HIGH ENERGY ASTROPHYSICS Gamma-ray Bursts (GRBs)

1. HIGH ENERGY ASTROPHYSICSGamma-ray Bursts (GRBs) • Historical background • Main observational properties • spatial distribution • temporal • spectral • afterglows; optical counterparts • Some basic constraints • Scenario: fireball model • internal and external shocks • inner engine • unsolved problems

2. Historical background In October of 1963 the US Air Force launched the first in a series of satellites inspired by a recently signed nuclear test ban treaty. Signatories of this treaty agreed not to test nuclear devices in the atmosphere or in space. “Vela” satellites (from the Spanish verb velar, to watch) were launched and operated in pairs with two identical satellites on opposite sides of a circular orbit 250,000 kilometers in diameter (about a 4 day orbit) so that no part of the earth was shielded from direct observation. With the timing accuracy of the later Vela satellites (1965) Klebesadel and colleagues at LANL were able to deduce the directions to the events with sufficient accuracy to rule out the sun and earth as sources. They concluded that the gamma-ray events were "of cosmic origin". In 1973, this discovery was announced in Ap.J. letters by Klebesadel, Strong, and Olsen. Their paper discusses 16 cosmic gamma-ray bursts observed by Vela 5a,b and Vela 6a,b between July 1969, and July 1972.

3. Historical background

4. Historical background

7. Observationsspatial distribution Observed burst rate with BATSE: ~1 burst/day => one event/106yr/galaxy Locations of a total of 2704 GRBs recorded with BATSE onboard CGRO during 9 years of operation: burst spatial distribution is isotropic

8. Observations: spatial distribution BUT the spatial distribution is inhomogeneous Intensity distribution: log N – log P P: peak flux; N(P): number of bursts brighter than P For an homogeneous distribution: V r3; Flux  r-2 => N  V  F-3/2 => log N= - 3/2 log F + ct. expected slope of log N – log P curve : - 3/2

9. - 3/2 slope Peak flux in phot/cm2/s

10. Observations: spatial distribution BUT the spatial distribution is inhomogeneous: Intensity distribution: log N – log P P: peak flux; N(P): number of bursts brighter than P For an homogeneous distribution: V r3; Flux  r-2 => N  V  F-3/2 => log N= - 3/2 log F + ct. expected slope of log N – log P curve : - 3/2 Lack of faint sources (P<10) is observed: inhomogeneous distribution => cosmological effects if sources of weak bursts are located at large redshifts

11. Observations: spatial distribution Another proof of the inhomogeneity of the spatial distribution: V/Vmax test: quantitative evaluation of the uniformity of the radial distribution of a sample. For each object, its radial location within the volume available to it as determined by the sample limits is computed. Uniform space distribution => uniform distribution of V/Vmax between 0 and 1 => <V/ Vmax > = 0.5 <V/ Vmax > smaller than 0.5 for GRBs (~0.3-0.4);

12. Observations: temporal properties • Great diversity of time profiles • Variability down to ms scale • Burst composed of individual pulses • Typical individual pulse: “FRED” (fast rise exponential decay; duration: 1:3)

14. Pulse width average: 1s Pulse separation average: 1.3s

15. Observations: spectral properties Uniform spectra Non thermal, well fitted with power laws Peak at some hundred keV No lines GeV photons (EGRET) in some cases TeV photons (Milagrito) possibly in one case

16. A sample of BATSE spectra: uniformity; non thermal; broken power-laws Kouveliotou 1994

17. Non thermal spectra, well fitted using two power laws with a smooth transition at the peak energy Ep E exp(-E/E0) E< Eb N(E)= A E [ Eb- exp(-Eb/E0) ] E  Eb withbreak energyEb = (-) E0 and E0 = Ep / (2+ ) Band et al. 1993

18. Eb : 100-400 keV • Average Eb: 250 keV • => not many soft GRBs • not real: related to lower BATSE sensitivity at low E • BUT: X-ray flashes, discovered in 2001: • strong non-thermal emision (2-20 keV) • weak emission in the “GRB” band 50-300 keV • short durations < some 1000 s • 17 detected by Beppo-SAX WFC in 5 years Preece et al. 2000

19. Preece et al. 2000 Average  : -1 Average  : -2.25

20. Observations: temporal properties; populations t: 10-3-103s typical duration  10 s Bimodal distribution: T90< 2s T90> 2s N(short) ~1/3 N(long) (but BATSE was less sensitive to short bursts: short burts were detected to smaller d) Durations of the GRBs recorded with CGRO/BATSE. T90: time needed to accumulate from 5% to 95% of the counts in the 50-300 keV band.

21. Observations: some correlations • Hardness ratio – duration correlation • Hardness ratio - intensity correlation: pulses become softer during the pulse decay • Pulses are narrower at higher E

22. Piran 2004 (review) Short bursts harder than long bursts Hardness ratio (HR) versus duration of the GRBs recorded with CGRO/BATSE. HR:ratio of fluence between channels 3 (100-300 keV) and 2 (50-100 keV)

23. Previous results with a smaller sample (1994)

24. Observations: some correlations between temporal and specral properties • Hardness ratio – duration correlation • Hardness ratio - intensity correlation: pulses become softer during the pulse decay • Pulses peak earlier and are narrower at higher E bands

25. Relationship between temporal and spectral structure: narrowing with E: W  E-0.4 Fenimore 1995

26. Observations: afterglows No known counterparts of GRBs at lower energies were known before 1997: no idea of distance, energy budget not known, … Both a spherical source distribution limited in spatial extent and a cosmological population could be adopted (but cosmological preferred) February 28, 1997: Italian-Dutch satellite BeppoSAX detected the X-ray afterglow from GRB970228

27. Observations: counterparts; X-ray afterglows

28. Observations: optical counterparts

29. Observations: optical counterparts

30. Observations: X-ray, optical and radio counterparts 44 (end of 2003) GRBs with optical afterglows, 33 with known redshifts: z: 0.1-4.5; typical z  1 Before 2000: only 12 with optical afterglows were known

31. X-ray afterglow • First (hours) and strongest afterglow signal (in around 90% of GRBs) • Energy emitted: a few % of GRB energy • Lines seen in some cases, but not clearly confirmed

32. Optical afterglow • Seen in 50% of (well localized) GRBs • Fast fading in general (several weeks) • Dark GRBs: high absorption? very large z? intrinsically very faint?

