Transcript E - indico in2p3

Theory of prompt and afterglow emission
Robert Mochkovitch (IAP)
Gamma-Ray Bursts in the Multi-Messenger Era (Paris, 16-19 June 2014)
What we know for (almost) sure….
central engine
What we know for (almost) sure….
central engine
Unsettled issues:
• acceleration/energy content of the jet: thermal/magnetic?
• dissipation mechanism at work?
• respective contributions of the forward and reverse shocks to the afterglow
• surprises in the early afterglow
The prompt emission
Brief observational summary
• Temporal properties: hard X-rays
diversity of light curves
bimodal duration distribution
long bursts: collapsars
short bursts: merging of NS
variability down to the ms time scale
• Temporal properties: optical
→ diversity of behaviors
GRB 990123
GRB 041219
GRB 080319B
(the naked eye burst)
RAPTOR
GRB 990123 : optical not correlated to hard X-rays
GRB 041219 : optical and hard X-rays correlated
optical flux consistent with extrapolation of the hard X-ray spectrum at low energy
GRB 080319b : correlated (?)
optical flux 100 times brighter than extrapolation of the hard X-ray spectrum at low energy
• Spectral properties
GRB spectra are (too) simple : broken power-laws
a
b
a+2
b+2
Ep
Phenomenological Band function:
grey area: bright BATSE bursts
solid line: Fermi data
(Lu et al, 2012)
• Spectral properties
Going beyond the Band function:
→ indications of the presence of an underlying thermal (photospheric) component
GRB 100724B
(Guiriec et al, 2010)
GRB 110721A
Additional power-law in some cases (excess at low (keV) and high (>10 MeV) energy)
• Polarization
positive detection in a few events:
- GRB 041219A: P ~ 4 – 40% (IBIS; Götz et al, 2009)
- GRB 100826A, 110301A, 110721A: P ~ 20 – 80% (GAP; Yonetoku et al, 2011,2012)
Models
• Basic requirements
z → DL + observed flux → Eg,iso = 1051 – 1054 erg
short time scale variability → compact source
Gmin ~ 100 – 1000
Relativistic outflow to avoid
opacity problem: gg → e+e-
(Hascoët et al, 2012)
• Acceleration of the flow
Thermal :
(fireball model)
r r
r
E
G  G0      for     
mc2
 r0   r0 
 r0 
m: entrained mass
r
  for    
 r0 
 = 400
.
rphot  31013
Tobs
Eiso,53
G
 rphot 

 T0 
r
 acc 
3
2
racc ~ r0
cm
2 / 3
.  rphot  2 / 3

L ph  Eth 
r
 acc 
T0 ~ a few MeV
• Acceleration of the flow
  emag / eK
Magnetic :
slower acceleration:
1/ 3
r
G  G0  
 r0 
rphot  31013
initially
.
Eiso,53
(1   )G23
r1/3
cm
.  rphot  2 / 3

L ph  Eth 
  r0 
(Tchekhovskoy et al, 2010)
→ Acceleration may not be completed at the photosphere


→ Thermal emission can be much reduced if  th  Eth / E  1
→ Remaining magnetization   at infinity ?
• Dissipation processes
below the photosphere: “photospheric models”
np collisions
shocks
reconnection
→ energetic electrons (and positrons)
IC on thermal photons at a few optical depths below the photosphere
+ synchrotron contribution/geometrical effect at low energy
E2N(E)
IC
syn
Planck → Band
E
(Vurm et al, 2011)
• Dissipation processes
above the photosphere: internal shocks
variable Lorentz factor in the outflow
G
G12G22
14  tV   G2 
rIS  2ctV 2

