Stars Nibbled Again and Again by Supermassive Black Holes

Two teams of astronomers have observed repeated bursts of light from dormant black holes that partially destroy stars over and over again.

At the center of most galaxies is a supermassive black holesupermassive black holehundreds of thousands to billions of times more massive than the SoleilSoleil. These objects are difficult to detect and observe, the intensity of their gravitational field preventing any form of mattermatter or radiation to escape.

However, when a star passes near such a giant, thestarstar luminous can find itself torn apart by the strong tidal forces of the black hole, forming a disc of debris on which the dark star feeds: this is called a tidal rupture event. The rayons Xrayons Xthe UV, the lightlight optics and waves radioradio emitted on this occasion can then be detected and reveal to us the presence of the black hole. Since their discovery in the 1990s, nearly 100 tidal rupture events have been observed.

Nibbled bite by bite

These events usually only happen once, when the black hole devours the star. However, two teams ofastronomersastronomers using XMM-NewtonXMM-Newtonspace observatory of the European Space Agency (ESAESA) intended for the observation of soft X-rays, observed the repetition of bursts of light coming from black holesblack holes inactive. The unexpected recurrence of these events would be due to the fact that these black holes only partially destroy these starsstars each time they pass by.

In a typical tidal rupture event, the brightnessbrightness increases significantly during the few months during which the black hole devours the star. However, two events have a particular behavior: they repeat themselves, the luminosity increasing several times in the X-rays and in ultravioletultraviolet. This suggests that the stars were not totally destroyed during the initial encounter with the black holes, continue their orbitorbit and encounter the black hole again, resulting in recurring flares. This activity is called a partial tidal rupture event.

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Some galaxiesgalaxies are constantly active as their supermassive black hole continually draws gaseous matter towards it, but the two new events come from black holes that generally remain dark and quiet, until a star approaches. It is the first time that jerksjerks repeating lights have been detected for inactive galaxies.

eRASSt J045650.3−203750 was discovered by eROSITA, an X-ray telescope aboard the Russian-German Spektr-RG mission, in a galaxy nearly 1.2 billion light-years away. Observations with XMM-Newton in 2021 and 2022, by a team led by Zhu Lui, from the Max-Planck Institute of physiquephysique alien, revealed that the original eruption was followed by repeated explosions approximately every 223 days.

AT2018fyk was discovered by the All-Sky Automated Survey for Supernovae (Asas-SN) in the core of a galaxy about 900 million light-years away. It shone in UV and X-rays for at least 500 days, before suddenly fading. In May 2022, Thomas Wevers of the European Southern Observatory and his colleagues used XMM-Newton to study the dramatic increase in X-ray and UV luminosity 1,200 days after its first appearance.

Animation describing the scientific result published by Wevers et al., (2022). A mysterious flash of electromagnetic radiation from the center of a galaxy was first detected in 2018. The eruption lasted about 2 years and then disappeared. This behavior is consistent with the supermassive black hole at the center of this galaxy disrupting a passing star. However, surprisingly, the center of this galaxy brightened again in 2022. A new study suggests that this may be explained by a partial tidal rupture event. © BlackHoleExplosions

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Important observations to study the influence of supermassive black holes

In total, more than five days of observations with XMM-Newton were used to monitor the evolution of X-rays from these sources. The extremely sensitive European Photon Imaging Camera (Epic) on board XMM-Newton has made it possible to study in detail the hot matter surrounding black holes.

« During typical tidal rupture events, we don’t expect to see a second eruption for a few thousand years.explains William Alston, a researcher at ESA. These new studies suggest that the affected star is being pulled into a close orbit [du trou noir supermassif] after being torn from a star systemstar system binary through the central supermassive black hole».

In addition to XMM-Newton and eROSITA, the studies involve other missions, in particular the Neil Gehrels Swift Observatory from NasaNasal’Australia Telescope Compact Array (Atca) and the Neutron Star Interior Composition Explorer (nicer) on the international space stationinternational space station. Both partial tidal rupture events will be closely monitored during the forecasted periods of the next episodes, in order to confirm the results and make further discoveries. Observers may not detect anything, indicating that the star was completely engulfed in the previous episode. The hunt is also beginning to find similar partial tidal rupture events.



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