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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. 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

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  1. 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

  2. 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

  3. 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

  4. 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

  5. - 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

  6. Magnetization: Synchrotron peak: Magnetized flows • εB ≠ 0 ⇒ synchrotron emission from non-thermal pairs

  7. Magnetization: Synchrotron peak: Magnetized flows • εB ≠ 0 ⇒ synchrotron emission from non-thermal pairs • εB <<1 • softer low-energy slopes • soft excess below ~50 keV

  8. Magnetization: Synchrotron peak: Magnetized flows • εB ≠ 0 ⇒ synchrotron emission from non-thermal pairs • εB <<1 • softer low-energy slopes • soft excess below ~50 keV

  9. 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

  10. 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

  11. 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)

  12. 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

  13. 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

  14. 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

  15. Radiative efficiency Collisional dissipation retains its efficiency in magnetized flows Heated flows ϵ = Lγ/L ~ 0.5 Lγ – radiative luminosity L – kinetic luminosity flow

  16. 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

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