Monte-Carlo Simulation of Thermal
Radiation from GRB Jets
 Sanshiro Shibata (Konan Univ.)
Collaborator: Nozomu Tominaga (Konan Univ., IPMU)
Introduction
Models for the prompt emission
• Internal shock model
– A standard scenario for a long time.
– Low Radiative efficiency, line of death problem
• Photospheric (thermal emission) model
– Thermal emission from relativistic jets
– (possibly) high radiative efficiency
– Some GRBs exhibit blackbody
like feature (e.g., GRB090902B).
(Ryde et al 2010)
Thermal emission from GRB jets
• Thermal emission from GRB jets have been investigated
by performing hydrodynamical simulations.
(Lazzati+2009, Mizuta+11, Nagakura+11)
• They calculated the light curves and spectra by
superposing blackbody radiation emitted from the
photosphere with τ=1.
progenitor
jet
photon
photosphere
observer
Thermal emission from GRB jets
However
– The observed photons may be generated in the inner layer with
τ > 1 (e.g., εfree-free∝n2).
– Radiation intensity can be anisotropic even in the comoving
frame at τ～1. (Beloborodov 2011)
In order to treat the thermal radiation from GRB jets properly,
both the radiative transfer in the jet and complex structure of
the jet and cocoon should be taken into account.
We calculate the radiative transfer in the jet.
Method
Hydrodynamical simulation
2D relativistic hydrodynamics (Tominaga 2009)
Setup
– Progenitor: 14Msun WR star (Rstar～1.5×1010cm)
– Γ0=5
– Θjet=10°
θjet
– Ljet=5.3×1050 erg s-1
– fth=0.9925 (eint/ρc2=80)
Ljet, fth, Γ0
– (log r, θ) = (600, 150) grids
R0
from R0=109cm
Rstar
Hydrodynamical simulation
Density [g/cc]
Radiative transfer
Numerical code
observer
progenitor
– Monte Carlo method
– Calculate Compton scattering
– Photons are injected at τ= τinj
jet
photon
τinj
photosphere
Photon injection
– 4 models with τinj =1, 5, 7, 10
– Planck distribution with local plasma temperatures
τinj=1
=10
– Isotropic in the comoving frame
τinj
We use a snapshot at t=40s as
the jet and cocoon structure.
τ=1
τ=10
Results
Energy spectra
• Epeak is higher for τinj=5 than for τinj=1.
• The models with τinj=10 have wide shape.
Black Body
Energy spectra of τinj=10
• Photons with large number of scatterings are tend to have
lower energies.
The spectrum becomes wide.
Comparison to observations
α=0
Black Body
(Kaneko et al. 2006)
Summary
Summary
 We develop a numerical code to calculate radiative
transfer in the relativistic jet.
 We perform radiative transfer calculation in relativistic
jet and cocoon with complicated structure, which is
obtained by 2D relativistic hydrodynamical calculation.
 The emergent spectra with different τinj have different
properties; e.g., Epeak and peak width.
We should treat the radiative transfer in the jet
properly in order to constrain the GRB prompt
emission models.