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γ -ray bursts: the most brilliant explosions in the universe

γ -ray bursts: the most brilliant explosions in the universe. Dimitrios Giannios Department for Astrophysical sciences, Princeton Princeton, 06/30/09. EM Radiation: many ways to describe the same thing. Electromagnetic waves can be characterized by Wavelength λ Frequency ν

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γ -ray bursts: the most brilliant explosions in the universe

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  1. γ-ray bursts: the most brilliant explosions in the universe Dimitrios Giannios Department for Astrophysical sciences, Princeton Princeton, 06/30/09

  2. EM Radiation: many ways to describe the same thing • Electromagnetic waves can be characterized by • Wavelength λ • Frequency ν • Energy e=h·ν(energy of the “quantum” of light-photon) • Temperature T=e/k • Units used here: CGS • Energy (erg) • Time (sec) • Distance (cm) • ...

  3. The electromagnetic waves:Our main window to the universe • Until 1950’s we had only access to optical and radio wavelengths • Limited by our atmosphere • Satellites opened new exciting windows filling the gaps in the ’60s Higher Energies

  4. Why? Look at the something familiar! Different views of the known objects Our understanding is significantly advanced Exciting new phenomena undetected in other wavelenths High-energy Astrophysics is born! Observing in different wave lengths

  5. High Energy Astrophysics The Astrophysics of X-rays and γ-rays; neutrinos; cosmic rays Physics of compact objects (neutron stars, black holes-small and big); Supernovae remnants; γ-ray bursts… X-ray binaries Active galactic nuclei Supernovae remnants γ-ray bursts

  6. What are γ-rays? • The most energetic photons • Energy of photon e >100 keV (~105 that of the optical) • Or corresponding T >109 K • Or frequency ν >3 1019 Hz • Or wavelength λ <10-9 cm γ-rays

  7. γ-ray bursts: the discovery • Discovered by one of the VELA satellites • Looking for explosions on earth • Found them on space!!! • Results declassified in 1973 • 35 years of exploration

  8. N(t) t (Almost) all the information we had for 20 years! • Rate: a couple of them per day • Poor localization on sky • But not coming from sun, planets • They do not come from the same location • Seen only in γ rays • Fluence F~10-5erg/cm2 • Last for ~seconds • Variable on short timescales (msec)

  9. E f(E)(erg/cm2/s) E (MeV) The typical spectrum of γ-ray bursts • The typical GRB spectrum has non-thermal appearance • Low energy power law slope α • High energy power law slope β • Connected at ~a few hundred keV (spectral break)

  10. What can we tell from this? Part I • The variability constrains the size of the source • a source with typical scale of Rscannot vary faster that timescales tvar ~ Rs/c • Variability of msec indicates a very compact source of Rs < ctvar ~ 3·107 cm • That’s 300 km not much larger than the radius of a neutron star Observer Rs f(t) t Rs/c

  11. What can we learn from this? Part II • We do not know the distance d but for observed F~10-5 erg/cm2 the closer the burst the easier to explain E=4π d2F • If in our solar system d ~1014 cm E ~1024 erg • If in the Galaxy d ~3·1022 cm E ~1041 erg (solar output in 1 year) • If in a far away galaxy d ~1028 cm E ~1052 erg (solar output in 300 billion years!) Comparison: the total energy in a supernova is E ~1051 erg

  12. What can they be? We have imagination! • In our solar system? • fast dust grains breaking up, magnetic reconnection in heliopause… • In our Galaxy? • matter falling to white dwarf/neutron star/black hole from companion, superflares from stars, comet strikes neutron star/white dwarf … • In far away galaxies? • supernova-star collision, Kerr white hole, collision of neutron stars/strange stars/white dwarfs, collapse of spinning massive star …

  13. 135 models (1993) Note: most are Galactic

  14. BATSE: the revolution of the 1990s • BATSE satellite was launched in 1991 • Detected ~3000 bursts in 10 years • Localized them within a degree • Measured their duration distribution • Put their locations on the sky…

  15. Lesson I: They come in two kinds Long Duration T > 2 sec Short Duration T < 2 sec

  16. Lesson II: they are isotropic on the sky

  17. Isotropy = Cosmological distance Objects that follow the Galactic distribution (of mass, stars etc) look different GRBs are NOT Galactic They are cosmological Sun

  18. The last 10 years, it is verified that γ-ray bursts are cosmological • Detecting emission that follows the burst in the X-rays, optical, radio • Good localization (less than arcmin) • Detecting the galaxies they come from • Measuring the redshift of the galaxies

  19. How to measure distance (part I) • BeppoSax satellite made the breakthrough in 1997 • Detected in low-energy γ rays • Localized in X rays at the same time • People found optical counterpart ~1 day later (arcsec resolution)

  20. How to measure distance (part II) • A month later the region is observed with a large telescope and the host galaxy is detected • From lines in the spectrum of the galaxy the redshift is measured • Today we have more than ~200 redshift measurements • The furthest away at z~8.2! (600 Myr after the big Bang) Number

  21. So they are Cosmological after all • Implications: • Huge energy release Eγ ~1053 erg ~100 times that of Supernovae • With luminosities L ~Eγ/T ~1052 erg/sec (106 that of powerful AGN; 1012 that of ULXs) • Big Questions • Where does the energy come from? • How much “fuel” is needed? • We need to take mfuel ~0.1 M⊙ and make it γ rays in ~10 sec if E=mfuelc2 • Why we see γ rays? • They should not escape the source!

