Analysis of the X-ray Emission of Nine Swift Afterglows

Abstract

The X-ray light-curves of 9 Swift XRT afterglows (050126, 050128, 050219A, 050315, 050318, 050319, 050401, 050408, 050505) display a complex behaviour: a steep t^{-3.0 \pm 0.3} decay until ~400 s, followed by a significantly slower t^{-0.65+/-0.20} fall-off, which at 0.2--2 d after the burst evolves into a t^{-1.7+/-0.5} decay. We consider three possible models for the geometry of relativistic blast-waves (spherical outflows, non-spreading jets, and spreading jets), two possible dynamical regimes for the forward shock (adiabatic and fully radiative), and we take into account a possible angular structure of the outflow and delayed energy injection in the blast-wave, to identify the models which reconcile the X-ray light-curve decay with the slope of the X-ray continuum for each of the above three afterglow phases. By piecing together the various models for each phase in a way that makes physical sense, we identify possible models for the entire X-ray afterglow. The major conclusion of this work is that a long-lived episode of energy injection in the blast-wave, during which the shock energy increases at t^{1.0+/-0.5}, is required for five afterglows and could be at work in the other four as well. Optical observations in conjunction with the X-ray can distinguish among these various models. Our simple tests allow the determination of the location of the cooling frequency relative to the X-ray domain and, thus, of the index of the electron power-law distribution with energy in the blast-wave. The resulting indices are clearly inconsistent with an universal value.