6
10
 
 cm
G1  G22
1
s
100

 
2
G
G1>G2
Gr
Ediss  m1G1  m2 G2  (m1  m2 )Gr  c 2
g
r
dissipate 10 – 20 % of the flow KE
r
Redistribution of the dissipated energy :
e x Ediss : into a non thermal (power law)
distribution of electrons
B x Ediss : in magnetic energy
→ synchrotron emission
(Daigne & Mochkovitch, 1998)
Above the photosphere: reconnection
∞ > 1
difficult and uncertain physics
→ few predictions except ICMART
(Internal-Collision-induced MAgnetic Reconnection and Turbulence)
(Zhang & Zhang, 2014)
Potentially large efficiency : 30 – 50 % ?
• Evaluating the models
Photon flux [ph/cm2/s]
8-260 keV GBM
260 keV-5 MeV
>100 MeV LAT
>1 GeV
Time lags Pulse width
Internal shocks : many predictions in good agreement with observations :
hardness – duration , HIC , HFC , W(E)
Potential problems
Time [s]
(Bošnjak & Daigne 2013)
Energy [keV]
• efficiency requires cooling electrons in “fast cooling regime”
→ low energy slope a = -1.5 while <a>obs ~ -1
(see however Derishev, 2007; Daigne et al, 2011; Uhm & Zhang, 2013)
• acceleration of electrons: much energy into a small (1%) fraction of the electrons
• magnetic acceleration required to avoid bright photospheric emission
but then what about ∞ and the existence/efficiency of shocks ?
Reconnection: natural model if magnetic acceleration with large ∞ ?
• uncertainties with the spectrum: general shape, low energy spectral slope a
Photospheric models: less uncertain input physics
• requires an “adaptable dissipative process”
should work for a full range of L, Ep (X-ray flashes, X ray emission during quiescence in gamma)
→ Looking for tests of the various models…
• Temporal tests
steep decay at the end of the prompt phase
a ~ -3
tb
R
high latitude emission
L(t ) ~
1
 t  tb 
1 

t 

3
a (tb ) 
t 
R
R

c
2cG 2
t
dLogL
 3 b
dLogt tb
t
IS, ICMART : t ~ tb → a ~ -3
Photospheric models : t << tb
In photopheric models the initial decay must correspond
to an effective behavior of the central engine
(Hascoët et al, 2012)
tb
• Spectral tests
additional thermal (photospheric) component in the spectra ?
expected in internal shock, reconnection models…
(Guiriec et al, 2013)
…but a priori not in photospheric models where the spectrum is the (modified)
photospheric emission
Optical emission
Bursts where g/opt are correlated suggest similar emission radii : Rem,g ~ Rem,opt
→ risk of self-absorption in photospheric models
High energy emission
GRB 080916C (Abdo et al. 2009)
GBM : keV-MeV
gg → e+e- : Gmin depends on RGeV
LAT
>100 MeV
>1 GeV
RGeV ~ RMeV possible with IS, ICMART
but not in photospheric models
• Polarization
Models with synchrotron emission (internal shocks and reconnection)
Large P possible if:


- Bordered in emission region i.e. qB > 1/G ( B lines anchored at the source)

- Jet viewed on the edge : qv ~ qj (within 1/G) (random B )
Photospheric models
Polarization averages to ~ 0 except if the jet is viewed on the edge
(but synchrotron contribution can be present → P ≠ 0 )
(Toma, 2014)
Conclusions (prompt emission)
Best and worst for each model:
• Photospheric emission
B: reliable physics; good spectra
W: early steep decay ≠ high latitude emission ; optical prompt self-absorbed
• Internal shocks
B: large set of predictions agrees with observations
W: acceleration of electrons ; ∞ ; low energy spectral index
• Reconnection
B: natural if ∞ is large ; possibly large efficiency
W: few predictions ; spectra ?
Model geography
territories:
friendly
hostile
Terra incognita
internal shocks
large
quite large
small
photospheric
medium
medium
medium
reconnection
small
small
large
The afterglow
… results from the deceleration of the flow by the external medium (uniform or stellar wind)
• The pre-Swift era: afterglows looked pretty simple !
Forward shock dynamics described by the Blandford-McKee solution
 R 

G  G0 
R
 dec 
R
 t 

 G0 
t
 dec 
t
uniform medium
(at deceleration radius: swept up mass M sw 
M ej
G0

