An international team of astronomers have detected a pulsar at the center of a globular cluster called NGC 6624, and discovered that it is likely orbiting an intermediate-mass black hole (IMBH) – an important “missing link” object.
NGC 6624, located 26,000 light-years away in the constellation of Sagittarius, is only a few light-years across and contains millions of stars. These conditions are ideal for the formation of extreme objects, and thus it's a perfect location for astronomers to look.
The team, led by The University of Manchester, found evidence of a millisecond pulsar called PSR B1820-30A, a highly magnetized neutron star spinning on its axis hundreds of times every second. Dynamical consideration suggests that the pulsar is orbiting an incredibly dense object, which packs in its small volume the mass of 7,500 stars.
"Pulsars like PSR B1820 30A act as fantastically accurate clocks and allow us to determine precisely their distance from the Earth in the same way that global positioning satellites work," said Professor Andrew Lyne, from the University of Manchester, in a statement. "The pulsar is therefore very sensitive to any motion arising from the gravity of other nearby massive objects, such as black holes, making it easier for us to detect them."
This is a discovery of records. The pulsar is the closest ever found to the center of a globular cluster (occupied by the IMBH) and it’s also the first pulsar to be found orbiting a black hole. If that wasn’t enough, the IMBH in NGC 6624 is one of only a handful of candidate and confirmed objects.
Black holes are most commonly found in two categories: stellar mass black holes, which form when the most massive stars go supernova, and supermassive black holes that are located at the center of galaxies and are millions of times the mass of the Sun. IMBHs are believed to have formed from stellar mass black hole mergers. In turn, the IMBHs merge to become supermassive black holes.
"High stellar densities towards the centre of globular clusters provide a likely environment for the formation of massive black holes. The detection of IMBHs is important for understanding the missing link between the different kinds of black holes,” team leader Dr Benetge Perera added. "It is generally thought that they could be formed by the direct collapse of very massive primordial stars or successive mergers of stellar-mass black holes and runaway collisions in dense young star clusters."
Follow-up observations will provide more insight into this currently unique globular cluster.