Frédéric Daigne (Institut d’Astrophysique de Paris)
with Z. Bošnjak, G. Dubus and R. Mochkovitch.

2009 Fermi Symposium – David Band session – Washington DC – November 2-5, 2009

Prompt GRB emission

Pre-Fermi observations

From Briggs et al. 1999

GRB 990123 :BATSE
50‐300 keVCGROPrompt GRB emission

Pre-Fermi observations

From Briggs et al. 1999

GRB 990123 :BATSE
50‐300 keV

CGROαβ« Peak energy » Ep

4‐parameters « Band spectrum » :Ep, α, β and normalizaMon
Band et al. 1993

Prompt GRB emission

Pre-Fermi observations

From Briggs et al. 1999

GRB 990123 :BATSE
50‐300 keV

CGROαβ« Peak energy » Ep

In most GRBs, the spectral parameters (Ep, α, …) evolve with Mme.

Scenario

Internal vs externalLog(R) [m]Photosphere

Central engine
RelaMvisMc ejecMon

Prompt emission(internal)
AYerglow(deceleraMon phase)Origin of the prompt GRB emission

QuestionsShort Mmescale variability in the lightcurve : internal origin of the prompt GRB emission(i.e. : emission is radiated from the relaMvisMc ou\low)

Two quesMons :‐ physical mechanism ?

Internal shocks ?
Photosphere ?
MagneMc dissipaMon ?

‐ dominant radiaMve process ?

Synchrotron ?
SSC ?
Others ?

To understand many observed features (spectral evoluMon, « delays », …),
both quesMons must be considered simultaneously.

Dominant radiative process

Synchrotron vs SSC

GBM LATSSC :?

?

IC2SynIC1
IC sca'. (Thomson)‐Where is the strong IC2 component ?
or the strong syn component ?
‐Energy crisis

GBM LAT
Synchrotron :Fermi‐LAT detecMon rate and observaMons
clearly favor the synchrotron process.
SynIC sca'. (Klein‐Nishina)
IC(see e.g. Bošnjak, Daigne & Dubus 09; Piran, Sari & Zou 09)

Spectral BATSE catalog(Preece et al. 2000; Kaneko et al. 2006)

Synchrotron radiation

Low‐energy slope α

The low-energy spectral slope α‐2/3

The low-energy spectral slope α‐3/2Spectral BATSE catalog(Preece et al. 2000; Kaneko et al. 2006)

Synchrotron radiation

γm : minimum Lorentz factor at injecMon
γc : radiaMve Mmescale = dynamical Mmescale
Synchrotron frequencies : γm↔νm and γc↔νcSynchrotron spectrum : fast cooling (γc < γm)νc νm‐2/3Low‐energy slope αDensity

RelaMvisMc electrons :‐pγmLorentz factor

Photon ﬂux‐3/2‐(p+2)/2Frequency

Synchrotron radiation

The low-energy spectral slope αNormalized E2 N(E)Band funcMon
α = ‐1.5
β = ‐2.25

Observed energy E / EpeakSynchrotron radiation

The low-energy spectral slope α

Normalized E2 N(E)Band funcMon
α = ‐1.5 → ‐1
β = ‐2.25~ 20 % of theradiated energy

Observed energy E / EpeakSynchrotron radiation

The low-energy spectral slope α

Normalized E2 N(E)Band funcMon
α = ‐1.5 → ‐1
β = ‐2.25~ 20 % of theradiated energy

Observed energy E / Epeak
Inverse Compton in Klein‐Nishina regime has an impact on the synchrotron slope α(see Derishev et al. 01 ; Bošnjak, Daigne & Dubus 09 ; Nakar, Ando & Sari 09)

Synchrotron radiation
Examplewm : importance of KN

Normalized ν FνThe low-energy spectral slope α

Y : importance of IC vs syn!
hνmwm = Γmme c2Lic4"e∼ Y = τ T Γm Γc "Lsyn
3"BFrequency
Exact calculaMon with synchrotron + IC only (no adiabaMc cooling; syn. self‐abs; γγ annihilaMon, …)

Synchrotron radiation

The low-energy spectral slope αParameter
space

wm : importance of KN

Thomson Y parameter!
hνmwm = Γmme c2

Y : importance of IC vs synLic
4"e∼ Y = τ T Γm Γc "
Lsyn
3"B

Klein‐Nishina parameter
Exact calculaMon with synchrotron + IC only (no adiabaMc cooling; syn. self‐abs; γγ annihilaMon, …)

Synchrotron radiation

The low-energy spectral slope αParameter
space
Thomson Y parameter

Steep slopes α = ‐1
can be obtainedin fast cooling regime
Lic/Lsyn ~ 0.1‐1
in this region

Klein‐Nishina parameter
Exact calculaMon with synchrotron + IC only (no adiabaMc cooling; syn. self‐abs; γγ annihilaMon, …)

Origin of the prompt GRB spectrum

Internal shocks

To go further, one needs a physical model for the emission region.
An oYen discussed possibility : internal shocks
Γ(Rees & Meszaros 1994)

