GRB Physics: an Overview Bing Zhang University of Nevada Las Vegas

1. GRB Physics: an Overview Bing Zhang University of Nevada Las Vegas Kavli Institute of Astronomy and Astrophysics & Astronomy Department, Peking University Nov. 8, 2012, Tsinghua SN and Transient Workshop, Beijing, Nov. 5-9. 2012

2. Gamma-ray bursts: the most luminous explosions in the universe

3. Light curves and spectra

4. What do we know for certain about GRBs? • They are at cosmological distances, and hence, very luminous and energetic • The emitters are moving towards earth with a relativistic speed • The outflow is very likely collimated • There are at least two types of progenitor: one is related to deaths of massive stars, the other is not.

5. 1. Distance and Energetics

6. Distance and Energetics Galactic halo: Cosmological:

7. Discovery of afterglow with BeppoSAX GRB 970228 X-ray afterglow: Costa et al. 1997 Optical afterglow: van Paradijs et al. 1997

8. GRB afterglow/host galaxy spectra are red-shifted: cosmological origin! GRBs are at cosmological distances, and GRBs are the most luminous explosions in the universe. Metzger et al. 2007

9. Distance and Energetics Galactic halo: Cosmological: For comparison: Energy emitted by a GRB in one second (assuming isotropic) is comparable to energy of Sun emitted in the entire life time: 10333.151071011~31051 erg

10. 2. Relativistic Motion

11. c δT

12. Relativity at Work • Solution: • In the comoving frame, the size of emission increases by a factor Γ. • In the comoving frame, the observed photon energy is systematically de-blueshifted by a factor Γ. Gamma-rays are now X-rays. The fraction of photons that are above the pair production threshold is greatly reduced. • Putting the two effects together, the optical depth is reduced by a factor Γ2β-2. For a typical bright GRB, one gets typical speed: v ~ 0.999995 c, or  ~ 300

13. 3. Collimation

14. Arguments for collimation • Eddington limit argument: no sustained accretion is possible if isotropic • Energetics argument: some GRBs have isotropic energy 1055 erg, which is 5 solar mass rest energy. Collimation greatly eases the energy budget • Afterglow lightcurve break: “jet break”

15. 4. Two types of progenitor

16. GRB/SN associations Bloom et al. (1999) Hjorth et al. (2003) Pian’s talk next!

17. GRB from a collapsing star

18. Swift & 2005 Discoveries GRB 050509B (z=0.225) Gehrels et al. 2005; Fox et al. 2005; Barthelmy et al. 2005; Berger et al. 2005 GRB 050724 (z=0.258)

19. NS-NS merger

20. NS-BH merger

21. GRBs & Astrophysics Stellar Astronomy GRBs Cosmology - Probing the dark era - Reionization history - Standard candle - Massive star physics - Supernova physics - Compact star physics - BH accretion physics - NS physics & strong B - Population synthesis Interstellar Medium Galactic Astronomy - Interstellar medium density - Stellar wind - Shock physics - Dust … - Star forming galaxies - Local star forming regions - Damped Lyman alpha systems

22. GRBs & Physics Quantum Gravity, New Physics Strong Interaction Weak Interaction Constrain LIV Neutrino Oscillation Hadronic (pp, pn, p) interactions Quark novae, QCD phase diagram Neutrino production GRBs Electromagnetic Gravity Synchrotron radiation Inverse Compton Pair production … Core collapse Accretion Compact star merger Gravitational waves Quantum Mechanics Relativity QED processes Hadronic processes Relativistic bulk motion Relativistic particles GR near the BH/NS

23. The GRB field • An active, exciting field • Due to their elusive nature, it is very difficult to observe GRBs in all the temporal and spectral regimes • the mystery of GRBs is gradually unveiled when new temporal or spectral windows are opened • GRBs may be also strong emitters of non-electromagnetic signals (e.g. high energy neutrinos, gravitational waves) • A sketch of physical picture is available, but many details remain vague – many open questions

24. Physical Picture: A Sketch Afterglow Progenitor Central Engine GRB prompt emission photosphere internal (shock) external shocks (reverse)(forward) Increasingly difficult to diagnose with electromagnetic signals

25. Open Questions in GRB Physics • Progenitors & classification(massive stars vs. compact stars; others? how many physically distinct types?) • Central engine (black hole, magnetar?) • Ejecta composition (baryonic, leptonic, magnetic?) • Energy dissipation mechanism (shock vs. magnetic reconnection) • Particle acceleration & radiation mechanisms (synchrotron, inverse Compton, quasi-thermal) • Afterglow physics (medium interaction vs. long-term engine activity)

26. Open Question 1: Origin of Afterglow

27. Physical Picture: A Sketch Afterglow Progenitor Central Engine GRB prompt emission photosphere internal (shock) external shocks (reverse)(forward)

28. Standard afterglow model Sari, Piran & Narayan (1998) Synchrotron emission from external forward shock: Meszaros & Rees (1997); Sari et al. (1998)

