At the beginning of the year, the Zwicky Transient Facility (ZFT) of the Palomar Observatory in California (USA) detected an extraordinary flash in a part of the sky where no such light had been observed the night before. With an intensity equivalent to a 1,000 trillion suns, this phenomenon was studied by different scientific groups. Now, two articles have been published in Nature and Nature Astronomy involving the Institute of Astrophysics of Andalusia (IAA-CSIC) and the Institute of Space Sciences (ICE-CSIC), both pertaining to the Spanish National Research Council (CSIC), together with the Institute of Space Studies in Catalonia (IEEC), that agree on the origin of the flash: it is a jet or relativistic jet produced by a supermassive black hole when devouring a star.
Most galaxies are home to supermassive black holes in their central regions, which contain up to billions of times the mass of the Sun. These are objects with such an intense gravitational field that not even light can escape, and they show a structure formed by a disk of gas and dust (accretion disk) that absorbs material from its surroundings. "Most of the time, however, supermassive black holes do not devour anything," explains Miguel Pérez-Torres, a researcher at the IAA-CSIC who participates in the Nature Astronomy article. "Thus, a phenomenon like this, which we know as tidal disruption event (TDE), may provide us with a unique opportunity to study the vicinity of these powerful objects."
The scenario is relatively well known: the star is torn apart by the tidal forces of the black hole, becomes part of its disk, and ends up being swallowed by it. "However, in some extremely rare cases, the black hole expels jets that travel almost at the speed of light in the process of destroying and accreting the material of a star," says José Feliciano Agüí Fernández, researcher at the IAA-CSIC and co-author of the work published in Nature–. Estimates suggest that these jets occur in only 1% of cases, and that is precisely what we observed."
In fact, the bright flash from the event named AT2022cmc is among the brightest ever observed. The source is also the most distant yet detected, at about 8.5 billion light-years away. It appears to be at the center of a galaxy that is not yet visible because the light from AT2022cmc eclipsed it. Nevertheless, when the episode ends, it could be photographed by the Hubble or James Webb space telescopes.
How could such a distant event appear so bright from the Earth? The two articles analyzing the phenomenon conclude that the jet from the black hole could be pointing directly towards the Earth, which makes the signal appear brighter than if the jet were pointing in any other direction. The effect is “Doppler boosting” – it is similar to the amplified sound of a siren and refers to the modification of the frequency of photons -and, therefore, of the luminosity- generated by matter that moves at speeds close to that of the light (relativistic matter). In this case, researchers found that the speed of the jet is 99.9% percent the speed of light.
In order to generate such an intense jet, the black hole must be in an extremely active phase, what Dheeraj Pasham, scientist from the Massachusetts Institute of Technology (MIT), calls “a hyper-feeding frenzy. It’s probably swallowing the star at the rate of half the mass of the Sun per year”.
This finding is the first Doppler-enhanced TDE observation since 2011, the first such event discovered using an optical sky survey and the fourth one ever detected. "A constant monitoring of the skies is essential to detect this type of event as soon as possible, in order to then closely monitor it with multiple telescopes," says the postdoctoral researcher at the Institute of Space Sciences (ICE-CSIC) and the Institute of Space Studies of Catalonia (IEEC) Tomás E. Müller Bravo, co-author of the study published in Nature Astronomy.
In the upcoming years, as more powerful telescopes are available, it could be possible to detect more tidal disruption events, which in turn could shed light on how supermassive black holes grow and shape the galaxies around them. “The discovery of this very distant TDE shows that we can find more in the future at these distances with the LSST project, an 8-meter telescope that is being built in Chile and that will start taking data at the end of 2024”, adds Lluís Galbany (ICE-CSIC, IEEC), who also participates in the study published in Nature Astronomy.
However, it is still unknown why some tidal disruption events produce jets and others do not. “Our work suggests that the difference probably lies in how the supermassive black hole rotates, and that a high rotation rate is a necessary ingredient for launching jets, an idea that brings us closer to understanding the physics of supermassive black holes in the centre of galaxies that are thousands of light years away”, concludes Agüí Fernández.
IAA-CSIC / ICE-CSIC Communication