[Paper Review] Subphotospheric heating in GRBs: analysis and modeling of GRB090902B as observed by Fermi

T. Nymark, M. Axelsson+4 more|arXiv (Cornell University)|Nov 1, 2011

Gamma-ray bursts and supernovae3 citations

TL;DR

This paper proposes that subphotospheric dissipation—energy release below the photosphere—can explain the spectral evolution in GRB 090902B, transforming a Planck-like thermal spectrum into a Band-like non-thermal shape. Using numerical modeling of radiative processes, it demonstrates that varying dissipation strength and location can produce observed spectral shapes, confirming photospheric emission as a key component in GRBs.

ABSTRACT

We analyze the spectral evolution of GRB 090902B and show that subphotospheric dissipation can explain both the spectra and the spectral evolution. The emission from a GRB photosphere can give rise to a variety of spectral shapes. The spectrum can have a shape close to that of a Planck function (as is observed during the first half of GRB090902B) or be broadened, resembling a typical Band function (as is observed during the second half of GRB090902B). The shape mainly depends on the strength and location of the dissipation in the jet, the ratio of the energy densities of thermal photons and of the electrons at the dissipation site, as well as on the strength of the magnetic field. We further discuss numerical models of the dissipation and relate these to the observed spectra.

Motivation & Objective

• To investigate the origin of spectral diversity in GRB 090902B, particularly the transition from a Planck-like to a Band-like spectrum.
• To determine whether subphotospheric dissipation can account for the observed spectral evolution without requiring a dominant non-thermal component from the start.
• To model the radiative processes occurring below the photosphere and assess their impact on emergent spectra.
• To test whether changes in the Lorentz factor or dissipation strength can explain the observed spectral evolution in GRB 090902B.
• To establish that photospheric emission can naturally produce non-thermal-like spectra under realistic dissipation conditions.

Proposed method

• Numerical modeling of radiative processes in relativistic outflows using a self-consistent code that includes synchrotron emission, synchrotron self-absorption (SSA), Compton scattering, pair production/annihilation, and electromagnetic cascades.
• Modeling dissipation at optical depth τ = 10, assuming a fraction εd of kinetic energy is dissipated, with εe and εB allocating energy to electrons and magnetic fields.
• Simulating the emergent spectrum from the photosphere under varying dissipation strengths (εd = 0.1 and 0.2) to reproduce observed spectral shapes.
• Comparing simulated spectra with time-resolved Fermi data from GRB 090902B, focusing on spectral evolution over time.
• Using a multicolor blackbody model to fit the early, steep component and a Band function for the later, broader component.
• Analyzing how changes in Lorentz factor affect the photospheric location and shift dissipation from above to below the photosphere.

Experimental results

Research questions

• RQ1 Can subphotospheric dissipation explain the transition from a Planck-like to a Band-like spectrum in GRB 090902B?
• RQ2 What physical conditions—such as dissipation strength, magnetic field, and electron energy fraction—are required to produce a Band-like shape from photospheric emission?
• RQ3 How does the location of dissipation (above or below the photosphere) affect the observed spectral shape?
• RQ4 What role does the Lorentz factor variation play in shifting the dissipation site below the photosphere and altering the spectrum?
• RQ5 Is photospheric emission alone sufficient to explain the observed spectral components in GRB 090902B, or is a separate non-thermal component required?

Key findings

• The early phase of GRB 090902B exhibits a steep, narrow spectral component consistent with a multicolor blackbody, strongly indicating photospheric emission.
• The late phase shows a broadened spectrum well-fit by a Band function, indicating significant modification of the thermal component by subphotospheric dissipation.
• Numerical models show that strong subphotospheric dissipation (εd = 0.2) can transform a Planck-like spectrum into a Band-like shape, matching observations.
• Weak dissipation (εd = 0.1) results in a spectrum close to a Planck function, while stronger dissipation leads to spectral broadening and non-thermal characteristics.
• The spectral evolution in GRB 090902B is best explained by a shift in dissipation location from above to below the photosphere, likely due to changes in the Lorentz factor.
• The study concludes that photospheric emission is a common and plausible origin of GRB spectra, even when they appear non-thermal, due to subphotospheric energy dissipation.