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Gamma-ray bursts from magnetized collisionally heated jets. Indrek Vurm (Hebrew University of Jerusalem) in collaboration with Andrei Beloborodov (Columbia University) Juri Poutanen (University of Oulu). Raleigh 2011. τ γγ =1. R * ~10 12 cm. τ T =1. R s. R n. R 0. DISSIPATION.

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gamma ray bursts from magnetized collisionally heated jets

Gamma-ray bursts from magnetized collisionally heated jets

Indrek Vurm

(Hebrew University of Jerusalem)

in collaboration with

Andrei Beloborodov (Columbia University)

Juri Poutanen (University of Oulu)

Raleigh 2011

dissipation in compound flows

τγγ=1

R*~1012 cm

τT=1

Rs

Rn

R0

DISSIPATION

RADIATION DOMINATION

ACCELERATION

MATTER DOMINATION

τn=1

Γp~500

COASTING

Γn< Γp

Dissipation in compound flows

(Beloborodov 2010)

= MeV

  • Protons and neutron flows decouple at Rn
  • Proton flow accelerates until Rs at the expense of radiation
  • Γn< Γp  n-p collisions  dissipation of bulk kinetic energy
  • Two branches
    • Elastic: heats the proton component
    • Inelastic: pion production  muons  electron-positron pairs

= GeV

slide3

Numerical method: kinetic equations

Mihalas (1980), Beloborodov (2011)

Radiative transfer equation in the flow frame:

- specific intensity

- photon frequency

- emissivity

- angle relative to radial direction

- bulk Lorenz factor

- opacity

Kinetic equation for pairs:

Processes: Compton, synchrotron, pair-production/annihilation, Coulomb collisions

- proper time

- pair density

- heating/cooling rate

- electron Lorentz factor

simulation setup
Simulation setup

τγγ=1

τT=1

  • Simulations run in the comovingframe, starting at Rn
  • RTE and pair kinetic equationsevolved in comoving time
  • Initial conditions at Rn fromrelativistic fluid-dynamics
  • Evolution of particle and photon distributions followed self-consistently until τT«1, τγγ «1.
  • Model parameters:
    • Lp, Ln – kinetic luminosities of the proton and neutron flows
    • Γp, Γn – corresponding Lorentz factors
    • εB – magnetization (fraction of flow kinetic energy in B-field)
    • R0 – radius at the base of the flow

Rs

Rn

R0

DISSIPATION

τn=1

Γn< Γp

spectra non magnetized flows

- Monte Carlo (Beloborodov 2010)

red

-kinetic

blue

Spectra: non-magnetized flows

pairs

MeV

GeV

Heating-cooling balance

cooling,

pair cascades

injection

Lp=1052 erg/s

Ln=2x1051 erg/s

Γp=600, Γn=100

r0=107 erg/s

Annihilationline

Non-thermalCompton

Thermal

Compton

Thermal

γγ - absorption

magnetized flows

Magnetization:

Synchrotron peak:

Magnetized flows
  • εB ≠ 0 ⇒ synchrotron emission from non-thermal pairs
magnetized flows1

Magnetization:

Synchrotron peak:

Magnetized flows
  • εB ≠ 0 ⇒ synchrotron emission from non-thermal pairs
  • εB <<1
    • softer low-energy slopes
    • soft excess below ~50 keV
magnetized flows2

Magnetization:

Synchrotron peak:

Magnetized flows
  • εB ≠ 0 ⇒ synchrotron emission from non-thermal pairs
  • εB <<1
    • softer low-energy slopes
    • soft excess below ~50 keV
magnetized flows3

Magnetization:

Synchrotron peak:

Magnetized flows
  • εB ≠ 0 ⇒ synchrotron emission from non-thermal pairs
  • εB <<1
    • softer low-energy slopes
    • soft excess below ~50 keV
  • εB ≈1
    • suppression of pair cascades
    • steep high-energy slopes
    • distinct GeV component
magnetized flows4

Magnetization:

Synchrotron peak:

Magnetized flows
  • εB ≠ 0 ⇒ synchrotron emission from non-thermal pairs
  • εB <<1
    • softer low-energy slopes
    • soft excess below ~50 keV
  • εB ≈1
    • suppression of pair cascades
    • steep high-energy slopes
    • distinct GeV component
magnetized flows5

Magnetization:

- GRB 090902B

red

Synchrotron peak:

- simulation

black

Magnetized flows
  • εB ≠ 0: synchrotron emission from non-thermal pairs
  • εB <<1
    • softer low-energy slopes
    • soft excess below ~50 keV
  • εB ≈1
    • suppression of pair cascades
    • steep high-energy slopes
    • distinct GeV component

GRB 090902B

Abdo et al. (2009)

low energy slope
Low-energy slope

Photon index vs magnetization

Low-energy photon indices in the commonly observed range for wide range of magnetizations

Nava et al. 2011

soft excess
Soft excess
  • Significant excess below ~15 keV in 14% of bright BATSE bursts

86 bright bursts (BATSE)

Excesses relative to PL

50/(1+z) keV

15 keV

Preece et al. 1996

low energy emission
Low-energy emission
  • Partially self-absorbed synchrotron emission predicts a universal power-law α = -1
  • Can extend to the optical band, typical delay ~1 sec

- SSA energy

- emissivity near Es

radiative efficiency
Radiative efficiency

Collisional dissipation retains its efficiency in magnetized flows

Heated flows

ϵ = Lγ/L ~ 0.5

Lγ – radiative luminosity

L – kinetic luminosity

flow

summary
Summary
  • Collisional dissipation in magnetized flows:
    • Band shape preserved
    • Low-energy photon indices in the commonly observed range over several orders in magnetization
    • Soft excess, distinct high-energy emission component
    • Robust prediction of low-energy emission with α = -1
    • High radiative efficiency maintained