  22. 4 protons: mass = 4.029 2 neutrinos, photons (light) 4He nucleus: mass = 4.0015 The energy issue • How much Fuel is needed to release 1053 erg? • How is it done? • Chemical energy (oil, coal…) • 1 gr gives ~3·1011 erg, we need ~3·108M⊙ • Nuclear energy • 1 gr gives ~3·1018 erg, we need ~ 20 M⊙ • Gravitational energy (neutron star or black hole) • 1 gr gives ~1020 erg, we need ~2 M⊙ E=δm·c2

  23. Gravitational energy release • Let mass m drop from large distance onto a star of mass M and radius R • The m reaches the surface of the star with kinetic energy E=GMm/R • The energy is released with efficiency • For a white dwarf R=5000 km, M~1 M⊙η~0.0003 • For a neutron star R=10 km, M~1.5 M⊙η~0.2 • For a black hole R=2GM/c2 η~0.5

  24. Concluding for energy source • Gravitational energy released by matter falling to a black hole is promising • Still we need to throw ~a solar mass in a few sec! • An alternative: A fast rotating (Ω~104 rad/sec) neutron star has enough rotational energy • Very powerful magnetic fields can extract it in the short time needed

  25. Another problem: how the γ rays escape the source to arrive to us? • γ rays with energy e ≥ mec2 can annihilate creating pairs γ + γ→ e- + e+ • But if you put ~1053 erg of γ rays in 3·107 cm nothing will escape • A very dense soup of γ, e-, e+ will form (fireball)

  26. Relativistic effects help • For observed variability δt, the radius of emission is constrained to be Rs <2· Γ2·c·δt Much weaker than the non-relativistic constaint Rs <c·δt • Larger radius of emission smaller density of γ-rays in the source less optical depth for annihilation 1/Γ Still Γ >100 are needed !

  27. Putting everything together • Central engine: Most likely a black hole or a neutron star • Fed with ~1 solar mass in ~seconds • Ejects a very fast flow that produces the burst • At large distance the fast material collides with whatever is around the burst • Leading to a shock into the external medium, afterglow emission

  28. Afterglow g-rays Central Engine Acceleration & internal dissipation External Shock ~1016-1018cm ~106cm r~1011-1016cm The GRB framework GRB emission X-rays optical radio

  29. Models today: from 135 down to a few Short Bursts Long Bursts

  30. Main model for long-duration bursts

  31. And for short ones…

  32. Open questions/Challenges • Which are the progenitors? • Massive stars, double NSs, neutron star+black hole, flares in magnetars, something else... • How is the flow launched/accelerated? • Neutrinos Vs Magnetic fields; thermal Vs magnetic pressure • How are the γ rays produced? • Shocks Vs magnetic reconnection • What produces the afterglow emission? Many, many exciting topics open to fresh ideas!

  33. The driving mechanism: MHD energy extraction and/or neutrino annihilation Neutrino annihilation energy deposition rate (erg cm –3 s-1) B ~ 1015 gauss for 3 solar mass black hole Well below equipartition in the disk! Blandford & Znajek 1977 Koide et al. 2001 van Putten 2001 Lee et al. 2001 Barkov & Komissarov 2008 Eichler 1989; Ruffert & Janka 1999; Popham et al. 1999; Aloy et al. 2000; Chen & Beloborodov 2007

  34. Basic radiative processes • Coming from fast electrons and magnetic fields • Synchrotron emission • Inverse Compton scattering e+γ e+γ

  35. How to accelerate electrons:Internal shocks vs Magnetic dissipation Γ2υ2m2 Γ1υ1m1 Γυmtot • Internal shocks • Unsteady flow composed by shells • A fast shell with Γ2> Γ1 collides with a slower one dissipating their relative kinetic energy • Magnetic dissipation • Magnetic fields carry most of the energy of the flow • The magnetic energy is dissipated internally through, e.g., reconnection g-rays FB FB Fk Fk I vote for Reconnection!

  36. E f(E)(erg/cm2/s) E (MeV) Spectra from magnetic dissipation models Γ=1000 FERMI Mostly observed Currently available Γ=250 Giannios 2006; 2008; Giannios & Spruit 2007

  37. Comparison with multi-band observations GRB 061121; Page et al. 2007 Γ=500 Γ=300

  38. Afterglows Burst Afterglow • The bursts are followed by prolonged softer emission (X-ray, optical, radio) • It is believed to come from the fast material that collides with the external medium • Comes mainly from synchrotron emission of fast electrons f (t) t

  39. Can we say something for the ejecta from the afterglow? • If magnetic fields are responsible for launching the material then it should be strongly magnetized • Does this affect the afterglow emission? • Yes, it makes big difference in the early afterglow (minutes after the burst)

  40. Relativistic MHD simulations of the early deceleration of magnetized ejecta Mimica, Giannios & Aloy 2008

  41. Summarizing: High Energy Astrophysics • In the 1960’s a revolution came in Astrophysics • The X-ray/γ-ray sky was revealed by satellites • A large variety of new objects observed • Neutron stars (isolated; in binary systems) • Black holes (in binaries; in galactic centers) • γ-ray bursts • Objects where special and general relativity make their most extreme appearance • ultrarelativistic plasma-jets • black-hole physics; • neutron star structure, strongest known magnetic fields

  42. Summarizing: γ-ray bursts • The most powerful known explosions (some 10 times more powerful than supernovae) • They come from distant galaxies up to z>8 • Appear in the γ-rays and come into two kinds: • the short (~0.1 sec) and long duration (~10 sec) ones • Connected to the death of massive stars/neutron star mergers • They are produced by relativistic plasma (Γ>100) • Because of internal collisions or magnetic reconnection • Result of photospheric emission, synchrotron and/or Compton scattering

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