One-man band: x-ray source plays two tunes.

It pulses, it flickers, and some 20 times a day it emits a torrent of X rays more intense than the combined radiation of a million suns. Since bursting on the scene in early December, this bizarre astronomical object, a Milky Way resident that lies toward our galaxy's center, has ranked as the brightest known X-ray source in the heavens.

Though it's still the champ at X-ray wavelengths, the source's intensity has halved, and in a month or two the fireworks will probably fizzle altogether. That's why researchers are scrambling to study this enigmatic system, which exhibits two types of X-ray activity-regular pulsing and sporadic bursting-never before seen in the same object.

Astronomers believe the object is a neutron star-a remnant of a collapsed star that packs the mass of the sun into a sphere no more than 16 kilometers in diameter. Some, perhaps even all, of the dazzling X-ray emissions stem from the violent interactions that ensue when the star steals mass from an orbiting companion.

Discovered Dec. 2, 1995, by NASA's Compton Gamma Ray Observatory (GRO), a spacecraft carrying X-ray and gamma-ray telescopes, this celestial source emits pulses of X rays every 2 seconds. At erratic intervals averaging about once an hour, it also emits an outburst of high-energy X rays. Rapidly rotating neutron stars that act as X-ray pulsars, beaming radiation at regular intervals, are familiar to astronomers. And the GRO has recorded thousands of so-called gamma-ray bursters-objects that explode like flashbulbs, emitting high-energy radiation and then vanishing without a trace.

But the new object, formally dubbed GRO J1744-28, manages to combine both behaviors.

"We've seen pulsars and we've seen bursters, but never together in the same source," says codiscoverer Chryssa Kouveliotou of the Universities Space Research Association (USRA) and NASA's Marshall Space Flight Center in Huntsville, Ala.

"We have some objects that play the drums, some that crash the cymbals . . . but this is a one-man band," declares Frederick K. Lamb of the University of Illinois at Urbana-Champaign, who has studied the system, informally known as the bursting pulsar, with the recently launched X-ray Timing Explorer Satellite.

Kouveliotou and her colleagues report their work in the Feb. 29 Nature. She and Lamb presented further details last week at a press briefing in Washington, D.C.

When the GRO first spied the bursting pulsar, it detected 140 powerful bursts of high-energy X rays in a single day. In mid-December, the craft discovered that the object also emits X rays of a lower intensity at a steady rate. Mark H. Finger of USRA found evidence that the bursting pulsar consists of a neutron star and a lower-mass, ordinary star that orbit each other every 12 days.

Lamb notes that most researchers agree on the origin of the pulsed X rays. Rapidly rotating neutron stars have strong magnetic fields. When such a star steals mass from a companion, the magnetic field funnels the material onto two tiny spots, the magnetic poles, at opposite sides of the star's surface. Crashing into the poles at half the speed of light, the stolen matter emits X rays, which sweep across the sky like a lighthouse beam as the star rotates. Astronomers speculate that the intermittent outbursts may stem from a variation on this theme. If material torn from the companion star is sometimes delayed from immediately crashing into the neutron star, enough matter could accumulate near the star to produce a huge outburst of X rays when it finally strikes. Such a delay might come about if the outward pressure exerted by the radiation streaming from the neutron star temporarily holds its own against gravity, keeping material away from the star's surface.

Racing against time, observers on the ground are striving to detect the bursting pulsar at other wavelengths. Using the Very Large Array Radio Telescope at the National Radio Astronomy Observatory near Socorro, N.M., Dale A. Frail has found an intriguing radio-emitting source in the same general part of the sky.

But Frail says he'll require the finer eye of another X-ray satellite, ROSAT, scheduled to observe the bursting pulsar on March 12, before he can determine whether the radio source coincides with the X-ray-emitting object.