22,579 research outputs found
Resolving discrete pulsar spin-down states with current and future instrumentation
An understanding of pulsar timing noise offers the potential to improve the
timing precision of a large number of pulsars as well as facilitating our
understanding of pulsar magnetospheres. For some sources, timing noise is
attributable to a pulsar switching between two different spin-down rates
. Such transitions may be common but difficult to resolve using
current techniques. In this work, we use simulations of -variable
pulsars to investigate the likelihood of resolving individual
transitions. We inject step-changes in the value of with a wide
range of amplitudes and switching timescales. We then attempt to redetect these
transitions using standard pulsar timing techniques. The pulse arrival-time
precision and the observing cadence are varied. Limits on
detectability based on the effects such transitions have on the timing
residuals are derived. With the typical cadences and timing precision of
current timing programs, we find we are insensitive to a large region of
parameter space which encompasses small, short timescale
switches. We find, where the rotation and emission states are correlated, that
using changes to the pulse shape to estimate transition epochs, can
improve detectability in certain scenarios. The effects of cadence on detectability are discussed and we make comparisons with a known
population of intermittent and mode-switching pulsars. We conclude that for
short timescale, small switches, cadence should not be compromised when new
generations of ultra-sensitive radio telescopes are online.Comment: 19 pages, 11 figure
Variations in propagation delay times for line ten (TV) based time transfers
Variation in the propagation delay for a 30 km TV (Line Ten) radio link was evaluated for a series of 30 independent measurements. Time marks from TV Channel 5 WTTG in Washington, D.C. were simultaneously measured at the Johns Hopkins University Applied Physics Laboratory and at the United States Naval Observatory against each stations' local cesium standard clocks. Differences in the stations' cesium clocks were determined by portable cesium clock transfers. Thirty independent timing determinations were made. The root mean square deviation in the propagation delay calculated from the timing determinations was 11 ns. The variations seen in the propagation delays are believed to be caused by environmental factors and by errors in the portable clock timing measurements. In correlating the propagation delay variations with local weather conditions, only a moderate dependence on air temperature and absolute humidity was found
Laser Interferometer Gravitational-Wave Observatory beam tube component and module leak testing
Laser Interferometer Gravitational-Wave Observatory (LIGO) is a joint project of the California Institute of Technology and the Massachusetts Institute of Technology funded by the National Science Foundation. The project is designed to detect gravitational waves from astrophysical sources such as supernova and black holes. The LIGO project constructed observatories at two sites in the U.S. Each site includes two beam tubes (each 4 km long) joined to form an "L" shape. The beam tube is a 1.25 m diam 304 L stainless steel, ultrahigh vacuum tube that will operate at 1×10^–9 Torr or better. The beam tube was manufactured using a custom spiral weld tube mill from material processed to reduce the outgassing rate in order to minimize pumping costs. The integrity of the beam tube was assured by helium mass spectrometer leak testing each component of the beam tube system prior to installation. Each 2 km long, isolatable beam tube module was then leak tested after completion
Nose controls on delta wings at supersonic speeds
Expressions are derived for lξ and a2 of nose ailerons and nose elevators on a delta wing, as depicted in Fig. 1, in supersonic flight. Nose and trailing edge controls on delta wings in supersonic flight are compared
Throat stability-by pass systems to increase the stable airflow range of a Mach 2.5 inlet with 60-percent internal contraction
The results of an experimental investigation to increase the stable airflow range (without unstart) of a supersonic mixed-compression inlet are presented. Various stability bypass entrances were located on the cowl side of the inlet throat. The types of entrance were distributed porous (normal holes), forward-slanted slot, and distributed educated slots. A large stable airflow range was obtained for each entrance type if a constant pressure was maintained in the stability bypass plenum. The distributed porous entrance provided the largest stable airflow range. Inlet unstart angle of attack was unaffected by the entrances
Distributed porous throat stability bypass to increase the stable airflow range of a Mach 2.5 inlet with 60 percent internal contraction
The results of an experimental investigation to increase the stable airflow operating range of a supersonic, mixed-compression inlet with 60-percent internal contraction are presented. Various distributed-porous, throat stability-bypass entrance configurations were tested. In terms of diffuser-exit corrected airflow, a large inlet stable airflow range of about 25 percent was obtained with the optimum configuration if a constant pressure was maintained in the by-pass plenum. The location of the centerbody bleed region had a decided effect on the overall inlet performance. Limited unstart angle-of-attack data are presented
Effects of systematic errors on the mixing ratios of trace gases obtained from occulation spectra
The influence of systematic errors in the parameters of the models describing the geometry and the atmosphere on the profiles of trace gases retrieved from simulated solar occultation spectra, collected at satellite altitudes, is investigated. Because of smearing effects and other uncertainties, it may be preferable to calibrate the spectra internally by measuring absorption lines of an atmospheric gas such as CO2 whose vertical distribution is assumed rather than to relay on externally supplied information
Distributed educated throat stability bypass to increase the stable airflow range of a Mach 2.5 inlet with 60-percent internal contraction
The results of an experimental investigation to increase the stable airflow operating range of a supersonic mixed-compression inlet are presented. A distributed educated throat stability-bypass entrance configuration was tested. In terms of diffuser-exit corrected airflow, a large inlet stable airflow range of about 16.1 percent was obtained if a constant pressure was maintained in the bypass plenum. Limited unstart angle of attack data are presented
The glitch activity of neutron stars
We present a statistical study of the glitch population and the behaviour of
the glitch activity across the known population of neutron stars. An unbiased
glitch database was put together based on systematic searches of radio timing
data of 898 rotation-powered pulsars obtained with the Jodrell Bank and Parkes
observatories. Glitches identified in similar searches of 5 magnetars were also
included. The database contains 384 glitches found in the rotation of 141 of
these neutron stars. We confirm that the glitch size distribution is at least
bimodal, with one sharp peak at approximately , which we
call large glitches, and a broader distribution of smaller glitches. We also
explored how the glitch activity , defined as the mean
frequency increment per unit of time due to glitches, correlates with the spin
frequency , spin-down rate , and various combinations of
these, such as energy loss rate, magnetic field, and spin-down age. It is found
that the activity is insensitive to the magnetic field and that it correlates
strongly with the energy loss rate, though magnetars deviate from the trend
defined by the rotation-powered pulsars. However, we find that a constant ratio
is consistent with the behaviour
of all rotation-powered pulsars and magnetars. This relation is dominated by
large glitches, which occur at a rate directly proportional to .
The only exception are the rotation-powered pulsars with the highest values of
, such as the Crab pulsar and PSR B054069, which exhibit a much
smaller glitch activity, intrinsically different from each other and from the
rest of the population. The activity due to small glitches also shows an
increasing trend with , but this relation is biased by selection
effects.Comment: Accepted for publication in A&
Cadaveric Renal Transplantation in Diabetics in the 1980's: with Special Reference to Cyclosporine.
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