1 / 38

Radiative transfer and photospheric emission in GRB jets

Radiative transfer and photospheric emission in GRB jets. Indrek Vurm ( Columbia University ) in collaboration with Andrei M. Beloborodov (Columbia University) Tsvi Piran ( Hebrew University ) Yuri Lyubarsky ( Ben-Gurion University ) Romain Hascoet (Columbia University). Moscow 201 3.

fahim
Download Presentation

Radiative transfer and photospheric emission in GRB jets

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Radiative transferand photospheric emissionin GRB jets Indrek Vurm (Columbia University) in collaboration with Andrei M. Beloborodov (Columbia University) Tsvi Piran (Hebrew University) Yuri Lyubarsky (Ben-Gurion University) Romain Hascoet (Columbia University) Moscow 2013

  2. Outline Prompt emission: optically thin vs. thick Photospheric emission from dissipative jets: Photon number and spectral peaks Non-thermal spectra GeV emission GeV flash from pair-loaded progenitor wind Example: 080916C

  3. GRB prompt emission:optically thin vs. thick Internal shocks Photospheric emission R0~107 cm Γf heating Γs L~1051 erg/s τT=1 ?

  4. EFE a=-2/3 a=-3/2 a synchrotron deathline cooling deathline E Hardness problem FORBIDDEN Nn~Fn/ n ~ na • Optically thin + radiatively efficient  > -1.5 (synch. or IC) Preece et al. (2000)

  5. Peak sharpness and position Blazars GRB 990123 Ghisellini (2006) Briggs et al. (1999) • GRB spectra narrow • Peak energies cluster Synch. peak Epk Goldstein et al. (2012)

  6. Photospheric emission • Spectral peaks • Narrow: can be as narrow as Planck • Position • Natural scale • Observed • Non-thermal shape:  photon production Dissipation

  7. Dissipative jets “Disturbed” jet • Jets could be dissipative throughout their expansion • Recollimation shocks • Internal shocks • Collisional dissipation • Magnetic reconnection • Emerging radiation shaped over a broad range of radii, i.e. knows about expansion history Recollimation shocks Morsony, Lazzati, Begelman (2007)

  8. Photon production and spectral peaks

  9. T=1 R0 PHOTON GENERATION Eph~5 MeV Epk~500 keV Observed photons must be produced in the jet

  10. Thermalization (e.g. Thompson, Meszaros, Rees 2007,Pe’er et al. 2007, Eichler & Levinson 2000)  “Yonetoku” - jet launch radius Photons from the central engine insufficient Thermalization/photon- production location • Blackbody relation • Observations

  11. Thermalization T=1 y~10 rbb abs=1 R0 PLANCK WIEN BB F BB F em/abs IC 4kTe h em/abs h 4kTe

  12. Thermalization T=1 T~102 y~10 rbb abs=1 R0 PLANCK WIEN BB F F Wien BB IC em/abs em/abs h 4kTe h Neither T»1 nor y»1 are sufficient conditions for thermalization

  13. Ne() nth 3kTe Photon sources • Non-magnetized flows: • Bremsstrahlung • Double-Compton scattering • Magnetized flows • Cyclotron • Synchrotron - thermal

  14. Photon production: summary T=1 bremsstrahlung double Compton cyclotron synchrotron rWien R0 WIEN PLANCK 1010 cm ~10 T~104 1012 cm y~103 T~102 y~10 • Photon production occurs in a limited range of radii, at T»1 • Observed Epk -s  modest ~10 at r~1011 cm • Most efficient mechanism: synchrotron • Number of photons at the peak established below/near the Wien radius

  15. Spectral shape Photospheric emission from a dissipative jetdoes NOT resemble a Planck spectrum • Spectrum broadened by: • Large-angle emission • `Fuzzy` photosphere • Diffusion in frequency space

  16. F F   Low-energy slope: dissipative jet DISSIPATION y~1 PLANCK WIEN τT=1 Low-energy spectrum is shaped in an extended region between the Wien radius and the Thomson photosphere

  17. DISSIPATION y~1 WIEN Low-energy slope: dissipative jet • Wien/Planck spectrum at y»1is broadened by the combinedeffect of Comptonizationand adiabatic cooling • Photospheric spectrumsubstantially softer than Planck τT=1

  18. Low-energy slope: dissipative jet; with a soft photon source • α=-1 slope is a slow attractor saturated Comptonization photon injection

  19. Dissipative jet: high-energy spectrum • Non-thermal spectrum above the peak: dissipation near τT~1 • Possible mechanism: collisional heating (Beloborodov 2010) • Proton and neutron flows decouple at T20 • Drifting neutron and proton flows nuclear collisions: • Elastic: Thermal heating of e± via Coulomb collisions • Inelastic: Injection of relativistic e± with ~300via pion production and decay Other models: Thompson (1994) Pe’er, Mészáros & Rees (2005) Giannios & Spruit (2006) Ioka et al. (2007) etc.

