Black holes come in three sizes: small, medium, and large. Small black holes have stellar mass. They form when a large star collapses at the end of its life. Large black holes lurk in the centers of galaxies and are millions or billions of solar masses. Medium-sized black holes are those between 100 and 100,000 solar masses. Known as Intermediate Mass Black Holes (IMBHs), they are the kind we least understand.
One of the biggest difficulties in studying IMBHs is that they are hard to find. We believe that they arise when large stars or black holes with stellar mass merge in the centers of globular clusters, so that they are more obscured in a dense cluster of bright stars. Medium black holes are usually not active, so we cannot identify them by their rays or intense X-rays. But they should be quite common. It is estimated that there could be around 45,000 medium-mass black holes in the vicinity of our galaxy.
M45 is a globular cluster that could harbor a black hole. Photo credit: ESA / Hubble & NASA
Recently, a team used a new technique to find one of these. Their method uses gamma-ray bursts and gravitational lenses. A gamma ray burst (GRB) is a bright flash of gamma rays that occurs from time to time. They are likely caused when a large star explodes as a hypernova, or when two large stars collide and merge. Usually a GRB occurs as a single flash that lasts about half a second, but sometimes we see two flashes that are in the same general region of the sky at almost the same time. This might just be a coincidence, but it is more likely that the two flashes were caused by the same GRB but appear as multiple flashes due to the gravitational lens.
How gravitational lenses can make a flash appear as multiple flashes. Photo credit: NASA, ESA and D. Player (STScI)
The team analyzed the pattern and spectrum of the double burst to show that the second was an echo of the first. This confirmed that the event was gravitationally affected by a mass between us and the GRB. They then used the timing of the two bursts to determine the cause. They found that the data was consistent with a mean mass black hole around 55,000 solar masses.
This method cannot be used to find many medium-mass black holes, but each IMBH we find gives us a different source of information. Intermediate mass black holes in the early Universe may have been the seeds of the supermassive black holes we see in galaxies today. The more we know about the middle children of the black hole family, the more we can know how black holes have shaped our modern universe.
Reference: Paynter, James, Rachel Webster, and Eric Thrane. “Evidence of a medium-mass black hole from a gamma-ray burst with gravitational lenses.” Natural Astronomy (2021): 1-9.