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Gamma-ray Bursts or things that go BOOM in the night David Bersier Astrophysics Research Institute, Liverpool John Moores University What is a GRB? Gamma-rays: ν=1019 Hz and above (optical ν=5×1014 Hz) E = 100 keV up to TeV (1 eV 1.6×10−19 J) Discovery and basic facts o Bursts of gamma-rays detected by Vela satellites in the 1960’s, monitoring nuclear testing activity o Extra-terrestrial (1973) o Uniformly distributed on the sky o “Curiosity” until CGRO (BATSE), launched in 1989 Short GRB XRF Size of the source Timescale of variability in light curve says that source is small: Take source of area A emitting a flux FA (power). Assume a fraction a doubles in flux. Total flux is F=0.99FA+2×a×FA . A The physical mechanism causing the flux increase can only act over an region of size x within a time cx. If a/A=0.01, the flux increase is 1%. a For the whole emitting region to double in flux (as is seen in some GRBs), A has to be small (to be causally connected). Variability on milliseconds implies size is light-milliseconds across (3×108 m/s × 10-3 s = 3×105 m = 300 km). Two groups of bursts Blue Red 3/4 are longer than 2 seconds, 1/4 are shorter than 2 seconds (rest of the talk on long GRBs). Where? Uniformity on the sky Cosmological origin? Not in solar system, not in Galactic disk. Searches (unsuccessful) for counterparts at other wavelengths error boxes too large, coming too late Do they emit radiation at other wavelengths: constrain the nature of GRBs Lack of data is never a problem for theorists: Plethora of models, from solar system origin to cosmological: Stellar flares, comets crashing on neutron stars, stellar explosions, mergers of two stars, … (100+ in 1994) Prediction: Afterglow radiation over the whole electromagnetic spectrum, lasting days/weeks/months. Would allow us to narrow down error boxes/search areas. Accurate position: Afterglow of 970228 X-ray and optical afterglow: decays with time (gets fainter) Distances Next step: spectrum of the afterglow to settle the distance problem Spectrum looks like it’s a star, means it’s a star. If very distant, typical signatures in the spectrum First distance: 970508. Very far away, 6.7 billion light years. Knowing the distance, we can determine its brightness: Typical brightness is E ~ 1046 J in gamma-rays. GRB 990123: afterglow was 1016 times brighter than the Sun! (power ~ 1026 W). “Jets solve energy crisis” Problem: Enormously large energy. No know physical process able to liberate so much energy in such a short time. Optical light curve of an afterglow: bending after ~ 1 day (decay becomes faster). Typical signature of a jet: light emitted only in a narrow cone. Energy becomes 100s of times less, comparable to a supernova: exploding massive star (10+ M). E ~ 1044 J. We can make that! Hint on nature of progenitors: massive stars that explode? Afterglow light curve ~ Log10(Brightness) ÷10 Decreases as a power law (straight line in log-log plot) Emission mechanism General physical considerations lead to a basic model: dissipation of kinetic energy of a relativistically expanding plasma of photons, e+, e-, a fireball (matter content very small E/m0c2 >>1). Emission: synchrotron or inverse Compton (photons bumped to high energy by electrons). Synchrotron = relativistic version of cyclotron. Cyclotron: because of Lorentz force, a charged particle travels in a circle of radius r=mV/qB, with frequency vc=Bq /2π m. Spectrum of synchrotron is a power law E(ν) ~ να (α<0). Spectra of GRB (i.e. in gamma rays) are in agreement with this; emission mechanism is not thermal. Relativistic beasts Mass: about the mass of the Earth, ~ 1024 kg. Relativistic emission is beamed towards us, similar to aberration of light. Light is emitted in a cone of opening angle Γ where Γ=1/(1-v2/c2)1/2 ≥1. Typically Γ = a few 100, v/c = 0.99999. This means that we see a small fraction of the fireball. Whether the source is a cone or is spherical, the observer sees the same. Another relativistic effect: source seems to be expanding faster than speed of light. Superluminal motion. Toy model of GRBs Different shells travelling at different speeds, catching up: GRB Jet “hits” and lights up interstellar matter: afterglow Jet slows down, afterglow gets fainter. ~1% of Sun-Earth distance 10x Sun-Earth distance GRB afterglow GRB 030329 Proof that there is a supernova, i.e. massive star exploding. afterglow Brightness supernova afterglow + supernova But what is a supernova? Stars as nuclear reactors Stars shine by burning nuclear fuel. Fusing 4H into He yields ~ 8 MeV/nucleon. The Sun converts 5×1011 kg of H into He each second. When a star runs out of hydrogen, it starts burning He, making C and O; when it runs out of helium, it burns CO, etc. Massive stars (M>8 M) keep burning heavier and heavier elements until the core (region where nuclear reactions occur) is made of iron, the most stable element. Exploding stars Massive stars take nuclear fusion all the way to iron, where it all stops suddenly: no more energy available. Source of pressure disappears, the core collapses in a very short time to a proto-neutron star (size ~ 10 km), a ~1M giant nucleus. The rest of the star is in free fall. Eventually, all the stellar envelope will be blown apart: supernova. Bottom line: reservoir of gravitational potential energy of ~ 1046 J. That’s plenty to explode the star. Energy used ~ a few 1043 J. Physical mechanism: unkown Most of the energy comes out in the form of neutrinos, some 10% used to move several times the mass of the Sun to speeds of ~ 10,000 km/s. The supernova (SN) becomes as bright as a billion suns. Powerful nuclear reactors: elements heavier than Fe synthesized. In particular, lots of 56Ni, which decays quickly (Ni -> Co -> Fe). This powers the light curve (makes the SN shine). Observational signature of supernovae Flux scale is log; late decay looks like a straight line because it is exponential (radioactive). Seen from much closer: GRB and SN For gamma-ray bursts, we think that the core will eventually collapse to a black hole. Matter surrounding the core forms an accretion disk, feeding the hungry black hole 20 seconds after collapse; accreting at a rate of 0.1 M/s. The black hole reaches a mass of several times the mass of the Sun. Density 1800 km GRB and SN (2) Magic happens! A jet is launched, plows through the star, lives its own life. Jet becomes a GRB (far from the star), rest of the star explodes (supernova). The SNe tend to be very bright/energetic (i.e. large kinetic energies with expansion velocities up to 30000 km/s): A hypernova. Most massive stars (>99%) do not end their lives as GRB, a SN is no guarantee of a GRB happening. What decides if a GRB can occur? Rotation (fast), heavy element content (low). Why these two phenomena have roughly similar energies? Summary • Long GRBs have jets, small amount of mass, relativistic ejecta emitted inside a narrow cone. • They are associated with supernovae, non-relativistic ejecta, several times the mass of the Sun, emitted isotropically. Found by devices supposed to detect nuclear explosions. Most of them ARE nuclear explosions.
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