1,484 research outputs found
Energy injection episodes in GRBs: The case of GRB 021004
A number of GRB afterglow light curves deviate substantially from the power law decay observed in most bursts. These variations can be accounted for by including refreshed shocks in the standard fireball model previously used to interpret the overall afterglow behavior. We show that the light curves of GRB 021004 can be accounted for by four energy injection episodes in addition to the initial
event. The polarization variations are shown to be a consequence of the injections
Star formation efficiency and host galaxies of Gamma-Ray Bursts
Gamma-ray bursts are likely to be associated with regions of active star formation and may therefore be useful as cosmological tracers of galaxy formation. Using hydrodynamical/N-body simulations the cosmological properties of host galaxies are investigated. Assuming the hosts to be efficient star-forming objects, i.e. with high specific star formation rates, the majority of the candidate
hosts are found to below mass, young galaxies with a moderate star formation rate. This picture, consistent with most observations of GRB host galaxies, corroborates the idea that gamma-ray bursts do not follow the cosmic star formation rate. More importantly the hosts should contribute to unveil the faint end of the galaxy luminosity function
Energy Injection Episodes in Gamma Ray Bursts: The Light Curves and Polarization Properties of GRB 021004
Several GRB afterglow light curves deviate strongly from the power law decay
observed in most bursts. We show that these variations can be accounted for by
including refreshed shocks in the standard fireball model previously used to
interpret the overall afterglow behavior. As an example we consider GRB 021004
that exhibited strong light curve variations and has a reasonably well
time-resolved polarimetry. We show that the light curves in the R-band, X-rays
and in the radio can be accounted for by four energy injection episodes in
addition to the initial event. The polarization variations are shown to be a
consequence of the injections.Comment: 4 pages, 2 figures. To appear in ApJ
Lens magnification by CL0024+1654 in the U and R band
[ABRIDGED] We estimate the total mass distribution of the galaxy cluster
CL0024+1654 from the measured source depletion due to lens magnification in the
R band. Within a radius of 0.54Mpc/h, a total projected mass of
(8.1+/-3.2)*10^14 M_sol/h (EdS) is measured, which corresponds to a mass-
to-light ratio of M/L(B)=470+/-180. We compute the luminosity function of
CL0024+1654 in order to estimate contamination of the background source counts
from cluster galaxies. Three different magnification-based reconstruction
methods are employed using both local and non-local techniques. We have
modified the standard single power-law slope number count theory to incorporate
a break and applied this to our observations. Fitting analytical magnification
profiles of different cluster models to the observed number counts, we find
that the cluster is best described either by a NFW model with scale radius
r_s=334+/-191 kpc/h and normalisation kappa_s=0.23+/-0.08 or a power-law
profile with slope xi=0.61+/-0.11, central surface mass density
kappa_0=1.52+/-0.20 and assuming a core radius of r_core=35 kpc/h. The NFW
model predicts that the cumulative projected mass contained within a radius R
scales as M(<R)=2.9*10^14*(R/1')^[1.3-0.5lg (R/1')] M_sol/h. Finally, we have
exploited the fact that flux magnification effectively enables us to probe
deeper than the physical limiting magnitude of our observations in searching
for a change of slope in the U band number counts. We rule out both a total
flattening of the counts with a break up to U_AB<=26.6 and a change of slope,
reported by some studies, from dlog N/dm=0.4->0.15 up to U_AB<=26.4 with 95%
confidence.Comment: 19 pages, 12 figures, submitted to A&A. New version includes more
robust U band break analysis and contamination estimates, plus new plot
Transient regime in non-linear transport through many-level quantum dots
We investigate the nonstationary electronic transport in noninteracting
nanostructures driven by a finite bias and time-dependent signals applied at
their contacts to the leads. The systems are modelled by a tight-binding
Hamiltonian and the transient currents are computed from the non-equilibrium
Green-Keldysh formalism. The numerical implementation is not restricted to weak
coupling to the leads and does not imply the wide-band limit assumption for the
spectral width of the leads. As an application of the method we study in detail
the transient behavior and the charge dynamics in single and double quantum
dots connected to leads by a step-like potential, but the method allows as well
the consideration of non-periodic potentials or short pulses. We show that when
the higher energy levels of the isolated system are located within the bias
window of the leads the transient current approaches the steady state in a
non-oscillatory smooth fashion. At moderate coupling to the leads and fixed
bias the transient acquires a step-like structure, the length of the steps
increasing with the system size. The number of levels inside a finite bias
window can be tuned by a constant gate potential. We find also that the
transient behavior depends on the specific way of coupling the leads to the
mesoscopic system.Comment: RevTeX, 12 pages, 11 include .eps figure
Thomas-Fermi Calculations of Atoms and Matter in Magnetic Neutron Stars II: Finite Temperature Effects
We present numerical calculations of the equation of state for dense matter
in high magnetic fields, using a temperature dependent Thomas-Fermi theory with
a magnetic field that takes all Landau levels into account. Free energies for
atoms and matter are also calculated as well as profiles of the electron
density as a function of distance from the atomic nucleus for representative
values of the magnetic field strength, total matter density, and temperature.
