The origin of the shallow decay segment in Swift XRT light curves remains a puzzle. We analyze the properties of this segment with a sample of 53 long Swift GRBs detected before 2007 February. We show that the distributions of the sample's characteristics are lognormal or normal, and its isotropic X-ray energy (E iso;X ) is linearly correlated with the prompt gamma-ray energy but with a steeper photon spectrum, aside from some X-ray flashes. No significant spectral evolution is observed from this phase to the following phase, and the latter is usually consistent with externalshock models, implying that the shallow decay is also of external-shock origin, likely a refreshed external shock. Within the refreshed-shock model, the data are generally consistent with a roughly constant injection luminosity up to the end of this phase, t b . A positive correlation between E iso;X and t b also favors this scenario. Among the 13 bursts that have well-sampled optical light curves, six have an optical break around t b and the breaks are consistent with being achromatic. However, the other seven either do not show an optical break or have a break at an epoch different from t b . This raises a concern for the energy injection scenario, suggesting that the optical and X-ray emission may not be the same component, at least for some bursts. There are four significant outliers in the sample, GRBs 060413, 060522, 060607A, and 070110. The shallow decay phase in these bursts is immediately followed by a very steep decay after t b , which is inconsistent with any external-shock model. The optical data for these bursts evolve independently from the X-ray data. These X-ray plateaus likely have an internal origin and demand continuous operation of a long-term central engine. We conclude that the observed shallow decay phase likely has diverse physical origins.

The onset of GRB afterglow is characterized by a smooth bump in the early afterglow lightcurve caused by the deceleration of the gamma-ray burst (GRB) fireball by the circumburst medium. We make an extensive search for such a deceleration feature, either from the literature for optical lightcurves, or from the X-ray afterglow lightcurve catalog established with the Swift/XRT. Twenty optically selected GRBs and 12 X-ray selected GRBs are found to show the onset signature, among which 17 optically selected GRBs and 2 X-ray-selected GRBs have redshift measurements. We study the optical z-known sample by fitting the lightcurves with a smooth broken power-law and measure the width (w), rising timescale (t r ), and decaying timescale (t d ) at full-width-at-half-maximum (FWHM). Strong mutual correlations among these timescales and with the peak time (t p ) are found. The optical peak luminosity (L p,O ) at the lightcurve bump is anti-correlated with t p and correlated with w, indicating a dimmer and broader bump at a later peak time. The ratio t r /t d is almost universal among bursts, but the ratio t r /t p varies from 0.3 ∼ 1. The isotropic gamma-ray energy (E γ,iso ) is tightly correlated with L p,O and t p in the burst frame. Assuming that the bumps signal the deceleration of the GRB fireballs in a constant density medium, we calculate the initial Lorentz factor (Γ 0 ) and the deceleration radius (R d ) of the GRBs in the optical-selected sample. It is found that Γ 0 are typically a few hundreds, and the typical deceleration radius is R dec ∼ 10 17 cm. More intriguingly, a tight correlation between the initial Lorentz factor and the isotropic gamma-ray energy is found, namely Γ 0 ≃ 195E 0.27 γ,iso,52 (satisfied for both the optical and X-ray z-known samples). This correlation is helpful to understand GRB physics, and may serve as an indicator of Γ 0 for other long GRBs. We find that the early bright X-rays are usually dominated by a different component from the external shock emission, but occasionally (for one case) an achromatic deceleration feature is observed. Components in X-rays would contribute to the diversity of the observed X-ray lightcurves.

The absence of a supernova accompanying the nearby long GRB 060614 poses a great puzzle about the progenitor of this event and challenges the current GRB classification scheme. This burst displays a short-hard emission episode followed by extended soft emission with strong spectral evolution. Noticing that this burst has an isotropic gamma-ray energy only ∼8 times that of GRB 050724, a good candidate of merger-type short GRBs, we generate a "pseudo" burst that is ∼8 times less energetic than GRB 060614 based on the spectral properties of GRB 060614 and the E p ∝ E 1/2 iso (Amati) relation. We find that this pseudo-burst would have been detected by BATSE as a marginal short-duration GRB, and would have properties in the Swift BAT and XRT bands similar to GRB 050724. This suggests that GRB 060614 is likely a more intense event in the traditional shorthard GRB category as would be detected by BATSE. Events like GRB 060614 that seem to defy the traditional short vs. long classification of GRBs may require modification of our classification terminology for GRBs. By analogy with supernova classifications, we suggest that GRBs be classified into Type I (typically short and associated with old populations) and Type II (typically long and associated with young populations). We propose that GRB 060614 belongs to Type I, and predict that similar events will be detected in elliptical galaxies.