High-energy emission from Gamma-Ray Bursts Frdric Daigne Institut - - PowerPoint PPT Presentation

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High-energy emission from Gamma-Ray Bursts Frdric Daigne Institut - - PowerPoint PPT Presentation

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High-energy emission from Gamma-Ray Bursts Frdric Daigne Institut dAstrophysique de Paris, Universit Pierre et Marie Curie HEPRO III High Energy Phenomena in Relativistic Outflows Barcelona, June 27 July 1, 2011 Gamma-Ray

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High-energy emission from Gamma-Ray Bursts

Frédéric Daigne Institut d’Astrophysique de Paris, Université Pierre et Marie Curie

HEPRO III – High Energy Phenomena in Relativistic Outflows – Barcelona, June 27 – July 1, 2011

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Duration : ms → 1000 s (2 groups) Highly variable lightcurve Non-thermal spectrum (peak ~ keV – MeV) Distance : zmax,obs = 8.2 ! Eγ,iso ~ 1051 – 1054 erg ! Afterglow : minutes → weeks Flux detection : X, optical, radio Fast decay : Fν ∝ t-α ν-β

Gamma-Ray Bursts

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Relativistic ejection Acceleration : Γ > 100 Photosphere Internal dissipation (shocks, reconnection ?) Prompt γ-rays External shock Afterglow Reverse shock

Contact discontinuity Lateral expansion Non-relativistic regime

Log( R ) [meters]

The physics of GRBs

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  • Fermi-LAT : ~ 10 GRBs / year (to compare to GBM : ~ 250 GRBs / year)
  • 4 brightest bursts :
  • Low detection rate by the LAT : - no bright component in the GeV range
  • need for a cutoff at ~ 100 MeV ?

(see e.g. Le & Dermer 2009 ; Granot et al. 2010 ;

Guetta et al. 2011 ; Beniamini et al. 2011) GRB z Eγ,iso Group

Refs

080916C 4.35 8.8 1054 erg long

Abdo et al. (2009a)

090510 0.9 1.1 1053 erg short ?

Ackermann et al. (2010)

090902B 1.8 3.6 1054 erg long

Abdo et al. (2009b)

090926A 2.1 2.2 1054 erg Long

Ackermann et al. (2011)

(1) Detection of GRBs at high energy (GeV)

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  • An example : GRB 080916C

GRB 080916C (Abdo et al. 2009)

(1) Detection of GRBs at high energy (GeV)

GBM : keV-MeV LAT >100 MeV >1 GeV

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Constraints on the Lorentz factor

Alternative : the emitting source moves at a relativistic speed Size of the emitting region is larger ➔ lower photon densities Photons paths are almost parallel ➔ photon interaction less efficient

2Γ2c tvar > R

For a static source: γ-rays should not be able to escape due to photon photon annihilation: γγ → e+e- (above mec2 = 511 keV for head-on collisions)

(Rees 1966)

c tvar > R

  • Compactness problem : short variability timescale + huge luminosities

GRBs : pre-Fermi estimates (MeV observations) Γmin ~ 100-300

(see e.g. Baring & Harding 1997; Lithwick & Sari 2001)

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Constraints on the Lorentz factor

  • Fermi-LAT detections in the GeV range :

GRB 080916C (Abdo et al. 2009) Stricter Lorentz factor constraints

  • GRB 080916C : Γmin ≥ 887

(Abdo et al. 2009)

  • GRB 090510 : Γmin ≥ 1200

(Ackerman et al. 2010) Such values of the Lorentz factor :

  • are challenging for most models
  • f the central engine ;
  • have strong consequences on

the GRB scenario (photospheric radius, deceleration radius, …). However, these estimates are based

  • n simplified single zone models.
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Constraints on the Lorentz factor

