189,402 research outputs found
The Angular Momentum Distribution within Halos in Different Dark Matter Models
We study the angular momentum profile of dark matter halos for a statistical
sample drawn from a set of high-resolution cosmological simulations of
particles. Two typical Cold Dark Matter (CDM) models have been analyzed, and
the halos are selected to have at least particles in order to
reliably measure the angular momentum profile. In contrast with the recent
claims of Bullock et al., we find that the degree of misalignment of angular
momentum within a halo is very high. About 50 percent of halos have more than
10 percent of halo mass in the mass of negative angular momentum . After the
mass of negative is excluded, the cumulative mass function follows
approximately the universal function proposed by Bullock et al., though we
still find a significant fraction of halos () which exhibit
systematic deviations from the universal function. Our results, however, are
broadly in good agreement with a recent work of van den Bosch et al.. We also
study the angular momentum profile of halos in a Warm Dark Matter (WDM) model
and a Self-Interacting Dark Matter (SIDM) model. We find that the angular
momentum profile of halos in the WDM is statistically indistinguishable from
that in the CDM model, but the angular momentum of halos in the SIDM is reduced
by the self-interaction of dark matter.Comment: 23 pages, 10 figures, 2 tables. Revised version, added a new table,
accepted for publication in MNRA
A model of a dual-core matter-wave soliton laser
We propose a system which can generate a periodic array of solitary-wave
pulses from a finite reservoir of coherent Bose-Einstein condensate (BEC). The
system is built as a set of two parallel quasi-one-dimensional traps (the
reservoir proper and a pulse-generating cavity), which are linearly coupled by
the tunneling of atoms. The scattering length is tuned to be negative and small
in the absolute value in the cavity, and still smaller but positive in the
reservoir. Additionally, a parabolic potential profile is created around the
center of the cavity. Both edges of the reservoir and one edge of the cavity
are impenetrable. Solitons are released through the other cavity's edge, which
is semi-transparent. Two different regimes of the intrinsic operation of the
laser are identified: circulations of a narrow wave-function pulse in the
cavity, and oscillations of a broad standing pulse. The latter regime is
stable, readily providing for the generation of an array containing up to
10,000 permanent-shape pulses. The circulation regime provides for no more than
40 cycles, and then it transforms into the oscillation mode. The dependence of
the dynamical regime on parameters of the system is investigated in detail.Comment: Journal of Physics B, in pres
H-Alpha and Hard X-Ray Observations of a Two-Ribbon Flare Associated with a Filament Eruption
We perform a multi-wavelength study of a two-ribbon flare on 2002 September
29 and its associated filament eruption, observed simultaneously in the H-alpha
line by a ground-based imaging spectrograph and in hard X-rays by RHESSI. The
flare ribbons contain several H-alpha bright kernels that show different
evolutional behaviors. In particular, we find two kernels that may be the
footpoints of a loop. A single hard X-ray source appears to cover these two
kernels and to move across the magnetic neutral line. We explain this as a
result of the merging of two footpoint sources that show gradually asymmetric
emission owing to an asymmetric magnetic topology of the newly reconnected
loops. In one of the H-alpha kernels, we detect a continuum enhancement at the
visible wavelength. By checking its spatial and temporal relationship with the
hard X-ray emission, we ascribe it as being caused by electron beam
precipitation. In addition, we derive the line-of-sight velocity of the
filament plasma based on the Doppler shift of the filament-caused absorption in
the H-alpha blue wing. The filament shows rapid acceleration during the
impulsive phase. These observational features are in principal consistent with
the general scenario of the canonical two-ribbon flare model.Comment: 15 pages, 5 figures, accepted for publication in Ap
Polarization-Tailored Raman Frequency Conversion in Chiral Gas-Filled Hollow Core Photonic Crystal Fibers
Broadband-tunable sources of circularly-polarized light are crucial in fields
such as laser science, biomedicine and spectroscopy. Conventional sources rely
on nonlinear wavelength conversion and polarization control using standard
optical components, and are limited by the availability of suitably transparent
crystals and glasses. Although gas-filled hollow-core photonic crystal fiber
provides pressure-tunable dispersion, long well-controlled optical
path-lengths, and high Raman conversion efficiency, it is unable to preserve
circular polarization state, typically exhibiting weak linear birefringence.
Here we report a revolutionary approach based on helically-twisted hollow-core
photonic crystal fiber, which displays circular birefringence, thus robustly
maintaining circular polarization state against external perturbations. This
makes it possible to generate pure circularly-polarized Stokes and anti-Stokes
signals by rotational Raman scattering in hydrogen. The polarization state of
the frequency-shifted Raman bands can be continuously varied by tuning the gas
pressure in the vicinity of the gain suppression point. The results pave the
way to a new generation of compact and efficient fiber-based sources of
broadband light with fully-controllable polarization state.Comment: 5 pages, 4 figure
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