Long Time Scale Modulations

An example of the 8 GRBM 1 s light curves, plotted in fig. , shows long timescale ($10^4$-$10^5$ s) variations in the background level. In this figure, some typical properties of the GRBM ratemeters stand out: the periodic structure, with a period of $\sim$ 6000 s, reflects the background component induced by the Earth magnetic field cut-off rigidity, whose period is approximately equal to the BeppoSAX orbital one. The periodic gaps in the count rates correspond to the passages over the South Atlantic Geomagnetic Anomaly (SAGA), when the HVs of the detectors (except for the Particle Monitor) are turned off, to preserve the detectors from damages.

The modulation of the amplitudes is caused by the the modulation of the geomagnetic cut-off orbit with time: one property of such an orbit is that it drifts with time and, therefore, the detectors' noise caused by the particles trapped by the Earth magnetic field changes consequently.

It is worth emphasizing the greater number of spikes in the 40-700 keV light curves than in the $>$ 100 keV band: these spikes are caused by high-energy charged particles crossing the detector slabs. For a more detailed description of these transient events, that are the most frequent, see later on.

The mean count rate typically ranges between 700 and 1200 counts/s, depending on the energy band, on the threshold values (LLT, ULT, ACT) defining the bands, on the unit: e.g., GRBM 2 is always pointing to the Sun within $\sim$ 30-40$^\circ$, so that its GRBM band mean count rate strongly reflects the solar X-ray activity: in fig.  the mean rate is little greater than the other units. However, other sources, that may face the detectors, contribute the ratemeters' background and their contribution is often modulated by the periodic occultations behind the Earth.

While Fig.  shows an example of the GRBM ratemeters during several contiguous orbits, as they are stored in the FOT data, fig.  shows the case of a single orbit: each of these data sets is scanned, soon after download, for performing the search of transient events like GRBs or SGRs.

Figure: GRBM 1 s light curves, one for each energy band and for each unit (orbit n. 26855, August 2, 2001). The duration is $\sim$6000 s. These are the typical time profiles of the GRBM ratemeters during a single orbit. At $t\sim$30921 SOD, corresponding to 08:35:21 UT, the GRB010802 was detected by the automatic on-line quest (automatic mail n. lkGRB[268]): in this case, the GRB was bright enough to be detected in all units, in both energy bands.

Here a data gap due to the SAGA passage can be clearly seen: it lasts $\sim$900 s, and it usually ranges between 600 s and 1200 s. In this example, concerning orbit n. 26855, the GRB010802 is visible at $t\sim$31000 s, just after the SAGA transit. There is also an intense spike in the unit 2, GRBM band (GRBM 2) around $t\sim$29400 s: a smaller trace of it can be seen in AC 2, as well. The main difference between the GRB and the spike can be easily appreciated: while the GRB, very strong, is clearly seen in every unit, in both the energy bands, on the other side the spike is detected only in unit 2: although this feature does not suffice to exclude the possibility of a burst, its spectral softness with respect the usual burst hardness is conclusive (the main differences between the spectral and temporal peculiarities of the several classes of transient events detected by the GRBM are thoroughly examined later on).