Transcript What is a Gamma-Ray Burst

Pedro F. Guillén.
“Once again, I declare myself a non expert in this subject so,
knowledge and wisdom is not guaranteed in this talk. So I would
feel like a dinosaur about to be burnt by a flare of GRB at 1052 ergs”
-Paco Beretta-
Fotografía cortesía del Dr. Gerardo Ramos Larios

What about this question: Have someone observed or
detected an extrasolar extragalactic planet? HIP 13044 b
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I aim (at least try) to describe the paper of J. Michael
Burgess and collaborators entitled CONSTRAINTS ON THE
SYNCHROTRON SHOCK MODEL FOR THE FERMI GRB 090820A
OBSERVED BY GAMMA-RAY BURST MONITOR
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However, at the end of the talk I suggest to the audience to
establish a discussion about the origin of these objects.
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So lets begin.
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Gamma-ray bursts are short-lived bursts of gamma-ray photons, the most
energetic form of light. At least some of them are associated with a special type
of supernovae.
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These stars are so far away that we don't actually see the light from them before
they explode. They belong to an early generation of stars (e.g. maybe the
second or third generation of stars) in the Universe. Although such stars long
ago died, only now is the light from their explosive deaths reaching us.
(Credit: NASA / SkyWorks Digital)
(Laura Whitlock, GSFC und NASA)
Typical durations are
20 seconds but there is
wide variation both in timestructure and duration.
Some last only hundredths
of a second. Others last
thousands of seconds.
Typical power spectra
peak at 200 keV and
higher.
(Credit: J.T. Bonnell (NASA/GSFC))
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Gamma-ray bursts are separated into two classes: long-duration bursts
and short-duration bursts. Long duration ones last more than 2 seconds
and short-duration ones last less than 2 seconds. However, this doesn't tell
the whole story. That is because short duration bursts range from a few
milliseconds to 2 seconds with an average duration time of about 0.3
seconds (300 milliseconds). The long-duration bursts last anywhere from 2
seconds to a few hundreds of seconds (several minutes) with an average
duration time of about 30 seconds.
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Collapsar Model.
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Magnetar Model.
Is the proposed model that describes how gamma-ray
bursts originates in the collapse of massive star. (e.g. Woosley, 1993).
Tries to explain that relativistic jets are
formed… (neutron star, pulsar)
The cause of these energetic phenomena is star death that involves an
unusually large amount of angular momentum (j ~ 1016 – 1017 cm2 s-1)
and quite possibly, one way or another, ultra-strong magnetic fields
(~1015 gauss). These are exceptional circumstances. A neutron star or a
black hole is implicated.

SNe Explosion of Wolf-Rayet stars.
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How to obtain large energy releases, large
luminosities, short variability time scales?
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How does the kinetic energy of the ejecta get
converted to electromagnetic radiation?
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Fireball/blast wave model
UV/opt/IR/radio
gamma-ray
20 km
gamma-ray
X-ray
UV/optical
IR
mm
radio
Central
engine
Photosphere
internal
(shocks)
external shocks
(reverse) ( forward)
(Credit: NASA / SkyWorks Digital)
(Credit: NASA / SkyWorks Digital)
Observer
AFTERGLOW
INTENSITY
BREAK
TIME
It is a property of matter moving close to the speed of light that it emits its
radiation in a small angle along its direction of motion. The angle is inversely
proportional to the Lorentz factor
This offers a way of
measuring the beaming
angle. As the beam runs into
interstellar matter it slows
down.

