INTEGRAL and XMM-Newton observations of GRB040106

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 similar to 52 s with two distinct pulses separated by similar to 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 gamma-ray spectrum is consistent with a single power-law of photon index - 1.72 +/- 0.15. The fluence ( 20 - 200 keV) was 8.2 x 10(-7) erg cm(-2). The X-ray afterglow (F-v( t) proportional to gamma(-betaX t-delta)) was extremely hard with beta(X) = 0.47 +/- 0.01 and delta = 1.46 +/- 0.04. The 2 - 10 keV flux 11 h after the burst was 1.1 x 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 similar to0.9(-0.4)(+0.5) ( 1sigma) and a lower limit on the isotropic radiated energy of similar to5 x 10(51) erg in this burst.