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

¹ INAF – Istituto di Astrofisica spaziale e Fisica Cosmica, sez. di Milano “G. Occhialini”, via Bassini 15, 20133 Milano, Italy e-mail: deluca@mi.iasf.cnr.it
² INAF – Osservatorio Astronomico di Roma, via di Frascati 33, 00040 Monteporzio Catone (Rome), Italy
³ INAF – Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate (LC), Italy
⁴ Università degli Studi dell'Insubria, Dipartimento di Fisica e Matematica, via Valleggio 11, 22100 Como, Italy
⁵ Observatori Astrònomic, Universitat de València, 46100 València, Spain
⁶ International School for Advanced Studies (SISSA-ISAS), via Beirut 2-4, 34014 Trieste, Italy

Received: 15 February 2005
Accepted: 8 May 2005

Abstract

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 , with . Its spectrum is well described by a power law (photon index ) 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 (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 cm^-2 in the GRB host galaxy, likely located at a redshift . GRB 040827 stands out as the best example of an X-ray afterglow with intrinsic absorption.