1
1 v / c
 1 / 
2
2
,
E.g.,  100 v  0.99995 c
  10 v  0.995 c
Measurements give
an opening angle of
about 5 degrees.
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Synchrotron Band model. Fits for most GRB
PERFECTLY!
•Band – function Band•Spectral analysis (D. Band et al. 1993, ApJ 413, 281).
•time-averaged GRB-spectra of BATSE spectroscopy detectors
• Spectra are well described by:
•fat low energies by a powerlaw with an exponential cutoff
• NE (E) ∝ E α exp (-E/E0)
•fat high energies by a steeper powerlaw
NE (E) ∝ E β;
with α > β
“Band” – Function (empirical fit)
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Synchrotron Band model. Fits for most GRB
PERFECTLY!
GRB 090227B:
time integrated
spectrum
Band function
α = -0.41 ± 0.20
β = -3.2 (-0.3 +0.2)
EPeak = 1.97 ± 0.09 MeV
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The shock wave accelerates electrons (via the Fermi acceleration
process) to velocities very close to the speed of light. Until they
got caught in the shock wave and by rattling back and forth across
the shock wave, these particles gain energy and become gamma
rays. However, astronomers still debate to what produce such an
energy.
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This Fermi acceleration process is also known as
Diffusive Shock Acceleration (DSA)
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Fermi acceleration is the acceleration that charged particles
undergo when reflected by a magnetic mirror. This is thought to be
the primary mechanism by which particles gain energy beyond the
thermal energy in astrophysical shock waves.
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Fermi–Ulam model (“the pinball effect”)
un1  un  U wall (n )
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Efficient diffusive shock acceleration lowers the shock temperature and
raises the post-shock density (Jones & Ellison,1991; Berezhko & Ellison,
1999).
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The non-equilibrium ionization is dependent upon both the shock
temperature and the shock density through their relation to the electron
temperature, Te , and electron density, ne.
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Tavani (1996a, 1996b) proposed a shock emission model that invoked
such diffusive acceleration as the means for generating nonthermal
distributions.
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Accordingly, it is appropriate to use an electron distribution similar to
Tavani’s to add insight to the discussion. Here the quasi-isotropic
electron distribution:
(Baring & Braby 2004)
T is the post shock temperature  T  kT m c2 and the rest are
e
parameter such as  the nonthermal index,  acceleration
enfficiency, T defines the minimum Lorentz factor of the
power-law.
Where Bcr = 4.41 × 10 13 G is the quantum critical field. When convolved with the
electron distribution , the substitution γ → ηγT in the equation of critical energy
then defines the scale for the break energy of the synchrotron continuum resulting
from the truncated power-law portion of the distribution.
In modeling prompt burst emission, the relativistic nature of the outflow introduces
an extra parameter, the bulk Lorentz factor Γ of the flow, which blueshifts the
spectrum so as to introduce the Γ factor in equation of critical energy, so that
equation for the Flux then expresses the synchrotron flux in the observer’s frame.
Figure 1. Light curve of GRB 090820A as observed by GBM. The two panels
show the count rate in the two Na i detectors (top) and BGO (bottom). The
dashed lines indicate the time intervals (a, b, c, d) used for the timeresolved analysis. It is clear that the burst consists of two main peaks and
that this burst is very bright in the BGO detectors.
Figure 2. Integrated spectrum of GRB
090820A. We are able to resolve
three
components,
thermal
synchrotron, power-law synchrotron,
and a blackbody.
Energy channels near the Na i K-edge
are omitted. The deviations in the fit
residuals are the due to systematics in
the detector response resulting from
the high count rate and spectral
hardness of this burst. However,
deviations are never greater than 4σ
and do not significantly impact the
values of the best-fit parameters. The
multiple curves near the peak of the
spectrum are an artifact of the
effective-area correction applied to
each detector and not related to the
different fitted models.
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Its shown that thermal and non-thermal synchrotron photon
models, with an additional blackbody, are well consistent with the
emission spectra of GRB 090820A in various time intervals. These
are physical models that afford the ability to constrain
parameters that are physically meaningful, for example, key
descriptors of the electron distribution that is motivated by shock
acceleration theory: Strongly cooled synchrotron emission,
inverse Compton, and jitter radiation are popular candidates.
First Gamma-Ray Burst
The Vela 5 satellites functioned
from July, 1969 to April, 1979
and detected a total of 73
gamma-ray bursts in the energy
range 150 – 750 keV (n.b,. Greater
than 30 keV is gamma-rays).
Discovery reported Klebesadel,
Strong, and Olson (1973).
Oh oh…