Crab and CygX-1 Occultation Steps

Finally, there are several known X-ray sources which can be detected within the GRBM band ratemeters: the two brightest ones are the Crab Nebula and the black hole candidate CygX-1.

Because of their directions in the sky, these sources are periodically hidden by the Earth, while BeppoSAX moves along its orbit: this gives rise to periodic occultations, like those shown in fig. ).

Figure: Two single occultation steps in the GRBM band, unit 1: left panel: Crab ($\phi=273$$^\circ$, $\theta=7$$^\circ$, OP 03991); right panel: CygX-1 ($\phi=287$$^\circ$, $\theta=7$$^\circ$, OP 04421).

Moreover, like in the case of BATSE ([Zhang et al., 1993,Zhang et al., 1994,Zhang et al., 1995]), [Guidorzi et al., 1998] , exploited the Earth occultation technique to monitor such bright sources by means of the GRBM ratemeters: in particular, in order to reduce the statistical noise, several contiguous occultations are summed together, as long as the satellite attitude does not change (usually, 10-15 occultations are summed together); then, an occultation profile, based on a model of the terrestrial atmosphere, is fitted to the summed data, thus providing an estimate of the source mean rate.

Figure: Summed occultation steps of Crab in the GRBM band, unit 1: left panel: occ. beginning; right panel: occ. end (OP 03991).

Figure: Summed occultation steps of CygX-1 in the GRBM band, unit 1: left panel: occ. beginning; right panel: occ. end (OP 04421, May 1998).

This technique made it possible to test the GRBM on-ground calibrations by monitoring the Crab Nebula at different angles with respect to the GRBM detector units; besides, it allowed to monitor variable sources as bright as CygX-1 ([Guidorzi et al., 1998]). In particular, the mean rate due to Crab (figg. ), when the source is on axis to the units 1 or 3 (those with the better sensitivity), is $\sim 40$ counts/s in the GRBM band and $\sim 20$ in the AC band, while the mean rate from CygX-1 (figg. ), variable on timescales from days to months, ranges from 5 to 40 c/s in the GRBM band and from 0 to 20 in the AC band. While in figg.  the two Crab occultations, at the beginning and at the end of the occultation, respectively, suggest the same mean rate, i.e. $40 \pm 3$ c/s, for CygX-1 the result (see fig. ) is different: one time, the occultation gives an estimate of $55 \pm 5$ c/s, while another time the occultation gives $40 \pm 3$ c/s: this discrepancy is explained by the fact that the GRBM is an all-sky monitor: therefore, the occultation could concern several sources in addition to CygX-1.