Finding the middle ground
Could astronomers have finally found the ‘missing link’ of black holes?
“The burst of X-rays might indicate the presence of the elusive intermediate mass black hole”
We know that most, if not all, galaxies have supermassive black holes at their centres. Though they can’t be seen directly, evidence comes from observing the behaviour of stars and gas, most strikingly at the centre of our own Milky Way where observations taken each summer for more than 20 years reveal the orbits of stars about the galactic centre. With a mass equivalent to nearly four million Suns, this is truly a supermassive black hole, but astronomers have struggled to explain how it came into being.
We know how at least some black holes form. Rigel, in Orion, has a mass about 18 times that of the Sun. When, in a few million years’ time, it exhausts the fuel at its centre, a spectacular supernova will be the result. Most of the star’s material will be scattered to the galactic winds, but the core will collapse, forming a small black hole, one which will weigh in at a few times the mass of the Sun. Evidence of such stellar mass black holes has been gathered over the last few decades, most spectacularly in the minuscule ripples in space recently detected by gravitational wave experiments and due to the collision of black holes.
Black hole quest
Gravitational wave experiments are most sensitive to the collision of massive black holes but have not seen evidence for anything much more massive than maybe 50 solar masses. That fits with our understanding of stars and supernovae, which predicts a maximum mass which falls well short of that obtained by the sort of behemoths that lurk at the centre of galaxies. In seeking to understand how such massive objects form, astronomers have long sought our ‘missing link’ – a population of black holes intermediate in mass between the two. Now results from a new paper suggest that the quest might have succeeded.
The object in question is 3XMM J215022.4-055108 (catchy, I know), a source of X-rays which suddenly flared up in 2006. Such a flare indicates that something dramatic is happening; in this case, we seem to have captured the final moments of a star being ripped apart by the gravitational pull of a black hole. Such things happen from time to time – a disruption to the delicate dance in the Milky Way’s centre could send its orbiting stars into harm’s way – but this particular flare did not come from the centre of a galaxy where supermassive black holes may be assumed to be lurking, and it was bright enough that a stellar-sized black hole would be an unlikely progenitor. That burst of X-rays might therefore indicate the presence of the elusive intermediate mass black hole, but there remained the possibility of a false alarm. The signal could have been coming from a less luminous source in the Milky Way itself. Using the orbiting Hubble Space Telescope, the location of the X-ray source has now been pinned down. The emission comes from a star cluster on the edge of a distant galaxy, a likely home for intermediate black holes. The missing link may have been found at last.
Chris Lintott was reading… Multiwavelength Follow- up of the Hyperluminous Intermediate- mass Black Hole Candidate 3XMM J215022.4- 055108 by Dacheng Lin et al. Read it online at: https://arxiv.org/abs/2002.04618