In a stroke of good luck, a NASA satellite probing space for new planets provided astronomers with an unexpected glimpse of a black hole ripping a star to shreds.
This is one of the most detailed visual yet of the phenomenon known as tidal disruption event aka TDE. It was the first sighting for NASA’s Transiting Exoplanet Survey Satellite commonly called TESS.
The breakthrough finding was achieved with the help of a worldwide network of robotic telescopes based at The Ohio State University called ASAS-SN i.e. All-Sky Automated Survey for Supernovae. Astronomers from the Carnegie Observatories, Ohio State and others published their amazing findings just now in The Astrophysical Journal.
Patrick Vallely, a co-author of the study and National Science Foundation Graduate Research Fellow at Ohio State, said, “We’ve been closely monitoring the regions of the sky where TESS is observing with our ASAS-SN telescopes, but we were very lucky with this event in that the patch of the sky where TESS is continuously observing is small, and in that this happened to be one of the brightest TDEs we’ve seen. Due to the quick ASAS-SN discovery and the incredible TESS data, we were able to see this TDE much earlier than we’ve seen others—it gives us some new insight into how TDEs form.”
Tidal disruption events take place when a star gets too close to a black hole. Depending on certain factors such as the size of the star, the size of the black hole and the distance between the star and the black hole, the latter can either absorb the star or rip it apart into a long, spaghetti-like strand.
Thomas Holoien, a Carnegie Fellow at the Carnegie Observatories in Pasadena, California, who earned his Ph.D. at Ohio State, said, “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. Because we discovered the tidal disruption quickly with the ground-based 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.”
ASAS-SN was the first system to spot the black hole ripping the star apart. Holoien was busy at the Las Campanas Observatory in Chile on Jan. 29, 2019, when he received an alert from one of ASAS-SN’s robotic telescopes in South Africa. Holoien had focused two Las Campanas telescopes on the tidal disruption event with follow-up observations requests to other telescopes around the world.
TESS already chanced to be monitoring the exact section of the sky where the ASAS-SN telescope had first spotted the tidal disruption event. It was not just pure luck that the telescopes and satellite were aligned as after TESS’s launch in July 2018, the team operating ASAS-SN dedicated more of the ASAS-SN telescopes’ time to the portions of the sky that TESS was observing.
But it was definitely good fortunate that the tidal disruption event took place in the systems’ lines of sight, according to Chris Kochanek, professor of astronomy at Ohio State.
Tidal disruptions are rare cosmic events that occur once every 10,000 to 100,000 years in a galaxy the size of the Milky Way. By comparison, supernovae occur every 100 years or so. Throughout history, scientists have observed around 40 tidal disruption events. Kochanek said that the events are rare mostly because stars need to be really close to a black hole, approximately the same distance our Earth is from the sun, in order to create one.
He said, “Imagine that you are standing on top of a skyscraper downtown, and you drop a marble off the top, and you are trying to get it to go down a hole in a manhole cover. It’s harder than that.”
Since ASAS-SN caught the tidal disruption event in its early stage, Holoien had time to train additional telescopes on the event and eventually capturing a more detailed look that might not have been possible before. Astronomers could then examine the data from TESS which was transmitted via a satellite in space a few weeks after the event and finally see whether they could identify the event in the lead-up. Data from TESS implied that they could see the initial signs of the tidal disruption event in data from around 10 days before it occurred.
Vallely said, “The early TESS data allow ed us to see light very close to the black hole, much closer than we’ve been able to see before. 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.”
Holoien’s team worked on the UV data from NASA’s Neil Gehrels Swift Observatory which provided the earliest yet seen from a tidal disruption in order to determine that the temperature dropped by about 50%, from approximately 71,500 to 35,500 degrees Fahrenheit (40,000 to 20,000 degrees Celsius), in just a few days.
Holoien said, “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”.
The low level of X-ray emission seen by Swift was even more typical for these kinds of events. Scientists still don’t fully understand why tidal disruptions produce so much UV emission but such low X-rays.
Astronomers estimate that the supermassive black hole that generated ASASSN-19bt weighs about 6 million times the sun’s mass. It is located at the center of a galaxy referred to as 2MASX J07001137-6602251 which is around 375 million light-years away in the constellation Volans. The destroyed star is suspected to be similar in size to our sun.