Les sursauts gamma la phase des chocs internes

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Les sursauts gamma : la phase des chocs internes. Frédéric Daigne Atelier POLAR –

Les sursauts gamma : la phase des chocs internes. Frédéric Daigne Atelier POLAR – Annecy – 18 janvier 2008

Prompt emission — Internal shocks R in meters

Prompt emission — Internal shocks R in meters

Prompt emission — Internal shocks (Rees & Meszaros 94) Variability of the lightcurve Activity

Prompt emission — Internal shocks (Rees & Meszaros 94) Variability of the lightcurve Activity of the central engine Matter ejection by the central engine : energy injection rate and/or mass injection rate can vary on a dynamical timescale (ms) R in meters the final distribution of the Lorentz factor at the end of the acceleration phase can be highly variable shock waves propagate within the relativistic ejecta = internal shocks GRB = emission of the shocked material

Prompt emission — Internal shocks Ejection : G 321>G 21 Gamma-rays Obs. Racc Relativistic

Prompt emission — Internal shocks Ejection : G 321>G 21 Gamma-rays Obs. Racc Relativistic ejecta : -Width 3. 1010 tw, s cm - Variable Lorentz factor (G≥ 100) - Kinetic energy Lkin, 4 p 1051 - 1054 erg/s Rph Ris R : internal shocks Ris is : internal shocks 2 14 G R 6. 1014 cm Ris G 222 ttvar, s is 6. 10 var, s cm GRB time profile

Prompt emission — Internal shocks : dynamics Simple model : Daigne & Mochkovitch 2000

Prompt emission — Internal shocks : dynamics Simple model : Daigne & Mochkovitch 2000 Daigne & Mochkovitch 1998 Validation using hydrodynamical simulations (relativistic, Lagragian, 1 D in spherical symmetry).

Prompt emission — Internal shocks : dynamics The central source is ejecting relativistic matter

Prompt emission — Internal shocks : dynamics The central source is ejecting relativistic matter from t=0 to t=tw : * Shell ejected at tejec+tvar If contrast k = Gmax / Gmin > 1 shock at : Lorentz factor Gmin » 1 : Lorentz factor Gmax » 1 Rshock = 2 f Gmin 2 c tvar tshock = tejec + 2 f Gmin 2 tvar with f ≈ k 2/(k 2 -1) ≈ 1 for k > 2 -3 (for simplicity, assume the two shells have same mass M) two shells merge : * new mass 2 M * new Lorentz factor Gr ≈ (Gmin Gmax)1/2 = Gmin k 1/2 * dissipated energy : e ≈ (Gmin + Gmax -2 Gr ) Mc 2 * efficiency : fd ≈ (k 0. 5 -1)2 / (1+k) ≈ 10% - 40% for k = 3 -10

Prompt emission — Internal shocks : dynamics Shock : Rshock = 2 f Gmin

Prompt emission — Internal shocks : dynamics Shock : Rshock = 2 f Gmin 2 c tvar and tshock = tejec + 2 f Gmin 2 tvar with f ≈ k 2/(k 2 -1) ≈ 1 for k > 2 -3 shocked material : Lorentz factor Gr ≈ Gmin k 1/2 Observer time * arrival time of photons * angular spreading Lightcurve source activity ta = t – R / c ≈ tejec Dta = R / (2 G 2 c) ≈ tvar R 1/G Central source Observer DR = R ( 1 - cos(1/G)) = R / (2 G 2)

Prompt emission — Internal shocks : microphysics Microphysics : Internal shocks : mildly relativistic

Prompt emission — Internal shocks : microphysics Microphysics : Internal shocks : mildly relativistic Shocked material : density r* ≈ 7 r energy density e* / c 2 ≈ a few 100 Me. V/p Another possibility : Large-scale magnetic field (central engine) Equipartition parameters : Magnetic field e. B Electrons ee, z, p B 2/8 p ≈ e. B r* e* Density : z r* / mp Energy density : ee r* e* Distribution : n(Ge) Ge-p for Ge>Gm Lorentz factor : Gm ≈ (ee/z) (m. P/me) (e*/c 2)

Prompt emission — Internal shocks : magnetic field Large scale magnetic field : (Spruit,

Prompt emission — Internal shocks : magnetic field Large scale magnetic field : (Spruit, Daigne & Drenkhahn 01) Trapped field : (if no reconnection) magnetic energy = cst. “Passive” field (no dynamical effect)

Prompt emission — Internal shocks : magnetic field Large scale magnetic field : (Spruit,

Prompt emission — Internal shocks : magnetic field Large scale magnetic field : (Spruit, Daigne & Drenkhahn 01) Trapped field : (if no reconnection) magnetic energy = cst. “Passive” field (no dynamical effect) “Active” field (dynamical effect)

