The spectral-temporal properties of the prompt pulses and rapid decay phase of GRBs

Abstract

The prompt emission from GRBs is the brightest electromagnetic emission known yet it's origin is not understood. The flux density of individual prompt pulses of a GRB can be represented by an analytical expression derived assuming the emission is from a thin, ultra-relativistically expanding, uniform, spherical shell over a finite range of radii. We present the results of fitting this analytical expression to the lightcurves from the four standard Swift BAT energy bands and two standard Swift XRT energy bands of 12 bursts. The expression includes the High Latitude Emission (HLE) component and the fits provide a rigourous demonstration that the HLE can explain the Rapid Decay Phase (RDP) of the prompt emission. The model also accommodates some aspects of energy-dependent lag and energy-dependent pulse width, but there are features in the data which are not well represented. Some pulses have a hard, narrow peak which is not well fitted or a rise and decay which is faster than expected using the standard indices derived assuming synchrotron emission from internal shocks, although it might be possible to accommodate these features using a different emission mechanism within the same overall framework. The luminosity of pulses is correlated with the peak energy of the pulse spectrum, Lf ~ (Epeak(1+z))^1.8, and anti-correlated with the time since ejection of the pulse, Lf ~ (Tf/(1 + z))^-2.0.