Prior to this discovery, researchers could not rule out other explanations for the 2.7-million-solar-mass gravitational field at our galactic center. For example, a cluster of dead neutron stars or a massive ball of neutrinos could cause the pull at the galactic core. But the small area of the flare put these and other theories to rest, leaving a black hole as the only possibility. "At this time, we don't have another physical explanation," says Frederick Baganoff of the Massachussetts Institute of Technology, the head of the research team.
Although the flare provided crucial information about the Milky Way's black hole, scientists are still unsure what caused it. One possibility is that the giant magnetic fields thought to exist around the surface of the hole sparked the flare. Like the fields around our sun, these black hole's magnetic fields can periodically "reconnect," a process that could catapult material from near its surface. Another theory is that a comet-size object falling into the black hole sparked the x-ray storm. If that were true, it would be "extremely exciting," according to Baganoff. "It's as if the material there sent us a postcard just before it fell in," he says.
Sometime in the near future, astronomers hope to observe a second flare¿this time while both Chandra and radio telescopes are watching. The combined radio wave and x-ray information could help them to more precisely determine the location of the flare and would further corroborate the black hole theory. But ultimately, astronomers will be unable to prove that a black hole lurks at our galactic center until they have actually seen its surface, or event-horizon. "That, to me, would be definitive proof," Baganoff says. And it will take at least another 10 years before a new generation of observatories allows that to happen.