GRB 070707: the first short gamma-ray burst observed by INTEGRAL

Abstract

Context: INTEGRAL has observed 47 long-duration GRBs (T₉₀ ≳ 2 s) and 1 short-duration GRB (T₉₀ ≲ 2 s) in five years of observation since October 2002.
Aims: This work presents the properties of the prompt emission of GRB 070707, which is the first short hard GRB observed by INTEGRAL.
Methods: The spectral and temporal properties of GRB 070707 were determined using the two sensitive coded-mask γ-ray instruments on board INTEGRAL, IBIS and SPI.
Results: The T₉₀ duration was 0.8 s, and the spectrum of the prompt emission was obtained by joint deconvolution of IBIS and SPI data to yield a best fit power-law with photon index α = -1.19 ^+0.14_-0.13, which is consistent with the characteristics of short-hard γ-ray bursts. The peak flux over 1 s was 1.79 ^+0.06_-0.21 photons cm^-2 s^-1 and the fluence over the same interval was (2.07 ^+0.06_-0.32) × 10^-7 erg cm^-2 in the energy range 20-200 keV. The spectral lag measured between 25-50 keV and 100-300 keV is 20 ±5 ms, consistent with the small or negligible lags measured for short bursts.
Conclusions: The spectral and temporal properties of GRB 070707 are comparable to those of the short hard bursts detected by other γ-ray satellites, including BATSE and Swift. We estimate a lower limit on the Lorentz factor Γ ≳ 25 for GRB 070707, assuming the typical redshift for short GRBs of z = 0.35. This limit is consistent with previous estimates for short GRBs and is smaller than the lower limits of Γ ≳ 100 calculated for long GRBs. If GRB 070707 is a member of the recently postulated class of short GRBs at z ~ 1, the lower limit on Γ increases to Γ ≳ 35.

Based on observations with INTEGRAL, an ESA project with instruments and science data centre funded by ESA member states (especially the PI countries: Denmark, France, Germany, Italy, Switzerland, Spain), Czech Republic and Poland, and with the participation of Russia and the USA.