  • Detailed calculation : space/time/direction-dependent radiation field

the estimate of Γmin is reduced by a factor ~ 2-3 (see Granot et al. 2008; Hascoët, Daigne, Mochkovitch & Vennin to be submitted) Model of bins a+b in GRB 080916C : Γmin ~ 360 (Hascoët et al. to be submitted) instead of ~900 (Abdo et al. 2009). 1 ¡MeV ¡ Bin ¡b ¡

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Constraints on the Lorentz factor

  • Detailed calculation : space/time/direction-dependant radiation field

the estimate of Γmin is reduced by a factor ~ 2-3 (see Granot et al. 2008; Hascoët, Daigne, Mochkovitch & Vennin to be submitted)

  • If the GeV and the MeV emission are not produced at the same place :

the constraint is even further reduced. (see Zhao et al. 2011; Zou et al. 2011)

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Constraints on the Lorentz factor

  • Detailed calculation : space/time/direction-dependant radiation field

the estimate of Γmin is reduced by a factor ~ 2-3 (see Granot et al. 2008; Hascoët, Daigne, Mochkovitch & Vennin to be submitted)

  • If the GeV and the MeV emission are not produced at the same place :

the constraint is even further reduced. (see Zhao et al. 2011; Zou et al. 2011)

  • A new approximate formula, more general, more accurate :

(Hascoët, Daigne, Mochkovitch & Vennin to be submitted) single zone formula (Abdo et al. 2009) correction factor (detailed modeling) additional correction factor, if different MeV/GeV emitting regions = 1, if RGeV = RMeV < 1, if RGeV > RMeV

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(2) Dominant spectral component

  • The main component already known in the keV-MeV range is dominant :

GRB 080916C (Abdo et al. 2009)

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(2) Dominant spectral component

  • The main component already known in the keV-MeV range is dominant :

GBM ¡ LAT ¡ GRB 080916C (Abdo et al. 2009)

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GRB 100724B (Guiriec et al. 2011)

(3) A weak and soft thermal component ?

  • In at least one case, there is possibly the detection of a weak thermal

component :

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The physical origin of the prompt emission

  • Fast variability : the prompt emission has an internal origin.
  • Three possible reservoirs for internal dissipation :

Thermal energy : radiated at the photosphere Pros :

  • no large theoretical uncertainty
  • high efficiency

Cons : -prompt spectrum is non-thermal : additional mechanisms are needed

  • origin of the steep decay in the X-ray afterglow ?
  • there is a hint for a weak soft component in GRBs which

is indeed thermal (Guiriec et al. 2011) TH ¡ NT ¡ Standard fireball : hot and bright photosphere TH ¡ NT ¡ Cold photosphere : magnetized outflow ? (see GRB 100724B) ? ¡ t ¡ Flux ¡ (X-­‑rays) ¡

(Paczynski 86; Goodman 86; Shemi & Piran 90; Meszaros & Rees 00; Meszaros et al. 02; Daigne & Mochkovitch 02; Zhang & Meszaros 02; Rees & Meszaros 05; Pe’er et al. 06, 07, 08, 10; Ioka et al. 07; Beloborodov 10; Toma et al. 10; Vurm et al. 2011; …)

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The physical origin of the prompt emission

  • Fast variability : the prompt emission has an internal origin.
  • Three possible reservoirs for internal dissipation :

Thermal energy : radiated at the photosphere Kinetic energy : -dissipation in shocks

  • radiation from shock accelerated electrons

Pros :

  • can reproduce well the temporal and spectral properties
  • origin of the early steep decay (X-ray afterglow) :

high-latitude emission (Kumar & Panaitescu 2000)

  • no large theoretical uncertainties on the dynamics
  • the spectrum may have several components

Cons : -low efficiency (Daigne & Mochkovitch 98 ;

see however Beloborodov 00; Kobayashi & Sari 01)

  • large theoretical uncertainties for shock acceleration

(Rees & Meszaros 94 ; Paczynski & Xu 94; Kobayashi et al. 97 ; Daigne & Mochkovitch 98, 00, 03 ; Meszaros & Rees 00; Pe’er et al. 06; Bosnjak, Daigne & Dubus 09 ; … )

