Jan 28 2015
A five-year analysis of an event captured by a tiny telescope at McDonald Observatory and followed up by telescopes on the ground and in space has led astronomers to believe they witnessed a giant black hole tear apart a star. The work is published this month in The Astrophysical Journal.
When a star encounters a black hole, tidal forces stretch the star into an elongated blob before tearing it apart, as seen in these images from a computer simulation by Wheeler's collaborator James Guillochon of Harvard University.
On Jan. 21, 2009, the ROTSE IIIb telescope at McDonald caught the flash of an extremely bright event. The telescope’s wide field of view takes pictures of large swathes of sky every night, looking for newly exploding stars as part of the ROTSE Supernova Verification Project (RSVP). Software then compares successive photos to find bright “new” objects in the sky — transient events such as the explosion of a star or a gamma-ray burst.
With a magnitude of -22.5, this 2009 event was as bright as the “superluminous supernovae” (a new category of the brightest stellar explosions known) that the ROTSE team discovered at McDonald in recent years. The team nicknamed the 2009 event “Dougie,” after a character in the cartoon “South Park.” (Its technical name is ROTSE3J120847.9+430121.)
The team thought Dougie might be a supernova and set about looking for its host galaxy, which would be too faint for ROTSE to see. They found that a sky survey had mapped a faint red galaxy at Dougie’s location. They used one of the giant Keck telescopes in Hawaii to pinpoint its distance: 3 billion light-years.
These deductions meant Dougie had a home — but just what was he? To narrow it down from four possibilities, they studied Dougie with the orbiting Swift telescope and the giant Hobby-Eberly Telescope at McDonald, and they made computer models. These models showed how Dougie’s light would behave if created by different physical processes. The astronomers then compared the different theoretical Dougies to their telescope observations of the real thing.
"When we discovered this new object, it looked similar to supernovae we had known already,” said lead author Jozsef Vinko of the University of Szeged in Hungary. “But when we kept monitoring its light variation, we realized that this was something nobody really saw before.”
Team member J. Craig Wheeler, leader of the supernova group at The University of Texas at Austin, said they got the idea they might be witnessing a “tidal disruption event,” in which the enormous gravity of a black hole pulls on one side of a star harder than the other side, creating tides that rip the star apart.
“These especially large tides can be strong enough that you pull the star out into a noodle” shape, said Wheeler. The star “doesn’t fall directly into the black hole,” Wheeler said. “It might form a disk first. But the black hole is destined to swallow most of that material.”
Astronomers have seen black holes swallow stars about a dozen times before, but this one is special even in that rare company: It’s not going down easily.
Models by team members James Guillochon of Harvard University and Enrico Ramirez-Ruiz of the University of California at Santa Cruz showed that the disrupted stellar matter was generating so much radiation that it pushed back on the infall. The black hole was choking on the rapidly infalling matter.
Based on the characteristics of the light from Dougie and their deductions of the star’s original mass, the team has determined that Dougie started out as a star like our sun before being ripped apart.
Their observations of the host galaxy, coupled with Dougie’s behavior, led them to surmise that the galaxy’s central black hole has the “rather modest” mass of about a million suns, Wheeler said.
Delving into Dougie’s behavior has unexpectedly resulted in learning more about small, distant galaxies, Wheeler said, musing “Who knew this little guy had a black hole?”
The paper’s lead author, Joszef Vinko, began the project while on sabbatical at The University of Texas at Austin. The team also includes Robert Quimby of San Diego State University, who started the search for supernovae using ROTSE IIIb and discovered the category of superluminous supernovae while a graduate student at The University of Texas at Austin.