The GRBM & Well Localized GRBs

The number of well localized GRBs detected also by the GRBM amounts to 79; except for one GRB, namely the WFC GRB010304 already discussed in section , the arrival directions are known with high accuracy, typically with few arcminutes uncertainties. Out of 79 GRBs, 46 are in common with the WFCs aboard BeppoSAX; actually, during the time interval scanned by the present GRBM off-line quest, lasting from July 03, 1996 to October 03, 2001, the WFCs detected 52 GRBs; among these, 45 have been automatically caught, only 1 did not trigger any SWTCs: GRB990704, UT 17:30:20, to date the most X-ray rich BeppoSAX burst ([Feroci et al., 2001]), and it was thus recovered after recognition by visual inspection (this GRB triggered the on-board logic). Another one, i.e. GRB000424, UT 18:18:07, occurred when the GRBM had been already switched off owing to the SAGA; the remaining 5 WFC GRBs show no clear signal in the GRBM energy bands, in agreement with their classification as X-ray rich GRBs: GRB990520, GRB991106, GRB991617, GRB000416, and GRB000608.

The number of bursts localized with the only IPN and detected by the off-line quest within the GRBM data amounts to 20; nevertheless, the GRBM did not contribute to the prompt localization of all, because its data were sometimes not immediately available (when the on-line quest has not been operating, owing to the problems already discussed in the previous chapter). For a discussion of the only IPN GRBs, that have been on-line discovered by the GRBM, see section . The remaining 13 well localized bursts have been positioned by the following experiments: 7 by ASM, 2 by the PCA/Rossi-XTE in combination with BATSE (GRB990506, also triangulated by the IPN, and GRB991216), 1 by the two CGRO experiments COMPTEL and BATSE (GRB980706), and 3 by HETE-II (GRB010326, and two refined with the IPN: GRB010612, GRB010629).

The situation is summarized in table .

Table: Well Localized GRBs detected by the GRBM

Experiment(s)	# GRBM	# on-board	# off-line
	GRBs	trg GRBs (%)	missed GRBs
WFCs	46	27 ($59 \pm 11$)	1
IPN	20	17 ($85 \pm 21$)	0
ASM	7	4 ($60 \pm 30$)	0
PCA+BATSE	2	2 (100)	0
COMPTEL+BATSE	1	1 (100)	0
HETE-II	3	3 (100)	0
Total	78	54 ($68 \pm 9$)	1

In this table, there is one value, that is worth mentioning: the fraction ($68 \pm 9$%) of GRBs that triggered the on-board logic; this is perfectly consistent with the estimate of the on-board trigger efficiency expressed in eq.  ($64 \pm 3$%). Also the single subsets of GRBs show consistent fractions, although the statistical significance is low, owing to the small numbers. Once again, the on-board trigger efficiency comes out to be $\sim$ 2/3.