GRB 070912—A gamma-ray burst recorded from the direction to the galactic center

The detection of GRB 070912 recorded in the field of view of the SPI, IBIS/ISGRI, and JEMX telescope on September 12, 2007, at 07h32m19s (UT) when analyzing the INTEGRAL archival data is reported. The burst is one of the well-localized events closest to the direction toward the Galactic center (less than from the source Sgr A*) over the entire history of burst observations. Since it was not promptly revealed by the INTEGRAL Burst Alert System (IBAS), no information about its coordinates was disseminated and no search for optical and soft X-ray afterglows was conducted. The 3–200 keV fluence was 2.8 × 10−6 erg cm−2 and the peak flux was 1.8 × 10−7 erg cm−2 s−1 (1.9 ph cm−2 s−1). The burst was also observed in the KONUS/WIND experiment in the background mode, although it was not included in the list of recorded bursts. GRB 070912 is among a limited number of events for which a broadband (3 keV-2 MeV) spectrum of X-ray and gamma-ray emission has been obtained and their evolution from the first instants to complete decay has been traced. It shows how the fast evolution of its spectrum gives rise to absorption features at energies of ∼100 keV.Within the first seconds after the onset of the burst, its spectrum was a power law with a photon index of ∼0.8, but it exhibited a noticeable deficit of photons at energies below 20 keV. Such an initial deficit (a delay in appearance) of X-ray photons can be explained by their “high-latitude” origin relative to the line of sight. The spectrum rapidly softened and at the decay phase was well described by a blackbody (or Wien) law. This allows the distance (redshift) to the burst source to be estimated.