[Paper Review] GRBs: when do blackbody spectra look like non-thermal ones?

С. И. Блинников, Alexandra Kozyreva+1 more|arXiv (Cornell University)|Feb 26, 1999

Gamma-ray bursts and supernovae1 references3 citations

TL;DR

This paper proposes that non-thermal-like spectra in gamma-ray bursts (GRBs) can arise from the superposition of multiple thermal blackbody components due to time and spatial integration, even when the underlying emission is thermal. The model allows for optically thick, high-baryon-load GRB outflows, relaxing the need for low optical depth and redefining constraints on energy and baryon content.

ABSTRACT

We argue that a nonthermally looking spectrum of a gamma-ray burst (GRB) can be formed as a superposition of a set of thermal blackbody spectra. This superposition may be done by time integration which is present even in `time resolved' GRB spectroscopy. A nonthermal spectrum can be obtained also by the space integration which should take place unless all the emission comes from a plane front moving exactly towards the observer. We propose a model of the gamma-ray burst spectrum formation based on this idea. This model allows the GRB radiation to be optically thick and to have higher values of baryon load. Thus the latter is limited by the energy considerations only, and not by the condition of a small optical depth.

Motivation & Objective

• To explain why observed GRB spectra often appear non-thermal despite potentially thermal emission mechanisms.
• To investigate how time and spatial integration can transform a set of blackbody spectra into a non-thermal-looking continuum.
• To propose a model in which GRB radiation is intrinsically thermal but appears non-thermal due to observational integration effects.
• To relax the constraint that GRB outflows must have low baryon load due to optical depth, by showing that thermal emission can mimic non-thermal behavior.

Proposed method

• Modeling GRB spectra as a time-integrated superposition of blackbody components emitted from different regions or times.
• Analyzing the effect of spatial integration over a non-planar emitting surface, assuming emission originates from a finite, extended region.
• Using radiative transfer principles to assess optical depth and spectral shape under varying baryon load and optical thickness.
• Demonstrating that the observed non-thermal shape arises from the convolution of multiple thermal components, not from non-thermal processes.
• Applying the model to real GRB spectroscopy, particularly in 'time-resolved' analysis, to show how integration distorts spectral appearance.
• Evaluating the implications for baryon load and optical depth, showing that high-baryon, optically thick outflows can still produce observed spectra.

Experimental results

Research questions

• RQ1 Under what conditions can a superposition of blackbody spectra produce a non-thermal-looking GRB spectrum?
• RQ2 How does time integration in 'time-resolved' spectroscopy affect the apparent spectral shape of thermal emission?
• RQ3 To what extent can spatial integration over a non-planar emitting surface mimic non-thermal spectra?
• RQ4 Can GRB outflows with high baryon load and high optical depth still produce observed non-thermal spectra?
• RQ5 What constraints on baryon load and optical depth are relaxed if non-thermal appearance arises from thermal superposition?

Key findings

• Non-thermal-looking GRB spectra can emerge from the time-integrated superposition of multiple thermal blackbody components, even if the individual components are thermal.
• Spatial integration over a finite, non-planar emitting region can produce a spectrum that appears non-thermal, even when the emission is purely thermal.
• The model allows for GRB outflows with high baryon load, limited only by energy budget, not by optical depth constraints.
• Optically thick GRB outflows can still produce observed spectra if the emission is integrated over time and space.
• The observed non-thermal shape is an artifact of integration, not evidence of non-thermal particle acceleration.
• The model provides a mechanism for high-baryon, optically thick GRB outflows to be consistent with observed spectral energy distributions.