9,293 research outputs found
A Typical Medium Dynamical Cluster Approximation for the Study of Anderson Localization in Three Dimensions
We develop a systematic typical medium dynamical cluster approximation that
provides a proper description of the Anderson localization transition in three
dimensions (3D). Our method successfully captures the localization phenomenon
both in the low and large disorder regimes, and allows us to study the
localization in different momenta cells, which renders the discovery that the
Anderson localization transition occurs in a cell-selective fashion. As a
function of cluster size, our method systematically recovers the re-entrance
behavior of the mobility edge and obtains the correct critical disorder
strength for Anderson localization in 3D.Comment: 5 Pages, 4 Figures and Supplementary Material include
A NuSTAR Observation of the Gamma-ray Emitting Millisecond Pulsar PSR J1723-2837
We report on the first NuSTAR observation of the gamma-ray emitting
millisecond pulsar binary PSR J1723-2837. X-ray radiation up to 79 keV is
clearly detected and the simultaneous NuSTAR and Swift spectrum is well
described by an absorbed power-law with a photon index of ~1.3. We also find
X-ray modulations in the 3-10 keV, 10-20 keV, 20-79 keV, and 3-79 keV bands at
the 14.8-hr binary orbital period. All these are entirely consistent with
previous X-ray observations below 10 keV. This new hard X-ray observation of
PSR J1723-2837 provides strong evidence that the X-rays are from the
intrabinary shock via an interaction between the pulsar wind and the outflow
from the companion star. We discuss how the NuSTAR observation constrains the
physical parameters of the intrabinary shock model.Comment: Accepted for publication in ApJ. 5 pages, 3 figure
Discovery of an unidentified Fermi object as a black widow-like millisecond pulsar
The Fermi Gamma-ray Space Telescope has revolutionized our knowledge of the
gamma-ray pulsar population, leading to the discovery of almost 100 gamma-ray
pulsars and dozens of gamma-ray millisecond pulsars (MSPs). Although the
outer-gap model predicts different sites of emission for the radio and
gamma-ray pulsars, until now all of the known gamma-ray MSPs have been visible
in the radio. Here we report the discovery of a "radio-quiet" gamma-ray
emitting MSP candidate by using Fermi, Chandra, Swift, and optical
observations. The X-ray and gamma-ray properties of the source are consistent
with known gamma-ray pulsars. We also found a 4.63-hr orbital period in optical
and X-ray data. We suggest that the source is a black widow-like MSP with a
~0.1 solar-mass late-type companion star. Based on the profile of the optical
and X-ray light-curves, the companion star is believed to be heated by the
pulsar while the X-ray emissions originate from pulsar magnetosphere and/or
from intra-binary shock. No radio detection of the source has been reported yet
and although no gamma-ray/radio pulsation has been found, we estimated that the
spin period of the MSP is ~3-5 ms based on the inferred gamma-ray luminosity.Comment: 6 pages, 2 figures; accepted for publication in ApJ
Swift, XMM-Newton, and NuSTAR observations of PSR J2032+4127/MT91 213
We report our recent Swift, NuSTAR, and XMM-Newton X-ray and Lijiang optical
observations on PSR J2032+4127/MT91 213, the gamma-ray binary candidate with a
period of 45-50 years. The coming periastron of the system was predicted to be
in November 2017, around which high-energy flares from keV to TeV are expected.
