An international team of astronomers has observed two unusual and extremely bright sources that might be a completely new type of cosmic explosion.

The scientists are calling them flares. These two objects became about a hundred times brighter in X-rays in less than a minute, and then returned to their original luminosity about an hour later.

"We've never seen anything like this," said lead author Jimmy Irwin of the University of Alabama in a statement. "Astronomers have seen many different objects that flare up, but these may be examples of an entirely new phenomenon."

The study, which appeared in the journal Nature, describes the flares as ultraluminous X-ray sources (ULXs), but their true nature is unknown.

"Now that we've discovered these flaring objects, observational astronomers and theorists alike are going to be working hard to figure out what's happening," said co-author Gregory Sivakoff of the University of Alberta.

^{Galaxy NGC 5128 with the ultraluminous X-ray flare. NASA/CXC/UA/J.Irwin et al}

One suggestion is that the objects are binary star systems made of a compact source like a black hole or a neutron star and with a normal star orbiting it. At least, that is what they appear to be like from the observations by the Chandra X-ray telescope.

But these binaries are not known to flare up like this. So, the researchers proposed a more eccentric orbit for their stellar companion; when it gets too close to the black hole (or neutron star), the gravity of the latter pulls away material. The intense forces heat this stuff to millions of degrees and it flares up in X-rays. As the material is absorbed, the luminosity is reduced again.

"These flares are extraordinary," said co-author Peter Maksym of the Harvard-Smithsonian Center for Astrophysics. "For a brief period, one of the sources became one of the brightest ULX to ever be seen in an elliptical galaxy."

One flare seems to be associated with NGC 4636, a galaxy 47 million light-years from Earth. The other is near NGC 5128 at a distance of 14 million light-years from us. This latter object was seen flaring five times.

The team looked at several thousand sources from nearby galaxies, but these two seem to be the only ones exhibiting this peculiar behavior.