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Various Optical Emission Components of Gamma-Ray Bursts

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Tutor: LiangEnWei
School: Guangxi University
Course: Theoretical Physics
Keywords: Gamma-ray Bursts,Optical Afterglow,FireballModel,Central Engine
CLC: P172.3
Type: Master's thesis
Year:  2012
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Abstract:
We summarize the observations of gamma-ray bursts and their optical afterglows and present our analysis results on various optical emission components of this phenomenon.By extensively searching from literature, we got well-sampled optical lightcurves of146gamma-ray bursts (GRBs) and fitted these lightcurves with the superposition of multiple broken power law functions to identify various emission components that may have distinct physical origins. We summarize the results in a "synthetic" optical lightcurve. Based on our statistical analysis results, we found that optical flares and an early optical shallow-decay component are likely related to a long-term central engine activity. Twenty-four optical flares are obtained from19GRBs. The flares peak at from tens of seconds to several days post the GRB trigger, and later flares tend to be wider and dimmer, following a power-law relation with an index of-1.15¡À0.15. The fraction of GRBs with detected optical flares is much smaller than that of X-ray flares. Associated X-ray flares are observed for4optical flares, and the optical flares usually lag behind the corresponding X-ray flares. An optical shallow decay segment is observed in39GRBs. Their break times range from tens of seconds to several days post the GRB trigger, with a typical value of104seconds. The break luminosity is anti-correlated to the break time with a power-law index of-0.78, similar to that derived from X-ray flares. The detection fraction of the optical shallow decay component is comparable to that in the X-ray band. The X-ray and optical breaks are usually chromatic, but a tentative correlation is found. We suggest that the optical flares are also related to the erratic behavior of the central engine and the shallow decay component, on the other hand, is likely due to energy injection into the blastwave, possibly related to a long-lasting spinning-down central engine or piling up of flare materials onto the blastwave.Clear afterglow onset humps, which may be due to the decelerateion of the GRB fireball by the embient medium, are observed in45GRBs in our sample. An onset hump peaks ayt later tends to be wider and dimmer, and the peak luminosity is tight correlated with the isotropic prompt gamma-ray energy. The decay slopes after the peak time of the humps are well consistent with the standard fireball, but the slopes of the rising phase are inconsistent with the expectation in both inter stelar medium (ISM) and wind medium. We infer the density profile surrounding GRBs from the rising slopes and find that nocr"k, where k=0.5-1.5. We then estimate the initial Lorentz factors of the GRB fireball based with these density profiles and re-visit the correlation between the isotropic gamma-ray energy the Lorentz factor.Late optical rebrightening is observed for30GRBs in our sample. The distributions of the rising and decaying slopes of the rebrightening bumps are well consistent with that of the onset bumps. A re-brightening hump peaks at later also tends to be wider and dimmer, but the peak luminosity is not correlated with the isotropic prompt gamma-ray energy. These results might suggest that the late rebrightening could be contributed by another jet component. Associated X-ray re-brightening bump is also detected in a few GRBs, such as100901A,060906, and080913. This indicates that the emission in the two energy bands should be from the same jet component.
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