In 2009, a very bright flash of light was caught by the ROTSE IIIb telescope, sparking confusion as to its origin. Now, a paper in The Astrophysical Journal proposes that the event was the death throes of a star being swallowed by a black hole. However, the consumption is proving unusually difficult for the black hole.
When the ROTSE3 J120847.9+430121 event was observed on January 21, 2009, as part of the ROTSE Supernova Verification Project (RSVP), four theories were offered about what could have caused such a brief, bright event. It could be the result of two neutron stars merging, a gamma-ray burst whose main radiation was directed away from us, a superluminous supernova, or a star getting eaten by the supermassive black hole at the center of its galaxy.
The event was certainly bright—its red shift placed it 2.9 billion light-years away, at which distance its observed brightness would translate to an absolute magnitude of -22.5, six times more luminous than the whole Milky Way and a match for the brightest known supernovae.
It has taken six years and a few days, but now an international team of astronomers believe they have solved the mystery, concluding that the event—nicknamed "Dougie" after a South Park character who serves as a sidekick to Professor Chaos—falls into the last category.
Extensive study of Dougie's rapidly fading afterglow, combined with modelling of each of the physical processes, suggested that the consumption of a star was the best fit. Witnessing a black hole in the process of destroying a star is not common, but such events have been seen several times. Part of the confusion about Dougie's identity, however, is that this didn't look like any of those previous events.
"We got the idea that it might be a 'tidal disruption' event," said study co-author J. Craig Wheeler of the University of Texas in Austin. "A star wanders near a black hole, the star's side nearer the black hole is pulled" on harder than the star's far side, Wheeler said. "These especially large tides can be strong enough that you pull the star out into a noodle" shape.
As a result, Wheeler says, the star “doesn't fall directly into the black hole. It might form a disk first. But the black hole is destined to swallow most of that material.”
However, like a crocodile fighting back at being eaten by a snake, the star isn't going quietly. Even when torn apart, it is radiating intensely and this radiation is pushing back against the hole.
The authors conclude that this is not because the star is particularly large—being only an estimated 80% of the mass of the sun—but because the black hole has the mass of about a million suns, which is “rather modest” in Wheeler's words, and less than a quarter that at the center of our own galaxy.