X-ray fireworks put on a stellar show.

X-ray fireworks put on a stellar show

Astronomers have detected a new, surprisingly strong source of X-rays, which appear to come from a pair of stars in the constellation Cygnus. The sudden onset of this radiation and the accompanying dramatic increase in the system's visible-light brightness signal violent activity within the system -- possibly the transfer of a massive glob of material from a large star to a disk surrounding its compact companion.

"What we think we're seeing is a close binary system in which one object is either a neutron star or a black hole," says Sumner G. Starrfield of Arizona State University in Tempe. But the strength, duration and variability of the X-ray and radio signals coming from the source show it to be unique.

Japan's Ginga satellite first detected the new source on May 22. The X-ray signals peaked at the beginning of June and are now starting to fade. Starrfield, R. Mark Wagner of Ohio State University in Columbus and Angelo Cassatella of the European Space Agency's International Ultraviolet Explorer observatory in Madrid traced the X-ray source to a system known as V404 Cygni, whose last sudden intensification occurred in 1938 when it became 2,000 times brighter than normal. Astronomers have seen a similarly strong brightening this time.

What makes the latest outburst especially interesting is that astronomers can track its behavior over a wide range of wavelengths. "There's a lot of activity," Wagner says. "This object is varying widely all over the spectrum." The strength of the X-ray signals fluctuates erratically by as much as a factor of 10 in a matter of seconds. Visible light shows similar flickering but on a scale of minutes.

Wagner and his co-workers have also detected a possible 10-minute periodic variation in the flickering. This suggests the compact companion may be a slowly rotating neutron star. However, astronomers need further observations to confirm the finding and to determine whether the compact object has a significant magnetic field.

"In this particular case, we have an event that last happened, as far as we know, 51 years ago," Wagner says. But, adds Starrfield, "the object is so strange that we have a feeling it may have gone off in the past and no one's noticed."

Researchers think they are probably seeing a sudden, huge transfer of material to a neutron star or a black hole. As the material falls, it gets compressed and heated to millions of degrees, generating X-rays and other radiation. Then this process of accretion abruptly stops, and the system winds down to a quiescent state, perhaps for decades.

V404's outburst has attracted considerable attention. Astronomers are closely monitoring the evolution of its X-ray, ultraviolet, visible-light, infrared and radio spectra. "This is a real multinational, multiwavelength campaign," Starrfield says. "It's a rare event." The last comparable X-ray outburst occurred in 1975 from a source designated A0620-00.

Astronomers hope to collect enough data to calculate the separation and masses of the two stars in the system, which lies about 3,000 light-years from Earth. If the compact object's mass turns out to be greater than twice the sun's mass, then it would be a black hole. If it has a smaller mass, the compact object may be a neutron star in the process of turning into a black hole as it collects material and gradually increases its mass.