1 ASM capabilities

The All-Sky Monitor on RXTE (Levine et al. 1996) has the capability to locate Gamma-Ray Bursts (GRBs) to within a few arcminutes in two dimensions. This can occur if the burst falls within the $\sim10^\circ$ by $\sim50^\circ$ parallelogram on the sky which is viewed simultaneously by the two azimuthal shadow cameras of the ASM (see Levine et al. 1996). The solid angle for such detections is actually somewhat larger in some cases because the burst intensity (or that of its immediate afterglow) may remain above ASM threshold as the ASM steps to its next celestial position, i.e. to its next "dwell''. This will sometimes bring the burst into the field of view of a camera that had not yet detected it. The ASM takes data for $\sim40\%$ of the orbital time.

It is more probable that the burst will fall within the FOV of only one of the three shadow cameras, each with a field of view of $\sim10^\circ \times \sim100^\circ$. In the case of a detection in only one collimator, the error region will be a few arcminutes wide and a few degrees long.

The RXTE does not carry a dedicated GRB detector, and the ASM detects only the X-ray portion of GRB spectra, 1.5-12 keV. It is therefore difficult to distinguish a rapid X-ray transient from a GRB. Our original method for securely identifying a new transient was to wait until several sightings confirmed the detection. This would distinguish a genuine transient from background events. In the case of GRB, one may have only one sighting. This requires one to impose a higher threshold for detection.

In addition to "position data'' that yields intensities and locations of a source based on 90-s integrations, the ASM records "time-series data'' in 1/8 s time bins for each of the three energy channels of each of the three detectors. If a new source is detected in the position data of a given ASM dwell, the presence of rapid variability in the corresponding time-series data improves the likelihood that the source is a GRB.