Abstract
GRB 050730 is a long duration high-redshift burst (z=3.967) discovered by Swift. The afterglow shows variability and is well monitored over a wide wavelength range. We present comprehensive temporal and spectral analysis of the afterglow of GRB 050730 including observations from the millimeter to X-rays. We use multi-wavelength afterglow data to understand the temporal and spectral decay properties with superimposed variability of this high redshift burst. Five telescopes were used to study the decaying afterglow of GRB 050730 in the B, V, r', R, i', I, J and K photometric pass bands. A spectral energy distribution was constructed at 2.9 hours post-burst in the K, J, I, R, V and B bands. X-ray data from the satellites Swift and XMM-Newton were used to study the afterglow evolution at higher energies. The early afterglow shows variability at early times and shows a steepening at ~0.1 days (8.6 ks) in the B, V, r', R, i', I, J and K passbands. The early afterglow light curve decayed with alpha_1 = -0.60+/-0.07 and alpha_2 = -1.71+/-0.06 based on R and I band data. A millimeter detection of the afterglow around 3 days after the burst shows an excess in comparison to predictions. The early X-ray light curve observed by Swift is complex and contains flares. At late times the X-ray light curve can be fit by a powerlaw alpha_x = -2.5+/-0.15 which is steeper than the optical light curve. A spectral energy distribution (SED) was constructed at \~2.9 hours after the burst. An electron energy index, p, of ~ 2.3 was calculated using the SED and the photon index from the X-ray afterglow spectra and indicates that the synchrotron cooling frequency nu_c is above observed frequencies.
Highlights
GRB 050730 is one of a growing number of known gamma-ray bursts (GRBs) at high redshift for which have peculiar superimposed variability from optical to X-rays (e.g. GRB050904: Böer et al 2005; Cusumano et al 2006; Watson et al 2005)
The observations presented in this figure are supplemented by photometric measurements of the afterglow published in GCN circulars (Fig. 3 and the caption)
The light-curves in Fig. 3 are presented relative to the GRB trigger time (T0 = 2005 July 30.8322 UT). It appears that the B and V light-curves show similar behaviour up to 0.1 days post-burst
Summary
GRB 050730 is one of a growing number of known gamma-ray bursts (GRBs) at high redshift for which have peculiar superimposed variability from optical to X-rays (e.g. GRB050904: Böer et al 2005; Cusumano et al 2006; Watson et al 2005). The observed variability contains a wealth of information about the nature of the burst and its afterglow (Burrows et al 2005). The very early X-ray and optical observations of GRB 050730 were possible due to the very fast slew time of Swift satellite (Gehrels et al 2004). The long-duration burst GRB 050730, was detected by the BAT instrument on Swift (trigger = 148225) at T0 = 19h58m23s UT on 30th July 2005 (Holland et al 2005). The X-ray and the optical afterglow of the burst were discovered by the onboard instruments XRT and UVOT respectively, after 132 s and 119 s after the BAT trigger (Holland et al 2005). The optical afterglow (OA) candidate was later confirmed by ground based
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