[show abstract][hide abstract] ABSTRACT: We present a comprehensive analysis of a bright, long duration (T90 ~ 257 s)
GRB 110205A at redshift z= 2.22. The optical prompt emission was detected by
Swift/UVOT, ROTSE-IIIb and BOOTES telescopes when the GRB was still radiating
in the gamma-ray band. Nearly 200 s of observations were obtained
simultaneously from optical, X-ray to gamma-ray, which makes it one of the
exceptional cases to study the broadband spectral energy distribution across 6
orders of magnitude in energy during the prompt emission phase. By fitting the
time resolved prompt spectra, we clearly identify, for the first time, an
interesting two-break energy spectrum, roughly consistent with the standard GRB
synchrotron emission model in the fast cooling regime. Although the prompt
optical emission is brighter than the extrapolation of the best fit X/gamma-ray
spectra, it traces the gamma-ray light curve shape, suggesting a relation to
the prompt high energy emission. The synchrotron + SSC scenario is disfavored
by the data, but the models invoking a pair of internal shocks or having two
emission regions can interpret the data well. Shortly after prompt emission (~
1100 s), a bright (R = 14.0) optical emission hump with very steep rise (alpha
~ 5.5) was observed which we interpret as the emission from the reverse shock.
It is the first time that the rising phase of a reverse shock component has
been closely observed. The full optical and X-ray afterglow lightcurves can be
interpreted within the standard reverse shock (RS) + forward shock (FS) model.
In general, the high quality prompt emission and afterglow data allow us to
apply the standard fireball shock model to extract valuable information about
the GRB including the radiation mechanism, radius of prompt emission R, initial
Lorentz factor of the outflow, the composition of the ejecta, as well as the
collimation angle and the total energy budget.
The Astrophysical Journal 11/2011; · 6.73 Impact Factor
[show abstract][hide abstract] ABSTRACT: The bright gamma-ray burst GRB 050525a has been detected with the Swift observatory, providing unique multiwavelength coverage from the very earliest phases of the burst. The X-ray and optical/UV afterglow decay light curves both exhibit a steeper slope 0.15 days after the burst, indicative of a jet break. This jet break time combined with the total gamma-ray energy of the burst constrains the opening angle of the jet to be 3 2. We derive an empirical “time-lag” redshift from the BAT data of , in good agreement with the spectroscopic redshift of 0.61. Prior to the jet break, the X-ray data can be modeled by a simple power law with index . However, after 300 s the X-ray flux brightens by about 30% compared to the power-law fit. The optical/UV data have a more complex decay, with evidence of a rapidly falling reverse shock component that dominates in the first minute or so, giving way to a flatter forward shock component at later times. The multiwavelength X-ray/UV/optical spectrum of the afterglow shows evidence for migration of the electron cooling frequency through the optical range within 25,000 s. The measured temporal decay and spectral indexes in the X-ray and optical/UV regimes compare favorably with the standard fireball model for gamma-ray bursts assuming expansion into a constant-density interstellar medium.
The Astrophysical Journal 02/2006; · 6.73 Impact Factor