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

¹ UCD School of Physics, University College Dublin, Dublin 4, Ireland
² Department of Physics, Royal Institute of Technology (KTH), AlbaNova University Center, 10691 Stockholm, Sweden e-mail: smcglynn@particle.kth.se
³ Max-Planck-Institut für extraterrestrische Physik, 85741 Garching, Germany

Received: 20 December 2007
Accepted: 9 May 2008

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 , which is consistent with the characteristics of short-hard γ-ray bursts. The peak flux over 1 s was  photons cm^-2 s^-1 and the fluence over the same interval was  erg cm^-2 in the energy range 20–200 keV. The spectral lag measured between 25–50 keV and 100–300 keV is  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 . 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.