Abstract
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.
Highlights
Gamma Ray Bursts (GRBs) are expected to be visible over a large range of redshifts with a potential upper limit of z ∼ 15 − 20 (Lamb & Reichart 2000)
We report the discovery and subsequent multi-wavelength afterglow behaviour of the high redshift (z = 4.27) Gamma Ray Burst GRB 050505
Two breaks were detected in the X-ray light curve, the first of which we consider to be due to the cessation of continued energisation of the ISM shock and the second is a jet break, caused by either a structured or uniform jet
Summary
Gamma Ray Bursts (GRBs) are expected to be visible over a large range of redshifts with a potential upper limit of z ∼ 15 − 20 (Lamb & Reichart 2000). The spectra were fit with a power law model (see Fig 4) with the absorption, NH, set at the Galactic column density (2.1 × 1020 cm−2, Dickey & Lockman 1990), and with power law models with excess absorption (either in our Galaxy or the GRB host galaxy) During our analysis both Wisconsin and Tubingen-Boulder ISM absorption models (Arnaud & Dorman 2003) were used; there was no significant difference in either the statistical quality of the fit or in the resulting derived parameters between the two. The spectrum is well modelled up to such high energies in the rest frame of the GRB, and the photon index is comparable to the values found from low redshift bursts. The sparse nature of this combined data-set naturally limits the knowledge that can be obtained
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