Pulse spectral evolution of GRBs: implication as standard candle

TL;DR

This study analyzes GRB pulses using empirical and physical spectral models, revealing strong correlations between spectral peak energy and luminosity, and explores their potential as standard candles for cosmology.

Contribution

It introduces an empirical spectral description of GRB pulses and demonstrates their use as luminosity indicators, extending their application as standard candles.

Findings

$E_{peak,0}$ correlates strongly with isotropic energy.

Peak energy/temperature decreases exponentially with fluence.

Spectral models fit well with acceptable residuals.

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.