with a tidal destruction event (known as TDE or tidal disruption event). The first is the destruction of an object, in this case a star, that passes too close to a black hole and with the latter who manages to tear away matter “fraying it” (and causing its temperature to rise) due to gravitational attraction. The second, on the other hand, is precisely linked to the gravitational attraction which, in areas where there are black holes, becomes extreme.
In June 2021 what had been a TDE defined “irrelevant” in 2018, it became very important with the black hole which showed signs of activity. Yvette Cendes (lead researcher of the study) said “this took us completely by surprise: no one had ever seen anything like it before”. Considering the particularity of the phenomenon, Cendes and colleagues managed to have additional observation time by different telescopes (continued until May 2022) in order to detect as much data as possible.
VLA, ALMA, MeerKAT and ATCA (which are located in different parts of the world) were thus used, adding the Chandra and Swift space telescopes (for X and gamma rays respectively). As also explained by Professor Edo Berger (co-author of the study), initially the black hole from AT2018hyz it did not seem to have generated any particular emissions. The scenario then changed about three years later showing particularly strong emissions in the radio waves.
The possible causes of the black hole’s behavior
The motivation for the abnormal behavior? Still not clear. Another anomaly is given by the fact that the expulsion took place at high speeds (up to half that of light) while TDEs usually have speeds five times lower. To get a better idea of the phenomenon it will be necessary to observe others. There are currently some hypotheses. The first is that the jet of material was too weak or inactive until about 750 days after the star’s destruction due to a change in the accretion disk surrounding the supermassive black hole.
The second is that the cause is related to the magnetic field ofevent horizon which would have been not particularly strong initially. This would not have allowed the creation of an emission of matter, which is possible after the matter has started to whirl around. Also linked to the magnetic field would also be the hypothesis that instead there was a previous active galactic nucleus which would have been attracted by the tidal destruction event but which would also have modified the magnetic field of the black hole.
Another possibility arises from the fact that emission in radio waves would have been delayed due to the structure and viscosity of the star’s matter. Initially, therefore, the parts of the star would have created a ring in the area of the accretion disk with emission mainly in the visible / UV. Only later would there be an evolution with emission in other wavelengths. Finally, the expelled matter would have always been present from the beginning of the event but due to too thick dust clouds and an unsuitable orientation this would not have been visible.
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