Recent studies with Chandra and Swift X-ray observations taken in 2015/16
showed that its X-ray emission has been brighter by a factors of ~10 than that
before 2013, probably revealing some on-going activities between the pulsar
wind and the stellar wind. Our new Swift/XRT lightcurve shows no strong
evidence of a single vigorous brightening trend, but rather several strong
X-ray flares on weekly to monthly timescales with a slowly brightening
baseline, namely the low state. The NuSTAR and XMM-Newton observations taken
during the flaring and the low states, respectively, show a denser environment
and a softer power-law index during the flaring state, implying that the pulsar
wind interacted with stronger stellar winds of the companion to produce the
flares. These precursors would be crucial in studying the predicted giant
outburst from this extreme gamma-ray binary during the periastron passage in
late 2017.Comment: 6 pages, including 3 figures and 2 tables. Accepted for publication
in Ap
High-Energy emissions from the Pulsar/Be binary system PSR J2032+4127/MT91 213
PSR J2032+4127 is a radio-loud gamma-ray-emitting pulsar; it is orbiting
around a high-mass Be type star with a very long orbital period of 25-50years,
and is approaching periastron, which will occur in late 2017/early 2018. This
system comprises with a young pulsar and a Be type star, which is similar to
the so-called gamma-ray binary PSR~B1259-63/LS2883. It is expected therefore
that PSR J2032+4127 shows an enhancement of high-energy emission caused by the
interaction between the pulsar wind and Be wind/disk around periastron. Ho et
al. recently reported a rapid increase in the X-ray flux from this system. In
this paper, we also confirm a rapid increase in the X-ray flux along the orbit,
while the GeV flux shows no significant change. We discuss the high-energy
emissions from the shock caused by the pulsar wind and stellar wind interaction
and examine the properties of the pulsar wind in this binary system. We argue
that the rate of increase of the X-ray flux observed by Swift indicates (1) a
variation of the momentum ratio of the two-wind interaction region along the
orbit, or (2) an evolution of the magnetization parameter of the pulsar wind
with the radial distance from the pulsar. We also discuss the pulsar wind/Be
disk interaction at the periastron passage, and propose the possibility of
formation of an accretion disk around the pulsar. We model high-energy
emissions through the inverse-Compton scattering process of the
cold-relativistic pulsar wind off soft photons from the accretion disk.Comment: 18 pages, 23 figures, 1 Table, accepted for publication in Ap
Intentions and Attitudes Towards Parenthood and Fertility Awareness Among Chinese University Students in Hong Kong: A Comparison with Western Samples
STUDY QUESTION What are the levels of awareness regarding female fertility and the intentions and attitudes towards parenthood among Chinese university students in Hong Kong compared with their counterparts in the West?
SUMMARY ANSWER Chinese university students in Hong Kong were similarly over-optimistic about the age-related fertility decline, although they were less inclined to have children and undergo fertility treatment compared with their Western counterparts.
WHAT IS KNOWN ALREADY Past studies of highly educated young adults in Europe and the USA have found that they are not sufficiently aware of the age-related decline in female fertility, and falsely believe that advanced reproductive treatments such as IVF will overcome fertility problems associated with age. Little is known about the perceptions of Chinese students in Hong Kong, a modernized Chinese city where the fertility rate is among the lowest in the world.
STUDY DESIGN, SIZE, DURATION An online cross-sectional survey of Chinese university students in Hong Kong was conducted in 2013. Results were compared with two similar studies in Sweden and the USA.
PARTICIPANTS/MATERIALS, SETTING, METHODS A total of 367 university students in Hong Kong (275 female, 92 male; mean age 23) responded to an e-mail invitation to participate in an online survey. Intentions and attitudes towards parenthood and awareness regarding female fertility were assessed using the Swedish Fertility Awareness Questionnaire.
MAIN RESULTS AND THE ROLE OF CHANCE Like their Western counterparts, a large proportion of Chinese university students underestimated the age-related fertility decline (92%) and overestimated the fertility treatment success rate (66%). However, they were less inclined to have children, were more aware of and less concerned with infertility and were less motivated to seek solutions in the event of a fertility problem. These comparisons were significant at P \u3c 0.05.
LIMITATIONS, REASONS FOR CAUTION Self-selection bias was inevitable in the questionnaire survey, and the anonymous nature of the survey did not permit the collection of characteristics of non-responders. International comparisons warrant caution because the Hong Kong sample was older than the US sample (mean age 20), but not older than the Sweden sample (mean age 24).
