1. 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 2013

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

3. R 0 ~10 7 cm τ T =1 L~10 51 erg/s ΓfΓfΓfΓf ΓsΓsΓsΓs Internalshocks Photosphericemission heating GRB prompt emission: optically thin vs. thick ?

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

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

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

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

8. Photon production and spectral peaks

9. R0R0R0R0 PHOTON GENERATION  T =1 E ph ~5 MeV Observed photons must be produced in the jet E pk ~500 keV

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

11. F F h 4kT e em/abs ICBBThermalization  abs =1 R0R0R0R0  T =1 y~10 F F h 4kT e em/abs BB PLANCK WIEN r bb

12. F F h BB Wien IC em/abs Thermalization  abs =1 R0R0R0R0  T =1 y~10 F F h 4kT e em/abs BB PLANCK WIEN r bb Neither  T »1 nor y»1 are sufficient conditions for thermalization  T ~10 2

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

14. Photon production: summary R0R0R0R0  T =1  ~10 10 10 cm 10 12 cm synchrotron bremsstrahlung double Compton cyclotron  T ~10 2  T ~10 4 y~10 3 y~10 r Wien Photon production occurs in a limited range of radii, at  T »1 Photon production occurs in a limited range of radii, at  T »1 Observed E pk -s  modest  ~10 at r~10 11 cm Observed E pk -s  modest  ~10 at r~10 11 cm Most efficient mechanism: synchrotron Most efficient mechanism: synchrotron Number of photons at the peak established below/near the Wien radius Number of photons at the peak established below/near the Wien radius PLANCK WIEN

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

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

17. Low-energy slope: dissipative jet Wien/Planck spectrum at y»1 is broadened by the combined effect of Comptonization and adiabatic cooling Wien/Planck spectrum at y»1 is broadened by the combined effect of Comptonization and adiabatic cooling Photospheric spectrum substantially softer than Planck Photospheric spectrum substantially softer than Planck τ T =1 WIEN DISSIPATION y~1

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

19. Dissipative jet: high-energy spectrum Non-thermal spectrum above the peak: dissipation near τ T ~1 Non-thermal spectrum above the peak: dissipation near τ T ~1 Possible mechanism: collisional heating (Beloborodov 2010) Possible mechanism: collisional heating (Beloborodov 2010) Proton and neutron flows decouple at  T  20 Proton and neutron flows decouple at  T  20 Drifting neutron and proton flows  nuclear collisions: Drifting neutron and proton flows  nuclear collisions: Elastic: Thermal heating of e ± via Coulomb collisions Elastic: Thermal heating of e ± via Coulomb collisions Inelastic: Injection of relativistic e ± with  ~300 via pion production and decay Inelastic: Injection of relativistic e ± with  ~300 via 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 ThermalCompton Non-thermal Compton γγ - absorption GeV MeV kT=15 keV Heating-cooling balance injection cooling, pair cascades Pairs

21. Dissipative jet: summary F F h 4kT e Wien R0R0R0R0 PH. GENERATION  T =1  ~10 SPECTRUM FORMATION  T ~10 2 y~10 DISSIPATION r Wien F F h E pk F F h E pk

22. Generic model for a dissipative jet τ T =1 r coll WIEN  (r coll )~10 Continuous dissipation throughout the jet Continuous dissipation throughout the jet Thermal and non-thermal channels: Thermal and non-thermal channels: Acceleration: Acceleration: Magnetization: Magnetization: Initial radius r coll =10 11 cm Initial radius r coll =10 11 cm DISSIPATION ACCELERATION - terminal Lorentz factor

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

24. Spectral formation Spectra at different stages of expansion r coll =10 11 cm  T (r coll )=400  (r coll )~50  =300 Initial spectrum: Wien Initial spectrum: Wien Peak shifted to lower energies due to photon production Peak shifted to lower energies due to photon production Broadening starts near the Wien radius, proceeds through the photosphere Broadening starts near the Wien radius, proceeds through the photosphere Final spectrum: Band Final spectrum: Band r Wien τ T =1 r coll WIEN Parameters:

25. Spectra: varying LF at the base τ T =1 r coll WIEN  (r coll ) r coll =10 11 cm Canonical Band shape Canonical Band shape Low-energy slope stays near  1 Low-energy slope stays near  1 Spectral peak sensitive to  (r coll ) via photon production efficiency Spectral peak sensitive to  (r coll ) via photon production efficiency  B = 10 -2

26. Photospheric emission typically NOT thermal-looking Photospheric emission typically NOT thermal-looking Dissipative jets Dissipative jets Naturally lead to Band-like spectra Naturally lead to Band-like spectra Photon index  =-1 is an attractor for the Comptonization problem Photon index  =-1 is an attractor for the Comptonization problem Typical E pk -s require Typical E pk -s require efficient dissipation at r~10 11 cm. Recollimation shocks? efficient dissipation at r~10 11 cm. Recollimation shocks? bulk Lorentz factor  ~10 at the same radii bulk Lorentz factor  ~10 at the same radii At least moderate magnetization  B > 10 -3 At least moderate magnetization  B > 10 -3 Summary Continuous dissipation throughout the jet?

27. GeV flashes with Andrei Beloborodov and Romain Hascoet

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

29. Observations: LAT lightcurves 080916C 090902B 090926A T 95 (GBM) T 95 (GBM) Fermi LAT collaboration (2013) ‘Regular’ behaviour: ‘Regular’ behaviour: external origin (forward shock)? external origin (forward shock)? LAT emission peaks during the prompt: LAT emission peaks during the prompt: likely not assoc. with deceleration likely not assoc. with deceleration Lasts well beyond T 95 Lasts well beyond T 95

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

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

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

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

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

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

36. GRB 080916C: lightcurve Delayed rise Delayed rise Peak during the prompt Peak during the prompt Persists well after T 95 Persists well after T 95 T 95 (GBM) Flux above 100 MeV Wind parameter Peak radius R  10 16 cm Non-thermal acceleration NOT required

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

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