All the black holes they are not the same: there are ‘small’ ones, barely five times the size of our Sun; and there are other gigantic ones, called supermassive, with masses of millions of suns. However, the classification does not end there: there are other intermediates, very difficult to discover, and of which there are only a handful of candidates. Now, project researchers Young Supernova Experiment (YSE), a collaboration of astronomers that mainly looks for stars exploding at the end of their lives (supernovae), has just added another possible intermediate mass hole to the list that, moreover, is very special: its ‘hunger’ is so voracious that it is eating a star and flinging its glowing ‘crumbs’ after it. The results have just been published in the journal ‘Nature Astronomy’.
Using the Pan-STARRS observatory (Panoramic Survey Telescope and Rapid Response System), located in Hawaii, the team was able to observe the same portion of the sky every few days; the hope was to catch a supernova explosion in the first hours or days after the astronomical phenomenon had occurred.
But in June 2020 they caught something unexpected in their data: a fast-brightening object in a dwarf galaxy nearly a billion light-years away. “We were very, very lucky,” he explains. Charlotte Angus, from the University of Copenhagen and first author of the study. After this unusual find, they continued to observe the object, called AT 2020neh, over the next few days and weeks, not only with Pan-STARRS, but also with other ground-based observatories and even the Hubble Space Telescope. Its light curve – that is, how its brightness changes over time – peaked after just over 13 days. Afterwards, a slow and prolonged diminution of its luminosity began.
A spaghetti star
The shape of the light curve and the characteristics of the light spectrum did not match those of a supernova; It seemed more like a tidal disruption event (TDE), a phenomenon that occurs when a star approaches the event horizon of a black hole, and the gravity of the ‘monster’ is so strong that part of the star’s material is drawn to its center, causing the star to ‘spaghetti’ and an arc of bright light.
However, AT 2020neh held even more surprises: it reached its maximum brightness more than twice as fast as in a typical TDE. Theorists who model these phenomena predict that smaller black holes produce fast-spiking TDEs. Using such models, the team calculated that this hole’s light curve could have been produced by a black hole with a mass between 100,000 and 1 million suns. However, very little is still known about these events.
Is it a seed for a supermassive black hole?
Astronomers believe that most normal-sized galaxies, such as the Milky Way, have a supermassive black hole at their center. But there are other smaller ones, such as the one found at AT 2020neh, in which there is still debate as to whether there are intermediate-mass black holes like the one detected at their centers, which could be proof that, indeed, these objects of intermediate mass from which nothing escapes, not even light, they would be a sort of ‘seed’ from which the supermassive would grow.
Regardless, the team notes, finding medium-sized TDEs may be a new way to find the elusive. intermediate mass black holes. And, if they can find a large enough sample, they could study whether the central holes grow at the same rate as the galaxy, as their supermassive ‘cousins’ do.
If that premise holds true for everyone, it would support the theory that galaxies grow by merging with one another, rather than originating from a single, gigantic cloud of cosmic dust. The next ground-based observatories to come, such as the Vera C. Rubin in Chile, or the recently released James Webb, will be able to point with greater resolution towards these ‘elusive’ black holes.