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 $t^{-\delta}$, with $\delta=1.41\pm0.10$. Its spectrum is well described by a power law (photon index $\Gamma=2.3\pm0.1$) 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 $K_{\rm s} = 19.44 \pm 0.13$ (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 $(0.4{-}2.6)\times10^{22}$ cm -2 in the GRB host galaxy, likely located at a redshift $0.5