Konus-WIND observation of the ultra-luminous GRB110918A

Abstract

The exceptionally intense long GRB 110918A was discovered by several GRB observingmissions: INTEGRAL (SPI-ACS), Konus-WIND, MarsOdyssey (HEND), and MESSENGER (GRNS) on September 18, 2011.This GRB was localized by the Interplanetary Network (IPN) and its bright X-raycounterpart was found in close vicinity of the IPN box in the Swift/XRTfollow-up observations starting 1.2 days after the trigger. The optical afterglow wasdiscovered by the Isaac Newton Telescope and its spectroscopic redshift z = 0.982 wasmeasured with the GMOS spectrograph mounted on the Gemini-N telescope. GRB 110918A is thebrightest burst detected by Konus-WIND for more than 17 years of itscontinuous observations. The instrument’s light curves in three energy bands covering22–1450 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 thelight-curves is determined, showing a substantial increase in the course of the burst. Theemission is detected up to 12 MeV. Modeling the time-integrated energy spectrum with theBand function yields a moderate value ofE_peak = 340keV, while the time-resolvedspectral analysis reveals strong hardness-intensity correlation and a hard-to-softevolution 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 theburst. The total 20 keV–10 MeV energy fluence amounts toS = (7.8 ± 0.4) × 10^-4erg cm^-2 and a 64-ms peakflux 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 andthe record-breaking peak luminosity L_iso;max = (4.7 ± 0.2) × 10⁵⁴erg s^-1.