Intermediate black holes have so far been regarded as a “missing link” because only a few candidates of such medium-sized black holes have been discovered so far. Now a team of astronomers has tracked down another specimen of this intermediate stage between stellar and supermassive black holes. This succeeded because this object acted as a gravitational lens and its mass distorted the radiation from a distant burst of gamma rays in a characteristic way. According to this, this object has a mass of 55,000 solar masses and can therefore neither have been formed in a simple star explosion, nor can it sit as a super massive gravity giant in the center of a galaxy.
Almost all black holes so far discovered by astronomers fall into two categories. Either they are stellar black holes with a maximum of around 50 solar masses, such as those formed in the supernova of a massive star. Or they are supermassive giants with a mass of one million to ten billion solar masses that sit in the center of galaxies. In the mass range in between, however, there is a gap, although according to the theory there should also be intermediate black holes. These could result from multiple mergers of stellar black holes, as they probably take place in the heart of globular clusters, or from the merger of massive stars into a megastar with a mass of around 1000 solar masses, which then collapses into a black hole. So far, however, there are only a few candidates for such “missing links” in the black hole population.
Tell-tale echo in the gamma-ray pulse
Now astronomers around James Paynter of the University of Melbourne could have tracked down another candidate for an intermediate black hole – in a new way. For their study, the team evaluated subtle delays in light from distant gamma-ray bursts. These short but extremely strong bursts of rays occur, among other things, in supernovae or collisions of stars. Part of the energy released in the process radiates out into space in the form of short-wave, very high-energy gamma-ray flashes, which can still be detected with gamma-ray telescopes in Earth orbit even from a distance of millions or even billions of light years. When these gamma rays pass an object with a large, compact mass on their way to Earth, for example a galaxy, a star cluster or a larger black hole, its gravitational effect slightly deflects the radiation.
This leads to a gravitational lens effect: With visible light, the distortion of the light caused by gravity can result in enlarged double images or rings of light; it is similar with gamma rays. But because gamma ray telescopes do not have a sufficiently high spatial resolution, such a gravitational lens effect shows up in them as a characteristic splitting and delay of the signal: “The observation signature of such a lens event is an initial gamma ray pulse followed by an ‘echo'”, explain Paynter and his team. The exciting thing about it: From the delay and the characteristics of this echo, it is possible to read off the mass and properties of the cosmic object that deflected the gamma radiation. The researchers therefore searched a data set of 2,700 gamma-ray bursts for an echo signal that has the characteristics of an intermediate black hole.
Lens flare matches intermediate black hole
The astronomers found what they were looking for in a gamma ray burst: The short burst GRB 950830 showed a radiation curve that indicated the effect of a gravitational lens with around 55,000 solar masses. “In this mass range there are three astrophysical objects that can serve as lenses: globular clusters, halos of dark matter or black holes,” write Paynter and his colleagues. However, according to your calculations, globular clusters are too rare and halos do not match the internal distribution of their mass to the observed distortion of the gamma rays. In the opinion of astronomers, there is therefore much to suggest that there is a black hole behind this gravitational lens effect during the gamma-ray burst. And because of its mass, it would have to be one of the long-sought intermediate black holes.
Also exciting: “This newly discovered black hole could be an old relic – a primordial black hole that was formed in the early universe by the first stars and galaxies,” explains co-author Eric Thrane of Monash University. “These early black holes could be the nucleus of the supermassive black holes that are located in the heart of the galaxies today.” Because how these heaviest representatives of the black holes are formed has only been partially clarified. Astronomers assume, however, that they are formed by the amalgamation of smaller precursors and further growth. “We do know that these supermasive black holes are in the centers of most, if not all, galaxies. But we still don’t understand how these giants could get so big since the beginning of the universe, ”adds Paynter.
Source: Nature Astronomy, doi: 10.1038 / s41550-021-01307-1