GRB emission mechanisms
Frédéric Daigne(Institut d’Astrophysique de Paris; Université Pierre et Marie Curie-UMPC)
with R. Mochkovitch, R. Hascoët, Z. Bosnjak

GRB Conference – Kyoto, November 11-15, 2013

Possible emission sites in GRBsLog( R )[meters]

Central engine
Relativistic ejection

Photosphere
Internal dissipation in
optically thin regime(shocks or reconnection)

External shock
Reverse shock

Contribution of each region ?
Dissipation mechanism ?
Radiative process ?
Internal dissipation: prompt

Deceleration: afterglow

Internal dissipation (1) photosphere-The relativistic outflow becomes transparent

Rph	  -Internal energy can be released as radiation-Almost no theoretical uncertainties(still: lateral geometry of the jet; initial magnetization)-Spectrum is quasi-thermal: exp. cutoff at high-energy
PL at low-energy with α ≈ +0,4Spectrum	  	  PHOTOSPHERE:

Planck → PhotospheregeometryE	  Internal dissipation (1) photosphere-The relativistic outflow becomes transparent

Rph	  -Internal energy can be released as radiation-Almost no theoretical uncertainties(still: lateral geometry of the jet; initial magnetization)-Spectrum is quasi-thermal: exp. cutoff at high-energy
PL at low-energy with α ≈ +0,4Spectrum	   PHOTOSPHERE:

Planck → Photosphere → Dissipative Photospheregeometry
synchrotron

ComptonizationE	   DISSIPATIVE PHOTOSPHERE:-Sub-photospheric dissipation: non-thermal electrons-Large uncertainties: details of the dissipation process
neutron heating ? internal shocks ? reconnection ? …
-Spectrum is non-thermal:
Comptonization: high-energy tail
Synchrotron radiation: modifies the low-energy slope

Internal dissipation (2) optically thin
Non-thermal emission can be produced above the photosphere if there are
dissipation processes producing non-thermal electrons.
SSC is ruled out by Fermi observations – Synchrotron ?
 INTERNAL SHOCKS:-Assumes:

RIS	  

-Large uncertainties:
microphysics (B amplification, e acceleration) ?
-Non-thermal spectrum, several components (syn, IC) RECONNECTION:

Variability of the central engine+ low magnetization at large distance-Assumes: Variability + large mag. at large distance-Large uncertainties:
radius ? microphysics ?
-Non-thermal spectrum

Rrec	  Prompt soft gamma-ray emission

Light curves
All possible sites for the prompt emission can reproduce the observed variable
light curves, but with important differences: (DISSIPATIVE) PHOTOSPHERE: INTERNAL SHOCKS:✔	   -Low radius: curvature effect is negligible(except for peculiar lateral distribution)-The light curve directly traces the activityof the central engine✔	  -The light curve is also tracing the central activity-Additional effects:
shock propagation & curvature effect RECONNECTION:✔	   -The light curve is also tracing the central activity-Additional effects:
reconnection process (fast variability)& curvature effectOpen issue with observations:continuum of variability timescales or two components ?

Spectrum (1) models

νFν	Epα	β	ν	General shape (“Band”) / Low-energy photon index α (obs: α ≈ -1)
 PHOTOSPHERE:?	  

 DISSIPATIVE PHOTOSPH.:✔?	   -α correct (depends on magnetization)-Instantaneous spectrum is narrow

 INTERNAL SHOCKS:?	  

Time-integ.
spec. ?

-α too large except for peculiar lateral struct.
-Instantaneous spectrum is narrow-Synchrotron only: α = -3/2 (fast cooling)-Possible mechanisms to increase α
IC in KN regime ; B decay in shocked region-Other process ?
-Spectrum is too broad around the peak ?

 RECONNECTION:?	  

-α correct ? (slow heating in turbulent acc.)-Spectrum is probably much too broad(multi emitters)

Spectrum (2) observations
 Should we believe the distribution of α ? the Band shape ?
-Fermi bursts: multi-component spectra (2, 3 components)-Parameters of the “Band” component vary when the othercomponents are taken into account Should we believe that the spectrum is narrow around the peak ?
-Spectral evolution in GRBs

(Briggs	  et	  al.	  1999)	  GRB 990123

Epeak	  α	Spectrum (2) observations
 Should we believe the distribution of α ? the Band shape ?
-Fermi bursts: multi-component spectra (2, 3 components)-Parameters of the “Band” component vary when the othercomponents are taken into account Should we believe that the spectrum is narrow around the peak ?
-Spectral evolution in GRBs
-The integration of a time-evolving Band function is not a Band function(it is broader)PulseTypical pulse decay

Luminosity	  Peak	  energy	  Time	  Time	  evolving	  spectrum	  Time-­‐integrated	  spectrum	  Pulse
Typical pulse decay

Luminosity	  Time-­‐integrated	  spectrum	  Peak	  energy	  Band	  with	  same	  Epeak	  Time	  The integration of a time-evolving Band function is not a Band function.

