Astronomers have found a pair of extraordinary cosmic objects that dramatically burst in X-rays. This discovery, obtained with NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton observatory, may represent a new class of explosive events found in space. The mysterious X-ray sources flare up and become about a hundred times brighter in less than a minute, before returning to original X-ray levels after about an hour. At their peak, these objects qualify as ultraluminous X-ray sources (ULXs) that give off hundreds to thousands of times more X-rays than typical binary systems where a star is orbiting a black hole or neutron star.
“We’ve never seen anything like this,” said Jimmy Irwin of the University of Alabama, who led the study that appears in the latest issue of the journal Nature. “Astronomers have seen many different objects that flare up, but these may be examples of an entirely new phenomenon.” While magnetars – young neutron stars with powerful magnetic fields – have been known to produce bright and rapid flares in X-rays, these newly discovered objects are different in key ways. First, magnetars only take a few seconds to tens of seconds to decline in X-rays after a flare. Secondly, these new flaring objects are found in populations of old stars in elliptical galaxies, which are spherical or egg-shaped galaxies that are composed mostly of older stars. This makes it unlikely that these new flaring objects are young, astronomically speaking, like magnetars are thought to be. Also, these objects are brighter in X-rays during their “calm” periods.
“These flares are extraordinary,” said Peter Maksym, a co-author from 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.” When they are not flaring, these sources appear to be normal binary systems where a black hole or neutron star is pulling material from a companion star similar to the Sun. This indicates that the flares do not significantly disrupt the binary system.