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Gamma Ray Bursts: open issues

Gamma Ray Bursts: open issues. Gabriele Ghisellini – Osservatorio di Brera. Brief history Power Short history of the paradigm: internal vs external shocks Afterglows: external shocks The spectral-energy relations GRBs for cosmology. Gamma-Ray Bursts: The story begins.

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Gamma Ray Bursts: open issues

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  1. Gamma Ray Bursts: open issues Gabriele Ghisellini – Osservatorio di Brera • Brief history • Power • Short history of the paradigm: internal vs external shocks • Afterglows: external shocks • The spectral-energy relations • GRBs for cosmology

  2. Gamma-Ray Bursts: The story begins Treates banning nuclear tests between USA and USSR in early 60s VELA Satellites: X and soft g-ray detectors Brief, intense flashes of g-rays Klebesadel R.W., Strong I.B., Olson R., 1973, Astrophysical Journal, 182, L85 `Observations of Gamma-Ray Bursts of Cosmic Origin’

  3. Paciesas et al (2002) Briggs et al (2002) Koveliotou (2002) Shortest 6 ms GRB 910711 Longest ~2000 s GRB 971208 Two flavours, long and short SHORT LONG Short – Hard Long - Soft

  4. Epeak featureless continuum power-laws - peak in F F ~ Eb F ~ Ea Spectra Non thermal spectra

  5. Italian-Dutch “Satellite per l’AstronomiaX” NFI Instruments Wide Field Cameras: 5% of sky – positioning ~ 4’ + Narrow Field Instruments arcmin resolution 1997: The BeppoSAX satellite Slewing in several hours

  6. Discovery of first afterglow! 28 February 3 March GRB 28 97 02

  7. Optical id. host galaxy: redshifts Cosmological origin ! ~120 / 3000 with z: <0.1 – 6.3 (Batse, SAX, HETE-II, Integral, Swift, …)

  8. Energy and Power Huge isotropicequivalent energy! Assume Isotropy GRB typical Fluence (i.e. time int. flux) is 10-8 – 10-4 erg/cm2 (1keV – 10 MeV) 119 GRBs with z

  9. Mc2c5 Planck power: = = 3.6x1059 erg/s Rg/c G GRB are powerful • AGN: L < 1048 erg/s • SN: L < 1045 erg/s (in photons) • GRB: L < 1053 erg/s

  10. “first light” & PopIII chemical evolution large scale structures cover the epoch of re-ionization Star Formation Rate Probes of far universe SNIa

  11. Huge energySmall VolumeFireball Invented even before knowing that GRBs are cosmological….

  12. A short history of fireballs 1978 Cavallo & Rees: fireball: photons trapped by their own pairs 1978 Rees: internal shocks in M87 to transport energy along the jet 1986 Paczynski: Cosmological GRB L=1051 erg/s and T~1 MeV 1986 Goodman: Tobs remains T during expansion. Doppler balances adiabatic cooling 1992 Pure fireball made by n n  e+e- . Focussing by gravitation

  13. e+e- n NS n

  14. A short history of fireballs 1978 Cavallo & Rees: fireball: photons trapped by their own pairs 1978 Rees: internal shocks in M87 to transport energy along the jet 1986 Paczynski: Cosmological GRB L=1051 erg/s and T~1 MeV 1986 Goodman: Tobs remains T during expansion. Doppler balances adiabatic cooling 1992 Pure fireball made by n n  e+e- . Focussing by gravitation 1992 Dirty fireball polluted by baryons. Re-conversion of bulk kinetic into radiation through shocks with external medium 1994 Internal shocks due to shells moving with different G

  15. Why internal shocks? A process that repeats itself Spikes have same duration

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

  17. Matheson et al. 2003 SN afterglow Afterglow re-brightening Host galaxies Faint (mR~ 25 ) galaxies Sites of star formation Low metallicities Bloom et al. 2002 Progenitors GRBs associated with SN (Ib,c) A few spectroscopic ident. (underluminous?)

