GRB 050505 a high-redshift burst discovered by Swift

We report the discovery and subsequent multiwavelength afterglow behaviour of the high-redshift (z= 4.27) Gamma Ray Burst (GRB) 050505. This burst is the third most-distant burst, measured by spectroscopic redshift, discovered after GRB 000131 (z= 4.50) and GRB 050904 (z= 6.29). GRB 050505 is a long GRB with a multipeaked gamma-ray light curve, with a duration of T<SUB>90</SUB>= 63 +/- 2s and an inferred isotropic release in gamma-rays of ~ 4.44 10<SUP>53</SUP>erg in the 1-10<SUP>4</SUP> keV rest-frame energy range. The Swift X-Ray Telescope followed the afterglow for 14 d, detecting two breaks in the light curve at 7.4 <SUP>+1.5</SUP><SUB>-1.5</SUB> and 58.0 <SUP>+9.9</SUP><SUB>-15.4</SUB> ks after the burst trigger. The power-law decay slopes before, between and after these breaks were 0.25<SUP>+0.16</SUP><SUB>-0.17</SUB>, 1.17<SUP>+0.08</SUP><SUB>-0.09</SUB> and 1.97<SUP>+0.27</SUP><SUB>-0.28</SUB>, respectively. The light curve can also be fitted with a `smoothly broken' power-law model with a break observed at ~T+ 18.5 ks, with decay slopes of ~0.4 and ~1.8, before and after the break, respectively. The X-ray afterglow shows no spectral variation over the course of the Swift observations, being well fitted with a single power law of photon index ~1.90. This behaviour is expected for the cessation of the continued energization of the interstellar medium shock, followed by a break caused by a jet, either uniform or structured. Neither break is consistent with a cooling break. The spectral energy distribution, indeed, shows the cooling frequency to be below the X-ray but above the optical frequencies. The optical-X-ray spectrum also shows that there is significant X-ray absorption in excess of that due to our Galaxy but very little optical-ultraviolet extinction, with E(B-V) ~ 0.10 for a Small Magellanic Cloud like extinction curve.