11,417 research outputs found
On gravitational wave-Cherenkov radiation from photons when passing through diffused dark matters
Analogy to Cherenkov radiation, when a particle moves faster than the
propagation velocity of gravitational wave in matter (), we
expect gravitational wave-Cherenkov radiation (GWCR). In the situation that a
photon travels across diffuse dark matters, the GWCR condition is always
satisfied, photon will thence loss its energy all the path. This effect is long
been ignored in the practice of astrophysics and cosmology, without
justification with serious calculation. We study this effect for the first
time, and shows that this energy loss time of the photon is far longer than the
Hubble time, therefore justify the practice of ignoring this effect in
astrophysics context.Comment: 3 pages; We add a citation to Caves (1980) to the original journal
version, after kindly reminde
Neurodegeneration induced by general anaesthetics in the developing brain: Mechanism and prevention
In the early development of brain, there is an important period called the
synaptogenesis period (also known as the brain growth-spurt period) which occurs in
different mammalian species at different times relative to birth. In rodents, it begins
a day or two before birth and ends 2 weeks after birth, whereas in humans it starts
at the beginning of the third trimester and ends several years after birth. This is a
critical period during neural development as neurons are establishing
communications with their environment and other neurons to ensure that they
develop and function appropriately; disruption at this stage by toxic agents including
anaesthetic agents may compromise the normal neuronal development of the
central nervous system.
During the course of my PhD study, I found that inhalational agents isoflurane (ISO)
alone or in combination with nitrous oxide (N2O) caused a wide spread
neurodegeneration in the brain including the cingulated cortex, hippocampus,
substantia nigra and basal forebrain in postnatal day 7 Sprague-Dawley rat pups. In
those brain regions, the cells involved included glutamatergic, GABAergic and
dopaminergic neurons. However, the cholinergic neurons in the basal forebrain were
spared from anaesthesia-induced neuroapoptosis. My studies also showed that the
neurodegeneration was not only confined to the higher centres, but was also
observed in the spinal cord. These data indicated that the anaesthetics studied acted
at molecular level and induced neuronal apoptosis by activating the intrinsic
apoptotic pathway. It has been well documented that preconditioning with hypoxia or other
interventions including pharmacological agents can protect against subsequent brain
injury induced by hypoxia/ischemia. However, my data showed that hypoxic
preconditioning exacerbated N2O+ISO-induced neonatal neurodegeneration, while
xenon preconditioning protected against anaesthetics-induced neuroapoptosis and
subsequent neurocognitve impairment.
Most studies in this field including my own study described above are more relevant
to the clinical situation where neonates are managed in the critical care unit, i.e. in
the absence of surgery. For this reason, I implemented another experimental setting
to mimic the clinical situation in the operating room where neonates receiving
anaesthetic drugs are also subjected to nociceptive surgical stimuli. Therefore, in
another experiment, rat pups in addition to receiving anaesthetic drugs were also
subjected to standardised nociceptive stimuli. The results suggested that nociceptive
stimuli enhanced neuronal apoptosis induced by anaesthetic drugs in the brain and
the spinal cord and consequently impaired neurological function.
Were the data reported in my thesis to be extrapolated to the clinical setting, it
would have very important implications for the clinical management of paediatric
population receiving general anaesthesia
Analysis of association studies and inference of haplotypic phase using hidden Markov models
In this thesis I focus on the development and application of hidden Markov model
(HMM) to solve problems in statistical genetics. Our method, based on a HMM,
models the joint haplotype structure in the samples, where the parameters in the
model are estimated by the Baum-Welch EM algorithm. Also, the model does not
require pre-defined blocks or a sliding window scheme to define haplotype boundaries. Thus our method is computationally efficient and applicable for either the
whole genome sequence or the candidate gene sequence.
The first application of this model is for disease association testing using inferred
ancestral haplotypes. We employed a HMM to cluster haplotypes into groups of
predicted common ancestral haplotypes from diploid genotypes. The results from
simulation studies show that our method greatly outperforms single-SNP analyses
and has greater power than a haplotype-based method, CLADHC, in most simulation scenarios. The second application is for inferring haplotypic phase and to
predict missing genotypes in polyploid organisms. Using a simulation study we
demonstrate that the method provides accurate estimates of haplotypic phase and
missing genotypes for diploids, triploids and tetraploids. The third application is
for joint CNV/SNP haplotype and missing data inference. The results are very
encouraging for this application.
With the increasing availability of genotype data in both diploid and polyploid
organisms, we believe that our programs can facilitate the investigation of genetic
variations in genome-wide scale studies
A new approach to the GeV flare of PSR B1259-63/LS2883
PSR B1259-63/LS2883 is a binary system composed of a pulsar and a Be star.
The Be star has an equatorial circumstellar disk (CD). The {\it Fermi}
satellite discovered unexpected gamma-ray flares around 30 days after the last
two periastron passages. The origin of the flares remain puzzling. In this
work, we explore the possibility that, the GeV flares are consequences of
inverse Compton-scattering of soft photons by the pulsar wind. The soft photons
are from an accretion disk around the pulsar, which is composed by the matter
from CD captured by the pulsar's gravity at disk-crossing before the
periastron. At the other disk-crossing after the periastron, the density of the
CD is not high enough so that accretion is prevented by the pulsar wind shock.
This model can reproduce the observed SEDs and light curves satisfactorily.Comment: 14 pages, 8 figures, 1 table. Accepted for publication in Ap
Probing the properties of the pulsar wind via studying the dispersive effects in the pulses from the pulsar companion in a double neutron-star binary system
The velocity and density distribution of in the pulsar wind are
crucial distinction among magnetosphere models, and contains key parameters
determining the high energy emission of pulsar binaries. In this work, a direct
method is proposed, which might probe the properties of the wind from one
pulsar in a double-pulsar binary. When the radio signals from the first-formed
pulsar travel through the relativistic flow in the pulsar wind from the
younger companion, the components of different radio frequencies will be
dispersed. It will introduce an additional frequency-dependent time-of-arrival
delay of pulses, which is function of the orbital phase. In this paper, we
formulate the above-mentioned dispersive delay with the properties of the
pulsar wind. As examples, we apply the formula to the double pulsar system PSR
J0737-3039A/B and the pulsar-neutron star binary PSR B1913+16. For PSR
J0737-3039A/B, the time delay in 300\,MHz is s near the
superior-conjunction, under the optimal pulsar wind parameters, which is
half of the current timing accuracy. For PSR B1913+16, with the assumption that
the neutron star companion has a typical spin down luminosity of
\,ergs/s, the time delay is as large as s in 300\,MHz.
The best timing precision of this pulsar is s in 1400\,MHz.
Therefore, it is possible that we can find this signal in archival data.
Otherwise, we can set an upper-limit on the spin down luminosity. Similar
analysis can be apply to other eleven known pulsar-neutron star binariesComment: 6 pages, 6 figures, accepted for publication in MNRA
- …