Star shredded by Black Hole in a ‘Cosmic Crime Scene’

Astronomers have witnessed the final moments of a star violently ripped apart by the tidal forces of a supermassive black hole — shedding light on a mysterious and rare phenomenon.

After passing too close to a supermassive black hole, the star in this artist’s conception is torn apart into a thin stream of gas, which is then pulled back around the black hole and slams into itself, creating a bright shock and ejecting more hot material. (Illustration by Robin Dienel, courtesy of the Carnegie Institution for Science.)

NASA’s Transiting Exoplanet Survey Satellite (TESS) has become a witness to the aftermath of violent death — a star ripped asunder by the tremendous tidal forces of a supermassive black hole. And now a team of astronomers must act as cosmic sleuths to attempt to piece together the events that led to the star’s messy demise.

The study conducted by a team of astronomers led by Carnegie’s Thomas Holoien and published in the Astrophysical Journal details the tidal disruption event (TDE) which led to the star’s destruction.

The team were able to study the event because of the light emitted during the TDE — which increased to peak brightness and then tapered off — gaining a better understanding of the physics behind the black hole and the forces that drive such occurrences.

“Only a handful of TDEs have been discovered before they reached peak brightness and this one was found just a few days after it started to brighten,” Holoien says.

The researcher, a founding member of the international network of telescopes that made the discovery — the Ohio State University-based All-Sky Automated Survey for Supernovae (ASAS-SN), says that this makes the newly discovered TDE — named ASASSN-19bt — a “poster-child” for such research.

NASA/CXC/U. Michigan/J. Miller et al.; Illustration: NASA/CXC/M. Weiss

Though not the first TDE to be observed by astronomers, the discovery of ASASSN-19bt marks the first time that the evolution of such an event has been thoroughly detailed. This is in larger part thanks to TESS’s extremely wide field of view and its ability to provide continuous coverage — ideal in tracking the development of TDEs.

“Thanks to it being in what’s called TESS’ ‘Continuous Viewing Zone,’ we have observations of it every 30 minutes going back months — more than ever before possible for one of these events,” elaborates Holoien.

Quickly after the discovery of ASASSN-19bt both space and ground-based telescopes were deployed to conduct follow-up observations of the TDE — thus building an impressively complete picture of the event.

“I was actually observing at Carnegie’s Las Campanas Observatory on the night of the discovery,” Holoien continues. “So, I was able to take spectra with our du Pont and Magellan telescopes less than a day after the event was first seen in South Africa by part of ASAS-SN’s network.”

The gory details of the star’s remains are assessed by separating the light emitted during the TDE into a distinct spectrum — much like light passing through a prism. This reveals information about the star’s chemical composition and the speed at which it was torn apart.

The team backtracked the evolution of the TDE from the night it was discovered to 42 days before it reached peak brightness — and then forward from the discovery for 37 days. They supplemented this with a vast amount of subsequent observations conducted over the months that followed this period.

What the team discovered was that this TDE is significantly different than other such events.

Not all TDEs are created equal

Astronomers had previously believed that all TDEs would look the same, but this research seems to cast that into doubt.

“Astronomers just needed the ability to make more detailed observations of them,” says Patrick Vallely of Ohio State, second author on the paper. “Recent sky survey projects like ASAS-SN have revealed new features of TDEs that we have not seen before.”

The team doesn’t yet have enough information to tell if the variances observed in ASASSN-19bt are common, he adds. “We have so much more to learn about how they work, which is why capturing one at such an early time and having the exquisite TESS observations was crucial.”

As it happens, ASASSN-19bt appears to be quite unique in a number of ways. For example, its host galaxy is younger and more dust-filled than has previously been observed for other TDE events. It also experienced a short blip of cooling and fading before its temperature levelled off and its luminosity continued to build toward its peak.

The increase in brightness as ASASSN-19bt approached its peak was extremely smooth with very little variation — something that not observed in TDEs before TESS enabled one to examined in such detail. Armed with this information, astronomers’ can now identify TDEs and differentiate them from other celestial events that have a much less smooth light emission fingerprint.

Carnegie’s Decker French, part of the research team concludes: “Having so much data about ASASSN-19bt will allow us to improve our understanding of the physics at work when a star is unlucky enough to meet a black hole.”