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Peer-reviewedCC BY 4.0The Astrophysical Journal

E peak – α Correlation in Time Resolved GRB Spectra: A Bottom-up Approach with Optically Thin Inverse Compton Scattering Model

  • Pragyan Pratim Bordoloi1
  • Ayush Shivkumar1
  • Shabnam Iyyani1
  1. 1Indian Institute of Science Education and Research Thiruvananthapuram
Open at sourcePDFDOICite

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.

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Cite

BibTeXRIS
BibTeX
@article{bordoloi2026peak,
  title = {{E 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},
  journal = {The Astrophysical Journal},
  year = {2026},
  month = oct,
  doi = {10.3847/1538-4357/aea6a8},
  url = {https://iopscience.iop.org/article/10.3847/1538-4357/aea6a8},
  note = {CC BY 4.0},
}
RIS
TY  - JOUR
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  - 2026/10/16
JO  - The Astrophysical Journal
DO  - 10.3847/1538-4357/aea6a8
UR  - https://iopscience.iop.org/article/10.3847/1538-4357/aea6a8
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 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.
SN  - 0004-637X
SN  - 1538-4357
ER  -