Intermediate-mass black holes are a void in black hole research, a mysterious missing-link between stellar-mass and supermassive black holes that have, until recently, managed to avoid detection. Yet, an international team of astronomers, led by the University of Manchester, might now have uncovered one in the Sagittarius constellation.
The team observed a pulsar at the centre of a globular cluster known as NGC 6624, and found it appeared to be orbiting an intermediate-mass black hole. “Millisecond pulsars, like the one we observed, are very stable rotators” study lead Dr Benetge Perera told I,Science, “They’re as precise as atomic clocks. Because of this, you can measure the gravitational effects on pulsar timing.”
The team monitored the beats of radiation emitted from the pulsar PSR B1820-30A, a collapsed star so dense that a grain of sand made of the same material would weigh approximately 60 million tonnes, and spinning so rapidly on its axis that it makes hundreds of rotations per second. By analysing variation in the timing of the emitted radiation, the researchers were able to calculate the distance and mass of nearby objects exerting gravitational forces on the pulsar. Their evidence appeared to show that the pulsar was orbiting an object 7,500 times the mass of the Sun, corresponding to an IMBH.
“This is an important discovery as it is the first time a pulsar so close to the centre of a globular cluster has been measured in this way” Dr Perera added.
Globular clusters, millions of tightly packed old stars, are good candidates for harbouring IMBHs as they have high stellar densities and so provide a favourable environment for forging these enigmatic objects. Although it is not well understood how IMBHs form, it is currently thought that they could be produced by the collapse of extremely massive primordial stars or through the merging of stellar mass black holes and runaway collisions in dense young star clusters. However, this discovery could aid understanding of how IMBHs form and evolve.
The team behind this discovery plan on observing more pulsars to collect further evidence for intermediate-mass black holes, although as Dr. Perera pointed out “You have to collect very large datasets. My colleagues have been taking the data since 1990, more than 25 years now.” So, it might be a little while till they detect the next one…
The globular cluster NGC 6624, which contains the pulsar PSR B1820-30A. Observations from this pulsar are consistent with the presence of a central intermediate-mass black hole.
Madeleine Finlay is studying for an MSc in Science Communication