The Landau shell structure, which is so prominent in cold dense matter in high
magnetic fields, is still clearly present at finite temperature as long as it
is less than approximately one tenth of the cyclotron energy. This structure is
reflected in an oscillatory behaviour of the equation of state and other
thermodynamic properties of dense matter and hence also in profiles of the
density and pressure as functions of depth in the surface layers of magnetic
neutron stars. These oscillations are completely smoothed out by thermal
effects at temperatures of the order of the cyclotron energy or higher.Comment: 37 pages, 17 figures included, submitted to Ap
Spin effects in a confined 2DEG: Enhancement of the g-factor, spin-inversion states and their far-infrared absorption
We investigate several spin-related phenomena in a confined two-dimensional
electron gas (2DEG) using the Hartree-Fock approximation for the mutual Coulomb
interaction of the electrons. The exchange term of the interaction causes a
large splitting of the spin levels whenever the chemical potential lies within
a Landau band (LB). This splitting can be reinterpreted as an enhancement of an
effective g-factor, g*. The increase of g* when a LB is half filled can be
accompanied by a spontaneous formation of a static spin-inversion state (SIS)
whose details depend on the system sision state (SIS) whose details depend on
the system size. The coupling of the states of higher LB's into the lowest band
by the Coulomb interaction of the 2DEG is essential for the SIS to occur. The
far-infrared absorption of the system, relatively insensitive to the spin
splitting, develops clear signs of the SIS.Comment: 7 figure
Manifestation of the Hofstadter butterfly in far-infrared absorption
The far-infrared absorption of a two-dimensional electron gas with a
square-lattice modulation in a perpendicular constant magnetic field is
calculated self-consistently within the Hartree approximation. For strong
modulation and short period we obtain intra- and intersubband magnetoplasmon
modes reflecting the subbands of the Hofstadter butterfly in two or more Landau
bands. The character of the absorption and the correlation of the peaks to the
number of flux quanta through each unit cell of the periodic potential depends
strongly on the location of the chemical potential with respect to the
subbands, or what is the same, on the density of electrons in the system.Comment: RevTeX file + 4 postscript figures, to be published Phys. Rev. B
Rapid Com
Luminosity Functions of Gamma-Ray Burst Afterglows
Aims: Use the standard fireball model to create virtual populations of
gamma-ray burst afterglows and study their luminosity functions.
Methods: We randomly vary the parameters of the standard fireball model to
create virtual populations of afterglows. We use the luminosity of each burst
at an observer's time of 1 day to create a luminosity function and compare our
results with available observational data to assess the internal consistency of
the standard fireball model.
Results: We show that the luminosity functions can be described by a function
similar to a log normal distribution with an exponential cutoff. The function
parameters are frequency dependent but not very dependent on the model
parameter distributions used to create the virtual populations. Comparison with
observations shows that while there is good general agreement with the data, it
is difficult to explain simultaneously the X-ray and optical data. Possible
reasons for this are discussed and the most likely one is that the standard
fireball model is incomplete and that decoupling of the X-ray and optical
emission mechanism may be needed.Comment: 5 pages, 4 figures; accepted for publication in A&
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