XMM-Newton and Swift observations prove GRB090709A to be a distant, standard, long GRB

Abstract

GRB090709A is a long gamma-ray burst (GRB) discovered by Swift, featuring a bright X-ray afterglow as well as a faint infrared transient with very red and peculiar colours. The burst attracted a large interest because of a possible quasi-periodicity at P = 8.1s in the prompt emission, suggesting that it could have a different origin with respect to standard, long GRBs. In order to understand the nature of this burst, we obtained a target of opportunity observation with XMM-Newton. X-ray spectroscopy, based on XMM-Newton and Swift data, allowed us to model the significant excess in photoelectric absorption with respect to the Galactic value as due to a large column density (~6.5 × 10²²cm^-2) in the GRB host, located at z ~ 4.2. Such a picture is also consistent with the infrared transient's properties. Re-analysis of the prompt emission, based on International Gamma-Ray Astrophysics Laboratory and on Swift data, excludes any significant modulation at P = 8.1s. Thus, we conclude that GRB090709A is a distant, standard, long GRB.