E peak – α Correlation in Time Resolved GRB Spectra: A Bottom-up Approach with Optically Thin Inverse Compton Scattering Model
Year: 2026
Type: article
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 ( 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 analyzed 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.
Source: The Astrophysical Journal
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: gold
Estimated APC paid: $4,499
OpenAlex ID: W7222343329