10,687 research outputs found
Radio Pulse Properties of the Millisecond Pulsar PSR J0437-4715. I. Observations at 20cm
We present a total of 48 minutes of observations of the nearby, bright
millisecond pulsar PSR J0437-4715 taken at the Parkes radio observatory in
Australia. The data were obtained at a central radio frequency of 1380 MHz
using a high-speed tape recorder that permitted coherent Nyquist sampling of 50
MHz of bandwidth in each of two polarizations. Using the high time resolution
available from this voltage recording technique, we have studied a variety of
single-pulse properties, most for the first time in a millisecond pulsar. We
find no evidence for "diffractive" quantization effects in the individual pulse
arrival times or amplitudes as have been reported for this pulsar at lower
radio frequency using coarser time resolution (Ables et al. 1997). Overall, we
find that the single pulse properties of PSR J0437-4715 are similar to those of
the common slow-rotating pulsars, even though this pulsar's magnetosphere and
surface magnetic field are several orders of magnitude smaller than those of
the general population. The pulsar radio emission mechanism must therefore be
insensitive to these fundamental neutron star properties.Comment: 24 Postscript pages, 11 eps figures. Accepted for publication in the
Astrophysical Journal. Abbreviated abstract follow
Discovery of Pulsed X-ray Emission from the SMC Transient RX J0117.6-7330
We report on the detection of pulsed, broad-band, X-ray emission from the
transient source RX J0117.6-7330. The pulse period of 22 seconds is detected by
the ROSAT/PSPC instrument in a 1992 Sep 30 - Oct 2 observation and by the
CGRO/BATSE instrument during the same epoch. Hard X-ray pulsations are
detectable by BATSE for approximately 100 days surrounding the ROSAT
observation (1992 Aug 28 - Dec 8). The total directly measured X-ray luminosity
during the ROSAT observation is 1.0E38 (d/60 kpc)^2 ergs s-1. The pulse
frequency increases rapidly during the outburst, with a peak spin-up rate of
1.2E-10 Hz s-1 and a total frequency change 1.8%. The pulsed percentage is
11.3% from 0.1-2.5 keV, increasing to at least 78% in the 20-70 keV band. These
results establish RX J0117.6-7330 as a transient Be binary system.Comment: 17 pages, Latex, aasms, accepted for publication in ApJ Letter
A descriptive analysis of admissions to Amanuel Psychiatric Hospital in Ethiopia
Background: The care of patients with severe mental disorder in Ethiopia remains centralized in the capital city.
Objective: To assess pattern of psychiatric admission and its implication for service provision.
Methods: A descriptive analysis of one-year admission data was undertaken from the only psychiatric hospital in Ethiopia.
Results: The annual age-sex standardized admission rate was 4 per 100 000 (n=1564). Two-thirds of the patients came from Addis Ababa and the immediate surrounding areas. Nearly three-quarters were men, and aged 30 years or younger. Schizophrenia and bipolar disorder were the top two discharge diagnoses. The median length of hospital stay
(LOS) was 63 days. Diagnosis of schizophrenia and place of residence, i.e. living outside of Addis Ababa independently predicted LOS above the median.
Conclusion: Admission data demonstrated a huge mental health unmet need in Ethiopia. Providing adequate resources for modernization and well-supervised decentralization may be vital steps in the quest for accessible and equitable psychiatric care.The Ethiopian Journal of Health Development Vol. 21 (2) 2007: pp. 173-17
Hexagonal Uniformly Redundant Arrays for Coded-Aperture Imaging
Uniformly redundant arrays are used in coded-aperture imaging, a technique for forming images
without mirrors or lenses. This technique is especially important for the high energy x-ray
and y-ray region above 20 keV. In this technique, a mask consisting of opaque (closed) and transparent
(open) areas is placed between the photon sources to be imaged and a position sensitive
detector or a detector array. Each source casts a shadow pattern of the mask or aperture onto the
detector. This shadow pattern may be viewed as an encoded signal for that source direction. If
each possible source code is unique, the detected composite of overlapping shadow patterns may
be decoded to produce an image of the source distribution
Optimality Theory as a Framework for Lexical Acquisition
This paper re-investigates a lexical acquisition system initially developed
for French.We show that, interestingly, the architecture of the system
reproduces and implements the main components of Optimality Theory. However, we
formulate the hypothesis that some of its limitations are mainly due to a poor
representation of the constraints used. Finally, we show how a better
representation of the constraints used would yield better results
Pulse source for 80 Gb/s systems using a gain-wwitched laser diode followed by a nonlinearly chirped grating
This work presents the generation of 3.5 ps pulses at a repetition rate of 10 GHz and the optimization of the pulse spectrum. The output pulses are near transform limited and have pulse pedestals that are virtually eliminated to 35 dB down from the peak of the pulse, thus providing a source suitable for use in 80 Gb/s OTDM systems
Gravitational recoil from spinning binary black hole mergers
The inspiral and merger of binary black holes will likely involve black holes
with both unequal masses and arbitrary spins. The gravitational radiation
emitted by these binaries will carry angular as well as linear momentum. A net
flux of emitted linear momentum implies that the black hole produced by the
merger will experience a recoil or kick. Previous studies have focused on the
recoil velocity from unequal mass, non-spinning binaries. We present results
from simulations of equal mass but spinning black hole binaries and show how a
significant gravitational recoil can also be obtained in these situations. We
consider the case of black holes with opposite spins of magnitude
aligned/anti-aligned with the orbital angular momentum, with the
dimensionless spin parameters of the individual holes. For the initial setups
under consideration, we find a recoil velocity of V = 475 \KMS a.
Supermassive black hole mergers producing kicks of this magnitude could result
in the ejection from the cores of dwarf galaxies of the final hole produced by
the collision.Comment: 8 pages, 8 figures, replaced with version accepted for publication in
Ap
A Group Theoretical Identification of Integrable Equations in the Li\'enard Type Equation : Part II: Equations having Maximal Lie Point Symmetries
In this second of the set of two papers on Lie symmetry analysis of a class
of Li\'enard type equation of the form ,
where over dot denotes differentiation with respect to time and and
are smooth functions of their variables, we isolate the equations which
possess maximal Lie point symmetries. It is well known that any second order
nonlinear ordinary differential equation which admits eight parameter Lie point
symmetries is linearizable to free particle equation through point
transformation. As a consequence all the identified equations turn out to be
linearizable. We also show that one can get maximal Lie point symmetries for
the above Li\'enard equation only when (subscript denotes
differentiation). In addition, we discuss the linearising transformations and
solutions for all the nonlinear equations identified in this paper.Comment: Accepted for publication in Journal of Mathematical Physic
Optimized pulse source employing an externally injected gain-switched laser diode in conjunction with a nonlinearly chirped grating
In this paper, we demonstrate the generation of transform-limited short optical pulses, which display excellent spectral and temporal qualities by employing a novel technology, based on an externally injected gain-switched laser in conjunction with a nonlinearly chirped grating. Using this technique, 3.5-ps optical pulses exhibiting a time-bandwidth product (TBP) of 0.45 are generated, which are suitable for use in high-speed 80 Gb/s optical time-division multiplexing (OTDM) communications systems. The numerical integration of a set of rate equations using suitable parameters for the devices used in the experiments were carried out to further confirm the feasibility of the proposed method for developing an optimized pulse source for high-speed photonic systems
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