GRBM Average Spectrum

In spite of the two possible arrival directions found with the localization technique, the two corresponding spectral response matrices do not look significantly different, thanks also to the relatively small angular distance between the two directions ( $\sim 24\rm ^{\circ}$). Thus, the response matrix corresponding to the first direction, with the best $\chi^2$ has been chosen.

The 128 s GRBM spectrum including the GRB, started $\sim$ 46 s before the GRB onset, and ended at $t\sim$ 80 s, i.e. just at the end if the burst itself (fig. ). The spectrum from GRBM unit 2 has been chosen, since it shows the best SNR.

Figure: GRBM average spectra of GRB980516. Top panel: fit with the Band Law ($\chi^2 = 1.02$); middle panel: fit with a broken power law (unfolded spectrum, $\chi^2 = 1.03$); bottom panel: fit with a single power law (unfolded spectrum, $\chi^2 = 1.25$).

The GRBM spectrum has been grouped, so that each energy channel has at least 4$\sigma$ significance. Three different models have been used to fit the GRB spectrum: the Band Law, the broken power law and the single power law (fig. ). All of them give acceptable results and are quite consistent among each other: the break energy coming out from the Band fit looks quite high, and this explains the fact that also a single powerlaw fit is quite good (tab. ).

Table: Parameters obtained by fitting the GRBM spectrum of GRB980516 with three laws: apart from one value ($1\sigma$), all uncertainties are $3\sigma$. (Courtesy of E. Montanari).

Parameter	Band Law	Broken Power Law	Single Power Law
PhotonIndex1	$-0.9^{+0.2}_{-0.1}$	$1.13\pm 0.1$ ($1\sigma$)	$1.28^{+0.2}_{-0.1}$
BreakE (keV)	$540.0^{+420}_{-240}$	$190 \pm 30$	--
PhotonIndex2	$-1.38 \pm 0.04$	$1.37^{+0.5}_{-0.3}$	--
DoF	218	218	220
$\chi^2$	$1.02$	$1.03$	$1.25$

The Band Law and the broken power law fits give consistent results for the two power law indices ($\sim 0.9$ and $\sim 1.4$, respectively).

Finally, the total fluence has turned out to be $\sim (2.0 \pm 0.2)\times 10^{-4} \rm erg\ \rm cm2$ in the 50-700 keV.