  20. Spectra: non-magnetized flows Pairs MeV GeV Heating-cooling balance kT=15 keV cooling, pair cascades injection Non-thermalCompton Thermal Compton γγ - absorption

  21. Dissipative jet: summary F Wien F F Epk Epk 4kTe h h h T=1 T~102 y~10 ~10 R0 SPECTRUM FORMATION PH. GENERATION DISSIPATION rWien

  22. Generic model for a dissipative jet ACCELERATION DISSIPATION WIEN rcoll (rcoll)~10 τT=1 • Continuous dissipationthroughout the jet • Thermal and non-thermalchannels: • Acceleration: • Magnetization: • Initial radius rcoll=1011 cm - terminal Lorentz factor

  23. Radiative transfer - intensity - photon angle Processes: Compton, synchrotron, pair-production/annihilation

  24. Spectral formation τT=1 rWien rcoll WIEN • Initial spectrum: Wien • Peak shifted to lower energiesdue to photon production • Broadening starts nearthe Wien radius, proceedsthrough the photosphere • Final spectrum: Band rcoll=1011 cm T(rcoll)=400 (rcoll)~50 =300 Parameters: Spectra at different stages of expansion

  25. (rcoll) WIEN rcoll τT=1 Spectra: varying LF at the base rcoll=1011 cm B= 10-2 • Canonical Band shape • Low-energy slope stays near 1 • Spectral peak sensitive to (rcoll)via photon production efficiency

  26. Summary • Photospheric emission typically NOT thermal-looking • Dissipative jets • Naturally lead to Band-like spectra • Photon index =-1 is an attractor for the Comptonization problem • Typical Epk -s require • efficient dissipation at r~1011 cm. Recollimation shocks? • bulk Lorentz factor ~10 at the same radii • At least moderate magnetization B>10-3 Continuous dissipation throughout the jet?

  27. GeV flashes with Andrei Beloborodov and Romain Hascoet

  28. Observations: GRB 080916C GRB 080916C GBM LAT Fermi collaboration (2013)

  29. Observations: LAT lightcurves • ‘Regular’ behaviour: • external origin (forward shock)? • LAT emission peaks during the prompt: • likely not assoc. with deceleration • Lasts well beyond T95 080916C T95 (GBM) 090902B 090926A Fermi LAT collaboration (2013)

  30. Emission mechanism Kumar & Barniol Duran (2009) Asano et al. (2009) Razzaque et al. (2010) Ghisellini (2010) • Synchrotron? • Theoretical limit: a few 10 MeV (comoving) ~ 10 GeV (observed); limit tighter at late times • Observed: 95 GeV (GRB 130427A) • Inverse Compton • GeV peak during prompt  intense IC cooling by prompt radiation e.g. Nakar & Piran (2010)

  31. Number of IC photons Bright GeV flashes: No. of emitted IC photons: Photon multiplicity Wind velocity Required pair multiplicity:

  32. Proposed mechanism: inverse Compton scattering of prompt MeV radiation in the forward shockin a pair-enriched external medium Forward shock EXTERNAL MEDIUM GeV PROMPT RADIATION

  33. Pair-enrichment of the external medium PROMPT RADIATION e± e- Prompt radiation pair-loads and pre-accelerates the ambient medium ahead of the FS e- FS e.g Thompson & Madau (2000) Beloborodov (2002) Kumar & Panaitescu (2004) Z±,pre 1. ISM particle scatters a prompt photon 2. Scattered photon pair-produces with another prompt photon 3. New pairs scatter further photons etc. Loading and pre-acceleration controlled by the column density of prompt radiation

  34. GRB 080916C:pair-loading and pre-acceleration Pre-acceleration and blastwave Lorentz factors Pair loading at the forward shock Beloborodov, Hascoet, IV (2013)

  35. GRB 080916C:thermal injection Lorentz factor Thermal heating: • Flash peaks when: • Early decay due to fast evolution of inj and Z± - pair loading

  36. GRB 080916C: lightcurve Flux above 100 MeV T95 (GBM) • Delayed rise • Peak during the prompt • Persists well after T95 Wind parameter Peak radius R1016 cm Non-thermal acceleration NOT required

  37. -2 GRB 080916C: spectra LAT photon index Spectra -2 Fermi LAT collaboration (2013)

  38. Summary • Proposed mechanism: GeV flashes from FS running into pair-loaded external medium • Radiative mechanism: IC of prompt MeV photons • Standard wind medium consistent with observations

More Related