Gamma-ray Bursts or

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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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