INTEGRAL and XMM-Newton observations of GRB 040106

Abstract

On January 6th 2004, the IBAS burst alert system triggered the 8th gamma-ray burst (GRB) to be located by the INTEGRAL satellite. The position was determined and publicly distributed within 12 s, prompting ESA's XMM-Newton to execute a ToO observation just 5 h later, during which an X-ray afterglow was detected. The GRB had a duration ∼52 s with two distinct pulses separated by ∼42 s. Here we present the results of imaging and spectral analyses of the prompt emission from INTEGRAL data and the X-ray afterglow from XMM-Newton data. The γ-ray spectrum is consistent with a single power-law of photon index −1.72 ± 0.15. The fluence (20-200 keV) was 8.2 × 10 −7 erg cm −2 . The X-ray afterglow (Fν(t) ∝ ν −βX t −δ )w as extremely hard with βX = 0.47 ± 0.01 and δ = 1.46 ± 0.04. The 2-10 keV flux 11 h after the burst was 1.1 × 10 −12 erg cm −2 s −1 . The time profile of the GRB is consistent with the observed trends from previous analysis of BATSE GRBs. We find that the X-ray data are not well-fit by either a simple spherical fireball or by a speading jet, expanding into a homogeneous medium or a wind environment. Based on previously determined correlations between GRB spectra and redshift, we estimate a redshift of ∼0.9 +0.5 −0.4 (1σ) and a lower limit on the isotropic radiated energy of ∼5 × 10 51 erg in this burst.