‘Blackhole shredding apart a star observed for the first time’

PTI, Sep 27, 2019, 1:38 PM IST

Washington: In a first, researchers watched a supermassive black hole — weighing about 6 million times the Sun’s mass — shred a star apart in a cosmic cataclysm called a tidal disruption event.

The discovery made using NASA’s planet-hunting Transiting Exoplanet Survey Satellite (TESS) — with follow-up observations by NASA’s Neil Gehrels Swift Observatory, and other facilities — produced the most detailed look of the star-destroying event.

NASA said that tidal disruptions are incredibly rare, occurring once every 10,000 to 100,000 years in a galaxy the size of our own Milky Way, with only 40 such events observed so far.

“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 Thomas Holoien, lead author of the study from the Carnegie Observatories in California.

“Because we identified the tidal disruption quickly with the ground-based All-Sky Automated Survey for Supernovae (ASAS-SN), we were able to trigger multiwavelength follow-up observations in the first few days. The early data will be incredibly helpful for modeling the physics of these outbursts,” Holoien said.

The findings, published in the Astrophysical Journal, noted that the observed supermassive black hole was present at the centre of a galaxy called ‘2MASX J07001137-6602251’ which is located around 375 million light-years away in the constellation Volans.

The shredded star, the study noted, may have been similar in size to the Sun.

NASA said that the tidal disruption event was discovered by the ASAS-SN — a worldwide network of 20 robotic telescopes headquartered at Ohio State University (OSU) in the US on January 29.

When Holoien received the alert from the project’s South Africa instrument, he quickly trained two robotic telescopes in Las Campanas, Chile, to lock in on the location in space where the colossal shredding event happened.

He also requested follow-up observations by NASA’s Swift space telescope, the European Space Agency’s XMM-Newton space telescope, and telescopes in the global Las Cumbres Observatory network in California.

TESS first saw the tidal disruption on January 21 — more than a week before it was bright enough for ASAS-SN to detect it.

However, TESS transmitted data to Earth only once every two weeks, and the received signals had to be processed at NASA’s Ames Research Center in Silicon Valley, California.

Because of this, NASA said that data from TESS on the cosmic event was not available until March 13 making them dependent on coordination with ground-based telescopes like ASAS-SN.

Since the tidal disruption was also observable from TESS’s southern continuous viewing zone, the space telescope switched to monitoring the northern sky at the end of July.

The location of the cosmic event allowed Holoien and his colleagues to follow the event across several telescope sectors.

If the cosmic cataclysm had occurred outside this zone, NASA said that TESS might have missed the beginning of the outburst.

“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 Patrick Vallely, a co-author and National Science Foundation Graduate Research Fellow at The Ohio State University (OSU).

“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,” Vallely said.

Holoien’s team found by processing data on the ultraviolet light detected by the telescopes that the temperature at the location of the event dropped by about 50 per cent, from around 40,000 to 20,000 degrees Celsius in just over a few days.

Holoien said that it was the first time such an early temperature decrease was seen in a tidal disruption, even if a few theories had predicted it.

The researchers said that the more typical feature for such cataclysmic star shredding events was the low level of X-ray emission which was observed by both Swift and XMM-Newton space telescopes.

The researchers added that it was not fully known why tidal disruptions produced 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 S. Bradley Cenko, Swift’s principal investigator at NASA’s Goddard Space Flight Center in the US.

Observing these events, much earlier as they happened, may provide answers to some of these questions, said Cenko.

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