On the nature of X-ray flashes

¹ Laboratoire d'Astrophysique de Toulouse-Tarbes, OMP, 31400 Toulouse, France
² Institut d'Astrophysique de Paris, 98 bis boulevard Arago, 75014 Paris, France e-mail: mochko@iap.fr
³ Université Pierre et Marie Curie – Paris VI, 4 Place Jussieu, 75005 Paris, France

Received: 1 July 2004
Accepted: 8 April 2005

Abstract

We discuss the origin of X-Ray Flashes (XRFs), a recently discovered class of Gamma-Ray Bursts (GRBs). Using a simplified model for internal shocks we check if XRFs can be intrinsically soft due to some specific values of the parameters describing the relativistic outflow emerging from the central engine. We generate a large number of synthetic events and find that XRFs are obtained when the contrast  of the Lorentz factor distribution is small while the average Lorentz factor  is large. A few XRFs may be GRBs at large redshifts but we exclude this possibility for the bulk of the population. If outflows with a small contrast are commonly produced, even a large population of XRFs could be explained. If conversely the Lorentz factor distribution within the wind is broad, one should then rely on extrinsic causes, such as viewing angle effects or high redshift.