Ehud Nakar

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Theoretical models of high energy emission from
GRBs
Ehud Nakar California Institute of Technology
Energy Budget in the High Energy Universe
Workshop ICRR, Kashiwa, Feb. 23
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• Outline
• Observed nonthermal radiation properties and
  energy output
• The fireball model
• Predicted nonthermal radiation
• Ultra-high energy protons
• Nuetrinos
• GeV-TeV photons
• Summary

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Observations 10 keV-1 MeV photons (prompt
emission)

• Non-thermal spectrum (best fitted by a broken
  power law, Peaking at 0.1-1Mev)
• Highly variable temporal structure

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Longs shorts
Kouveliotou et al. 1993
?
Longs Collapsar (Woosley et al.) Associated with
SNe (Galama et al. 1998 Stanek et al. 2003
Hjorth et al. 2003 )
Shorts Gyrs old progenitors (Nakar et al
2005). A merger of compact binary ??? (Eichler
et al 1989 )
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Observations lt10 keV photons (afterglow)
The soft g-rays are followed by non-thermal x-ray
(minutes-hours), optical (hours-days) and radio
(days-years) counterpart emission
(Afterglow). Ek Eafterglow Eprompt
Palazzi et al 99
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Observations 10 MeV-100 GeV photons
Hurley et al 1994
EGRET observed GeV emission from several GRBs. In
GRB 940217 A 18GeV photon was observed 90min
after the burst
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GRBs (longs shorts) are highly relativistic
sources
High Luminosity 1052erg/s
Hard (gtMeV) nonthermal Spectrum
Rapid Variability dt10ms


The emitting source must be relativistic in order
to avoid high pair-production opacity! Sari
Lithwick (2001) find Ggt300 for several bursts for
which very hard photons are observed.
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The Fireball Model Prompt emission
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Afterglow (in the fireball model)
Relativistic ejecta
X-rays Optical Radio
Baryonic flow
Forward shock (1017-1018 cm)
External medium
Meszaros Rees 92 Meszaros Rees 92
Katz 94 Sari Piran 95 Luytikov Blandford
2002
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OR

• A reverse shock (? Baryonic flow) has a
  distinctive optical and radio signature (Sari
  Piran 99 Nakar Piran 2004).
• Swift did not detect any burst with clear reverse
  shock signature.
• Why?
• Optical flash is fainter than early estimates
  (Nakar Piran 2004)
• IC cooling may suppress the optical emission
  (Beloborodov 2005)
• The early x-ray aftergow do not behave as
  expected (energy injection ?)
• Highly magnetized flow (Lutikov 2005 Zhang
  Kobayashi 2005)

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Energy per burst
Beaming
The jet opening angle is estimated by the time of
the monochromatic break in the afterglow
Etot 1051 erg
(Frail et al 2001 Bloom et al 2003)
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No significant energy is carried by a low G flow

• Radio Calorimetry
• After about a year the blast wave is Newtonian
  and roughly spherical. Radio observations at
  this time provide an independent estimate of the
  total energy.
• Observations of several radio bright bursts show
  a total energy of 1051 erg (e.g. Frail et al
  1997 Frail et al 2004)
• Orphan afterglows
• Analysis of ROSAT all sky x-ray survey shows that
  the total energy emitted in low G flows (G10)
  that do not emit g-rays is small (Nakar Piran
  2003) .

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Theoretical predictions (Calculated for long GRBs)
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Ultra High Energy Cosmic rays
GRBs are one of the most promising sources of
UHECR through diffusive shock acceleration DSA
(Waxman 1995 Vietri 1995 Milgrom Usov 1995)
Energy budget
Ok, given that UHECR production is as efficient
as g-rays production
But can GRBs accelerate UHECRs?
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Internal shocks and/or reverse shock

• Necessary conditions for DSA
• Confinement acceleration faster than adiabatic
  cooling (assuming coherent B over R/G)
• Larmour radius lt R/G ?

Wind luminosity
Magnetization parameter
CR energy
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2) acceleration faster than radiative cooling
tacclttsynch ?
Yes, if the relativistic wind is weakly
magnetized (not poynting flux dominated) and
100ltGlt1000
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External shock

• The external medium B is too low in order to
  confine the UHECRs to the shock (Gallant
  Achtenberg 1999)
• Amplified B cannot be coherent over scale larger
  than R/G2
• Even equipartition B with such coherence length
  cannot confine UHECRs to the shock
• UHECRS are NOT produced in External shocks
  through DSA (Milosavljevic Nakar 2005)

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Neutrinos
Neutrinos in GRBs are expected to be produced
during Internal shocks photopion production
Reverse shock photopion production
Jet-envelope interaction photopion and
inelastic nucleon-nucleon collisions Baryonic
wind acceleration inelastic nucleon-nucleon
collisions
All processes are expected only in a baryonic
dominated relativistic wind
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Internal shocks MeV g 1016 eV p ? 1014 eV
nm,nm,ne
Assumed Accelerated CRs
Observed radiation
A flat spectrum in the range 1014-1016 eV and
a total energy output (waxman Bahcall
1997,1999 Rachen Meszaros 1998)
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Reverse shock UV g 1020 eV p ? 1018 eV
nm,nm,ne
(waxman Bahcall 2000)
Optical-UV flash
Given the lack of bright optical-UV flashes in
Swift GRBs during the onset of the afterglow the
flux of these nutrinos is unlikely to be
detectable.
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Baryonic wind acceleration
neutrons
protons
photons
electrons
Collisions (first to break)
Thompson scattering
Collective EM forces
If decoupling takes place during acceleration
then inelastic nucleon-nucleon collisions produce
10GeV nutrinos (Derishev et al 1999 Bahcall
Meszaros 2000). About 10 of the nucleons energy
is radiated as nuetrinos
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Jet-envelope interaction (Meszaros Waxman 01)
Nucleons and x-ray photons

Internal shocks accelerated protons
gt5 TeV neutrinos
May be produced efficiently also in g-ray faint
GRBs (e.g. GRB 980425/SN98bw GRB 060218/SN06?)
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GeV-TeV photons

• Gev-TeV photons are expected to result from two
  processes
• Inverse compton
• Comptonization of the self synchrotron emission
  (SSC) in the internal, external and reverse
  shocks.
• IC of photons produced in one shock by electrons
  that are accelerated in another shock.
• p0 decay, proton synchrotron
• Expected to be fainter than IC component

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External shock SSC (Meszaros et al 1994 Dermer
2000 )
Multi-wavelength afterglow modeling (Panaitescu
Kumar 2002 Yost et al. 2003) finds ee10eB.
Suggestin an SSC component of 10GeV photons with
energy that is comparable or even larger than the
prompt emission.
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• Various IC components (some examples)
• Prompt emission photons on reverse shock
  electrons GeV-TeV flash (Beloborodov 2005).
• Reverse shock photons on forward shock electrons
  GeV-TeV flash (Peer Waxman 2004).
• X-ray flares photons on forward shock electrons
  GeV-TeV flare (Wang et al. 2006)
• Hypothesized UV flares photons on forward shock
  electrons sub-GeV flare (Fan Piran 2006)

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Summary (The energy pie)
Observed
Partially Observed
Predicted in a baryonic dominated Relativistic
outflow
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Thanks!
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There is a type of underluminos GRBs
GRB 980425/SN1998bw z0.0085 Eg,iso 1048erg
Rate 100 Gpc-3 yr-1
Ggt6
4 days ago GRB 060218/SN2006?? z0.03
Eg,iso ?
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