Near or far? Hubble view of the point source and adjacent fuzzy patch.

New Intrigue Surrounds Gamma-Ray Source

The long-running mystery of gamma-ray bursts (GRBs)--flashes from somewhere in space that periodically set detectors screaming--has taken another dizzying twist. After astronomers thought they had tracked a recent burst to a faraway source, a new analysis of observations by the Hubble Space Telescope suggests that the source may in fact be a fast-moving object a stone's throw away.

The question is whether GRBs originate in or near our galaxy or billions of light-years away, at cosmological distances, which would make them the brightest outpourings in the universe. Astronomers thought they were on the verge of an answer when an Italian-Dutch satellite saw a fading source of x-rays that seemed to be the afterglow of a GRB the satellite had detected on 28 February. Because x-ray detectors have much better spatial resolution than those for gamma rays, that helped pin down the burst's position for further observation. Hopes shot even higher when ground-based telescopes aimed at the spot then fished out both a point of light and a faint fuzzy patch next to it--possibly the GRB source and its host galaxy in the distant universe (Science, 21 March, p. 1738). The cosmological alternative seemed poised to carry the day. But then the Hubble got into the act.
The latest results of its scrutiny of the proposed source, reported on the Internet in International Astronomical Union (IAU) circulars, have thrown the debate wide open again. Patrizia Caraveo at the Istituto di Fisica Cosmica in Milan, Italy, and several collaborators claim to have found something startling: The point source was moving across the sky. The angular motion was so quick, they say, that the object might have to be within a few hundred light-years of Earth--much closer than even the proponents of a galactic origin for GRBs have been suggesting recently. The fuzzy object could then be a transient cloud of gas associated with the burst or a background galaxy, aligned by chance with the pointlike object.

Another group--looking at the same data--saw nothing of the kind. An analysis by Kailash Sahu of the Space Telescope Science Institute (STScI) in Baltimore and several collaborators suggests the point source is stationary. "We cannot reproduce what Caraveo says in the IAU circular in spite of our best efforts," Sahu told ScienceNOW.

Resolving these issues, experts say, is likely to require a third Hubble observation when the point source, now drawing close to the sun, reemerges from its glow in a few months. If the object is still visible, the next glimpse of it should settle the question of whether it's moving. In the meantime, many astronomers share the sentiments of Chryssa Kouveliotou of NASA's Marshall Space Flight Center in Huntsville, Alabama: "I'm more confused than anything," she says.

Posted in Space, Physics