The exceptionally intense long GRB 110918A was discovered by several GRB observing
missions: INTEGRAL (SPI-ACS), Konus-WIND, Mars
Odyssey (HEND), and MESSENGER (GRNS) on September 18, 2011.
This GRB was localized by the Interplanetary Network (IPN) and its bright X-ray
counterpart was found in close vicinity of the IPN box in the Swift/XRT
follow-up observations starting 1.2 days after the trigger. The optical afterglow was
discovered by the Isaac Newton Telescope and its spectroscopic redshift z = 0.982 was
measured with the GMOS spectrograph mounted on the Gemini-N telescope. GRB 110918A is the
brightest burst detected by Konus-WIND for more than 17 years of its
continuous observations. The instrument’s light curves in three energy bands covering
22–1450 keV range show an extremely bright, short, hard pulse followed by three weaker,
softer, partly overlapping pulses within next 25 seconds. A spectral lag between the
light-curves is determined, showing a substantial increase in the course of the burst. The
emission is detected up to 12 MeV. Modeling the time-integrated energy spectrum with the
Band function yields a moderate value of
Epeak = 340keV, while the time-resolved
spectral analysis reveals strong hardness-intensity correlation and a hard-to-soft
evolution of the emission: Epeak falls from
~ 4 MeV at the onset of the huge initial pulse to ~50 keV at the final stage of the
burst. The total 20 keV–10 MeV energy fluence amounts to
S = (7.8 ± 0.4) × 10-4erg cm-2 and a 64-ms peak
flux Fmax = (9.2 ± 0.4) × 10-4erg cm-2s-1, which corresponds to a huge isotropic-equivalent energy release Eiso = (2.1 ± 0.1) × 1054erg and
the record-breaking peak luminosity Liso;max = (4.7 ± 0.2) × 1054erg s-1.