Cosmic gamma-ray burst 060428C detected in the fields of view of the IBIS and SPI telescopes of the INTEGRAL observatory and its early afterglow

Abstract

Analyzing archival data of the INTEGRAL observatory we discovered and localized a cosmic gamma-ray burst detected within fields of view of the IBIS/ISGRI and SPI telescopes on April 28, 2006. The burst has not been revealed by the INTEGRAL Burst Alert System (IBAS), so the information on its coordinates has not been distributed in time and search for the afterglow has not been carried out. The burst has been also detected by the KONUS/WIND and RHESSI spacecrafts. Its fluence was 2.3x10^{-6} erg/cm^2 in the 20-200 keV energy band, the maximum flux was 3.6x10^{-7} erg/cm^2/s (3.9 phot/cm^2/s). The burst had a complicated multi-peak profile and was outstanding of the typical bursts by increasing its spectral hardness with time. The emission spectrum near the flux maximum was characterized by the photon index alpha ~ -1.5 and the peak energy Ep ~ 95 keV. The burst lasted ~12 s, then we observed its afterglow at energies 15-45 keV decaying according to a power law with the index ~ -4.5. The spectral hardness decreased substantially during the afterglow.