Direct and bulk-scattered forward-shock emissions: sources of X-ray afterglow diversity

I describe the modifications to the standard forward-shock model required to account for the X-ray light-curve features discovered by Swift in the early afterglow emission and propose that a delayed, pair-enriched, and highly relativistic outflow, which bulk-scatters the forward-shock synchrotron emission, yields sometimes a brighter X-ray emission, producing short-lived X-ray flares, X-ray light-curve plateaus ending with chromatic breaks, and fast post-plateau X-ray decays. The hundreds of X-ray afterglow light-curves measured by Swift/XRT during the last years display three phases of power-law decay Fx ∝ t−αx: a plateau (slow-decay), a normal decay, and a steep falloff (e.g. [1]). Only rarely, the 0.3-10 keV flux exhibits an extremely rapid drop by 1-2 decades at the end of the plateau. Normal decay. During the second phase, the X-ray decay index αx ∈ (0.75,1.5) is similar to that measured for ∼ 40 pre-Swift optical light-curves [2], a majority (70-90%) of X-ray decay indices being compatible [3] with the expectations of the standard FORWARD-SHOCK model (e.g. [4]), for the measured X-ray spectral slope βx (with Fν ∝ ν−βx). In this model, the afterglow is identified with the synchrotron emission from ambient electrons accelerated to energies γmec ∼ GeV energies (comoving frame) at the ultrarelativistic shock driven by the GRB ejecta into the circumburst medium. In its standard form, the forward-shock model assumes that (1) no energy is added to the blastwave, (2) its kinetic energy is uniformly distributed with angle (dEk/dΩ = 0), and (3) electrons and magnetic fields acquire fractions (εe and εB) of the post-shock energy that are constant. With these assumptions, the power-law deceleration of the blast-wave (Lorentz factor Γ ∝ t−g, where g depends on the ambient medium radial stratification) and the power-law distribution of shock-accelerated particles (dN/dγ ∝ γ−p) lead to a power-law decay of the synchrotron flux at photon energies above the peak of the synchrotron spectrum, with αx = 1.5βx +c. Compatibility of the αx and βx measured for Swift X-ray afterglows and the standard forward-shock model predictions is obtained if the X-ray domain is, for some afterglows, below the cooling frequency of the synchrotron spectrum (see [5] for definition) and above it for others. Steep fall-off. For this phase, αx ∈ (1.75,2.75), which is compatible with the post-break decay indices measured for ∼ 15 pre-Swift optical light-curves [2]. Such a light-curve break was predicted to arise from the finite angular extent of the forward-shock [6] and was identified in about 3/4 of pre-Swift optical afterglows well-monitored until after a few days. There has been a suggestion [7] that Swift X-ray afterglows do not display "jet-breaks" as often as pre-Swift optical light-curves. In a sample of about 100 Swift X-ray afterglows with good temporal coverage, I find roughly equal numbers of light-curves with good evidence for a jet-break and without a break until at least a few days, indicating that the fraction of X-ray light-curves with jet-breaks is around 1/2 [8]. The