Pulse spectral evolution of GRBs: implication as standard candle

Abstract: Using an \emph{empirical} description of a prompt GRB pulse, we analyze the individual pulses of all Fermi/GBM GRBs with known redshifts, till July 2009. This description is simultaneous in time and energy and allows one to determine the peak energy of Band spectrum at zero fluence ($E_{peak,0}$). We demonstrate, for the first time, that the $E_{peak,0}$ bears a very strong correlation with the isotropic energy of the individual pulses, and hence, each pulse can be used as a luminosity indicator. As a physical description is needed in order to use GRB pulses for cosmological purposes, we explore other physical spectral models. As pulses are the building blocks of a GRB, we choose another sample of Fermi/GBM GRBs having bright, long and single/ separable pulse(s) and fit the time-resolved spectra of the individual pulses with the Band model and a model consisting of a blackbody and a power-law. Both these models give acceptable fits. We find that the peak energy/ temperature always decreases exponentially with fluence in the later part of a pulse. We investigate multiple spectral components in the initial rising part and provide a comprehensive empirical description of the spectral and timing behaviour of prompt GRB pulses. This work strongly extends the possibility of using GRB pulses as standard candles and the spectral parameters as proxy for redshift.