GRB 050505: a high-redshift burst discovered by Swift

We report the discovery and subsequent multi-wavelength 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 γ-ray light curve, with a duration of T90 = 63±2 s and an inferred isotropic release in γ-rays of ∼ 4.44×10 ergs in the 1−10 keV rest frame energy range. The Swift X-Ray Telescope followed the afterglow for 14 days, detecting two breaks in the light curve at 7.4 −1.5 ks and 58.0 +9.9 −15.4 ks after the burst trigger. The power law decay slopes before, between and after these breaks were 0.25 −0.17, 1.17 −0.09 and 1.97 +0.27 −0.28 respectively. The light curve can also be fit 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 fit with a single power law of photon index ∼ 1.90. This behaviour is expected for the cessation of continued energisation of the ISM 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 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/UV extinction, with E(B − V ) ≈ 0.10 for a SMC-like extinction curve.