"Technically you cannot say you have discovered a black hole until you show that what you have discovered an object that is massive and that has an event horizon," said astronomy professor Ramesh Narayan, one of the researchers.

Narayan called the evidence that he and his colleagues had for event horizons "fairly compelling" but "not conclusive." For instance, other scientists say the difference in brightness could be caused by less matter flowing toward dense neutron stars, not by black holes devouring the matter.

"The specific details of our theory may or may not be correct," Narayan said, "but whatever explanation one comes up with will very likely have to invoke the event horizon to explain this dramatic difference in the brightness of these two systems."

Researchers used the sophisticated Chandra X-ray Observatory, an orbiting satellite launched about a year and a half ago by the National Aeronautics and Space Administration (NASA).

"It's a billion dollar mission. It's a technological marvel," Narayan said.

Chandra has cleaner and more precisely aligned mirrors than any other telescope. According to Garcia, the improvement means Chandra can see objects that are 100 times dimmer than what previous telescopes could see--and that difference is enough to detect the factor of 100 in brightness between neutron stars and black holes.

The namesake of the Chandra observatory--Subrahmanyan Chandrasekharan, an Indian astronomer who won the Nobel Prize in 1983 for his work on the evolution of stars--also studied black holes.

"The black hole is, I think, one of the most amazing objects we study in astrophysics," Narayan said.