The GRBM Data

During the normal mode, the following housekeeping data are continuously acquired by each of the GRBM units (all have 16 bits/counter):

• 1 s ratemeters in the energy band between the LLT and ULT thresholds (approx. to the 40-700 keV band); this is called ``GRBM band'';
• 1 s ratemeters in the energy band above the ACT threshold (approx. to the $>$ 100 keV band); this is called ``AC band'';
• 128 s, 256 channel PHA spectra in the GRBM band, whose start times are independent from the above ratemeters; the channels from 241 to 256 do not contain scientific data.

The dead times due to ADC are $4 \mu$s for the ratemeters and $14 \mu$s for the spectra. When the HTR Mode starts, besides the above described data, the following HTR count rates (only in the GRBM band) are available for a limited time interval: let $t_0$ be the trigger time (i.e., the time when the required OBTCs become verified):

1. count rates (9 bits/counter) in the interval [$t_0$-8s, $t_0$] with a time res. of $7.8125$ ms;
2. count rates (6 bits/counter) in the interval [$t_0$, $t_0$+10s] with a time res. of $0.48828125$ ms;
3. count rates (10 bits/counter) in the interval [$t_0$+10s, $t_0$+98s] with a time res. of $7.8125$ ms.

Table: GRBM main characteristics.

Energy range	40-700 keV
CsI(Na) thickness	10 mm
Geometrical area per detecting unit	1136 cm$^{2}$
	$\sim$85 cm$^{2}$@40 keV
	$\sim$200 cm$^{2}$@60 keV
LS1/LS3 On-axis effective area$^{(1)}$	$\sim$500 cm$^{2}$@120 keV
	$\sim$590 cm$^{2}$@300 keV
	$\sim$280 cm$^{2}$@600 keV
Field of view (FWHM)	open (2$\pi$)
Energy resolution	from 43% @40 keV to 15% @600 keV
	0.488 ms in HTR Mode
Maximum time resolution	1 s for GRBM and AC ratemeters
	128 s for PHA spectra
Energy spectra	256 channels
Minimum channel width of energy spectra	2.7 keV
ADC dead time	4 $\mu$s for the ratemeters
	14 $\mu$s for PHA spectra
	16 bit for each ratemeter bin
	16 bit for each PHA spectra channel
Memory allocation	HTR Pre-Trigger Mode (first 8 s)
	1024 9-bit counters for 7.8125 ms bins
	HTR Trigger Mode (middle 10 s)
	20480 6-bit counters for 0.48828125 ms bins
	HTR Post-Trigger Mode (last 88 s)
	11264 10-bit counters for 7.8125 ms bins
On-Board Trigger Dead Time	$\ge$ 7 min

$^{(1)}$

From [Amati, 1998].