A temporal and spectral analysis of gamma-ray bursts observed with BATSE

The connection between Gamma-Ray Bursts (GRBs) and their X-ray, optical, and radio afterglows is a fundamental question in the GRB mystery without an answer. Afterglow models of synchrotron emission generated by external shocks in the GRB fireball model predict emission that may initially be detectable in gamma-rays ($\gtrsim$25 keV). Observations of GRB spectra that span the hard X-ray to gamma-ray regime with the BeppoSAX satellite suggest that in some cases the afterglow may overlap the burst, or may be begin at a later time (∼10 − 102 s) after the GRB.; The primary aim of this dissertation is to search for signatures of the onset of the afterglow during the GRB in the 25–2000 keV range, i.e., the “early high-energy afterglow” emission with peak frequency ν _m that initially begins in the gamma-ray phase and subsequently evolves into X-Ray, optical, and radio emission as the fireball is decelerated by the ambient medium. Our motivation for this analysis was sparked by an extraordinary burst detected by BATSE, GRB980923, in which a period of rapid variable emission lasting ∼40 s was followed by a smooth power-law emission tail lasting ∼400 s. We present a temporal and spectral analysis that reveals that the spectral evolution in the tail of GRB980923 mimics that of a cooling synchrotron spectrum, similar to the spectral evolution of the low-energy afterglows for GRBs. This evidence for a separate emission component is consistent with the internal-external shock scenario in the relativistic fireball picture. In particular, it illustrates that the external shocks can be generated during the gamma-ray emission phase, as in the case of GRB990123.; We also present a temporal and spectral analysis of a subset of GRBs observed with BATSE that exhibits smooth extended emission tails that are characteristic of the power-law decay behavior of the late-time long wavelength afterglows. We find that their temporal decays are best described with a power-law ∼ t−β, rather than an exponential, with a mean index ⟨β⟩ ≈ 2. Using spectral modeling techniques and color-color diagrams to characterize the spectral evolution, we find that ∼20% of these events are consistent with a fast-cooling synchrotron spectrum for an adiabatic blast wave, three of which are consistent with the blast wave evolution of a jet-like outflow. This behavior suggests that in some bursts the emission may orginate from a jet consisting of “nuggets” whose angular size are less than 1/Γ, where Γ is the bulk Lorentz factor of the flow.