Astronomers presenting at the American Astronomical Society (AAS) meeting in Washington D.C. on Jan. 4, have reported the detection of the emission generated by a black hole as it devoured a white dwarf star in the elliptical galaxy NGC 1399.
This may not appear to be a huge deal to begin with — stars being eaten by supermassive black holes in the centers of galaxies have been detected before — but it would appear that this particular white dwarf was ripped apart and then devoured by a mysterious “intermediate-mass” black hole.
Black holes are thought to come in all sizes and astronomers are very familiar with stellar-mass black holes (i.e. black holes that are created from single massive star supernovae) and supermassive black hole behemoths (weighing in at several million solar masses) confirmed to be living in the centers of most galaxies.
But what about the black holes in between?
Black holes measuring in the range of hundreds to thousands of solar masses have only existed in theory up till now and observational evidence of these “medium-sized” singularities has been very hard to come by. However, the NGC 1399 observation may shed some light on the existence of a particular intermediate-mass black hole in the Fornax cluster, 65 million light years from Earth.
“Astronomers have made cases for stars being torn apart by supermassive black holes in the centers of galaxies before, but this is the first good evidence for such an event in a globular cluster,” said Jimmy Irwin of the University of Alabama who led the study.
Using X-ray data from NASA’s Chandra space telescope and optical data from the Magellan I and II telescopes in Las Campanas, Chile, Irwin and his colleagues were able to study an abnormally bright X-ray emission in the globular cluster.
This is known as an “ultraluminous X-ray source” (or ULX for short). By analyzing the ULX, the team was able to work out that a white dwarf had been shredded by the extreme tidal forces of a black hole and the material was being eaten by the black hole.
The observations suggest oxygen is in abundance, but there is a deficiency in hydrogen, suggesting that the material was being stripped from an old, white dwarf star. (The lack of hydrogen shows that the stellar object has burnt up its fuel, indicating the butchered star did not belong in the Main Sequence.)
Nitrogen was also detected in the spectrum, a factor that has yet to be explained.
By studying the characteristics of the material as it orbited and fell into the black hole — generating the X-rays as it was accelerated into the event horizon — the researchers were also able to deduce the black hole’s mass: approximately 1,000 suns, a mass that corresponds to a black hole in “Medium.”
Is this definitive proof of the existence of an intermediate-mass black hole killing a careless white dwarf star? Possibly, but further examination of the ULX will no doubt be forthcoming as it is thought that the X-ray emission from NGC 1399 should be persistent for some time yet.