33. Radio afterglow • ~80% of GRBs with optical afterglow have a radio aterglow • Size of the emitting region deduced from radio emission fluctuations: ~1017cm at ~4 weeks after outburst => proof of relativistic expansion • Longer timescale: direct estimate of the total energy in the ejecta

34. Some basic constraints (from main observational properties) • Energy released in gamma-rays E :1051 – 1054 (/4) erg • (beaming accounted => effective E :5 1050 – 1051 erg; collimation: 10<<200) • Short time scale (ms) variability => small size (<c.t), compact source (of stellar M) • Super-Eddington L: no static envelope => wind • compactness problem : number of photons with E>500keV & size of the source imply that the source is extremely optically thick to pair creation (  e+e-) • solution: relativistic wind (Lorentz factor >100) – relativistic fireball model • Frequency of the events: ~10-6 per year (per galaxy) • Non thermal spectrum: observed emission should emerge from an optically thin region

35. Fireball model: sources • Merging of compact stars (10-5/yr): • Neutron star + Neutron star • Black Hole + Neutron star • (Collapse of a White Dwarf to a Neutron Star - accretion induced collapse: does not work because of baryon pollution - see later) • Collapse of a massive star (10-3/yr): failed supernova, collapsar, hypernova ... • afterglow seen well inside host galaxies (in coinidence with SNe in some cases) . Could explain some long bursts but not the short ones • FINAL CONFIGURATION: BH + thick disk

36. Fireball model: formation of the relativistic wind • Available energy (BH+disk) • mass accretion: gravitational energy of the disk • black hole angular momentum extraction by a magnetic field (Blandford-Znajek process) 1053-1054 erg (OK) • Energy extraction:   annihilation  e+e-  magnetic processes • Problem of baryonic pollution: E degraded from  to UV... • unless = E/Mc2 > 100 (E,M: wind mass and energy) • solved if injection of E along the system axis (baryon free region because of centrifugal forces)

37. GRBs as collimated jets from neutron star/black hole mergersMochkovitch, Hernanz, Isern, Martin, Nature, 1993

38. GRBs from relativistic beams in neutron star mergersMochkovitch, Hernanz, Isern, Loiseau, A&A, 1995

39. Fireball model, with relativistic internal-external shocks • Three main stages: • Inner engine (not observed directly, hidden):produces a relativistic energy flow. Observed rapid fluctuations and huge E released => compact source • Energy transferred relativistically to optically thin regions (distances ~ 1013 cm) • Relativistic ejecta is slowed down: internal shocks convert KE to internal E of accelerated particles which emit the observed  rays • Afterglow: External shocks due to interaction of relativistic matter with surrounding matter – ISM, circumstellar wind

40. Internal shocks: irregular flow, where faster shells catch up and collide with slower shells: kinetic E  internal E Highly variable temporal structure is well reproduced Shocks take place at ~ 1013 cm Duration of the GRB: t during which “inner engine” is active

41. (If) GRB arise from internal shocks and aterglow from external shocks: no direct scaling between GRB and its afterglow => confirmed by observations

42. Emission mechanism (GRB and afterglow): synchrotron X ray to radio spectrum of GRB 970508 m : sync. freq. of an e with Emin (low-E cutoff) c : sync. freq. of an e that cools during the local hydrodynamic time scale. For slow cooling c> m

43. Sari et al, 1998

44. OPEN QUESTIONS • Energetics : is there an energy crisis? What’s the energy distribution between the GRB and its afterglow? • Unknown micropysics: how does the inner engine accelerate the ejecta to relativistic velocities? • Observations can help to understand: • the sources: GRB-SN association, GRB and star forming regions? Detection of gravitational waves coincident with GRBs would demonstrate the connection with NS-NS mergers, for instance • physical processes: detailed (multi-) information about the promt emission and early afterglow (HETE-II, SWIFT)

45. REFERENCES • Piran, T., 1999, Gamma-ray bursts and the fireball model, Physics Reports, 314, 575-667 • Piran, T., 2000, Gamma-ray bursts – a puzzle being resolved, Physics Reports, 333-334, 529-553 • Piran, T., 2004, The physics of gamma-ray bursts,Reviews of Modern Physics (astro-ph/0405503) • Fishman, G.J, Meegan, C.A., 1995, Gamma-ray bursts, Annual Review of Astronomy & Astrophysics, 33, 415-458 • Kouveliotou, C. et al., 1993, Identification of two classes of gamma-ray bursts, Astrophys. J., 413, L101-L104 • Sari et al., Spectra and light curves of gamma-ray burst afterglows, Astrophys. J., 497, L17-L20 • Preece, R.D. et al., 2000, The BATSE gamma-ray burst spectral catalog. I., Astrophys. J. Supplement, 126, 19-36