Article

# Early Afterglows in Wind Environments Revisited

Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.52). 08/2005; DOI:10.1111/j.1365-2966.2005.09411.x
Source: arXiv

ABSTRACT When a cold shell sweeps up the ambient medium, a forward shock and a reverse shock will form. We analyze the reverse-forward shocks in a wind environment, including their dynamics and emission. An early afterglow is emitted from the shocked shell, e.g., an optical flash may emerge. The reverse shock behaves differently in two approximations: relativistic and Newtonian cases, which depend on the parameters, e.g., the initial Lorentz factor of the ejecta. If the initial Lorentz factor is much less than $114 E_{53}^{1/4} \Delta_{0,12}^{-1/4} A_{*,-1}^{-1/4}$, the early reverse shock is Newtonian. This may take place for the wider of a two-component jet, an orphan afterglow caused by a low initial Lorentz factor, and so on. The synchrotron self absorption effect is significant especially for the Newtonian reverse shock case, since the absorption frequency $\nu_a$ is larger than the cooling frequency $\nu_c$ and the minimum synchrotron frequency $\nu_m$ for typical parameters. For the optical to X-ray band, the flux is nearly unchanged with time during the early period, which may be a diagnostic for the low initial Lorentz factor of the ejecta in a wind environment. We also investigate the early light curves with different wind densities, and compare them with these in the ISM model. Comment: 14 pages, 7 figures, 2 tables, accepted for publication in MNRAS

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##### Article: Lorentz Factor Constraint from the very early external shock of the gamma-ray burst ejecta
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ABSTRACT: While it is generally agreed that the emitting regions in Gamma-Ray Bursts (GRBs) move ultra relativistically towards the observer, different estimates of the initial Lorentz factors, $\Gamma_0$, lead to different, at times conflicting estimates. We show here that the quiet periods in which the signals goes down below the instrumental thresholds, put strong upper limits on the values of $\Gamma_0$. According to the standard internal-external shocks model an external shock should develop during the prompt stage. This external shock radiates in the hard X-rays to soft gamma-rays bands and this emission should be seen as a smooth background signal. The observed deep minima indicate that this contribution is negligible. This limits, in turn, $\Gamma_0$. We obtain upper limits on $\Gamma_0$ for several bursts with typical values around hundreds. We compare these values with those obtained by the other methods, which typically yield lower limits. The results are marginally consistent leaving only a narrow range of allowed values for $\Gamma_0$. Comment: 9 pages, 2 figures. Submitted to MNRAS
Monthly Notices of the Royal Astronomical Society 08/2009; · 5.52 Impact Factor
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##### Article: The initial Lorentz factors of fireballs inferred from the early X-ray data of SWIFT GRBs
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ABSTRACT: We intend to determine the type of circumburst medium and measure directly the initial Lorentz factor $\Gamma_0$ of GRB outflows. If the early X-ray afterglow lightcurve has a peak and the whole profile across the peak is consistent with the standard external shock model, the early rise profile of light curves can be used to differentiate whether the burst was born in interstellar medium (ISM) or in stellar wind. In the thin shell case, related to a sub-relativistic reverse shock, the peak time occurring after the end of the prompt emission, can be used to derive an accurate $\Gamma_0$, especially for the ISM case. The afterglow lightcurves for a flat electron spectrum $1<p<2$ have been derived analytically. In our GRB sample, we obtain $\Gamma_0 \sim 300$ for the bursts born in ISM. We did not find any good case for bursts born in stellar wind and behaving as a thin shell that can be used to constrain $\Gamma_0$ reliably.
Astronomy and Astrophysics 03/2009; · 5.08 Impact Factor
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##### Article: The possible high‐energy emission from GRB 080319B and origins of the GeV emission of GRBs 080514B, 080916C and 081024B
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ABSTRACT: We calculate the high-energy (sub-GeV to TeV) prompt and afterglow emission of GRB 080319B that was distinguished by a naked-eye optical flash and by an unusual strong early X-ray afterglow. There are three possible sources for high-energy emission: the prompt optical and -ray photons IC scattered by the accelerated electrons, the prompt photons IC scattered by the early external reverse-forward shock electrons, and the higher band of the synchrotron and the synchrotron self-Compton emission of the external shock. There should have been in total hundreds of high-energy photons detectable for the Large Area Telescope onboard the Fermi satellite, and tens of photons of those with energy >10 GeV. The >10 GeV emission had a duration about twice that of the soft -rays. Astro-rivelatore Gamma a Immagini Leggero (AGILE) could have observed these energetic signals if it was not occulted by the Earth at that moment. The physical origins of the high-energy emission detected in GRB 080514B, GRB 080916C and GRB 081024B are also discussed. These observations seem to be consistent with the current high-energy emission models.
Monthly Notices of the Royal Astronomical Society 06/2009; 396(2):1163 - 1170. · 5.52 Impact Factor

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