XMM-Newton and VLT observations of the afterglow of GRB 040827

The field of the Gamma-Ray Burst GRB 040827 was observed with XMM-Newton and with the ESO/VLT starting 6 and 12 h after the burst, respectively. A fading X-ray afterglow is clearly detected with the XMM-Newton/EPIC instrument, with a time decay t^-δ, with δ=1.41±0.10. Its spectrum is well described by a power law (photon index Γ=2.3±0.1) affected by an absorption largely exceeding (by a factor 5) the expected Galactic one, requiring the contribution of an intrinsic, redshifted absorber. In the optical/NIR range, the afterglow emission was observed in the K_s band as a weak source superimposed on the host galaxy, with magnitude K_s = 19.44 ± 0.13 (12 h after the GRB, contribution from the host subtracted); in other bands the flux is dominated by the host galaxy. Coupling constraints derived from X-ray spectral fitting and from photometry of the host, we estimated a gas column density in the range (0.4{-}2.6)×10²² cm^-2 in the GRB host galaxy, likely located at a redshift 0.5<z<1.7. GRB 040827 stands out as the best example of an X-ray afterglow with intrinsic absorption.