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Register a new userThe Very Early Gamma-ray Burst Afterglows Powered by Structured Jets
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If X-ray flashes (XRFs) and X-ray rich Gamma-ray Bursts(XRRGs) have the same origin with Gamma-ray Bursts (GRBs) but are viewed from larger angles of structured jets, their early afterglows may differ from those of GRBs. When the ultra-relativistic outflow interact with the surrounding medium, there are two shocks formed, one is a forward shock, the other is a reverse shock. In this paper we calculate numerically the early afterglow powered by uniform jet, Gaussian jet and power-law jet in the forward-reverse shock scenario. A set of differential equations are used to govern the dynamical evolution and synchrotron self-Compton effect has been taken into account to calculate the emission. In uniform jets, the very early afterglows of XRRGs and XRFs are significantly lower than GRBs and the observed peak times of RS emission are longer in interstellar medium environment. The RS components in XRRGs and XRFs are difficult to be detected. But in stellar wind, the reduce of very early flux and the delay of RS peak time are not so remarkable. In nonuniform jet(Gaussian jet and power-law jet), where there are emission materials on the line of sight, the very early light curve resembles isotropic-equivalent ejecta in general although the RS flux decay index shows notable deviation if the RS is relativistic(in stellar wind).
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The structure of Gamma-Ray Burst (GRB) jets impacts on their prompt and afterglow emission properties. Insights into the still unknown structure of GRBs can be achieved by studying how different structures impact on the luminosity function (LF): i) we show that low ($10^{46} < L_{rm iso} < 10^{48}$ erg/s) and high (i.e. with $L_{rm iso} > 10^{50}$ erg/s) luminosity GRBs can be described by a unique LF; ii) we find that a uniform jet (seen on- and off-axis) as well as a very steep structured jet (i.e. $epsilon(theta) propto theta^{-s}$ with $s > 4$) can reproduce the current LF data; iii) taking into account the emission from the whole jet (i.e. including contributions from mildly relativistic, off-axis jet elements) we find that $E_{rm iso}(theta_{rm v})$ (we dub this quantity apparent structure) can be very different from the intrinsic structure $epsilon(theta)$: in particular, a jet with a Gaussian intrinsic structure has an apparent structure which is more similar to a power law. This opens a new viewpoint on the quasi-universal structured jet hypothesis.
We present a multiwavelength analysis of 63 Gamma-Ray Bursts observed with the worlds three largest robotic optical telescopes, the Liverpool and Faulkes Telescopes (North and South). Optical emission was detected for 24 GRBs with brightnesses ranging from R = 10 to 22 mag in the first 10 minutes after the burst. By comparing optical and X-ray light curves from t = 100 to about 10^6 seconds, we introduce four main classes, defined by the presence or absence of temporal breaks at optical and/or X-ray wavelengths. While 15/24 GRBs can be modelled with the forward-shock model, explanation of the remaining nine is very challenging in the standard framework even with the introduction of energy injection or an ambient density gradient. Early X-ray afterglows, even segments of light curves described by a power-law, may be due to additional emission from the central engine. 39 GRBs in our sample were not detected and have deep upper limits (R < 22 mag) at early time. Of these, only ten were identified by other facilities, primarily at near infrared wavelengths, resulting in a dark burst fraction of about 50%. Additional emission in the early time X-ray afterglow due to late-time central engine activity may also explain some dark bursts by making the bursts brighter than expected in the X-ray band compared to the optical band.
We show that the jet structure of gamma-ray bursts (GRBs) can be investigated with the tail emission of the prompt GRB. The tail emission which we consider is identified as a steep-decay component of the early X-ray afterglow observed by the X-ray Telescope onboard Swift. Using a Monte Carlo method, we derive, for the first time, the distribution of the decay index of the GRB tail emission for various jet models. The new definitions of the zero of time and the time interval of a fitting region are proposed. These definitions for fitting the light curve lead us an unique definition of the decay index, which is useful to investigate the structure of the GRB jet. We find that if the GRB jet has a core-envelope structure, the predicted distribution of the decay index of the tail has a wide scatter and has multiple peaks, which cannot be seen for the case of the uniform and the Gaussian jet. Therefore, the decay index distribution tells us the information on the jet structure. Especially, if we observe events whose decay index is less than about 2, both the uniform and the Gaussian jet models will be disfavored according to our simulation study.
The discovery of multiband afterglows definitely shows that most $gamma$-ray bursts are of cosmological origin. $gamma$-ray bursts are found to be one of the most violent explosive phenomena in the Universe, in which astonishing ultra-relativistic motions are involved. In this article, the multiband observational characteristics of $gamma$-ray bursts and their afterglows are briefly reviewed. The standard model of $gamma$-ray bursts, i.e. the fireball model, is described. Emphasis is then put on the importance of the nonrelativistic phase of afterglows. The concept of deep Newtonian phase is elaborated. A generic dynamical model that is applicable in both the relativistic and nonrelativistic phases is introduced. Based on these elaborations, the overall afterglow behaviors, from the very early stages to the very late stages, can be conveniently calculated.
We study thermal emission from circumstellar structures heated by gamma-ray burst (GRB) radiation and ejecta and calculate its contribution to GRB optical and X-ray afterglows using the modified radiation hydro-code small STELLA. It is shown that thermal emission originating in heated dense shells around the GRB progenitor star can reproduce X-ray plateaus (like observed in GRB 050904, 070110) as well as deviations from a power law fading observed in optical afterglows of some GRBs (e.g. 020124, 030328, 030429X, 050904). Thermal radiation pressure in the heated circumburst shell dominates the gas pressure, producing rapid expansion of matter similar to supenova-like explosions close to opacity or radiation flux density jumps in the circumburst medium. This phenomenon can be responsible for so-called supernova bumps in optical afterglows of several GRBs. Such a `quasi-supernova suggests interpretation of the GRB-SN connection which does not directly involve the explosion of the GRB progenitor star.
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