11,395 research outputs found
How does relativistic kinetic theory remember about initial conditions?
Understanding hydrodynamization in microscopic models of heavy-ion collisions
has been an important topic in current research. Many lessons obtained within
the strongly-coupled (holographic) models originate from the properties of
transient excitations of equilibrium encapsulated by short-lived quasinormal
modes of black holes. This paper aims to develop similar intuition for
expanding plasma systems described by a simple model from the weakly-coupled
domain, the Boltzmann equation in the relaxation time approximation. We show
that in this kinetic theory setup there are infinitely many transient modes
carrying information about the initial distribution function. They all have the
same exponential damping set by the relaxation time but are distinguished by
different power-law suppressions and different frequencies of oscillations,
logarithmic in proper time. We also analyze the resurgent interplay between the
hydrodynamics and transients in this setup.Comment: 11 pages, 4 figures; Published in Physical Review
Measuring social structure in a rural context Applying the SOCPO scheme to Scania, Sweden (17(th)-20(th) century)
Steep Slopes and Preferred Breaks in GRB Spectra: the Role of Photospheres and Comptonization
The role of a photospheric component and of pair breakdown is examined in the
internal shock model of gamma-ray bursts. We discuss some of the mechanisms by
which they would produce anomalously steep low energy slopes, X-ray excesses
and preferred energy breaks. Sub-relativistic comptonization should dominate in
high comoving luminosity bursts with high baryon load, while synchrotron
radiation dominates the power law component in bursts which have lower comoving
luminosity or have moderate to low baryon loads. A photosphere leading to steep
low energy spectral slopes should be prominent in the lowest baryon loadComment: ApJ'00, in press; minor revs. 10/5/99; (uses aaspp4.sty), 15 pages, 3
figure
Hydrodynamization in kinetic theory: Transient modes and the gradient expansion
We explore the transition to hydrodynamics in a weakly-coupled model of
quark-gluon plasma given by kinetic theory in the relaxation time approximation
with conformal symmetry. We demonstrate that the gradient expansion in this
model has a vanishing radius of convergence due to the presence of a transient
(nonhydrodynamic) mode, in a way similar to results obtained earlier in
strongly-coupled gauge theories. This suggests that the mechanism by which
hydrodynamic behaviour emerges is the same, which we further corroborate by a
novel comparison between solutions of different weakly and strongly coupled
models. However, in contrast with other known cases, we find that not all the
singularities of the analytic continuation of the Borel transform of the
gradient expansion correspond to transient excitations of the microscopic
system: some of them reflect analytic properties of the kinetic equation when
the proper time is continued to complex values.Comment: 6 pages, 2 figures, v2: author added, major rewrite, mysterious off
real axis singularities in the Borel plane explained (!), see also
arXiv:1802.08225 [nucl-th] by Heller and Svensson; v3: references added,
minor improvements in the text, first 426 terms from Eq. (8) included in the
submission; v4: title changed, matches published versio
Closure Relations for Electron-Positron Pair-Signatures in Gamma-Ray Bursts
We present recipes to diagnose the fireball of gamma-ray bursts (GRBs) by
combining observations of electron-positron pair-signatures (the
pair-annihilation line and the cutoff energy due to the pair-creation process).