Distribution of Epeak"
Hardness-Duration correlation
 Epeak varies a lot :-from a GRB to another (XRF, XRR, GRBs, short GRBs)-within a GRB (spectral evolution)-dissipative photosphere:✔?	  (depends on the details of the heating)✔	  -reconnection: ?	  
-internal shocks:

 Short bursts have usually higher peak energies-dissipative photosphere: change in properties of central engine-internal shocks: natural explanation-reconnection:?	  

✔	  ?	  Hardness-Duration in internal shocks

Effect of duration:-hardness-duration correlation-lags become shortand tend to zero-pulses become
more symmetric

Pulse calculation: the only varying parameter is the duration(Bosnjak & Daigne submitted)

Spectral evolution

Ep evolution (intensity tracking)Hardness Intensity correlation (HIC)
Hardness Fluence correlation (HFC)
Pulse width vs Energy ; Time lags ; etc.
 Dissipative photosphere: details of the dissipative process Internal shocks:

qualitative agreement ;✔	   -natural-constraints on microphysics
for a quantitative agreement

 Reconnection:?	  

?	  Dissipative photosph.: spectral evolution(Beloborodov	  2013)	  Dissipative photosph.: spectral evolution

What are the constraints
on the dissipative process ?
How does the dissipativeprocess adjust its radius tothe photospheric radius ?
Typical evolution
within a pulse(Beloborodov	  2013)	  Internal shocks: spectral evolution
Example of a simulated pulse (internal shocks with full radiative calculation)

Evolution of Epeak and α	Time-evolving spectrum(Bosnjak & Daigne submitted ; see also Asano & Meszaros)

Additional PL
component withindex ~-2 ?

Internal shocks: spectral evolution
Example of a simulated pulse (internal shocks with full radiative calculation)

Light curve in BATSE range :channels 1 (blue) to 4 (red)Pulse width and time lags

W (E) ∝ E −aa � 0.2 − 0.3

Delayed onset ? γγ ?
(Bosnjak & Daigne submitted ; see also Asano & Meszaros) (Hascoet et al. 2012)

The end of the prompt emission:"
X-ray early steep decay
 A natural explanation: high-latitude emission from the prompt (fits well XRT data)✗	  -(Dissipative) photosphere:(radius is too small)-Internal shocks:(final radius of the order of Γ2 c tburst)✔	  -Reconnection: ✔?	   (final radius ?)GRB061121(Page et al. 2007)

High-latitude emission in internal shocks(Hascoët et al. 2012)

The end of the prompt emission:"
X-ray early steep decay
 A natural explanation: high-latitude emission from the prompt (fits well XRT data)-(Dissipative) photosphere:-Internal shocks:✔	  -Reconnection: ✔?	  

✗	   (radius is too small) Alternative explanation: late evolution of the central engine?	  (inefficient ?)GRB061121- Dissipative photosphere: ?	  (constraints on dissipative process ?)- Photosphere:

Dissipative ph.: X-ray early steep decay

More severe constraint
than for the spectral
evolution in a pulse

Typical evolution
within a pulse
Typical X-ray
early steep decay(Beloborodov	  2013)	  Electron acceleration in intern. shocks ?
Microphysics is the main source of uncertainties in internal shocks. It is sometimes
proposed that they play only a dynamical role, without associated emission.
 Acceleration is difficult (PIC simulations, …) :However, if electron acceleration does not work in the mildly relativisticregime, then:Major crisis !
-no emission from internal shocks, even in non GRB sources-no emission from the reverse shock-no emission from the late forward shock Shock acceleration leads to Maxwellian+Power-law tail:one should detect the Maxwellian !
Same question should be asked for the Forward and Reverse Shock
Small εe(Maxwell) / εe(PL) ?

Dissipative photosphere:"emission above the photosphere ?
 Several possible dissipation process have no reasons to stop close
to the photosphere:
e.g. internal shocks, reconnection
 Hidden component in the spectrum ?
No detection: constraints on the dissipation efficiency ?

Photosphere+internal shocks/reconn.

