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Jan 13, 2023
Two teams of astronomers using ESA’s XMM-Newton space telescope have observed repeated outbursts of light from inactive black holes that partially destroy stars again and again. This discovery is unexpected, since outbursts of black holes usually appear only once when a black hole consumes a star.
What is a black hole?
Supermassive black holes lie at the centres of most galaxies. Their masses range from hundreds of thousands to billions of times the mass of our Sun. Despite this, black holes are elusive, trapping light and remaining hard to detect.
A hidden supermassive black hole can be uncovered when a star veers on a close approach to it. The star gets ripped apart by strong tidal forces, forming a disk of stellar debris on which the black hole is feeding. Energetic X-rays, UV, optical and radio light can be detected during this process known as a tidal disruption event.
Not totally destroyed
Typical tidal disruption events exhibit a bright outburst of light, known as a flare, which lasts a few months during which the black hole consumes the star. However, two new flares with peculiar behaviour have been observed by XMM-Newton. These flares repeatedly shine bright in X-ray and UV light after the first outburst, suggesting that the stars have not been totally destroyed during the initial encounter with the black holes.
The studies led by astronomers Thomas Wevers from the European Southern Observatory, and Zhu Liu from the Max Planck Institute for Extraterrestrial Physics, Germany, reveal that part of the stars may have survived their first attack from the black holes. The X-ray and UV data suggest that parts of the stars are not entirely eaten up, continue their orbit and encounter the disruptive black hole again, leading to recurring flares. This activity is called a partial tidal disruption event.
A black hole eating a star again and again
The astronomers found repeated flares from two separate galaxies hosting supermassive black holes. These galaxies lie well beyond the outskirts of the Milky Way at distances of almost 900 million light-years and 1 billion light-years.
One of the re-brightening events, called eRASSt J045650.3−203750, was discovered by the X-ray telescope eROSITA on board the Spectrum-Roentgen-Gamma mission. XMM-Newton observations in 2021 and 2022 by a team led by Zhu found that the original flare was followed by repeated outbursts roughly every 223 days.
Zhu explains: “The results from our first XMM-Newton observation were surprising. The black hole showed an unusually drastic dimming of X-ray light, compared to when it had been discovered two weeks previously by the eROSITA telescope. Follow-up observations with XMM-Newton and other instruments confirmed our speculations that this behaviour was being caused by a partial tidal disruption event.”
The other tidal disruption event, called AT2018fyk, was discovered by the All-Sky Automated Survey for Supernovae. It shone bright in UV and X-rays for at least 500 days, followed by a sudden dimming. In May 2022, Thomas and colleagues used XMM-Newton to study the dramatic increase in X-ray and UV brightness 1200 days after it first appeared.
Back to the drawing board
“At first, we were absolutely puzzled by what the rebrightening could mean. We had to go back to the drawing board to assess all the possible options to explain the observed behaviour. It was a very exciting moment when we realised that the model for a repeating tidal disruption event could reproduce the observed data,” adds Thomas.
A black hole repeatedly destroying a star
Video above: A video animation of the partial tidal disruption event AT2018fyk depicts the model: A black hole repeatedly destroying a star.
In total, over five days of XMM-Newton observations were used to monitor the change of X-ray light coming from these sources. The extremely sensitive European Photon Imaging Camera on board XMM-Newton helped to study the hot material surrounding the black holes in great detail.
William Alston, ESA Research Fellow, explains the significance of the results. “These new observations are incredibly interesting for studying the influence of supermassive black holes. In typical tidal disruption events, we don’t expect to see a second flare for a few thousand years. With the flares recurring so quickly, the orbit of the disrupted star must have been bound close to the supermassive black hole. These new studies suggest that the disrupted star is pulled into a close orbit after it is ripped away from a binary star system by the central supermassive black hole.”
Artist's impression of XMM-Newton
The teams making the new discovery extend worldwide – in addition to XMM-Newton and eROSITA, the studies involve other missions including NASA’s Neil Gehrels Swift Observatory, The Australia Telescope Compact Array (ATCA) and the Neutron Star Interior Composition Explorer Mission payload on the International Space Station. Collaborations allowed for these unprecedented cosmic events to be observed, modelled, and understood in maximum detail.
Usually dark and quiet
Some galaxies are constantly active, emitting flares as the supermassive black hole continually pulls gaseous matter into its orbit. The two new events observed by XMM-Newton, however, come from black holes which usually lie dark and quiet, until a star approaches. These events are the first time that repeated outbursts of light have been detected from inactive galaxies. The results of these studies are published in two papers in Astronomy & Astrophysics and The Astrophysical Journal Letters.
Since their discovery in the 1990s, almost 100 tidal disruption events have been observed. XMM-Newton observations of tidal disruption events are vital to understand more about the otherwise hard to observe supermassive black holes which lie at the centre of large galaxies such as our own.
Both partial tidal disruption events will be monitored closely during the predicted periods of future re-brightening episodes to confirm the findings and make further discoveries. Observers may be met with silence, indicating that the star has been swallowed whole in the previous flaring episode. Turbulent times lie ahead for these events – and the hunt begins to find similar partial tidal disruption events.
Notes to Editor
‘Live to die another day: the rebrightening of AT2018fyk as a repeating partial tidal disruption event’ by T. Wevers et al. is published in The Astrophysical Journal Letters. Preprint: https://arxiv.org/abs/2209.07538
‘Deciphering the extreme X-ray variability of the nuclear transient eRASSt J045650.3−203750 : A likely repeating partial tidal disruption event’ by Z. Liu et al. is accepted for publication in Astronomy & Astrophysics. https://www.aanda.org/10.1051/0004-6361/202244805
The team’s results will be presented at a press conference of the American Astronomical Society (AAS) on Thursday 12 January 2023.
Related link:
ESA’s XMM-Newton space telescope: https://www.esa.int/Science_Exploration/Space_Science/XMM-Newton_overview
Images, Videos, Text, Credits: ESA, CC BY-SA 3.0 IGO/ESA/D. Ducros.
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