The Solution?

The Italian-Dutch X-ray satellite Beppo-SAX has linked a number of gamma-ray bursts to counterparts in other wavelength bands, and the mystery of the burst distance scale may have been solved. This satellite includes two wide-field cameras (WFCs) perpendicular to the coaligned narrow-field telescopes which are the mission's main instruments. These WFCs each have a field-of-view of 40 degrees; consequently they observe approximately 8 bursts per year. Because of this burst capability, the anti-coincidence shields surrounding the narrow-field instruments are also used as a burst detector. This array of instruments allows Beppo-SAX to localize gamma-ray bursts, and search the region where the burst occurred for an X-ray afterglow. Within ~4--8 hours of the burst the coordinates can be disseminated to observers in other wavelength bands, who can then search for the burst source.

The precise sequence of events is as follows: The burst detector detects a burst. When the telemetry from the time of the burst reaches the ground, images are constructed for the two WFCs using the photons detected while the burst was in progress (this maximizes the signal-to-noise ratio). If the burst was in the field-of-view of one of the WFCs, one of the images will contain an X-ray point source not present before the burst. The source in the WFC image can be localized to an error radius of 3 arcmin. The spacecraft is then reoriented (on a timescale of ~4--8 hours) so that the narrow-field instruments point at the location of the burst, and one or more X-ray sources may be found in the burst error box. Repeated observations over the next few days may identify a variable source which is likely to be the burst's afterglow. The narrow-field instruments can localize a source to an error radius of ~50 arcsec. Optical telescopes can then image these small error boxes over the next few days, searching for a variable source (variability is the expected signature of the burst counterpart).

These error boxes are unprecedented in the study of gamma-ray bursts. The WFC error box is comparable to the better burst error boxes resulting from the IPNs, and the position of the X-ray afterglow is comparable to the very best error boxes previously available. And the Beppo-SAX positions are available within hours of the burst, allowing the search for fading afterglows. The scientific bonanza resulting from the Beppo-SAX observations has led to other methods of rapidly localizing bursts. About one burst a month falls within the field of the All-Sky Monitor on the Rossi X-ray Timing Explorer (RXTE); RXTE raster-scans the resulting error box with its main detectors. Similarly, RXTE raster-scans small BATSE error boxes at the rate of ~once a month. Thus many afterglows, or limits on their presence, should be available over the next few years.

More than a dozen bursts have been localized by either Beppo-SAX or RXTE. In some cases no afterglow was observed, in others only an X-ray variable was identified, but in many cases optical transients were found, providing a wealth of data.

The significant conclusion drawn from the spectral lines is that at least some bursts are cosmological. Occam's Razor dictates that unless proven otherwise, we should assume that all bursts have the same origin and are thus cosmological. However, Loredo and Wasserman have shown that the data permit at least two burst populations, one of which could be cosmological and the other local.

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Page maintained by David Band.
Last revised January 21, 1999.