13,943 research outputs found
Prospects for detection of very high-energy emission from GRB in the context of the external shock model
The detection of the 100 GeV-TeV emission by a gamma-ray burst (GRB) will
provide an unprecedented opportunity to study the nature of the central engine
and the interaction between the relativistic flow and the environment of the
burst's progenitor. In this paper we show that there are exciting prospects of
detecting from the burst by MAGIC high-energy (HE) emission during the early
X-ray flaring activity and, later, during the normal afterglow phase. We also
identify the best observational strategy, trigger conditions and time period of
observation. We determine the expected HE emission from the flaring and
afterglow phases of GRBs in the context of the external shock scenario and
compare them with the MAGIC threshold. We find that an X-ray flare with the
average properties of the class can be detected in the 100 GeV range by MAGIC,
provided that z<0.7. The requested observational window with MAGIC should then
start from 10-20 s after the burst and cover about 1000-2000 s. Furthermore, we
demonstrate that there are solid prospects of detecting the late afterglow
emission in the same energy range for most of the bursts with z<0.5 if the
density of the external medium is n> a few cm^-3. In this case, the MAGIC
observation shall extend to about 10-20 ks. We provide recipes for tailoring
this prediction to the observational properties of each burst,in particular the
fluence in the prompt emission and the redshift, thus allowing an almost real
time decision procedure to decide whether to continue the follow-up observation
of a burst at late times.Comment: 6 pages, 2 color figures, accepted for the pubblication in A&
Bose Einstein Condensation of incommensurate solid 4He
It is pointed out that simulation computation of energy performed so far
cannot be used to decide if the ground state of solid 4He has the number of
lattice sites equal to the number of atoms (commensurate state) or if it is
different (incommensurate state). The best variational wave function, a shadow
wave function, gives an incommensurate state but the equilibrium concentration
of vacancies remains to be determined. In order to investigate the presence of
a supersolid phase we have computed the one--body density matrix in solid 4He
for the incommensurate state by means of the exact Shadow Path Integral Ground
State projector method. We find a vacancy induced Bose Einstein condensation of
about 0.23 atoms per vacancy at a pressure of 54 bar. This means that bulk
solid 4He is supersolid at low enough temperature if the exact ground state is
incommensurate.Comment: 5 pages, 2 figure
Bounds for the Superfluid Fraction from Exact Quantum Monte Carlo Local Densities
For solid 4He and solid p-H2, using the flow-energy-minimizing one-body phase
function and exact T=0 K Monte Carlo calculations of the local density, we have
calculated the phase function, the velocity profile and upper bounds for the
superfluid fraction f_s. At the melting pressure for solid 4He we find that f_s
< 0.20-0.21, about ten times what is observed. This strongly indicates that the
theory for the calculation of these upper bounds needs substantial
improvements.Comment: to be published in Phys. Rev. B (Brief Reports
μ+ → e+γ search with the MEG experiment: Results and perspectives
I present the preliminary results of the data collected by the MEG detector at the Paul Scherrer Institute in 2009 in search of the lepton flavour violating decay μ+ → e+γ with a sample of 6 × 1013 muon decays on target
Population III star formation in a Lambda CDM universe, II: Effects of a photodissociating background
We examine aspects of primordial star formation in the presence of a
molecular hydrogen-dissociating ultraviolet background. We compare a set of AMR
hydrodynamic cosmological simulations using a single cosmological realization
but with a range of ultraviolet background strengths in the Lyman-Werner band.
