We considered 292 GRBs observed by BATSE [(Brainerd 1998)] during occurred in the field of view of EASTOP with a zenith angle . For each event the measured number of counts C during the time interval in which BATSE detected 90% of the flux is compared with the average number of counts B expected from the background (evaluated in 600 s around the burst).
No significant excess has been observed and the distribution of the excesses C-B is fully explained by poissonian fluctuations (see Fig. 1). Looking for possible delayed or anticipated excesses with respect to the BATSE event, the same analysis procedure has been performed in a 2 hours interval centered on the BATSE time, changing the width of the time window from 1 to 200 s. Also in this case no excess was found.
|Figure 2: Upper limits on the energy fluence obtained for 292 BATSE events, as a function of the zenith angle. Full squares correspond to 4 GRBs observed by Beppo-Sax (GRB 971214, GRB 970508, GRB 971227 and GRB 980703)|
Figure 2 shows the obtained upper limits on the energy fluence in the range for the 292 bursts analyzed, in the time window ,as a function of the zenith angle. The fluence upper limits have been calculated at 5 standard deviations level, assuming a GRB spectrum dN/dE with , extending up to 1 TeV.
The gamma-ray absorption in the intergalactic space
through pair production
affects the high energy part of the spectra of GRBs located
at cosmological distances.
According to [(Salomon&Stecker)]
the gamma-ray flux from a source at redshift z=1
is reduced by a factor at E=50 GeV and by a factor
at E=100 GeV.
Assuming the GRB sources at z=0.5(1.0), the obtained
fluence upper limits have to be increased by a factor .
b) All sky survey
GRBs can be searched as short duration increases ( s) in the flux of secondary charged particles. In each second, the counting rate C is compared with the expected background B calculated averaging the counting rate in 15 minutes around. Figure 3 shows the distribution of the differences in unit of standard deviations, obtained assuming poissonian fluctuations, for a total time of measurement of 3.6 104 hours. The data are well fitted by a Gauss distribution with ,showing the stability and good performance of the detector over a long time exposure.
A single statistical significant excess (10.6 standard deviation) has been observed on 1992 July 15 at 13:22:26 UT [(Aglietta 1993)]. Assuming this excess due to a gamma-ray burst with a zenith angle , the correspondent energy fluence is .
|Figure 3: Distribution of the excesses of duration s in units of standard deviations, obtained in the "all sky" survey|
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