Epeak-α Correlation in Time Resolved GRB Spectra: A Bottom-Up Approach with Optically Thin Inverse Compton Scattering Model
Abstract
Gamma-ray bursts (GRBs) are the brightest explosions in the Universe, yet the origin of their emission remains uncertain. Time-resolved spectral analysis offers key insights into the evolution of spectral shapes, constraining both radiation mechanisms and emission-site microphysics. Observationally, GRB spectra are well described by the empirical Band function, characterized by the peak energy (Epeak) and low-energy spectral index (α). We investigate the temporal evolution of spectra produced by optically thin inverse-Compton scattering (ICS) within a standard fireball jet framework, focusing on the scenarios that can produce the two commonly observed spectral evolution patterns: hard-to-soft evolution and intensity tracking, within a single emission pulse. The evolution is analysed using both Bayesian block and constant-fluence binning, with the observed spectrum modeled consistently using the Band function. Using this bottom-up approach, we find that optically thin ICS yields a positive Epeak-α correlation, with α evolving from hard (Planck-like, > +0.5) to softer (< −0.67) values. Such hard α values are inconsistent with standard synchrotron emission. This characteristic evolution in the Epeak-α plane, therefore, provides a diagnostic signature of optically thin ICS as the dominant radiation mechanism during the prompt phase of GRBs. Furthermore, this type of smooth evolution of α within a single pulse does not require invoking a transition between different radiation mechanisms, unless additional observational evidence supports such a change.
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BibTeX
@misc{bordoloi2026epeakα,
title = {{$E_\rm peak$-$α$ Correlation in Time Resolved GRB Spectra: A Bottom-Up Approach with Optically Thin Inverse Compton Scattering Model}},
author = {Pragyan Pratim Bordoloi and Ayush Shivkumar and Shabnam Iyyani},
howpublished = {arXiv},
year = {2026},
month = sep,
doi = {10.48550/arxiv.2606.04894},
eprint = {2606.04894},
archivePrefix = {arXiv},
url = {https://arxiv.org/abs/2606.04894},
note = {CC BY 4.0},
}
RIS
TY - UNPB
TI - E_peak-α Correlation in Time Resolved GRB Spectra: A Bottom-Up Approach with Optically Thin Inverse Compton Scattering Model
AU - Pragyan Pratim Bordoloi
AU - Ayush Shivkumar
AU - Shabnam Iyyani
PY - 2026
DA - 2026/09/26
PB - arXiv
DO - 10.48550/arxiv.2606.04894
UR - https://arxiv.org/abs/2606.04894
AB - Gamma-ray bursts (GRBs) are the brightest explosions in the Universe, yet the origin of their emission remains uncertain. Time-resolved spectral analysis offers key insights into the evolution of spectral shapes, constraining both radiation mechanisms and emission-site microphysics. Observationally, GRB spectra are well described by the empirical Band function, characterized by the peak energy (E_peak) and low-energy spectral index (α). We investigate the temporal evolution of spectra produced by optically thin inverse-Compton scattering (ICS) within a standard fireball jet framework, focusing on the scenarios that can produce the two commonly observed spectral evolution patterns: hard-to-soft evolution and intensity tracking, within a single emission pulse. The evolution is analysed using both Bayesian block and constant-fluence binning, with the observed spectrum modeled consistently using the Band function. Using this bottom-up approach, we find that optically thin ICS yields a positive E_peak-α correlation, with α evolving from hard (Planck-like, > +0.5) to softer (< −0.67) values. Such hard α values are inconsistent with standard synchrotron emission. This characteristic evolution in the E_peak-α plane, therefore, provides a diagnostic signature of optically thin ICS as the dominant radiation mechanism during the prompt phase of GRBs. Furthermore, this type of smooth evolution of α within a single pulse does not require invoking a transition between different radiation mechanisms, unless additional observational evidence supports such a change.
ER -