Nasa scientists get unprecedented view as black hole rips apart a star

Scientists still don't fully understand the strange and powerful blasts that come out of such an event

Andrew Griffin
Thursday 26 September 2019 16:23 BST
NASA scientists get unprecedented view as black hole rips apart a star

Nasa has managed to get an unprecedented view of a black hole ripping apart a star.

The new observations – which allowed scientists to see the cataclysmic phenomenon as it happened – mark the first ever time that scientists have been able to watch the intense event directly in such a way.

Such events are called tidal disruptions and are incredibly rare, happening only once every 10,000 to 100,000 years in a galaxy the size of our Milky Way.

It was spotted by Nasa's Transiting Exoplanet Survey Satellite. That was sent into space to look for other worlds, but allows researchers to look at other phenomena in the universe.

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, a Carnegie Fellow at the Carnegie Observatories in Pasadena, 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 modelling the physics of these outbursts.”

The event was first spotted by ASAS-SN, a worldwide network of 20 different telescopes at the beginning of the year. As it did, it sent out an alert to international astronomers, so that they could track it themselves.

Holoien received that alert and pointed two telescopes towards the incident and asked for more to look that way too.

Nasa's TESS was already looking towards the right part of space. It spends its time watching vast parts of the sky known as sectors for 27 days each, with the aim of spotting moments where stars go dark that could indicate a planet has passed in front of them.

Astronomers thought it might also be able to catch the earliest moments of flare-ups around stars, such as supernovae and tidal disruptions. And it did see this one: first spotting a week before ASAS-SN saw it, but unable to tell Earth straight away because it sends back its data once every two weeks, and that must then be processed by Nasa.

Researchers could then look back at that data and see the star as it was being torn up by the black hole.

“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 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.”

Much about tidal disruptions is still mysterious to astronomers, but observations of this kind could shed more light on why they happen. They are unclear about why they throw out so much UV emissions but so few X-rays, for instance.

“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 Greenbelt, Maryland. “More early-time observations of these events may help us answer some of these lingering questions.”

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