In the optical thin scenario (internal shocks or reconnection), photosphericemission is expected, with a brightness depending on the composition of the jet.
 GBM observations: weak photospheric emission is detected ?

GRB 100724B(long)

Guiriec et al. (2011)

GRB 120323A(short)Guiriec et al. (2013)

 Favors magnetic acceleration, with a range of magnetization in the GRB
population, with a hint for a lower magnetization in short GRBs

Photosphere + internal shocks

Hascoet, Daigne & Mochkovitch 2013

Initial Lorentz factor

Light curveInternal shocks
Photosphere
Spectrum

Spectral evolution

Prompt GeV emission
Prompt optical emission

Prompt GeV emission (LAT) There is probably a prompt variable component in the LAT,
different from the long lasting emission (external origin)(1st GRB LAT catalog)

GRB 090902Btvar	  <	  100	  ms	  	   Strong constraint on the emission radius from γγ opacity- (Dissipative) photosphere:✔	  (IC)- Reconnection: ?	  
- Internal shocks:✗	  ✔	  Additional process is needed(e.g. scattering mechanism proposedby Beloborodov et al.)Prompt GeV emission in internal shocks(Bosnjak & Daigne submitted ; see also Asano & Meszaros)

Prompt optical emission
 The prompt optical emission can change a lot from a burst to another In optical bright burst, the optical emission is probably variable: internal origin(Racusin et al. 2008)

GRB 080319B @ z = 0.937

 Strong constraint on the radius from the synchrotron self-absorption- (Dissipative) photosphere:✗	  ✔	  (late collisions)- Reconnection: ?	  
- Internal shocks:

Additional process is needed(e.g. mechanism proposedby Beloborodov et al.)✔	  

Optical emission from internal shocksV	  band	  γ-­‐rays	  (Hascoët et al. 2011)(Racusin et al. 2008)

Afterglow

Deceleration: emission sites
 FORWARD SHOCK:-Dynamics is well understood-Main uncertainty: microphysics(but also: external medium) REVERSE SHOCK:-Assumes low magnetization at large distance-Main uncertainty: microphysics

In both cases: non-thermal radiation from shock-accelerated electrons
Density

Contact
discontinuity

ReverseShockΓΓ*	

Forwardshock

Γ*	

Shocked Shocked
ejecta externalmedium

Relativistic
ejectaExternal
mediumRadiusGRB emission
Log Flux	Prompt(soft γ-rays)

Afterglow(X-rays)initial steep decay: α = 3 - 5flares

Plateaushallow decay:
α = 0 – 0.5

normal decay: α = 1 - 1.5

steeper decay: α = 2 - 3
Log Time
Also: prompt
optical, GeV

Also: optical, radio afterglow
long-lasting LAT emission

GRB emission: « standard » model
Log Flux	

Internal origin

Log Time

GRB emission: « standard » model
Log Flux	

External origin

Log Time

Some issues with afterglow theory
 Early X-ray afterglow: plateaus- FS:✗	  (energy crisis)- late activity of the central engine ?
- varying microphysics?	  

- RS: OK if long lasting RS due to low-Γ tailRequires: - inefficient radiation from FS (acceleration in UR regime ?)?	  
- comparable efficiency prompt/RS (internal shocks ?) ✔?	  

 Variability in optical afterglows: bumps (e.g. GRB 030329)- FS:- density clumps✗	  - Refreshed shocks ?

✔?	   (requires very low ΔΓ/Γ: post IS ?)- RS: OK if long lasting RS due to low-Γ tail (same constraints as above)
 Variability in X-ray afterglows: flares- FS:✗	  - impossible- Requires late prompt emission?	  
- RS: OK if long lasting RS due to low-Γ tail ✔?	  
Requires:- over-densities in ejecta (a signature from internal shocks ?)SummarySummary
Understanding the physical origin of the GRB emission is difficult, especially forthe prompt emission.
Dissipative photospheres are promising, however:- strong constraints on the unknown dissipation process- “complicated” model: different mechanisms for different components
in the prompt (soft γ-rays, optical, GeV)
Reconnection above the photosphere looks promising, however:- uncertainties both on the dynamics and the microphysics- difficult to conclude without any predictions for the spectrum- potential problem with the spectral shape (broadening by multi-emitters)
Internal shocks can produce emission from optical to GeV. The model can beexplored in details (spectral evolution, etc.). Results are promising, however:- uncertainties on the microphysics- is there a problem with α ? With the efficiency ?
- is there a problem with the general shape of the spectrum ? (too broad ?)Obsevations: a better description of the spectral properties is needed(issues with the present method of analysis, based on the Band model)