Automatic GRB directions with GRBM on BACODINE

In order to exploit the GRBM localization capabilities not only for the catalog, but also for the on-line detected bursts, the possibility of automatically positioning at least some of them and of delivering their coordinates to all potential observers via BACODINE has been set up and tested.

In this section only some requirements on the GRB features automatically estimated by the on-line quest are discussed; a more detailed description of the conditions that have to be fulfilled to yield an acceptable position for a GRB, is given in the next sections, regarding, in particular, the off-line GRBM localization of bursts of this catalog.

When a GRB is promptly detected, a fixed format e-mail, like in the example of GRB010317 reported in section , is automatically sent to the GRBM alert mailing list. Since the localization technique needs estimates of the burst total counts in each detector unit, in both energy bands, together with their corresponding uncertainties, some criteria have been set up and refined, aimed to check the reliability of the automatically computed total counts (called ``fluences'' in the alert e-mail). These conditions are expressed by eq. , where $u = 1,\ldots,4$ is the unit number, (G) and (A) denote the two energy bands, i.e. GRBM and AC, respectively, and $F_u^{(G)}$ is the fluence, in the GRBM band, for the $u$-th unit. As the fluences mainly depend on the goodness of the background fit around the burst in each detector, for both energy bands, an upper threshold on the $\chi^2$ of each ratemeter has been set; (first condition in eq. ); then, a lower threshold on the total counts (or fluence) of the most illuminated unit in the 40-700 keV band has been used, to avoid useless localizations of faint bursts, which are difficult since their fluences are known with too low precision (second condition in eq. ; (see section , concerning the GRB directions with GRBM). Finally, last but not least is a further condition on the weighted average hardness ratio, which must overcome a higher threshold ($0.5$) than that automatically used during the GRB quest (eq. ) ($0.3$): this stronger requirement is aimed to further limit the presence of solar hard X-ray flares, which sometimes are automatically detected as GRBs, owing to their occasional spectral hardness (third condition in eq. ).

$$\textrm{} \left\{\begin{array}{l} \displaystyle \chi^2_G(u) \ < \... ...{1,\ldots,4\Big\}\\ \displaystyle \overline{HR_w} \ > \ 0.5 \end{array}\right.$$ (30)

When the criteria expressed by eq.  are fulfilled, i.e. when the background fit around the burst looks reliable, and the burst itself is bright and hard enough, then the localization technique applies to it, provided that the BeppoSAX attitude at the burst time is known accurately: this is not the case when, for instance, the spacecraft goes into GSM (Gyro Safe Mode) unexpectedly, as it sometimes happens.