EK
)
G02c 2
stellar wind
R
-3/2
-1/2
t
-3/8
-1/4
Shock dissipated energy injected into a non-thermal distribution of electrons: e , p , z
and amplifies the magnetic field: B
Injection Lorentz factor of the electrons:
gm 
Post-shock magnetic field: B   1B/ 2  1/ 2 G → g c
 e mp
G
z me
nm ~ Bgm2 G
nc ~ Bgc2 G
Afterglow spectra and light curves are made of consecutive power-law segments
nc < nm
nm < nc
spectra
(Sari, Piran, Narayan, 1998)
Hz
Hz
light curves
(Panaitescu & Kumar, 2000)
day
day
multi-wavelength fit of the afterglow → EK , e , B , p , n/A*
EK = 2.6 1053 erg ; n = 0.14 cm-3
e = 0.046 ; B = 8.6 10-4 ; p = 2
(Panaitescu & Kumar, 2002)
Concerns: robustness of the results
→ constant microphysics parameters ?
→ uniform external medium often found; at odds with expectation for a WR progenitor
• The Swift era: surprises in the early afterglow
plateau phase, flares, steep slopes, optical/X-rays: (a)chromatic behaviors…
t -3.2
New ingredients/paradigm needed !
New ingredients
Plateaus and flares: a late activity of the central engine ?
Extended plateaus require large amounts of energy to be injected into the forward shock
E0 = 1052 erg
Einj = k  E0 with k = 2 , 10 , 100
Efs = E0 + Einj
→ efficiency crisis for the prompt phase
f mes 
Eg
Eg  E fs
but
f true 
Eg
Eg  E0
Ex: fmes = 0.1 ; k = 10 , 100 → ftrue = 0.53 , 0.92 !
Flares : late internal shocks ? But how to explain that
~ 100 s
~ 1000 s
t
 0. 1 - 0. 3
t
~ 10000 s
New ingredients
Plateaus: an initially inefficient afterglow ?
Let us assume:
“missing” energy”
• a wind external medium
• e ∝ n-n (n > ncrit) and constant for n < ncrit
∝ R2n
→ flat plateau for n ~ 1
n=1 ; A*=1
Efs
(Hascoët et al, 2014)
(Margutti et al, 2012)
New paradigm
Making the early afterglow with a long-lived reverse shock
Standard picture: the reverse shock is short-lived; it rapidly crosses the high G ejecta, heats the electrons
→ slow cooling electrons radiate in optical/IR: early optical flash (GRB 990123; Sari & Piran, 1999)
Alternative proposal: ejecta has a low G tail → the reverse shock is long-lived
(Genet et al, 2007; Uhm & Beloborodov, 2007)
Emission from the reverse shock sensitive to the distribution of energy in the ejecta
→ great flexibility in light curve shapes (Uhm et al, 2012)
Plateaus
injected power in the tail
FS
Steep slopes
-2.9
-3
But what about the steepest slopes? (internal plateaus)
→ magnetar activity ? (Lyons, O’Brien, Zhang et al, 2010)
(luminosity/duration of the plateau ↔ energy reservoir of a magnetar)
-9
Flares
After completion of internal shocks the ejecta is highly structured
t
 Cst
t
FS
(simulation by F. Daigne)
Additional assumption:
anisotropy of the radiation
field in comoving frame
• High energy afterglow emission
Forward shock synchrotron emission…
+ inverse Compton (for the highest energy photons)
(95 GeV @ 244 s and 32 GeV @ 34.4 ks in GRB 130427)
Alternative: pair loading and heating at the blast wave (Vurm, Hascoët, Beloborodov, 2014)
GRB 130427
The pairs make:
synchrotron emission → optical flash
IC scaterring with prompt/early afterglow photons → GeV emission
Conclusion
How to make new progress?
• Expect a Rosetta stone burst: GRB 130427A ?
• Enter a new era: SVOM (2020)
an improved spectral coverage of the prompt emission
GWACs: a real-time coverage in optical of ECLAIRs fov
GFTs: dedicated follow-up telescopes
• The multi-messenger era
neutrinos, cosmic rays, gravitational waves…
→ new clues on GRB physics ? (shock waves, magnetization)
GWAC
GFT