Originally predicted by Einstein’s Theory of General Relativity, black holes are the most extreme object in the known Universe. These objects form when stars reach the end of their life cycle, blow off their outer layers, and are so gravitationally powerful that nothing (not even light) can escape their surfaces. They are also of interest because they allow astronomers to observe the laws of physics under the most extreme conditions. Periodically, these gravitational behemoths will devoir stars and other objects in their vicinity, releasing tremendous amounts of light and radiation.
In October 2018, astronomers witnessed one such event when observing a black hole in a galaxy located 665 million light-years from Earth. While astronomers have witnessed events like this before, another team from the Harvard & Smithsonian Center for Astrophysics noticed something unprecedented when they examined the same black hole three years later. As they explained in a recent study, the black hole was shining very brightly because it was ejecting (or “burping”) leftover material from the star at half the speed of light. Their findings could provide new clues about how black holes feed and grow over time.
The team was led by Yvette Cendes, a research associate with the CfA, who was joined by an international team of researchers from the Commonwealth Scientific and Industrial Research Organisation (CSIRO), the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), the Space Telescope Science Institute (STScI), the Columbia Astrophysics Laboratory, the Flatiron Institute’s Center for Computational Astrophysics, UC Berkeley, Radboud University (Netherlands), and York University in Toronto. The paper that describes their findings recently appeared in the Astrophysical Journal.
As they stated in their paper, the team observed the outburst while revisiting data on tidal disruption events (TDEs) that took place over the last few years. These occur when stars pass too close to black holes and are pulled apart during multiple passes, a process known as “spaghettification” because of how the stars are ripped into strands. In 2018, the TDE in question (dubbed AT2018hyz) was observed by astronomers at Ohio State University as part of the All-Sky Automated Survey for Supernovae (ASAS-SN).
Shortly thereafter, an international team examined AT2018hyz in the visible and ultraviolet wavelengths using the All-Sky Automated Survey for Supernovae, the Fred Lawrence Whipple Observatory, and the UV-Optical Telescope (UVOT) on the Neil Gehrels Swift Observatory. This team was led by Sebastian Gomez, a postdoctoral fellow at the Space Telescope Science Institute and co-author on the new paper. As he explained, the TDE was “unremarkable” at the time. In June of 2021, Cendes and her colleagues examined it again using radio data from the Very Large Array (VLA) in New Mexico.
To their surprise, they noticed that the black hole had mysteriously become reanimated. As Cendes explained in a CfA press release:
“This caught us completely by surprise — no one has ever seen anything like this before. We applied for Director’s Discretionary Time on multiple telescopes, which is when you find something so unexpected, you can’t wait for the normal cycle of telescope proposals to observe it. All the applications were immediately accepted.”
The team then conducted follow-up observations of AT2018hyz with multiple telescopes and in multiple wavelengths. This included radio observations made with the VLA, the Atacama Large Millimeter-submillimeter Array (ALMA) Observatory in Chile, the MeerKAT in South Africa, and the Australian Telescope Compact Array in Australia. These were combined with X-ray and Gamma-ray data obtained by the space-based Chandra X-Ray Observatory and the Neil Gehrels Swift Observatory (respectively).
According to Edo Berger, a professor of astronomy at Harvard University and the CfA and a co-author on the new study, the radio observations of the TDE proved to be the most striking:
“We have been studying TDEs with radio telescopes for more than a decade, and we sometimes find they shine in radio waves as they spew out material while the star is first being consumed by the black hole. But in AT2018hyz there was radio silence for the first three years, and now it’s dramatically lit up to become one of the most radio-luminous TDEs ever observed.“
The team concluded that this resulted from the black hole ejecting residual material from the star at relativistic speeds (a fraction of the speed of light). This is the first time astronomers have ever observed such a phenomenon, and the team is unsure why the outflow was delayed by several years. TDEs are well-known for emitting light when they occur since spaghettified material from the star becomes elongated around the black hole and heats up, creating a flash that astronomers can see millions of light-years away.
In some cases, spaghettified material will be thrown back into space, which astronomers liken to black holes being “messy eaters.” However, the outflow emissions normally develop quickly after a TDE occurs and not years later. In short, said Cendes, it’s as if this black hole began abruptly burping out a bunch of stellar material it ate years ago. What’s more, these “burps” were extremely energetic, with ejected material reaching speeds of up to 50% the speed of light – about five times what astronomers have observed with other TDEs. Said Berger:
“This is the first time that we have witnessed such a long delay between the feeding and the outflow. The next step is to explore whether this actually happens more regularly and we have simply not been looking at TDEs late enough in their evolution.”
These results, along with observations of similar events, will help astronomers to better understand the feeding behavior of black holes. This, in turn, could provide insight into how they grow and evolve with time and their role in galactic evolution.