TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3222 SUBJECT: GRB 050408: Swift XRT measurement of possible jet break DATE: 05/04/10 20:00:03 GMT FROM: David Burrows at PSU/Swift O. Godet, K. L. Page, M. R. Goad, J. P. Osborne (U. Leicester), M. Capalbi, P. Giommi (ASDC), D. Grupe, D. N. Burrows (PSU), S. Campana, G. Chincarini (INAF-OAB), V. La Parola, T. Mineo (INAF-IASF/Palermo), S. Barthelmy, L. Angelini, N. Gehrels (GSFC), and P. Meszaros (PSU) report on behalf of the Swift XRT team: The Swift XRT instrument observed GRB 050408 as a Target of Opportunity for 20 ks (exposure time) on 8/9 April 2005, beginning 40 minutes after the HETE trigger and lasting for 60 ks of real time . Analysis of the data shows evidence for a possible jet break at about 24 ks post-burst, with the following parameters: initial slope: -0.71 +0.10/-0.12 Break time : 24 +25/-12 ks later slope -1.43 +0.72/-0.40 Both single power-law and broken power-law fits to the lightcurve are statistically acceptable at this point. Further observations planned for April 11 will give more information to help distinguish between the two and determine the jet break time.