Februrary 16, 2015
Astronomers revealed last month that after five years of analysis, a distant light they once thought was a supernova was more likely caused by a supermassive black hole interacting with a normal star.
In January 2009, while searching for supernovae in distant galaxies with an 18-inch automatic telescope called ROTSE, or Robotic Optical Transient Search Equipment, a small research team of professors and graduate students led by professor J. Craig Wheeler at the University of Texas at Austin captured data from a relatively short-lived flash of light three billion light-years away.
According to San Diego State University’s Mount Laguna Observatory Director Robert Quimby, much of the team first assumed that the light belonged to a variety of supernovae called “superluminous” supernovae. Superluminous supernovae were first discovered by Quimby as a graduate student at the University of Texas.
“At least that seemed to explain the early data,” Quimby said.
A typical supernova is caused by either the total gravitational collapse of a massive star or the accidental explosion of a small, old star.
However, as the research team looked closer at the observed event, they realized it did not strictly fit into either category.
“When the new spectra arrived, all of us were scratching our heads, because the spectra were different from what we expected from a supernova,” said Jozsef Vinko, an astronomy professor from the University of Szeged in Hungary who joined Wheeler’s research team while on sabbatical at the University of Texas. “None of us saw anything like this before.”
Instead, observations seemed to fit the characteristics of a theoretical “tidal disruption,” a phenomenon where a black hole tears apart and swallows a passing star.
“Of course we saw only the light emitted by this event, and we were unable to see it directly because of the large distance from the Earth,” Vinko said. “So, we cannot be 100 percent sure that we proved that tidal disruption really exists. But we are close to that.”
Tidal disruptions occur when a star comes close enough to a black hole to be affected by its gravity. The black hole will pull on the star, distorting its shape and slowly accreting, or gathering, the star’s hot gas until the star falls apart.
“When the star comes apart, then that gas spiraling around the black hole is the light that we see,” Wheeler said.
In this case, according to Wheeler, the star was about the size of our sun, and the black hole was a supermassive black hole about a million times heavier than the sun, found at the center of a galaxy.
Not only are tidal disruptions relatively rare and have only been observed a handful of times, but this particular tidal disruption was much different because the intense light emitted from the dying star was strong enough to push back and resist the black hole’s gravitational pull.
“We know that phenomenon of intense radiation pushing back in other contexts, but it hadn’t been witnessed in this context,” Wheeler said. “It adds a little bit to the lore of how this can happen.”
Supernovae, and rarer events like tidal disruptions, are found by taking pictures deep into space periodically and comparing the series of pictures to determine if there are any new lights in the sky.
Then, astronomers will determine what kinds of elements are present in the object by splitting its light into its various wavelengths, creating a spectrum, much like how visible light can be separated into the colors of the rainbow, according to Vinko.
The spectra of a luminous object, and the resulting information about its chemical composition, can then be used to determine what kind of object or phenomenon it is. In this case, the spectra were the first clue that it was not a typical supernova.
“When I looked at the first spectrum of the new object, it was indeed similar to a young supernova shortly after explosion,” Vinko said.
“But as we continued to take follow-up observations, it did not evolve as I was expecting,” Quimby added. “We didn’t see the usual supernova signatures, like broad features in the spectra, showing up.”
As the research team continued to piece together the event’s characteristics, Wheeler said everything else “fell away” of being a possibility and the “best hypothesis” was a tidal disruption event.
“I’m 80 or 90 percent convinced that it’s right,” Wheeler said. “It’s very hard to be 100 percent convinced with one event.”
The uniqueness of the event has brought in publicity to the astronomy department, and, according to Wheeler, a story about black holes tends to “get the interest of people.”
“You do it partly for the fun of it, and it is kind of fun,” Wheeler said. “But you draw attention to your research group, to your university, to your department and to your observatory. We’re supported by public taxpayer money, and in some sense this is saying to the people out there, ‘this is how we’re using your tax dollars.'”
According to Wheeler, although observations are basically following the established theoretical computer models of tidal disruptions, each instance of the phenomenon discovered since proves more complicated than the simplistic models predict. These complications help clarify astronomers’ understanding of the physics behind black holes.
“Maybe nature showed us again how unbelievably wide variety of phenomena may exist out there, and we should be continuously prepared to see more wonders and surprises when looking up at the sky,” Vinko said.