Analysis of the X-ray emission of nine Swift afterglows

• Published in: Monthly notices of the Royal Astronomical Society 2006-03, Vol.366 (4), p.1357-1366
• Main Authors: Panaitescu, A., Mészáros, P., Gehrels, N., Burrows, D., Nousek, J.
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; gamma-rays: bursts; ISM: jets and outflows; Mathematical models; radiation mechanisms: non-thermal; shock waves; Stars & galaxies; X-ray astronomy
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2005.09900.x

The X-ray light curves of nine Swift XRT afterglows (050126, 050128, 050219A, 050315, 050318, 050319, 050401, 050408 and 050505) display a complex behaviour: a steep t−3.0±0.3 decay until ∼400 s, followed by a significantly slower t−0.65±0.20 fall-off, which at 0.2–2 day 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 t1.0±0.5, is required for five afterglows and could be at work in the other four as well. For some afterglows, there may be other mechanisms that can explain the t > 400 s fast falling-off X-ray light curve (e.g. the large-angle gamma-ray burst emission), the 400 s to 5 h slow decay (e.g. a structured outflow), or the steepening at 0.2–2 day (e.g. a jet-break, a collimated outflow transiting from a wind with a r−3 radial density profile to a homogeneous or outward-increasing density region). 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 a universal value.