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

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

Gamma-Ray Bursts Duration : ms → 1000 s (2 groups) Highly variable lightcurve Non-thermal spectrum (peak ~ keV – MeV) Distance : z max,obs = 8.2 ! E γ ,iso ~ 10 51 – 10 54 erg ! Afterglow : minutes → weeks Flux detection : X, optical, radio Fast decay : F ν ∝ t - α ν - β

The physics of GRBs Log( R ) [meters] Relativistic ejection Acceleration : Γ > 100 Photosphere Contact Internal dissipation Lateral expansion discontinuity (shocks, reconnection ?) Non-relativistic regime Prompt γ -rays Reverse shock External shock Afterglow

(1) Detection of GRBs at high energy (GeV)  Fermi-LAT : ~ 10 GRBs / year (to compare to GBM : ~ 250 GRBs / year)  4 brightest bursts : GRB z E γ ,iso Group Refs 080916C 4.35 8.8 10 54 erg long Abdo et al. (2009a) 090510 0.9 1.1 10 53 erg short ? Ackermann et al. (2010) 090902B 1.8 3.6 10 54 erg long Abdo et al. (2009b) 090926A 2.1 2.2 10 54 erg Long Ackermann et al. (2011)  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)

(1) Detection of GRBs at high energy (GeV)  An example : GRB 080916C GBM : keV-MeV GRB 080916C (Abdo et al. 2009) LAT >100 MeV >1 GeV

Constraints on the Lorentz factor  Compactness problem : short variability timescale + huge luminosities For a static source: γ -rays should not be able to escape c t var > R due to photon photon annihilation: γγ → e + e - (above m e c 2 = 511 keV for head-on collisions) Alternative : the emitting source moves at a relativistic speed (Rees 1966) Size of the emitting region is larger ➔ lower photon densities Photons paths are almost parallel ➔ photon interaction less efficient GRBs : pre-Fermi estimates (MeV observations) Γ min ~ 100-300 2 Γ 2 c t var > R (see e.g. Baring & Harding 1997; Lithwick & Sari 2001)

Constraints on the Lorentz factor  Fermi-LAT detections in the GeV range : Stricter Lorentz factor constraints • GRB 080916C : Γ min ≥ 887 (Abdo et al. 2009) • GRB 090510 : Γ min ≥ 1200 (Ackerman et al. 2010) Such values of the Lorentz factor : - are challenging for most models of the central engine ; - have strong consequences on the GRB scenario (photospheric radius, deceleration radius, …). GRB 080916C (Abdo et al. 2009) However, these estimates are based on simplified single zone models.

Constraints on the Lorentz factor  Detailed calculation : space/time/direction-dependent radiation field the estimate of Γ min is reduced by a factor ~ 2-3 (see Granot et al. 2008; Hascoët, Daigne, Mochkovitch & Vennin to be submitted) 1 ¡MeV ¡ Bin ¡b ¡ Model of bins a+b in GRB 080916C : Γ min ~ 360 (Hascoët et al. to be submitted) instead of ~900 (Abdo et al. 2009) .

Constraints on the Lorentz factor  Detailed calculation : space/time/direction-dependant radiation field the estimate of Γ min 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)

Constraints on the Lorentz factor  Detailed calculation : space/time/direction-dependant radiation field the estimate of Γ min 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) correction factor (detailed modeling) single zone formula (Abdo et al. 2009) additional correction factor, if different MeV/GeV emitting regions = 1, if R GeV = R MeV < 1, if R GeV > R MeV

(2) Dominant spectral component  The main component already known in the keV-MeV range is dominant : GRB 080916C (Abdo et al. 2009)

(2) Dominant spectral component  The main component already known in the keV-MeV range is dominant : GBM ¡ LAT ¡ GRB 080916C (Abdo et al. 2009)

(3) A weak and soft thermal component ?  In at least one case, there is possibly the detection of a weak thermal component : GRB 100724B (Guiriec et al. 2011)

The physical origin of the prompt emission  Fast variability : the prompt emission has an internal origin.  Three possible reservoirs for internal dissipation : Flux ¡ ? ¡ (X-­‑rays) ¡ Thermal energy : radiated at the photosphere t ¡ 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) (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; …) TH ¡ NT ¡ NT ¡ Standard fireball : Cold photosphere : hot and bright magnetized outflow ? photosphere (see GRB 100724B) TH ¡

The physical origin of the prompt emission  Fast variability : the prompt emission has an internal origin. ~R/2 Γ 2 c ¡  Three possible reservoirs for internal dissipation : Flux ¡ (X-­‑rays) ¡ Thermal energy : radiated at the photosphere t ¡ 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 ; … )

The physical origin of the prompt emission  Fast variability : the prompt emission has an internal origin. ~R/2 Γ 2 c ¡  Three possible reservoirs for internal dissipation : Flux ¡ (X-­‑rays) ¡ Thermal energy : radiated at the photosphere t ¡ 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

The physical origin of the prompt emission  Fast variability : the prompt emission has an internal origin. ~R/2 Γ 2 c ¡  Three possible reservoirs for internal dissipation : Flux ¡ (X-­‑rays) ¡ Thermal energy : radiated at the photosphere t ¡ 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)

The physical origin of the prompt emission  Dominant radiative process : synchrotron vs SSC ? (non photospheric models) LAT GBM SSC : ? ¡ - Where is the strong IC2 component ? ? ¡ or the strong syn component ? IC2 ¡ - Energy crisis IC1 ¡ IC ¡sca'. ¡(Thomson) ¡ Syn ¡ GBM ¡ Synchrotron : Fermi-LAT detection rate and observations LAT ¡ clearly favor the synchrotron process. Syn ¡ IC ¡sca'. ¡(Klein-­‑Nishina) ¡ IC ¡ (see e.g. Bo š njak, Daigne & Dubus 09; Piran, Sari & Zou 09)  Synchrotron + IC scatterings in KN regime : low-energy slope α ~ -3/2 → -1 (Derishev et al. 2001; Bosnjak et al. 2009 ; Nakar et al. 2009 ; Daigne et al. 2011)

(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)

(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)

(5) Additional components at high energy  In some cases, an additional component is needed GRB 000902B (Abdo et al. 2009)

(5) Additional components at high energy  In some cases, an additional component is needed GRB 000902B (Abdo et al. 2009)

GeV delayed onset & additional components  Intrinsic spectral evolution ? (e.g. emergence of an IC component) (Bosnjak, Daigne & Dubus 2009)