GRB 050505: a high-redshift burst discovered by Swift

• Published in: Monthly notices of the Royal Astronomical Society 2006-05, Vol.368 (3), p.1101-1109
• 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., Parola, V. La, Perri, M., Romano, P., Smith, R., Gehrels, N.
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; galaxies: high-redshift; galaxies: ISM; Gamma rays; gamma-rays: bursts; gamma-rays: observations; Red shift; Starbursts; X-ray astronomy
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2006.10188.x

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 γ-ray light curve, with a duration of T90= 63 ± 2 s and an inferred isotropic release in γ-rays of ∼ 4.44 × 1053 erg in the 1–104 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 +1.5−1.5 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 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.