India's AstroSat and NASA's space observatories have recorded intense explosions from the remnants of a star around a massive black hole.
A massive black hole has destroyed a star and is now using the debris to strike another star or smaller black hole that used to be in the clear. This breakthrough was achieved with the help of NASA's space observatories — Chandra, HST, NICER, Swift — and ISRO's AstroSat, as stated by the Indian space agency.
"It provides astronomers with valuable insights, linking two mysteries where there had previously only been hints of a connection", Bengaluru-headquartered ISRO (Indian Space Research Organisation) said in a statement.
In 2019, astronomers observed a star being torn apart by the gravitational forces of a black hole after it got too close. The remnants of the star formed a disk around the black hole, creating a stellar graveyard.
Over the course of a few years, the disk has expanded and is now in the direct path of a star, or possibly a stellar-mass black hole, that was previously orbiting the massive black hole at a safe distance, as per ISRO's report.
The orbiting star now intersects the debris disk approximately every 48 hours as it orbits, resulting in collisions that produce bursts of X-rays captured by Chandra, according to astronomers.
“Imagine a diver repeatedly going into a pool and creating a splash every time she enters the water,” Matt Nicholl of Queen's University Belfast, the United Kingdom, the lead author of the study that appears in the current issue of 'Nature' was quoted as saying in the ISRO statement.
“The star in this comparison is like the diver and the disk is the pool, and each time the star strikes the surface it creates a huge 'splash' of gas and X-rays. As the star orbits around the black hole, it does this over and over again," he said.
ISRO has reported numerous instances where an object has been ripped apart by a black hole when it comes too close, resulting in a sudden burst of light known as "tidal disruption events" (TDEs).
In recent times, astronomers have identified a fresh category of intense emissions emanating from the cores of galaxies, which are exclusively observed in X-rays and occur repeatedly. These occurrences are also associated with supermassive black holes, but the cause of the periodic X-ray bursts had previously eluded explanation. These events were termed "quasi-periodic eruptions," or QPEs.
“There had been feverish speculation that these phenomena were connected, and now we've discovered the proof that they are,” said co-author Dheeraj Pasham of the Massachusetts Institute of Technology. “It's like getting a cosmic two-for-one in terms of solving mysteries.”
The tidal disruption event, named AT2019qiz, was initially spotted by the Zwicky Transient Facility, a wide-field optical telescope at the Palomar Observatory, in 2019. Astronomers utilized both Chandra and NASA's Hubble Space Telescope in 2023 to examine the remnants resulting from the conclusion of the tidal disruption.
The Chandra data were obtained during three different observations, each separated by about four to five hours. The total exposure of about 14 hours of Chandra time revealed only a weak signal in the first and last chunk, but a very strong signal in the middle observation, it stated.
"From there Nicholl and collaborators used NASA's Neutron Star Interior Composition Explorer (NICER) to look frequently at AT2019qiz for repeated X-ray bursts. The NICER data showed that AT2019qiz erupts roughly every 48 hours. Observations from NASA''s Neil Gehrels Swift Observatory and India's AstroSat telescope cemented the finding," ISRO stated.
The Hubble's ultraviolet data, collected simultaneously with the Chandra observations, enabled the researchers to ascertain the dimensions of the disk surrounding the supermassive black hole. They determined that the disk had expanded to a size where any object orbiting the black hole within a week or less would collide with the disk, causing eruptions.
Gulab Dewangan, a co-author from the Inter-University Center for Astronomy and Astrophysics (IUCAA) in Pune, highlighted that India's AstroSat mission offers a distinctive capability for studying such phenomena in UV and X-ray wavelengths. Both AstroSat's Soft X-ray Telescope and the Ultra-Violet Imaging Telescope (UVIT) detected the source AT2019qiz, but only the X-ray emissions corresponded to the eruptions.
"Future sensitive simultaneous X-ray and UV observations of similar eruptions will enable a deeper investigation into their nature," he said.
“This is a huge breakthrough in our understanding of the origin of these regular eruptions,” said Andrew Mummery of Oxford University. “We now realise we need to wait a few years for the eruptions to ‘turn on’ after a star has been torn apart because it takes some time for the disk to spread out far enough to encounter another star," Dewangan stated.
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