Observations of soft gamma-ray or hard X-ray bursts in the GRIF experiment on the Mir orbiting station

Abstract

During the GRIF experiment onboard the Mir orbiting station, the sky was monitored with a PX-2 wide-field (∼1 sr) scintillation X-ray spectrometer to detect bursts in the photon energy range 10–300 keV. Because of the comprehensive instrumentation, which, apart from the X-ray and gamma-ray instruments, also included charged-particle detectors, the imitations of astrophysical bursts by magnetospheric electron precipitations and strongly ionizing nuclei were effectively filtered out. It was also possible to separate solar and atmospheric events. Several tens of bursts interpreted as being astrophysical were detected in the experiment at sensitivity levels S∼10−7 erg cm−2 (for bursts whose spectra were characterized by effective temperatures kT∼100 keV) and S∼3×10−8 erg cm−2 (for bursts with kT∼25 keV). Some of the soft gamma-ray or hard X-ray bursts with kT∼10–50 keV were identified with the bursting pulsar GRO J1744-28. Our estimate of the detection rate for cosmological soft gamma-ray or hard X-ray bursts from the entire sky suggests that the distributions of long-duration (>1 s) gamma-ray bursts (GRBs) in characteristic energy kT and duration are inconsistent with the steady-state cosmological model in which the evolution of burst sources is disregarded. Based on GRIF and BATSE/CGRO data, we conclude that most of the GRB sources originate at redshifts 1<z<5.