The All-Sky Monitor aboard Rossi-XTE

The All-Sky Monitor ([Levine, 1996], fig. ) is an experiment on board the Rossi X-ray Timing Explorer (Rossi-XTE) satellite ([Bradt et al., 1993,Swank et al., 1998,Bradt et al., 2001]), launched on December 30, 1995. It consists of three wide-angle shadow cameras equipped with proportional counters with a total collecting area of 90 cm$^2$ and with a $6\rm ^{\circ}\times 90\rm ^{\circ}$ field of view for each camera; these are mounted on a rocking bearing and have an angular resolution of $\sim 0.2\rm ^{\circ}$.

Figure: Schematic view of the Rossi-XTE payload.

Two cameras point normally to the rotation axis to the same direction, so that their error boxes cross each other, therefore reducing the positioning error to $0.2\rm ^{\circ}\times 1.0\rm ^{\circ}$ for weak sources and to $3\mbox{$^\prime$}\times 15\mbox{$^\prime$}$ for stronger sources; it has a 20 mCrab sensitivity. the third camera points along the rotation axis. Each observation lasts about 100 s; then, the next pointing is obtained with a $6\rm ^{\circ}$ rotation and so on, untill a complete rotation is performed (every 100 min), during which $\sim$ 80% of the whole sky has been monitored. This capability in monitoring possible X-ray transients and variations in flux of known sources, like CygX-1, makes the ASM experiment suitable for detecting GRBs; nevertheless, the small collecting area and the small field of view limit the number of localized bursts. Moreover, when a GRB is within the field of view of one camera, its direction is known only along one dimension, given the large width ( $90\rm ^{\circ}$) of the error strip; therefore, although it cannot precisely locate GRBs by itself, nevertheless it can be very important in reducing the error box, when other experiments detected the same burst: this occurred in several cases ([Smith et al., 1999]).