A giant black hole ripped apart a star and then gorged on its remains for about a decade, according to astronomers. This is more than ten times longer than any observed episode of a star’s death by black hole.
Researchers made this discovery using data from NASA’s Chandra X-ray Observatory and Swift satellite as well as ESA’s XMM-Newton. They published their findings February 6 in Nature Astronomy.
The trio of orbiting X-ray telescopes found evidence for a “tidal disruption event” (TDE), wherein the tidal forces due to the intense gravity from a black hole can destroy an object – such as a star – that wanders too close. During a TDE, some of the stellar debris is flung outward at high speeds, while the rest falls toward the black hole. As it travels inwards to be ingested by the black hole, the material heats up to millions of degrees and generates a distinct x-ray flare.
“We have witnessed a star’s spectacular and prolonged demise,” said Dacheng Lin from the University of New Hampshire in Durham, who led the study. “Dozens of tidal disruption events have been detected since the 1990s, but none that remained bright for nearly as long as this one.”
According to coauthor Enrico Ramirez-Ruiz, professor and chair of astronomy and astrophysics at UC Santa Cruz, the observations of this TDE with a long-lasting bright phase fit the theoretical predications made in a 2015 paper he wrote with a former graduate student, coauthor James Guillochon, now at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass.
"We correctly predicted in advance of observations that some TDEs would be discovered with long-lived light curves," Ramirez-Ruiz said. "The pieces of the star have to make an accretion disk promptly in order for the material to be digested by the black hole. With a lower mass black hole, stars can get disrupted farther away from the event horizon, and as a result we expect circularization and accretion to be slow."
The extraordinary long bright phase of this event spanning over ten years means that a large amount of mass had to be digested by the black hole. The theoretical model supports either the complete digestion of a star with twice the mass of the sun, or the partial disruption of a much more massive star, Ramirez-Ruiz said, adding that he favors the full disruption of a lower mass star.
The X-ray source containing this force-fed black hole, known by its abbreviated name of XJ1500+0154, is located in a small galaxy about 1.8 billion light years from Earth.
The source was not detected in a Chandra observation on April 2nd, 2005, but was detected in an XMM-Newton observation on July 23rd, 2005, and reached peak brightness in a Chandra observation on June 5, 2008. These observations show that the source became at least 100 times brighter in x-rays. Since then, Chandra, Swift, and XMM-Newton have observed it multiple times.
The sharp x-ray vision of Chandra data shows that XJ1500+0154 is located at the center of its host galaxy, the expected location for a supermassive black hole. The x-ray data also indicate that radiation from material surrounding this black hole has consistently surpassed the so-called Eddington limit, defined by a balance between the outward pressure of radiation from the hot gas and the inward pull of the gravity of the black hole.
“For most of the time we’ve been looking at this object, it has been growing rapidly,” Guillochon said. “This tells us something unusual—like a star twice as heavy as our Sun—is being fed into the black hole.”
This TDE may help answer the question as to how supermassive black holes in the early universe grow. If supermassive black holes can grow from TDEs or other means faster than the Eddington limit, this could explain how supermassive black holes were able to reach masses about a billion times higher than the sun when the universe was only about a billion years old.
“This event shows that black holes really can grow at extraordinarily high rates,” said co-author Stefanie Komossa of QianNan Normal University for Nationalities in Duyun City, China. “This may help understand how precocious black holes came to be.”
Based on the modeling by the researchers, the black hole’s feeding supply should be significantly reduced in the next decade. This would result in XJ1500+0154 fading in x-ray brightness over the next several years.
NASA’s Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA’s Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra’s science and flight operations.