GRB 050505: A high redshift burst discovered by Swift

• Published in: arXiv.org 2006-02
• Main Authors: Hurkett, C P, Osborne, J P, Page, K L, Rol, E, Goad, M R, O'Brien, P T, Beardmore, A, Godet, O, Burrows, D N, Tanvir, N R, Levan, A, Zhang, B, Malesani, D, Hill, J E, Kennea, J A, Chapman, R, La Parola, V, Perri, M, Romano, P, Smith, R, Gehrels, N
• Format: Article
• Language: English
• Subjects: Breaking; Cooling; Decay; Extinction; Gamma ray astronomy; Gamma ray bursts; Gamma rays; Light curve; Milky Way Galaxy; Power law; Red shift; Slopes; Spectral energy distribution; X ray absorption; X ray telescopes
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.0602236

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 gamma-ray light curve, with a duration of T_90 = 63+/-2 s and an inferred isotropic release in gamma-rays of ~4.44 x 10^53 ergs in the 1-10^4 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 (+0.16/-0.17), 1.17 (+0.08/-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.