HR-Duration Correlation

Another result confirming a propery already known from the BATSE catalog is the correlation between hardness ratio (HR) and duration: in particular, short GRBs look harder than long ones, as it can be seen from fig. .

In the GRBM case, the HR here considered, is the weighted average of the HRs of the single GRBM units (eq. ), where each HR expresses the ratio of the $>$100 keV counts over the 40-700 keV counts; although these bands are strongly correlated (owing to the common 100-700 keV counts), nevertheless short GRBs are significantly harder, as it can be seen in fig. ; in this figure, the error bars have been omitted for clarity. In fig. , only the GRBs with HR at least $1.5\sigma$ significant and with HR uncertainties lower than $0.10$ have been included; here the error bars are reported as well.

Figure: HR-Duration Correlation of 501 ($T_{90}$) and 497 ($T_{50}$) GRBM bursts. In the upper panel, duration is expressed in terms of $T_{90}$, while $T_{50}$ has been used in the lower panel.

The mean HR has been computed for the different duration classes: the mean HR is $0.870 \pm 0.007$ and $0.734 \pm 0.001$ for short and long GRBs, respectively. This property, originally discovered by BATSE, is therefore confirmed independently by the GRBM catalog.