532 research outputs found
On the rapid orbital expansion in the compact low-mass X-ray binary 2A 1822371
The neutron star low-mass X-ray binary 2A 1822371 has an orbital period of
5.57 hr. Mass transfer in such short-period binaries is thought to be driven by
magnetic braking with orbital shrinking. However, 2A 1822371 shows a very
rapid orbital expansion, implying that mass transfer occurs rapidly in this
system. The accretion rate of the neutron star is observationally estimated to
be higher than the Eddington limit, which is also hard to be explained by the
standard magnetic braking mechanism. In this work, we construct a model to
account for the peculiar properties of 2A 1822371. We assume that the donor
star possesses a relatively strong magnetic field, which is coupled with the
stellar winds excited by the X-ray radiation from the neutron star. This would
generate efficient angular momentum loss, leading to a high mass transfer rate
and hence orbital expansion. We provide possible evolutionary tracks of 2A
1822371 and study how the input parameters affect the results. The
observational implications of the irradiation-driven mass loss are also briefly
discussed in the context of evolution of low-mass X-ray binaries and
millisecond pulsars.Comment: 14 pages, 10 figures, accepted for publication in Ap
Numerically Fitting The Electron Fermi Energy and The Electron Fraction in A Neutron Star
Based on the basic definition of Fermi energy of degenerate and relativistic
electrons, we obtain a special solution to electron Fermi energy, , and express as a function of electron fraction, ,
and matter density, . Several useful analytical formulae for and
within classical models and the work of Dutra et al. 2014 (Type-2) in
relativistic mean field theory are obtained using numerically fitting. When
describing the mean-field Lagrangian, density, we adopt the TMA parameter set,
which is remarkably consistent with with the updated astrophysical observations
of neutron stars. Due to the importance of the density dependence of the
symmetry energy, , in nuclear astrophysics, a brief discussion on the
symmetry parameters and (the slope of ) is presented. Combining
these fit formulae with boundary conditions for different density regions, we
can evaluate the value of in any given matter density, and
obtain a schematic diagram of as a continuous function of
. Compared with previous study on the electron Fermi energy in other
models, our methods of calculating are more simple and
convenient, and can be universally suitable for the relativistic electron
regions in the circumstances of common neutron stars. We have deduced a general
expression of and , which could be used to indirectly
test whether one EoS of a NS is correct in our future studies on neutron star
matter properties. Since URCA reactions are expected in the center of a massive
star due to high-value electron Fermi energy and electron fraction, this study
could be useful in the future studies on the NS thermal evolution.Comment: 30 pages, 14 figure
A quantum degenerate Bose-Fermi mixture of K and Li
We report a new apparatus for the study of two-species quantum degenerate
mixture of K and Li atoms. We develop and combine several advanced
cooling techniques to achieve both large atom number and high phase space
density of the two-species atom clouds. Furthermore, we build a high-efficiency
two-species magnetic transport system to transfer atom clouds from the 3D
magneto-optical-trap chamber to a full glass science chamber of extreme high
vacuum environment and good optical access. We perform a forced radio-frequency
evaporative cooling for K atoms while the Li atoms are
sympathetically cooled in an optically-plugged magnetic trap. Finally, we
achieve the simultaneous quantum degeneracy for the K and Li atoms.
The Bose-Einstein condensate of K has 1.410 atoms with a
condensate fraction of about 62%, while the degenerate Fermi gas of Li has
a total atom number of 5.410 at 0.25 Fermi temperature
Observation of the density effect on the closed-channel fraction in a Li superfluid
Atomic Fermi gases provide an ideal platform for studying the pairing and
superfluid physics, using a Feshbach resonance between closed channel molecular
states and open channel scattering states. Of particular interest is the
strongly interacting regime. We show that the closed-channel fraction
provides an effective probe for the important many-body interacting effects,
especially through its density dependence, which is absent from two-body
theoretical predictions. Here we measure as a function of interaction
strength and the Fermi temperature in a trapped Li superfluid
throughout the entire BCS--BEC crossover. Away from the deep BEC regime, the
fraction is sensitive to . In particular, our data show with at unitarity, in quantitative agreement with
calculations of a two-channel pairing fluctuation theory, and
increases rapidly into the BCS regime, reflecting many-body interaction effects
as predicted.Comment: Major changes have been made since last version. This submission
contains supplementary information as an ancillary fil
Coupled Dipole Oscillations of a Mass-Imbalanced Bose and Fermi Superfluid Mixture
Recent experimental realizations of superfluid mixtures of Bose and Fermi
quantum gases provide a unique platform for exploring diverse superfluid
phenomena. We study dipole oscillations of a double superfluid in a
cigar-shaped optical dipole trap, consisting of K and Li atoms
with a large mass imbalance, where the oscillations of the bosonic and
fermionic components are coupled via the Bose-Fermi interaction. In our
high-precision measurements, the frequencies of both components are observed to
be shifted from the single-species ones, and exhibit unusual features. The
frequency shifts of the K component are upward (downward) in the radial
(axial) direction, whereas the Li component has down-shifted frequencies
in both directions. Most strikingly, as the interaction strength is varied, the
frequency shifts display a resonant-like behavior in both directions, for both
species, and around a similar location at the BCS side of fermionic superfluid.