29. Afterglow Closure Relations Well-predicted temporal decay indices and spectral indices Sari, Piran & Narayan (1998) Chevalier & Li (2000) Dai & Cheng (2001) Zhang & Meszaros (2004)

30. Pre-Swift: Confronting data with theory Wijers & Galama 99 Panaitescu & Kumar (01, 02) Stanek et al. 99

31. Swift surprise Gehrels et al. (2004) Nousek et al. (2006), O’Brien et al. (2006)

32. Swift surprise Not predicted!

33. A Five-Component Canonical X-Ray Afterglow ~ -3 Zhang et al. (2006) I V II ~ -0.5 III 10^4 – 10^5 s ~ - 1.2 ~ -2 10^2 – 10^3 s 10^3 – 10^4 s IV

34. Canonical lightcurves: Internal or external?(Zhang et al. 2006; Nousek et al. 2006) “Curvature” tail Internal emission Late central engine activity I V II Continuous energy injection III Normal decay IV External forward shock emission? Maybe internal as well Post jet break decay

35. Puzzling fact: Chromatic breaks GeV light curve (external shock?, Kumar, Ghisellini) • At least 3 emission components: • Erratic flares • X-ray power law component • Optical power law component X-ray lightcurve I V II III • At least 3 emission sites? • External forward shock • External reverse shock • Internal dissipation site of a late jet from central engine • …… IV Optical light curve

36. Current afterglow picture • The so-called “afterglow” is a superposition of the traditional external shock afterglow and internal dissipation of a long-lasting wind launched by a gradually dying central engine. • The GRB cartoon picture no longer just describes a time sequence, but delineates an instantaneous spatial picture as well. • Observed emission comes from multiple emission sites!

37. Physical Picture: A Sketch Afterglow Progenitor Central Engine GRB prompt emission photosphere internal (shock) external shocks (reverse)(forward)

38. Open Questions 2, 3 & 4: Origin of Prompt Emission:Jet Composition (matter vs. magnetic) Energy dissipation (shock vs. reconnection) Radiation Mechanisms (thermal, synchrotron, inverse Compton)

39. Prompt GRB Emission: Still a Mystery ? central photosphere internal external shocks engine (reverse)(forward) Whatis the jet composition (baryonic vs. Poynting flux)? Whereis (are) the dissipation radius (radii)? Howis the radiation generated (synchrotron, Compton scattering, thermal)?

40. Fireball shock model(Paczynski, Meszaros, Rees, Piran …) Afterglow Progenitor Central Engine GRB prompt emission photosphere internal shocks external shocks (reverse)(forward)

41. Fireball Predictions: Internal shock vs. photosphere Meszaros & Rees (00) Pe’er et al. (06) Daigne & Mochkovitch (02)

42. Fermi Satellite:Broad-Band High Energy Observatory ?

43. Fermi surprise: GRB 080916C(Abdo et al. 2009, Science)

44. Fermi Surprise: Photosphere component missing Zhang & Pe’er (2009) Sigma: ratio between Poynting flux and baryonic flux:  = LP/Lb: at least ~ 20, 15 for GRB 080916C

45. The simplest fireball model does not work! Theorists’ view cannot be more diverse since the establishment of cosmological origin of GRBs! • Three distinct views: • The observed component is: • The internal shock component • The photosphere component • Neither (Poynting flux dissipation component)

46. Modified Fireball Model (1) central photosphere internal shocks external shocks engine (reverse)(forward) GRB prompt emission is from internal shocks Photosphere emission suppressed

47. Internal shock model • Pros: • Naturally expected • Variability reflects engine activity, supported by the data • Cons: • Bright photosphere, require a magnetized central engine and fast magnetic acceleration • Low efficiency • Only a fraction of electrons accelerated • Fast cooling problem • Ep – Eiso (Liso) correlation inconsistency Talks: Mochkovitch, Daigne, Hacoet …

48. Modified Fireball Model (2) central photosphere internal shocks external shocks engine (reverse)(forward) GRB prompt emission: from photosphere Internal shock emission suppressed

49. Photosphere model • Pros: • Naturally expected in a hot fireball • Roughly right Ep, narrow Ep distribution • Roughly right empirical correlations • Cons: • Low energy spectrum too hard (cf. Pe’er’s talk) • Inconsistent with the 3 independent constraints (X-ray, optical, GeV) of large GRB emission radius Pe’er, Ryde, Ioka, Beloborodov, Giannios, Lazzati, Toma, Ruffini …

50. Distance Scales in the ICMART Model (Internal Collision-induced MAgnetic Reconnection & Turbulence) Emission suppressed GRB At most 1/(1+σ) energy released At most 1/(1+σ) energy released 1/(1+σend) energy released photosphere R ~ 1011 - 1012 cm   0 early collisions R ~ 1013 - 1014 cm  ~ 1- 100 ICMART region R ~ 1015 - 1016 cm ini ~ 1- 100 end  1 External shock R ~ 1017 cm   1 central engine R ~ 107 cm  = 0 >> 1 Zhang & Yan (2011)