HR Threshold

Let ${\mathcal{S}}_u(i)$ be the set of indices of the bins containing counts from the burst, depending on the unit $u$ and on the triggering bin $i$; the criteria used for determining which bins belong to ${\mathcal{S}}_u(i)$, are widely described in the dedicated section  (eqq. , ). In particular, two quantities are defined there: the total counts, or count fluences, one for each energy band, for a given unit $u$ and triggering bin $i$: $F_u^{(G)}(i)$ for the GRBM band and $F_u^{(A)}(i)$, obtained by summing the net signals over the set ${\mathcal{S}}_u(i)$ (eq. ).

Here we use the concept of the net total counts, required for the definition of the fifth SWTCs, that must be always satisfied, in combination with anyone of the above four SWTCs.

For each transient event triggering at least one of the SWTCs at the $i$th time bin, for each detector unit $u$ that detected it, the ratio of the AC counts over the GRBM counts, called Hardness Ratio $HR_u$, is automatically computed, according to eq. .

To be tagged as good GRB candidate, the transient event must have the following property: the weighted average $\overline{HR_w}$ among the units that detected it (for which the above definition of $HR_u$ is meaningful), cannot be lower than a given threshold $HR_{min}$, that we fixed at 0.3:

$$\displaystyle \overline{HR_w} \ = \ \frac{\displaystyle \sum... ...{1}{\sigma_{{\tiny HR_u}}^2}} \quad > \quad HR_{min} \ = \ 0.3$$ (21)