These rich phenomena challenge theoretical understanding of superfluids
Oscillatory-like Expansion of a Fermionic Superfluid
We study the expansion behaviours of a Fermionic superfluid in a cigar-shaped
optical dipole trap for the whole BEC-BCS crossover and various temperatures.
At low temperature (), the atom cloud undergoes an anisotropic
hydrodynamic expansion over 30~ms, which behaves like oscillation in the
horizontal plane. By analyzing the expansion dynamics according to the
superfluid hydrodynamic equation, the effective polytropic index
of Equation-of-State of Fermionic superfluid is extracted. The
values show a non-monotonic behavior over the BEC-BCS crossover, and have a
good agreement with the theoretical results in the unitarity and BEC side. The
normalized quasi-frequencies of the oscillatory expansion are measured, which
drop significantly from the BEC side to the BCS side and reach a minimum value
of 1.73 around . Our work improves the understanding of the
dynamic properties of strongly interacting Fermi gas
Strongly Interacting Bose Gases near a -wave Shape Resonance
Many unconventional quantum matters, such as fractional quantum Hall effect
and -wave high-Tc superconductor, are discovered in strongly interacting
systems. Understanding quantum many-body systems with strong interaction and
the unconventional phases therein is one of the most challenging problems in
physics nowadays. Cold atom systems possess a natural way to create strong
interaction by bringing the system to the vicinity of a scattering resonance.
Although this has been a focused topic in cold atom physics for more than a
decade, these studies have so far mostly been limited for -wave resonance.
Here we report the experimental observation of a broad -wave shape resonance
in degenerate K gas. We further measure the molecular binding energy
that splits into three branches as a hallmark of -wave molecules, and find
that the lifetime of this many-body system is reasonably long at strongly
interacting regime. From analyzing the breathing mode excited by ramping
through this resonance, it suggests that a quite stable low-temperature atom
and molecule mixture is produced. Putting all the evidence together, our system
offers great promise to reach a -wave molecular superfluid
Feshbach spectroscopy of an ultracold K-Li mixture and K atoms
We have observed 69 K-Li interspecies Feshbach resonances
including 13 elastic p-wave resonances and 6 broad d-wave resonances of
K atoms in different spin-state combinations at fields up to 600~G.
Multi-channel quantum defect theory calculation is performed to assign these
resonances and the results show perfect agreement with experimental values
after improving input parameters. The observed broad p- and d- wave resonances
display a full resolved multiplet structure. They may serve as important
simulators to nonzero partial wave dominated physics
Observation of state-to-state hyperfine-changing collision in a Bose-Fermi mixture of Li and K atoms
Hyperfine-changing collisions are of fundamental interest for the studying of
ultracold heteronuclear mixtures. Here, we report the state-to-state study of
the hyperfine-changing-collision dynamics in a Bose-Fermi mixture of Li and
K atoms. The collision products are directly observed and the
spin-changing dynamics is measured. Based on a two-body collision model, the
experimental results are simultaneously fitted from which the spin-changing
rate coefficient of is gained,
being consistent with the multi-channel quantum defect theory calculation. We
further show that the contact parameter of Li-K mixture can be
extracted from the measured spin-changing dynamics. The obtained results are
consistent with the first order perturbation theory in the weakly-interacting
limit. Our system offers great promise for studying spin-changing interactions
in heteronuclear mixtures
A 30 W, sub-kHz frequency-locked laser at 532 nm
We report on the realization of a high-power, ultranarrow-linewidth, and
frequency-locked 532~nm laser system. The laser system consists of single-pass
and intra-cavity second harmonic generation of a continuous-wave Ytterbium
doped fiber laser at 1064~nm in the nonlinear crystal of periodically poled
lithium niobate and lithium triborate, respectively. With 47~W infrared input,
30~W green laser is generated through the type I critical phase matching in the
intracavity lithium triborate crystal. The laser linewidth is measured to be on
the order of sub-kHz, which is achieved by simultaneously locking the
single-pass frequency doubling output onto the iodine absorption line R69
(36-1) at 532~nm. Furthermore, the phase locking between the laser system and
another slave 1064~nm laser is demonstrated with relative frequency tunability
being up to 10~GHz. Our results completely satisfy the requirements of 532~nm
laser for quantum simulation with ultracold atoms
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