WIDER IMPLICATION OF FINDINGS While this study was consistent with past Western studies on the lack of fertility awareness among highly educated young people, the findings reveal significant cultural differences in family planning and responses to infertility between Asia and the West
Low-Reynolds number swimming in gels
Many microorganisms swim through gels, materials with nonzero zero-frequency
elastic shear modulus, such as mucus. Biological gels are typically
heterogeneous, containing both a structural scaffold (network) and a fluid
solvent. We analyze the swimming of an infinite sheet undergoing transverse
traveling wave deformations in the "two-fluid" model of a gel, which treats the
network and solvent as two coupled elastic and viscous continuum phases. We
show that geometric nonlinearities must be incorporated to obtain physically
meaningful results. We identify a transition between regimes where the network
deforms to follow solvent flows and where the network is stationary. Swimming
speeds can be enhanced relative to Newtonian fluids when the network is
stationary. Compressibility effects can also enhance swimming velocities.
Finally, microscopic details of sheet-network interactions influence the
boundary conditions between the sheet and network. The nature of these boundary
conditions significantly impacts swimming speeds.Comment: 6 pages, 5 figures, submitted to EP
The X-ray modulation of PSR J2032+4127/MT91 213 during the Periastron Passage in 2017
We present the Neil Gehrels Swift Observatory (Swift), Fermi Large Area
Telescope (Fermi-LAT), and Karl G. Jansky Very Large Array (VLA) observations
of the gamma-ray binary PSR J2032+4127/MT91 213, of which the periastron
passage has just occurred in November 2017. In the Swift X-ray light curve, the
flux was steadily increasing before mid-October 2017, however, a sharp X-ray
dip on a weekly time-scale is seen during the periastron passage, followed by a
post-periastron X-ray flare lasting for ~20 days. We suggest that the X-ray dip
is caused by (i) an increase of the magnetization parameter at the shock, and
(ii) the suppression due to the Doppler boosting effect. The 20-day
post-periastron flare could be a consequence of the Be stellar disk passage by
the pulsar. An orbital GeV modulation is also expected in our model, however,
no significant variability is seen in the Fermi-LAT light curve. We suspect
that the GeV emission resulted from the interaction between the binary's
members is hidden behind the bright magnetospheric emission of the pulsar.
Pulsar gating technique would be useful to remove the magnetospheric emission
and recover the predicted GeV modulation, if an accurate radio timing solution
over the periastron passage is provided in the future.Comment: 6 pages, including 2 figures. Accepted for publication in Ap
NuSTAR observations and broadband spectral energy distribution modeling of the millisecond pulsar binary PSR J1023+0038
We report the first hard X-ray (3-79 keV) observations of the millisecond
pulsar (MSP) binary PSR J1023+0038 using NuSTAR. This system has been shown
transiting between a low-mass X-ray binary (LMXB) state and a rotation-powered
MSP state. The NuSTAR observations were taken in both LMXB state and
rotation-powered state. The source is clearly seen in both states up to ~79
keV. During the LMXB state, the 3-79 keV flux is about a factor of 10 higher
that in the rotation-powered state. The hard X-rays show clear orbital
modulation during the X-ray faint rotation-powered state but the X-ray orbital
period is not detected in the X-ray bright LMXB state. In addition, the X-ray
spectrum changes from a flat power-law spectrum during the rotation-powered
state to a steeper power-law spectrum in the LMXB state. We suggest that the
hard X-rays are due to the intra-binary shock from the interaction between the
pulsar wind and the injected material from the low-mass companion star. During
the rotation-powered MSP state, the X-ray orbital modulation is due to Doppler
boosting of the shocked pulsar wind. At the LMXB state, the evaporating matter
of the accretion disk due to the gamma-ray irradiation from the pulsar stops
almost all the pulsar wind, resulting the disappearance of the X-ray orbital
modulation.Comment: 8 pages, 6 figures; accepted for publication in Ap
- …