~R/2Γ2c ¡ t ¡ Flux ¡ (X-­‑rays) ¡

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The physical origin of the prompt emission

  • Fast variability : the prompt emission has an internal origin.
  • Three possible reservoirs for internal dissipation :

Thermal energy : radiated at the photosphere Kinetic energy : -dissipation in shocks

  • radiation from shock accelerated electrons

Magnetic energy :

  • dissipation by magnetic reconnection
  • particle acceleration – radiation

Only toy models are available :

  • efficiency may be high (Thomson 94 ; Spruit et al. 01 ; Drenkhahn & Daigne

02 ; Giannios 06 ; Giannios & Spruit 07 ; Giannios 08 ; …)

  • spectrum may have several spectral components (Giannios 2008)
  • lightcurves may show two typical timescales (Zhang & Yan 2011)
  • lightcurves may show too symmetric pulses (Lazar et al. 2009)

More realistic and physically motivated simulations are needed ~R/2Γ2c ¡ t ¡ Flux ¡ (X-­‑rays) ¡

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The physical origin of the prompt emission

  • Fast variability : the prompt emission has an internal origin.
  • Three possible reservoirs for internal dissipation :

Thermal energy : radiated at the photosphere Kinetic energy : -dissipation in shocks

  • radiation from shock accelerated electrons

Magnetic energy : -dissipation by magnetic reconnection

  • particle acceleration – radiation

Combinations are possible :

  • photospheric emission + internal shocks
  • photospheric emission + magnetic dissipation
  • magnetic dissipation + internal shocks : unlikely (shocks cannot

propagate if the outflow is highly magnetized) ~R/2Γ2c ¡ t ¡ Flux ¡ (X-­‑rays) ¡

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The physical origin of the prompt emission

  • Dominant radiative process : synchrotron vs SSC ? (non photospheric models)
  • Synchrotron + IC scatterings in KN regime : low-energy slope α ~ -3/2 → -1

? ¡ ? ¡ SSC : Syn ¡ IC1 ¡ IC2 ¡ Synchrotron : Syn ¡ IC ¡

  • Where is the strong IC2 component ?
  • r the strong syn component ?
  • Energy crisis

Fermi-LAT detection rate and observations clearly favor the synchrotron process.

(see e.g. Bošnjak, Daigne & Dubus 09; Piran, Sari & Zou 09)

GBM LAT GBM ¡ LAT ¡ IC ¡sca'. ¡(Thomson) ¡ IC ¡sca'. ¡(Klein-­‑Nishina) ¡

(Derishev et al. 2001; Bosnjak et al. 2009 ; Nakar et al. 2009 ; Daigne et al. 2011)

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(4) Delayed onset of the GeV component

  • In several cases, there is a delayed onset of the GeV component :

GRB 080916C (Abdo et al. 2009)

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(4) Delayed onset of the GeV component

  • In several cases, there is a delayed onset of the GeV component :

GRB 080916C (Abdo et al. 2009)

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GRB 000902B (Abdo et al. 2009)

(5) Additional components at high energy

  • In some cases, an additional component is needed
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(5) Additional components at high energy

  • In some cases, an additional component is needed

GRB 000902B (Abdo et al. 2009)

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GeV delayed onset & additional components

  • Intrinsic spectral evolution ? (e.g. emergence of an IC component)

(Bosnjak, Daigne & Dubus 2009)

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GeV delayed onset & additional components

  • Intrinsic spectral evolution ? (e.g. emergence of an IC component)

Similar conclusions are obtained by Asano & Meszaros (Asano’s talk @ Fermi Symposium) They show that the late synchroton emission may explain the X-ray excess.

