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.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- August 2008
- DOI:
- 10.1051/0004-6361:20079295
- arXiv:
- arXiv:0805.2880
- Bibcode:
- 2008A&A...486..405M
- Keywords:
-
- gamma rays: bursts;
- gamma rays: observations;
- Astrophysics
- E-Print:
- 7 pages, 3 figures, accepted for publication in A&