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. jet progenitor photosphere observer photon

Thermal emission from GRB jets However – The observed photons may be generated in the inner layer with τ > 1 (e.g., ε free-free ∝ n 2 ). – Radiation intensity can be anisotropic even in the comoving frame at τ ～ 1. 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. (Beloborodov 2011) We calculate the radiative transfer in the jet.

Method

Hydrodynamical simulation R0R0 θ jet L jet, f th, Γ 0 R star 2D relativistic hydrodynamics (Tominaga 2009) Setup – Progenitor: 14M sun WR star (R star ～ 1.5×10 10 cm) – Γ 0 =5 – Θ jet =10° – L jet =5.3×10 50 erg s -1 – f th = (e int /ρc 2 =80) – (log r, θ) = (600, 150) grids from R 0 =10 9 cm

Hydrodynamical simulation Density [g/cc]

Radiative transfer jet τ inj observer photon photosphere progenitor τ=10 τ=1 We use a snapshot at t=40s as the jet and cocoon structure. τ inj =10 τ inj =1 Numerical code – Monte Carlo method – Calculate Compton scattering – Photons are injected at τ= τ inj Photon injection – 4 models with τ inj =1, 5, 7, 10 – Planck distribution with local plasma temperatures – Isotropic in the comoving frame

Results

Energy spectra E peak is higher for τ inj =5 than for τ inj =1. The models with τ inj =10 have wide shape. Black Body

Energy spectra of τ inj =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

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 τ inj have different properties; e.g., E peak and peak width. We should treat the radiative transfer in the jet properly in order to constrain the GRB prompt emission models.