Konus-WIND observation of the ultra-luminous GRB 110918A

¹ Ioffe Physico-Technical Institute, St. Petersburg 194021, Russia
² St. Petersburg State Technical University, St. Petersburg 195251, Russia
³ Emeritus: NASA’s Goddard Space Flight Center, Greenbelt, MD 20771, USA

Abstract

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 E_peak = 340keV, while the time-resolved spectral analysis reveals strong hardness-intensity correlation and a hard-to-soft evolution of the emission: E_peak 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 F_max = (9.2 ± 0.4) × 10^-4erg cm^-2s^-1, which corresponds to a huge isotropic-equivalent energy release E_iso = (2.1 ± 0.1) × 10⁵⁴erg and the record-breaking peak luminosity L_iso;max = (4.7 ± 0.2) × 10⁵⁴erg s^-1.