This allows us to study the effects of Lyman-Werner radiation on suppressing H2
cooling at low densities as well as the high-density evolution of the
collapsing core in a self-consistent cosmological framework. We find that the
addition of a photodissociating background results in a delay of the collapse
of high density gas at the center of the most massive halo in the simulation
and, as a result, an increase in the virial mass of this halo at the onset of
baryon collapse. We find that, contrary to previous results, Population III
star formation is not suppressed for J, but occurs even with
backgrounds as high as J. We find that H2 cooling leads to collapse
despite the depressed core molecular hydrogen fractions due to the elevated H2
cooling rates at K. We observe a relationship between the
strength of the photodissociating background and the rate of accretion onto the
evolving protostellar cloud core, with higher LW background fluxes resulting in
higher accretion rates. Finally, we find that the collapsing halo cores in our
simulations do not fragment at densities below cm
regardless of the strength of the LW background, suggesting that Population III
stars forming in halos with T K may still form in isolation.Comment: 46 pages, 14 figures (9 color). Accepted by the Astrophysical
Journal, some minor revision
Post-T Tauri stars: a false problem
We consider the problem of the apparent lack of old T Tauri stars in low-mass
star forming regions in the framework of the standard model of low-mass star
formation. We argue that the similarity between molecular cloud lifetime and
ambipolar diffusion timescale implies that star formation does not take place
instantaneously, nor at a constant rate. We conclude that the probability of
finding a large population of old stars in a star forming region is
intrinsically very small and that the post-T Tauri problem is by and large not
existent.Comment: 6 pages (LaTeX), no Figures to be published in The Astrophysical
Journal Letter
Exact ground state Monte Carlo method for Bosons without importance sampling
Generally ``exact'' Quantum Monte Carlo computations for the ground state of
many Bosons make use of importance sampling. The importance sampling is based,
either on a guiding function or on an initial variational wave function. Here
we investigate the need of importance sampling in the case of Path Integral
Ground State (PIGS) Monte Carlo. PIGS is based on a discrete imaginary time
evolution of an initial wave function with a non zero overlap with the ground
state, that gives rise to a discrete path which is sampled via a Metropolis
like algorithm. In principle the exact ground state is reached in the limit of
an infinite imaginary time evolution, but actual computations are based on
finite time evolutions and the question is whether such computations give
unbiased exact results. We have studied bulk liquid and solid 4He with PIGS by
considering as initial wave function a constant, i.e. the ground state of an
ideal Bose gas. This implies that the evolution toward the ground state is
driven only by the imaginary time propagator, i.e. there is no importance
sampling. For both the phases we obtain results converging to those obtained by
considering the best available variational wave function (the Shadow wave
function) as initial wave function. Moreover we obtain the same results even by
considering wave functions with the wrong correlations, for instance a wave
function of a strongly localized Einstein crystal for the liquid phase. This
convergence is true not only for diagonal properties such as the energy, the
radial distribution function and the static structure factor, but also for
off-diagonal ones, such as the one--body density matrix. From this analysis we
conclude that zero temperature PIGS calculations can be as unbiased as those of
finite temperature Path Integral Monte Carlo.Comment: 11 pages, 10 figure
Sub-structure formation in starless cores
Motivated by recent observational searches of sub-structure in starless
molecular cloud cores, we investigate the evolution of density perturbations on
scales smaller than the Jeans length embedded in contracting isothermal clouds,
adopting the same formalism developed for the expanding Universe and the solar
wind. We find that initially small amplitude, Jeans-stable perturbations
(propagating as sound waves in the absence of a magnetic field), are amplified
adiabatically during the contraction, approximately conserving the wave action
density, until they either become nonlinear and steepen into shocks at a time
, or become gravitationally unstable when the Jeans length
decreases below the scale of the perturbations at a time . We
evaluate analytically the time at which the perturbations enter
the non-linear stage using a Burgers' equation approach, and we verify
numerically that this time marks the beginning of the phase of rapid
dissipation of the kinetic energy of the perturbations. We then show that for
typical values of the rms Mach number in molecular cloud cores, is
smaller than , and therefore density perturbations likely dissipate
before becoming gravitational unstable. Solenoidal modes grow at a faster rate
than compressible modes, and may eventually promote fragmentation through the
formation of vortical structures.Comment: 8 pages, 4 figure
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