Black holes that are millions or billions of times more massive than the Sun lurk at the cores of large galaxies and can have profound effects on their surroundings. One of the more exciting of those effects comes when a star ventures too close to the black hole and falls victim to that monster’s powerful gravitational pull. The star is shredded by tidal forces in a process colorfully termed spaghettification.
When that happens, some of the star’s material is pulled into a disk that orbits the black hole, heating rapidly and launching jets of fast-moving particles outward in two opposite directions. This produces an outburst that can be observed with a variety of telescopes, including radio, visible, ultraviolet, and X-ray instruments.
Over the past couple of decades, astronomers have seen a number of outbursts that they have concluded are either the star-shredding Tidal Disruption Events (TDEs) or candidates for such events. In 2018, astronomers used the National Science Foundation’s Very Long Baseline Array (VLBA) to directly image the formation and expansion of a jet coming from a TDE.
The 22 February edition of Nature Astronomy includes reports on observations of two different TDEs, each of which adds to our knowledge of these phenomena but also raises new questions for scientists to tackle. The NSF’s Karl G. Jansky Very Large Array (VLA) was used to study both of these events, occurring in 2015 and 2019 respectively.
One of these star-shredding events is the first known to produce a high-energy neutrino — an elusive subatomic particle moving at nearly the speed of light. The other is the first seen to emit flares of radio waves long after the initial event. Both discoveries are forcing astronomers to rethink their explanations for some of the processes involved in TDEs.
The neutrino-producing TDE, called AT2019dsg, was discovered on 9 April 2019 by the Zwicky Transient Facility (ZTF), a robotic optical telescope at the Palomar Observatory in California. Astronomers subsequently observed it with the VLA, NASA’s Neil Geherels Swift Observatory, and the European Space Agency’s XMM-Newton. They found that it occurred in a galaxy called 2MASX J20570298+1412165, more than 690 million light-years from Earth in the constellation Delphinus.
On 1 October, 2019, the NSF’s IceCube Neutrino Observatory in Antarctica detected a high-energy neutrino that came from the same region of sky as the April TDE. Neutrinos are pervasive throughout the universe but are extremely difficult to detect because they very rarely interact with other matter. In fact, this is only the second high-energy neutrino to be linked to an object outside our Milky Way galaxy. The detection was surprising because astronomers had expected that if TDEs produced such neutrinos it would happen relatively soon after the start of the event.
“Astrophysicists have long theorized that tidal disruptions could produce high-energy neutrinos, but this is the first time we’ve actually been able to connect them with observational evidence,” said Robert Stein, a doctoral student at the German Electron-Synchrotron (DESY) research center in Zeuthen, Germany, and Humboldt University in Berlin. “But it seems like this particular event, called AT2019dsg, didn’t generate the neutrino when or how we expected. It’s helping us better understand how these phenomena work.”
The other TDE, called ASASSN-15oi, was discovered at visible-light wavelengths by the All-Sky Automated Survey for SuperNovae (ASASSN) on 14 August 2015, in a galaxy more than 700 million light-years from Earth. Astronomers began observing it with the VLA eight days after its discovery, expecting to detect radio emission in the early stages of the event. Instead, they saw no radio emission from the object until six months later, in February of 2016.
In addition, they later learned that the ongoing VLA Sky Survey observed the region in July of 2019 and found evidence of another radio flare then, nearly four years after the initial event. The astronomers called the two delayed flares “a new puzzling phenomenon in TDEs.”
“Flares with such delays have not been observed before. Moreover, the delayed flares exhibit peculiar properties currently not supported by theories of TDE radio emission,” said Assaf Horesh, of the Hebrew University of Jerusalem.
In both cases, the researchers look forward to studying future TDEs for clues that can help resolve the new mysteries their work has unveiled. These dramatic events are an excellent example of how we can advance our understanding of the universe through multimessenger astronomy — studies that use electromagnetic radiation (visible light, radio waves, ultraviolet, etc.), particles such as neutrinos, and even gravitational waves — ripples in spacetime — to learn how cosmic objects work.