Direction Distribution

For 441 out of 944 GRBs reported in out GRB catalog, an estimate of the arrival direction, even with variable accuracy, was available from other instruments (WFC, ...).

The localization technique based on the GRBM data, applied to all the 944 GRBs of the GRBM catalog, gave the following results: a unique and acceptable solution only for 446 out of the initial 944 GRBs ($\sim$47%). These 446 GRBM-localized GRBs split into the following classes: 152 in common with BATSE 4B, 10 with Kommers', 41 with Stern's, 45 with the well localized GRBs (mainly WFC + IPN); the remaining GRBs, amounting to 446-152-10-41-45=198, have been detected and positioned by the only GRBM, with no possibility of cross-checking with any other independent detections. In table  the situation is summarized.

Table: GRB positioned with the only GRBM.

Experiment	# GRBM-positioned	# Total	GRBM-positioned
	GRBs	detected	GRBs fraction
Well Loc.	45	79	57%
BATSE 4B	152	283	54%
Kommers'	10	18	56%
Stern's	41	61	67%
Only GRBM	198	503	39%
Total	446	944	47%

From table  it comes out that, on average, the GRBM localization technique can yield a unique and reliable position for $\sim$50% out of the GRBs included in our catalog. Out of the 503 GRBs detected by the only GRBM, 503-198=305 GRBs cannot be positioned with a unique and reliable direction, due to the GRBM response function ambiguity. In such cases, more than one direction is available. (sec. ).

Figure: Direction Distribution of 362 GRBs from the GRBM catalog, in common with BATSE 4B, Kommers' and Stern's catalogs. The directions are taken from these catalogs (galactic coordinates, aitoff projection).

In fig.  the arrival direction distribution of the common sample of GRBM-BATSE (both triggered and non-triggered: 4B+Kommers'+Stern's), therefore including 362 (283+18+61) GRBs, is shown in galactic coordinates (aitoff projection).

Figure: Direction Distribution of 639 GRBs from the GRBM catalog; 79 well localized GRB positions, 283 from BATSE 4B, 18 from Kommers', 61 from Stern's,and 198 GRBM positioned GRBs (galactic coordinates, aitoff projection).

In fig.  the most complete set of positioned GRBs, including 639 (944-503) GRBs (table ) out of 944 GRBs forming this GRBM catalog; although several bursts out of this set have been localized by different experiments, like some WFC, BATSE, the GRBM by itself, for these bursts the most accurate position has been taken (in order: first: well localized, i.e. WFC, IPN, etc...; second, BATSE 4B; third, Kommers'; fourth, Stern's; fifth, GRBM).

Figure: Direction Distribution of 362 GRBs from the GRBM catalog, in common with BATSE 4B, Kommers' and Stern's catalogs. The directions are taken from these catalogs (aitoff projection). Upper panel: BeppoSAX local coordinates; bottom panel: equatorial coordinates (in this case, only the 283 common BATSE 4B bursts are shown.

In fig.  the 362 GRBM-BATSE common bursts already shown in fig.  are here shown in both BeppoSAX local coordinates and equatorial coordinates. In the BeppoSAX local frame of reference (upper panel), the lack of GRBs in the vicinity of the local south pole, owing to the low GRBM efficiency, is apparent.