Using a trio of X-ray telescopes, astronomers have observed material being blown away from a supermassive black hole after it tore a star apart. The event is the closest so-called ‘tidal disruption’ discovered in ten years, and is described in a paper published this week in the journal Nature.
Astronomers report the detection of flows of hot, ionized gas in high-resolution X-ray spectra of a nearby tidal disruption event, ASASSN-14li in the galaxy PGC 43234. This artist’s impression shows a supermassive black hole at the center of PGC 43234 accreting mass from a star that dared to venture too close to the galaxy’s center. Image credit: ESA / C. Carreau.
When a star wanders too close to a black hole, intense tidal forces rip the star apart. In these ‘tidal disruptions’, some of the stellar debris is flung outward at high speed while the rest falls toward the black hole.
Astronomers have been studying this phenomenon for over 30 years, and in that time have detected dozens of black holes tearing apart and devouring stars in many galaxies, near and far.
Now, the new study offers the first close-up look into the formation of a black-hole accretion disc after a tidal disruption event.
“All previous observations of tidal disruption events revealed an already formed disc around the black hole. But this is the first time that we catch such a disc in its infancy, so we can study the details of how matter starts flowing from the shattered star towards the black hole and settles in circular orbits around it,” said Dr Jon Miller of the University of Michigan, lead author on the study.
Dr Miller and his colleagues from the Netherlands, the United States and the UK used three X-ray telescopes – ESA’s XMM-Newton observatory, NASA’s Chandra X-ray Observatory and the Swift Gamma Ray Burst Explorer – to observe a tidal disruption located in the center of PGC 43234, a galaxy that lies approximately 290 million light years from Earth. This makes this event the closest tidal disruption discovered in ten years.
This tidal disruption, dubbed ASASSN-14li, was discovered in an optical search by the All-Sky Automated Survey for Supernovae in 2014.
Theory predicts that early in the evolution of a tidal disruption, material from the star should be pulled towards the black hole at a high rate, generating a huge amount of light.
The amount of light should decline as the disrupted material falls onto the black hole. In the case of ASASSN-14li, the mass of the black hole is a few million solar masses.
Gas often falls toward black holes by spiraling inward in a disk. But how this process starts has remained a mystery.
This is an optical image, based on data from the Sloan Digital Sky Survey, showing some galaxies that are part of the Coma Supercluster. The yellow blob at the center is PGC 43234, a small galaxy in which astronomers witnessed stellar debris being blown away after a supermassive black hole destroyed a star. With PGC 43234 located 290 million light-years away from us, it was the closest tidal disruption event to be discovered in the past decade. Studying it with X-ray telescopes, the astronomers could measure, for the first time, the physical properties of a newly formed accretion disc around a black hole. Image credit: Sloan Digital Sky Survey.
In ASASSN-14li, Dr Miller and co-authors were able to witness the formation of such a disk by looking at the X-ray light at different wavelengths and tracking how that changed over time.
They determined the velocity of the material in the disc, its density and ionization properties (for the first time in a tidal disruption event).
The data also revealed signs of X-ray absorbing material beyond the disc. This could either be outflowing gas, leaving the disc via a wind, or a stream of gas flowing towards the black hole but temporarily set on a different orbit, not having settled yet in the disc with most of the leftover matter from the destroyed star.
The astronomers hope to find more events like ASASSN-14li to test theoretical models about how black holes affect their environments.
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Jon M. Miller et al. 2015. Flows of X-ray gas reveal the disruption of a star by a massive black hole. Nature 526, 542-545; doi: 10.1038/nature15708
