NASA’s Transiting Exoplanet Survey Satellite (TESS) gave astronomers an unexpected glimpse at a black hole violently ripping apart a Sun-sized star. It is one of the most detailed looks yet at a cataclysmic phenomenon called tidal disruption, and the first for TESS.
When a star strays too close to a black hole, intense tides break it apart into a stream of gas. The tail of the stream escapes the system, while the rest of it swings back around, surrounding the black hole with a disk of debris. Image credit: NASA’s Goddard Space Flight Center.
Tidal disruption events occur when a star gets too close to a supermassive black hole. The black hole’s forces overwhelm the star’s gravity and tear it to shreds. Some of its material gets flung out into space and the rest falls back onto the black hole, forming a disk of hot, bright gas as it is consumed.
By observing the light given off during this process, which increases to a peak brightness and then tapers off, astronomers can better understand the physics of the black hole and the forces driving these phenomena.
These cataclysmic events are incredibly rare, occurring once every 10,000 to 100,000 years in a galaxy the size of our own Milky Way. In total, astronomers have observed only about 40 tidal disruptions so far, and scientists predicted TESS would see only one or two in its initial two-year mission.
“TESS data let us see exactly when this destructive event, named ASASSN-19bt, started to get brighter, which we’ve never been able to do before,” said Dr. Thomas Holoien, an astronomer with Carnegie Observatories and the lead author of a paper published in the Astrophysical Journal.
“Because we identified the tidal disruption quickly with the ground-based All-Sky Automated Survey for Supernovae (ASAS-SN), we were able to trigger multi-wavelength follow-up observations in the first few days. The early data will be incredibly helpful for modeling the physics of these outbursts.”
Astronomers think the supermassive black hole that generated ASASSN-19bt had a mass of around 6 million solar masses. It sits at the center of 2MASX J07001137-6602251, a galaxy located around 375 million light-years away in the constellation Volans. The destroyed star may have been similar in size to our Sun.
TESS first saw ASASSN-19bt on January 21, 2019, over a week before the event was bright enough for ASAS-SN to detect it.
“The early TESS data allow us to see light very close to the black hole, much closer than we’ve been able to see before,” said co-author Dr. Patrick Vallely, an astronomer at the Ohio State University.
“They also show us that ASASSN-19bt’s rise in brightness was very smooth, which helps us tell that the event was a tidal disruption and not another type of outburst, like from the center of a galaxy or a supernova.”
The researchers used UV data from NASA’s Neil Gehrels Swift Observatory — the earliest yet seen from a tidal disruption — to determine that the temperature dropped by about 50%, from around 71,500 to 35,500 degrees Fahrenheit (40,000 to 20,000 degrees Celsius), over a few days.
“It’s the first time such an early temperature decrease has been seen in a tidal disruption before, although a few theories have predicted it,” Dr. Holoien said.
More typical for these kinds of events was the low level of X-ray emission. Astronomers don’t fully understand why tidal disruptions produce so much UV emission and so few X-rays.
“People have suggested multiple theories — perhaps the light bounces through the newly created debris and loses energy, or maybe the disk forms further from the black hole than we originally thought and the light isn’t so affected by the object’s extreme gravity,” said Dr. S. Bradley Cenko, Swift’s principal investigator at NASA’s Goddard Space Flight Center, who was not involved in the study.
“More early-time observations of these events may help us answer some of these lingering questions.”
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Thomas W.-S. Holoien et al. 2019. Discovery and Early Evolution of ASASSN-19bt, the First TDE Detected by TESS. ApJ 883, 111; doi: 10.3847/1538-4357/ab3c66