Our recipes are largely model-independent and extract information even from the
non-detection of either pair-signature. We evaluate physical quantities such as
the Lorentz factor, optical depth and pair-to-baryon ratio, only from the
observable quantities. In particular, we can test whether the prompt emission
of GRBs comes from the pair/baryonic photosphere or not. The future-coming
Gamma-Ray Large Area Space Telescope (GLAST) satellite will provide us with
good chances to use our recipes by detecting or non-detecting pair-signatures.Comment: 7 pages, 4 figures, accepted for publication in ApJ, with extended
discussions. Conclusions unchange
A Complexity-Brightness Correlation in Gamma Ray Bursts
We observe strong correlations between the temporal properties of gamma ray
bursts (GRBs) and their apparent peak brightness. The strongest effect (with a
significance level of 10^{-6}) is the difference between the brightness
distributions of simple bursts (dominated by a single smooth pulse) and complex
bursts (consisting of overlapping pulses). The latter has a break at a peak
flux of 1.5 ph/cm^2/s, while the distribution of simple bursts is smooth down
to the BATSE threshold. We also observe brightness dependent variations in the
shape of the average peak aligned time profile (ATP) of GRBs. The decaying
slope of the ATP shows time dilation when comparing bright and dim bursts while
the rising slope hardly changes. Both slopes of the ATP are deformed for weak
bursts as compared to strong bursts. The interpretation of these effects is
simple: a complex burst where a number of independent pulses overlap in time
appears intrinsically stronger than a simple burst. Then the BATSE sample of
complex bursts covers larger redshifts where some cosmological factor causes
the break in the peak brightness distribution. This break could correspond to
the peak in the star formation rate that was recently shown to occur at a
redshift of z~1.5.Comment: 13 pages; 11 figures; replaced with the published versio
Description of superdeformed bands in light N=Z nuclei using the cranked HFB method
Superdeformed states in light nuclei are studied by means of the
self-consistent cranking calculation (i.e., the P + QQ model based on the
cranked Hartree-Fock-Bogoliubov method). Analyses are given for two typical
cases of superdeformed bands in the mass region, that is, bands
where backbending is absent (Ca) and present (Ar). Investigations
are carried out, particularly for the following points: cross-shell excitations
in the sd and pf shells; the role of the g and d orbitals; the
effect of the nuclear pairing; and the interplay between triaxiality and band
termination.Comment: 17 pages, 18 figures, accepted in Phys. Rev.
Kinetics of electron-positron pair plasmas using an adaptive Monte Carlo method
A new algorithm for implementing the adaptive Monte Carlo method is given. It
is used to solve the relativistic Boltzmann equations that describe the time
evolution of a nonequilibrium electron-positron pair plasma containing
high-energy photons and pairs. The collision kernels for the photons as well as
pairs are constructed for Compton scattering, pair annihilation and creation,
bremsstrahlung, and Bhabha & Moller scattering. For a homogeneous and isotropic
plasma, analytical equilibrium solutions are obtained in terms of the initial
conditions. For two non-equilibrium models, the time evolution of the photon
and pair spectra is determined using the new method. The asymptotic numerical
solutions are found to be in a good agreement with the analytical equilibrium
states. Astrophysical applications of this scheme are discussed.Comment: 43 pages, 7 postscript figures, to appear in the Astrophysical
Journa
Quasi-thermal Comptonization and gamma-ray bursts
Quasi-thermal Comptonization in internal shocks formed between relativistic
shells can account for the high energy emission of gamma-ray bursts. This is in
fact the dominant cooling mechanism if the typical energy of the emitting
particles is achieved either through the balance between heating and cooling or
as a result of electron-positron pair production. Both processes yield sub or
mildly relativistic energies. In this case the synchrotron spectrum is
self-absorbed, providing the seed soft photons for the Comptonization process,
whose spectrum is flat [F(v) ~ const], ending either in an exponential cutoff
or a Wien peak, depending on the scattering optical depth of the emitting
particles. Self-consistent particle energy and optical depth are estimated and
found in agreement with the observed spectra.Comment: 10 pages, ApJ Letters, accepted for publicatio
Creation of Electron--Positron Wind in Gamma-Ray Bursts and Its Effect on the Early Afterglow Emission
We calculate the creation of electron--positron pairs in Gamma-Ray Bursts
(GRBs) resulting from the collision between scattered and outward moving
gamma-ray photons. The number of pairs exceeds the number of ambient medium
electrons encountered by the GRB ejecta up to ~ 10^{16} cm from the center of
explosion. The shock resulting from the interaction of the ejecta with the
pair-wind may brighten the afterglow synchrotron emission during the first few
minutes. Even without this effect, the peak intensity of the optical afterglow
increases with the density of the surrounding medium. Therefore, observations
of the optical flux at early times constrain the density of the circumburst
medium. If the electron and magnetic field energies behind the forward shock
sweeping-up the pair-wind and the circumburst medium are as inferred from fits
to the broadband afterglow emission at 0.5-100 days, then the current upper
limits on the optical counterpart emission, set by the ROTSE and LOTIS
experiments, indicate that the circumburst medium within 0.01 pc is less dense
than 100 cm^{-3} or, if a wind, corresponds to a progenitor mass-loss to wind
speed ratio below 10^{-6} M_sun/yr/(1000 km/s).Comment: 9 pages, submitted to MNRAS in 200
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