Γγ‐raysr

Detailed model :(Bošnjak, Daigne & Dubus 09)- Dynamics :mulM‐shell approximaMonr- Microphysics :magneMc ﬁeld (εB) ; non‐thermal populaMon of electrons (εe,ζ,p)- RadiaMon :solve Mme evoluMon of electrons and photons in the comoving
frame of each shocked region(adiabaMc cool.; synchrotron; syn. self‐absorpMon; IC; γγ→e+e‐)- Observed GRB :integraMon over equal‐arrival Mme surfaces

Detailed model

ExampleA single pulse burst (as a building block for more complex GRBs)- IniMal distribuMon of Lorentz factor :- EjecMon lasts for tw = 2s
- Constant energy injecMon rate : Lkin = 2×1052 erg/s

Detailed model

ExampleA single pulse burst (as a building block for more complex GRBs)- Dynamical evoluMon :Lorentz factor in the shocked region

Electron Lorentz factor

MagneMc ﬁeld

- Constant microphysics parameters : εe=εB=1/3 ; ζ = 0.01 ; p=2.5

Detailed model

ExampleA single pulse burst (as a building block for more complex GRBs)- Dynamical evoluMon :Lorentz factor in the shocked region

Electron Lorentz factorMagneMc ﬁeld

- Constant microphysics parameters : εe=εB=1/3 ; ζ = 0.01 ; p=2.5

Detailed model

ExampleA single pulse burst (as a building block for more complex GRBs)- Dynamical evoluMon :Lorentz factor in the shocked region

Electron Lorentz factor

MagneMc ﬁeld

- Constant microphysics parameters : εe=εB=1/3 ; ζ = 0.01 ; p=2.5

Detailed model

ExampleA single pulse burst (as a building block for more complex GRBs)- Dynamical evoluMon :Lorentz factor in the shocked region

Electron Lorentz factorMagneMc ﬁeld

- Constant microphysics parameters : εe=εB=1/3 ; ζ = 0.01 ; p=2.5

Synchrotron radiation in internal shocks
εB =εe= 0.1

ζ = 10‐3

Examples : a single pulse burst
EBL not included

Full calculaMon with all processes (adiabaMc cooling; sync; sync self‐abs; IC; γγ annihilaMon)
Dynamics : ejecMon Γ(t)=100→400 ; duraMon = 1 s ; kineMc energy injecMon rate Lkin = 1054 erg

Synchrotron radiation in internal shocks
εB =εe/10= 0.01

ζ = 5×10‐4

Examples : a single pulse burst
EBL not included

Full calculaMon with all processes (adiabaMc cooling; sync; sync self‐abs; IC; γγ annihilaMon)
Dynamics : ejecMon Γ(t)=100→400 ; duraMon = 1 s ; kineMc energy injecMon rate Lkin = 1054 erg

Synchrotron radiation in internal shocks
εB =εe/100= 0.001

ζ = 3×10‐4

Examples : a single pulse burst
EBL not included

Full calculaMon with all processes (adiabaMc cooling; sync; sync self‐abs; IC; γγ annihilaMon)
Dynamics : ejecMon Γ(t)=100→400 ; duraMon = 1 s ; kineMc energy injecMon rate Lkin = 1054 erg

Synchrotron radiation in internal shocks
εB =εe/1000= 0.0001

ζ = 2×10‐4

Examples : a single pulse burst
EBL not included

Full calculaMon with all processes (adiabaMc cooling; sync; sync self‐abs; IC; γγ annihilaMon)
Dynamics : ejecMon Γ(t)=100→400 ; duraMon = 1 s ; kineMc energy injecMon rate Lkin = 1054 erg

Synchrotron radiation in internal shocks

Examples : a single pulse burst
EBL not included

Delayed GeV emission
This feature is suppressed
for more rapid variaMons of Γ(t)Variable addiMonal
component at high‐energy

The origin of the prompt GRB spectrum

Summary‐Fermi/LAT observaMons (rate + HE spectrum of detected GRBs) :‐ synchrotron is favored‐ SSC seems in contradicMon with observaMons(Bosnjak, Daigne & Dubus 2009 ; Piran, Sari & Zou 2009)

‐IC scawerings in KN regime can aﬀect the synchrotron slope : ‐3/2 ≤ α ≤ ‐1
‐ this can reconcile the synchrotron process with observaMons‐ spectra with ‐1 ≤ α ≤ ‐2/3 ?

(Bosnjak, Daigne & Dubus 2009 ; Nakar, Ando & Sari 2009 ; Daigne et al. in preparaMon)

‐Shock acceleraMon physics in mildly relaMvisMc regime ? (low ζ ; low εB)

Are microphysics parameters constant ? (may change spectral evoluMon)

‐Internal shocks : spectral evoluMon is expected(Bosnjak, Daigne & Dubus 2009)‐ delayed GeV emission‐ variable addiMonal power‐law component at high‐energy(but component below 50 keV ?) Work in progress :‐ detailed modelling of LAT bursts‐ test of observed hardness‐intensity and hardness‐ﬂuence correlaMons