The existence of extended emission (EE) is an intriguing property of short-duration gamma-ray bursts, because the nature of the EE is still unclear. It might be a rising X-ray afterglow, or it could ...be a manifestation of the prolonged activity of a central engine. We consider short-duration gamma-ray bursts, emphasizing the common properties of both short bursts and short bursts with EE. Assuming that the EE with broad dynamic range is a common property of short bursts, we propose a two jet model which can describe both short main episode of hard spectra emission, specific for short bursts, and softer spectra EE by different off axis position of observer. The model involves a short-duration jet, which is powered by heating due to
annihilation, and a long-lived Blandford-Znajek jet with a significantly narrow opening angle. Our proposed model is a plausible mechanism for short-duration burst energization. It can explain short bursts both with and without EE within a single class of progenitor.
We present very early ground-based optical follow-up observations of gamma-ray burst (GRB) 140423A, which was discovered by the Swift/Burst Alert Telescope (BAT) and by the Fermi/Gamma-Ray Burst ...Monitor. Its broadband afterglow was monitored by the Swift/X-Ray Telescope and ground-based optical telescopes from T0 + 70.96 s to 4.8 days after the Swift/BAT trigger. This is one more case of a prompt optical emission observation. The temporal and spectral joint fits of the multiwavelength light curves of GRB 140423A reveal that achromatic behavior is consistent with the external shock model, including a transition from a stellar wind to the interstellar medium (ISM) and energy injection. In terms of the optical light curves, there is an onset bump in the early afterglow with a rising index (peaking at s). It then decays with a steep index , and shows a steeper to flatter "transition" with at around T0 + 5000 s. The observed X-ray afterglow reflects achromatic behavior, as does the optical light curve. There is no obvious evolution of the spectral energy distribution between the X-ray and optical afterglows, with an average value of the photon index . This "transition" is consistent with an external shock model having the circumburst medium transition from a wind to the ISM, by introducing a long-lasting energy injection with a Lorentz factor stratification of the ejecta. The best parameters from Monte Carlo Markov Chain fitting are erg, , , , , cm, , , , , and .
Abstract
We obtained well-sampled optical photometry of GRB 110213A, including Swift/UVOT and XRT. Combining our data from those of other ground-based telescopes, we present 15 optical multicolor ...light curves showing similar shapes with two peaks. In contrast, in the X-ray band, only a single peak is observed between the two optical peaks. Temporal and spectral analysis of GRB 110213A shows that the X-rays differ from the optical for Phases I–III (before the second peak of the optical band at ∼5.6 ks). Moreover, they have the same spectral behavior at late times (Phases IV–VI). These data indicate that the optical and X-ray emission are dominated by different components. The synchrotron-supported pair cascade emission is included in the standard external forward-shock model, which is dominated by synchrotron radiation and synchrotron self-Compton (SSC). We find that the optical bands of GRB 110213A are dominated by the cascade emission from synchrotron radiation + SSC at the early stage, while the primary synchrotron + SSC radiation dominates the X-ray band. At late stages, both the X-ray and optical bands are dominated by emission from primary synchrotron + SSC radiation. The cascade component can reasonably explain the first optical peak. In contrast, the primary synchrotron + SSC emission mainly contributes to the second peak.
Statistical studies of BATSE gamma-ray bursts are presented: we average the time profiles, aligning the events at their highest peaks. Using the time histories in different energy channels, we ...summarize the general features of the average intensity and spectral evolution of gamma-ray bursts (GRBS) and compare the average evolution of bright and dim events. While no differences are found between the average intensity histories, the average hardness ratio histories demonstrate a hardness/brightness correlation. The observed lack of differences between the intensity histories of bright and dim bursts is incompatible with cosmological time dilation effects predicted by simple cosmological models. The combined results of no intensity history difference but a hardness history difference between bright and dim bursts places constraints on galactic models for the origin of GRBS.
The Swift Gamma-Ray Burst Explorer performed its first autonomous, X-ray follow-up to a newly detected GRB on 2005 January 17, within 193 s of the burst trigger by the Swift Burst Alert Telescope. ...While the burst was still in progress, the X-Ray Telescope (XRT) obtained a position and an image for an uncataloged X-ray source simultaneously with the gamma-ray observation. The XRT observed flux during the prompt emission was 1.1 x 10 super(-8) ergs cm super(-2) s super(-1) in the 0.5-10 keV energy band. The emission in the X-ray band decreased by 3 orders of magnitude within 700 s, following the prompt emission. This is found to be consistent with the gamma-ray decay when extrapolated into the XRT energy band. During the following 6.3 hr, the XRT observed the afterglow in an automated sequence for an additional 947 s, until the burst became fully obscured by the Earth limb. A faint, extremely slowly decaying afterglow, a = -0.21, was detected. Finally, a break in the light curve occurred and the flux decayed with a < -1.2. The X-ray position triggered many follow-up observations: no optical afterglow could be confirmed, although a candidate was identified 3" from the XRT position.
We present very early ground-based optical follow-up observations of GRB~140423A, which was discovered by \emph{Swift}/BAT and by {\it Fermi}/GBM. Its broadband afterglow was monitored by {\it ...Swift}/XRT and ground-based optical telescopes from \(T_0+\)70.96~s to 4.8~d after the {\it Swift}/BAT trigger. This is one more case of prompt optical emission observation. The temporal and spectral joint fit of the multiwavelength light curves of GRB 140423A reveals that achromatic behavior is consistent with the external shock model including a transition from a stellar wind to the interstellar medium (ISM) and energy injection. In terms of the optical light curves, there is an onset bump in the early afterglow with a rising index \(\alpha_{\rm O,I} = -0.59 \pm 0.04\) (peaking at \(t_{\rm peak}-T_0 \approx 206\)~s). It then decays with a steep index \(\alpha_{\rm O,II} = 1.78 \pm 0.03\), and shows a steeper to flatter "transition" with \(\alpha_{\rm O,III} = 1.13 \pm 0.03\) at around \(T_0 + 5000\)~s. The observed X-ray afterglow reflects an achromatic behavior, as does the optical light curve. There is no obvious evolution of the spectral energy distribution between the X-ray and optical afterglow, with an average value of the photon index \(\Gamma \approx 1.95\). This "transition" is consistent with an external shock model having the circumburst medium transition from a wind to the ISM, by introducing a long-lasting energy injection with a Lorentz factor stratification of the ejecta. The best parameters from Monte Carlo Markov Chain fitting are \(E_{\rm K,iso} \approx 2.14\times10^{55}\) erg, \(\Gamma_0 \approx 162\), \(\epsilon_e \approx 0.02\), \(\epsilon_B \approx 1.7\times10^{-6}\), \(A_\ast \approx 1.0\), \(R_t \approx 4.1\times10^{17}\) cm, \(n \approx 11.0 \rm\ cm^{-3}\), \(L_0 \approx 3.1\times10^{52} \rm\ erg\ s^{-1}\), \(k \approx 1.98\), \(s \approx 1.54\), and \(\theta_j > 0.3\) rad.
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