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Delayed onset of the GeV emission

  • Intrinsic spectral evolution ? (e.g. emergence of an IC component)
  • External inverse Compton
  • seed photons = jet cocoon, photosphere, …
  • relativistic electrons = internal shocks

(Toma et al. 2009; Toma et al. 2011)

(Asano & Meszaros 2011 : Asano’s talk @ Fermi Symposium)

This model shows naturally several components in the spectrum.

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Delayed onset of the GeV emission

  • Intrinsic spectral evolution ? (e.g. emergence of an IC component)
  • External inverse Compton
  • Emergence of a hadronic component ?
  • delay = proton acceleration
  • GeV emission = hadronic cascade

(Bötcher & Dermer 1998 ; Gupta & Zhang 2007 ; Asano & Inoue 2007 ; Asano, Inoue & Meszaros 2009 ; etc…)

Efficiency is low and these models require a huge energy injected in protons.

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Delayed onset of the GeV emission

  • Intrinsic spectral evolution ? (e.g. emergence of an IC component)
  • External inverse Compton
  • Emergence of a hadronic component ?
  • Delay : opacity effect ? (Hascoët, Daigne, Mochkovitch & Vennin to be submitted)

GBM LAT GRB080916C : Fermi obs. (Abdo et al. 2009)

tdelay ≈ 5 s tvar ≈ 0.5 s

1 MeV 3 GeV

tdelay ≈ 5 s tvar ≈ 0.5 s

GRB080916C : model

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  • Except in one case, there is no clear signature for a cutoff at high energy
  • In GRB 090926A, the spectral shape of the cutoff cannot be well characterized:
  • γγ opacity (shape depends on the time interval) ?
  • intrinsic curvature of the HE component ?

GRB 090926A (Ackermann et al. 2011)

(6) Cutoff at high energy

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(7) Long lasting emission in the LAT

  • A long lasting emission is detected in many cases in the LAT after the end of the

GBM emission : high-energy afterglow ? GRB 080916C (Abdo et al. 2009)

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Long lasting emission

  • High-energy afterglow ?

Problem : no simultaneous observations of the early X-ray afterglow by Swift Where is the X-ray early steep decay ? (in the high latitude emission scenario : prompt/afterglow transition) The long lasting emission associated with standard early afterglow

  • bservation may help in distinguishing between afterglow models)
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Long lasting emission

  • High-energy afterglow ?
  • An intringuing possibility : may the whole GeV emission be due to the external

shock ?

(Kumar & Barniol Duran 2009, 2010 ; Gao et al. 2009 ; Corsi, Guetta & Piro 2010 ; de Pasquale 2010; Ghisellini et al. 2010 ; Ghirlanda et al. 2010 ; …) Ghisellini et al. 2010

Possible issues :

  • afterglow must be in radiative regime

pair enrichment ? (see e.g. Beloborodov 2002)

  • exceeds maximum energy of synchrotron ?

(Piran & Nakar 2010)

  • needs Γ > 1000 (independent from γγ constraint)
  • needs very low density and magnetization in

the external medium (Kumar & Barniol Duran 2009) A possible test ? Variability in GeV lightcurve

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Summary

  • Fermi observations :

(1) low detection rate by the LAT (4 bright bursts in the GeV range) (2) MeV non-thermal component is dominant (3) weak & soft thermal component ? (4) GeV delayed onset (5) additional HE component is some cases (6) HE cutoff in at least one case (7) long lasting emission in the LAT

  • Models for the prompt emission :
  • photosphere + internal shocks or - (photosph. ?) + magnetic dissipation ?
  • dominant photosphere : spectral shape ? Early X-ray afterglow ?
  • dominant internal shocks : efficiency ? Shock acceleration ?

synchrotron + IC in Klein Nishina regime is favored

  • dominant magnetic dissipation : more modelling is needed
  • Models for the high-energy emission :
  • hadronic models have a low efficiency
  • leptonic models seem to be able to reproduce most observed features by a

combination of intrinsic spectral evolution + opacity effects

  • an intringuing possibility : external origin of the whole GeV emission ?