  18. compact object mergers(NS-NS, NS-BH) short GRBs Progenitors core collapse of massive stars(M > 30 Msun) long GRBs Collapsar or Hypernova(MacFadyen & Woosley 1999) GRB simultaneous with SN Supranova – two-step collapse(Vietri & Stella 1998) GRB delayed by few months-years ? Discriminants: host galaxies, location within host, duration, environment, redshift distribution, ...

  19. Accreting torus The engine Formation of a spinning BH + dense torus, sustaining B ~ 1014-1015 G Extraction BH spin energy (0.29 MBHc2) Extract E > 1052 erg tGRB ~ 104 tdyn

  20. Jets

  21.   , Surf. Jet half opening angle Relativistc beaming: emitting surface  1/ Log(F) Jet break Log(t) Jet effect  >> 1/   1/

  22. Jet break time tbreak Jet opening angle  Israel et al. 1999 GRB Jet measure “Jet break”

  23. Bloom et al. 2003 “True” energetics Isotropic equivalent energy Epeak was not considered… Etrue = Eiso (1 – cos ) Frail et al. 2001

  24. Peak energy – Isotropic energy Correlation 9+2 BeppoSAX GRBs Epeak  Eiso0.5 Amati et al. 2002 Epeak(1+z)

  25. 1- cos qjet “Ghirlanda” (25) “Amati” (62) Nava et al. 2006; Ghirlanda et al. 2007

  26. GRBs GRBs can be used as cosmological RULERS ! Ghirlanda, Ghisellini, Lazzati & Firmani 2004 Luminosity distance Supernovae redshift

  27. Problems: 1: Efficiency

  28. Efficiency=Radiated/total energy Only the RELATIVE kinetic energy can be used! Shells of equal masses Dynamical efficiency (%) Shells of equal energies 5% Gfinal ~ (G1G2)1/2

  29. A lot of kinetic energy should remain to power the afterglow Prompt SAX X-ray afterglow light curve Piro astro-ph/0001436

  30. SWIFT Willingale et al. 2007

  31. Eafterglow < Eprompt Eafterglow ~ 0.1 Eprompt

  32. Problems:2: Early “afterglow”

  33. Good old times Piro astro-ph/0001436

  34. Now: a mess GRB 050904 z=6.29

  35. X Opt. Panaitescu 2006

  36. X-ray and optical behave differently Is this “real” afterglow? i.e. external shock? X-rays: steep-flat-steep TA

  37. Early (normal) prompt: G>>1/qj Late prompt: G>1/qj ”real” after-glow Late prompt: G=1/qj Late prompt: G<1/qj Ghisellini et al. 2007

  38. Long lasting engine?? • Rs/c ~ 10-4 s (for a 10 solar mass BH) • Even 10 s are 105 dynamical times • Two-phase accretion?

  39. Conclusions “Paradigm”: internal+external shocks, synchrotron for both: it helps, but it is limiting Efficiency is an issue Progenitors for long: done. For short: not yet Central engine? How long does it live? GRBs as probes of the far universe (continue…)

  40. There can be a Black Body … but Time integrated spectrum Time resolved spectra Ghirlanda et al. 2007b The same occurs for ALL GRBs detected by BATSE and with WFC

  41. Memory

  42. Epeak = 390 keV Epeak =509 keV Epeak = 416 keV Epeak =503 keV Ghirlanda PhD thesis cts/sec EF(E) Time [sec] EF(E) EF(E) GRB spectrum evolves with time within single bursts

  43. Hard to Soft evolution phot /cm^2 sec Epeak,a(t), b(t) Epeak Decrease independent of the rise and decay of the flux a b

  44. Tracking evolution Photon flux Correlated with Epeak Epeak(t), a(t) , b(t) a b time

  45. By construction, internal shocks should all be equal. Then, why does the spectrum evolve?

  46. Spectra Spectra na nb Fishman & Meegan 1995 Epeak

  47. Prompt radiation: Synchrotron?

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

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