A search for lines in the bright X-ray afterglow of GRB 120711A

Abstract

GRB120711A, discovered and rapidly localized by the INTEGRAL satellite, attracted particular interest due to its high gamma-ray fluence, very bright X-ray afterglow, and the detection of a prompt optical transient and of long-lasting emission at GeV energies. A follow-up observation carried out with the XMM-Newton satellite has provided an X-ray spectrum in the 0.3-10 keV with unprecedented statistics for a GRB afterglow 20 hours after the burst. The spectrum is well fit by a power-law with photon index 1.87+-0.01, modified by absorption in our Galaxy and in the GRB host at z=1.4. A Galactic absorption consistent with that estimated from neutral hydrogen observations is obtained only with host metallicity lower than 0.05 of the Solar value. We report the results of a sensitive search for emission and absorption lines using the matched filter smoothing method (Rutledge and Sako 2003). No statistically significant lines were found. The upper limits on the equivalent width of emission lines, derived through Monte Carlo simulations, are few tens of eV, a factor about 10 lower than that of the possible lines reported in the literature for other bursts.