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GRB emission models and jet properties

GRB emission models and jet properties. Gabriele Ghisellini INAF-Osservatorio Astronomico di Brera - Italy. with the help of: Z. Bosniak, D. Burlon, A. Celotti, C. Firmani, G. Ghirlanda, D. Lazzati, M. Nardini, L. Nava, F. Tavecchio. The “standard” model. “The” model: Internal/External Shocks.

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GRB emission models and jet properties

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  1. GRB emission modelsand jet properties Gabriele Ghisellini INAF-Osservatorio Astronomico di Brera - Italy with the help of: Z. Bosniak, D. Burlon, A. Celotti, C. Firmani, G. Ghirlanda, D. Lazzati, M. Nardini, L. Nava, F. Tavecchio

  2. The “standard” model

  3. “The” model:Internal/External Shocks Rees-Meszaros-Piran Shell still opaque Relativ. e- + B: synchrotron?? Relativ. e- + B: synchrotron

  4. Spectra Spectra na nb E F(E) Fishman & Meegan 1995 Epeak Energy [MeV]

  5. Prompt radiation: Synchrotron?

  6. Synchrotron g-ray emission? The shell itself carries B-field B can also be produced and amplified by the internal shock The shock can accelerate e- to relativistic energies Synchrotron seems a good choice hnsyn ~ a few hundreds keV seems reasonable

  7. Synchrotron g-ray emission? ee (G/100) 3 tcool ~ 10-7 sec 2 nMeV Radiative cooling is (and must be) very rapid • Extremely short - No way to make it longer • tcool << tdynamical ~ 10-2 sec • It must be short: if not, how can the flux vary?

  8. Energy spectrum of a cooling electron Fast cooling + synchro: E(n) n-1/2 N(n) n-3/2 Photon index a

  9. Kaneko+ 2006

  10. Kaneko+ 2006 Nava PhD thesis 2009 Line of death for cooling e- Line of death for non cooling e-

  11. Seeking alternatives… • Bulk Compton (Lazzati+ 2000; GG+ 2000) • Quasi-thermal Comptonization (GG & Celotti 1999) • Black-body from “deep impacts” (Thompson+2007; GG+ 2007; Lazzati+ 2009) • Reconnection and continuous heating (Giannios 2008)

  12. Seeking alternatives… • Bulk Compton (Lazzati+ 2000; GG+ 2000) • Quasi-thermal Comptonization (GG & Celotti 1999) • Black-body from “deep impacts” (Thompson+2007; GG+ 2007; Lazzati+ 2009) • Reconnection and continuous heating (Giannios 2008)

  13. “Deep impacts” Thompson, Meszaros & Rees 2007Lazzati, Morsony Begelman 2008GG+ 2007 At R ~ Rstar the fireball dissipates part of its energy  BB

  14. Dissipation Wolf Rayet (about to explode) BB Transparency G~1/qj (i.e very small)

  15. Problems • G fine tuned (and small) in Thompson+ 2008 • BB??

  16. There can be a Black Body … but BATSE Time integrated spectrum BB Time resolved spectra power law Cut off pl The same occurs for ALL GRBs detected by BATSE and with WFC 30 keV Ghirlanda+ 2007

  17. FERMI-GBM Cutoff PL Ghirlanda+ 2009 30 keV

  18. FERMI-GBM BB+PL Ghirlanda+ 2009 30 keV

  19. FERMI-GBM Cutoff PL Ghirlanda+ 2009

  20. FERMI-GBM BB+PL The same occurs for all time slices Ghirlanda+ 2009

  21. First conclusion • We do not know yet what is the emission mechanism of the prompt

  22. GeV emission Prompt or afterglow?

  23. All LAT GRBs GG+ 2009

  24. short

  25. a b G Log nFn GBM LAT Log n

  26. a G b Log nFn Log n LAT GBM b vs G a vs G G

  27. The 4 brightest LAT GRBs t-10/7 Spectrum and decay: afterglow Radiative!

  28. The 4 brightest LAT GRBs t-10/7 Radiative?

  29. e

  30. e

  31. e e- e+

  32. e e- e+

  33. Time Time

  34. GRB 090510 Fermi-LAT • Short • Very hard • z=0.903 • Detected by the LAT up to 31 GeV!! • Well defined timing • Delay: ~GeV arrive after ~MeV (fraction of seconds) • Quantum Gravity? Violation of Lorentz invariance?

  35. precursor 8-260 keV 0.26-5 MeV Delay between GBM and LAT Due to Lorentz invariance violation? LAT all Abdo et al 2009 >100 MeV >1 GeV 31 GeV 0.6s 0.5s Time since trigger (precursor)

  36. 0.1 GeV 30 GeV Average Different component Time resolved 0.5-1s 2 3 nF(n) [erg/cm2/s] If LAT and GBM radiation are cospatial: G>1000 to avoid photon-photon absorption 3 4 Abdo et al 2009 1 If G>1000: deceleration of the fireball occurs early  early afterglow! If G>1000: large electron energies  synchrotron afterglow! Energy [keV]

  37. Fermi-LAT t2 t-1.5 Ghirlanda+ 2009

  38. 0.1-1 GeV Ghirlanda+ 2009 >1 GeV T-T* [s]

  39. ~MeV and ~GeV emission are NOT cospatial. But the ~GeV emission is… No measurable delay in arrival time of high energy photons: tdelay<0.2 s  Strong limit to quantum gravity  MQG > 4.7 MPlanck Ghirlanda+ 2009 T-T* [s]

  40. Conclusions “Paradigm”: internal+external shocks, synchrotron for both: it does not work Fermi/LAT detection  large G Early high energy (and powerful) afterglow Decay suggests radiative afterglows GRB 090510: Violation of the Lorentz invariance? No (not yet)

  41. Why internal shocks? A process that repeats itself Spikes have same duration Counts/s Time [seconds]

  42. Kaneko+ 2006 [keV]

  43. Nava PhD thesis 2009 Kaneko+ 2006 [keV]

  44. Can it be rescued by: • Reacceleration?No, in IS e- are accelerated only once. More generally, only few selected e- (and always the same) must be (re)accelerated • Adiabatic losses?No, too small regions would be involved, large e- densities, too much IC • Fast decaying B-field?No, synchro not efficient and too much IC • Self absorption?No, lots of e- needed, too much IC • Self Compton?No, tcool too small even in this case • Small pitch angles? No,Very very small to avoid cooling, becomes inefficient • External shocks to avoid cooling? No,same reason

  45. Bulk Compton Seeds n Wolf Rayet (about to explode) G2n G~100

  46. Problems • Time to refill the funnel of seeds • Needs a lot of seeds, not clear if the funnel is sufficient • Does not work for short (they do not have a SN)

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