1 Introduction

The GRB detector onboard ULYSSES has been working almost continuously since it was turned on, on November 9, 1990. ULYSSES presents several characteristics that are of interest for the study of the brightness distribution of the rare, intense GRBs: continuous operation for several years, an almost unobstructed field of view and a constant energy threshold. In addition, the GRB detector is small, making it mostly insensitive to dead time and pileup effects that affect the study of very bright bursts with larger instruments. Finally, in April 1998, the sky coverage of ULYSSES amounted to 87.4 yr sr, a number which is comparable to the sky coverage of PVO over its entire lifetime ($\sim\!125$ yr sr, Fenimore et al. 1993).

We present here the peak flux distribution of 331 GRBs detected by ULYSSES from November 1990 to April 1998. The burst intensity is measured by the peak count rate over 1 s (hereafter $C_{\rm max}$), within the full ULYSSES energy range (approximately 20-150 keV). In order to facilitate the comparison with BATSE we also indicate the corresponding 50-300 keV peak flux. Finally, we have restricted our analysis to GRBs with durations >2 s.

We study 2 samples that we describe now.

Triggers: The GRBs that triggered ULYSSES. The sky coverage of ULYSSES for these bursts is $\rm 87.4\, yr\, sr$.This sample is complete down to $\rm 9 \,ph\, cm^{-2}\ s^{-1}$($C_{\rm max} =$ 250 s^-1) and contains 168 GRBs above this threshold.

Real Time events: The GRBs found in ULYSSES real time data from the Current BATSE Catalog. Starting from the BATSE Catalog allows us to use GRBs that are a factor of 2.5 fainter at the price of reducing the spatial coverage by a factor of 2. The sky coverage of ULYSSES for these bursts is 42.2 yr sr. This sample is complete down to $\rm 3.6\, ph \,cm^{-2}\, s^{-1}$($C_{\rm max}$ = 100 s^-1) and contains 331 GRBs above this threshold.

  

Figure 1: The GRB peak flux distribution measured by the gamma burst detector on ULYSSES (see text). The dotted lines have the slope -3/2 expected for a Euclidean distribution