Prompt emission — Internal shocks : magnetic field Large scale magnetic field : (Spruit,

Prompt emission — Internal shocks : magnetic field Large scale magnetic field : (Spruit, Daigne & Drenkhahn 01) Trapped field : (if no reconnection) magnetic energy = cst. “Passive” field (no dynamical effect) “Active” field (dynamical effect) BUT : Reconnection can modifiy this picture Early reconnection : acceleration and then “passive” field Late reconnection : an alternative to internal shocks for the prompt emission ? In shocks, a turbulent B seems necessary to accelerate particles…

Prompt emission — Internal shocks : radiative processes Synchrotron / IC : ■ if

Prompt emission — Internal shocks : radiative processes Synchrotron / IC : ■ if z ≈ 1 : Gm ≈ 100 -1000 : GRB = IC pb = low efficiency (low B is needed) ■ if z small : Gm is larger : GRB = synchrotron ; HE=KN efficiency is better Global efficiency = f(dissipation) (10 -40 %. . . ) x ee (10%-50% ? ? ? ) x f(rad) (close to 100% fast cooling) x f(BATSE) (close to 100% if syn)

Prompt emission — Internal shocks : pulses Ryde & Svensson 2002 Spectral evolution in

Prompt emission — Internal shocks : pulses Ryde & Svensson 2002 Spectral evolution in GRB pulses : ■ Favors a continuous outflow (vs single shells with initial large separations, e. g. Kobayashi et al. 1997) This avoids to be dominated by the “curvature effect” (see Fenimore 1994) G 1>G 2 g m ■ Evolution of microphysics parameters ? (e. g. more electrons accelerated in violent shocks) Daigne & Mochkovitch 2003 G m

Prompt emission — Internal shocks: high energy emission ANR project : high-energy gamma-ray emission

Prompt emission — Internal shocks: high energy emission ANR project : high-energy gamma-ray emission from relativistic jets With Z. Bosnjak (IAP), G. Dubus (LAOG), B. Giebels (LLR) and F. Piron (LPTA) Front An example : Initial distribution of the Lorentz factor Evolution of the physical conditions in the shocked medium

Prompt emission — Internal shocks: high energy emission ANR project : high-energy gamma-ray emission

Prompt emission — Internal shocks: high energy emission ANR project : high-energy gamma-ray emission from relativistic jets With Z. Bosnjak (IAP), G. Dubus (LAOG), B. Giebels (LLR) and F. Piron (LPTA) An example : Lightcurves Time integrated spectrum Time dependant spectrum

Prompt emission — Internal shocks: high energy emission ANR project : high-energy gamma-ray emission

Prompt emission — Internal shocks: high energy emission ANR project : high-energy gamma-ray emission from relativistic jets With Z. Bosnjak (IAP), G. Dubus (LAOG), B. Giebels (LLR) and F. Piron (LPTA) An other example : a more optimistic case for GLAST… Lightcurves Time dependant spectrum Time integrated spectrum

Prompt emission — Internal shocks : polarization Granot & Königl 2003 ; Granot 2003

Prompt emission — Internal shocks : polarization Granot & Königl 2003 ; Granot 2003 ; Nakar, Piran & Waxman 2003 ■ Synchrotron radiation : local polarization (with p ~ 2. 5) : P ~ 75 % ■ Observation : averaging over a region ~ 1/G ■ Necessary conditions to reach a high observed polarization : peculiar field geometry (ordered field) or peculiar geometry (off axis observation …) : not favored by statistics If we exclude off-axis observations : ■ No large-scale magnetic field or dominant random field : a small polarization is expected (0 to a few %) ■ Dominant large-scale magnetic field : a larger polarization (~ 10 -40 %) can be expected. Warning : if the large-scale magnetic field is very large (s ~ 1 or more), internal shocks disappear (as well as the reverse shock : see Robert’s talk). Then, the emission has to be explained by magnetic reconnection in the outflow. . .

Prompt emission — Internal shocks : conclusion Internal shocks Status of the model: R

Prompt emission — Internal shocks : conclusion Internal shocks Status of the model: R in meters ■Dynamics is well understood ■Microphysics is poorly understood (GLAST, SVOM, …) ■Many GRB properties are reproduced (variability, spectral evolution, …) ■The model can reproduce the diversity of the GRB population ■Some difficulties/problems: a low efficiency, the low-energy slope (see Ghisellini, Celotti & Lazzati 2000). In the future : ■origin of « Amati » relation ■ Optical prompt emission ■Diagnostics from the HE emission ■Polarization ? ■…