885 research outputs found
Electrical characteristics of amorphous silicon schottky barriers
The behaviour of the admittance of an a-Si Schottky barrier as a function of bias, small signal measuring frequency and temperature is not well understood. In this thesis model calculations are described which are both well defined and comprehensive in their description of the Schottky barrier admittance. These calculations allow a better understanding of experimental admittance plots. Various methods are developed for finding, from Schottky barrier admittance measurements, the density of states in the a-Si mobility gap. The methods are essentially developments of the model admittance calculations, and it should be stressed that the reliability of the deduced density of states depends on the correctness of the initial model premises. In particular it is assumed that the gap state capture cross-sections are all equal and independent of energy. Experimental admittance measurements are presented for an n-type doped a-Si Schottky barrier. The measurements are quite consistent with the developed theory and an estimate of the density of states in the upper half of the mobility gap is calculated. The average value is ~ 10(^17) cm(^-3)eV(^-1) and there is a minimum situated approximately at 0.3 eV below the conduction band mobility edge. This result is in approximate agreement with the density of states deduced by the DLTS technique. It is also deduced from current-voltage measurements that, of the existing theories, Diffusion Theory probably best describes the leakage current in a-Si Schottky barriers. This deduction is arrived at using some novel analysis
Discovery and Follow-up of Rotating Radio Transients with the Green Bank and LOFAR Telescopes
We have discovered 21 Rotating Radio Transients (RRATs) in data from the
Green Bank Telescope (GBT) 350-MHz Drift-scan and the Green Bank North
Celestial Cap pulsar surveys using a new candidate sifting algorithm. RRATs are
pulsars with sporadic emission that are detected through their bright single
pulses rather than Fourier domain searches. We have developed {\tt RRATtrap}, a
single-pulse sifting algorithm that can be integrated into pulsar survey data
analysis pipelines in order to find RRATs and Fast Radio Bursts. We have
conducted follow-up observations of our newly discovered sources at several
radio frequencies using the GBT and Low Frequency Array (LOFAR), yielding
improved positions and measurements of their periods, dispersion measures, and
burst rates, as well as phase-coherent timing solutions for four of them. The
new RRATs have dispersion measures (DMs) ranging from 15 to 97 pc cm,
periods of 240 ms to 3.4 s, and estimated burst rates of 20 to 400 pulses
hr at 350 MHz. We use this new sample of RRATs to perform statistical
comparisons between RRATs and canonical pulsars in order to shed light on the
relationship between the two populations. We find that the DM and spatial
distributions of the RRATs agree with those of the pulsars found in the same
survey. We find evidence that slower pulsars (i.e. ms) are
preferentially more likely to emit bright single pulses than are faster pulsars
( ms), although this conclusion is tentative. Our results are consistent
with the proposed link between RRATs, transient pulsars, and canonical pulsars
as sources in various parts of the pulse activity spectrum.Comment: 18 pages, 13 figures, 5 tables, published in Ap
A millisecond pulsar in a stellar triple system
Gravitationally bound three-body systems have been studied for hundreds of
years and are common in our Galaxy. They show complex orbital interactions,
which can constrain the compositions, masses, and interior structures of the
bodies and test theories of gravity, if sufficiently precise measurements are
available. A triple system containing a radio pulsar could provide such
measurements, but the only previously known such system, B1620-26 (with a
millisecond pulsar, a white dwarf, and a planetary-mass object in an orbit of
several decades), shows only weak interactions. Here we report precision timing
and multi-wavelength observations of PSR J0337+1715, a millisecond pulsar in a
hierarchical triple system with two other stars. Strong gravitational
interactions are apparent and provide the masses of the pulsar (1.4378(13)
Msun, where Msun is the solar mass and the parentheses contain the uncertainty
in the final decimal places) and the two white dwarf companions (0.19751(15)
Msun and 0.4101(3) Msun), as well as the inclinations of the orbits (both
approximately 39.2 degrees). The unexpectedly coplanar and nearly circular
orbits indicate a complex and exotic evolutionary past that differs from those
of known stellar systems. The gravitational field of the outer white dwarf
strongly accelerates the inner binary containing the neutron star, and the
system will thus provide an ideal laboratory in which to test the strong
equivalence principle of general relativity.Comment: 17 pages, 3 figures, 1 table. Published online by Nature on 5 Jan
2014. Extremely minor differences with published version may exis
A Radio Pulsar/X-ray Binary Link
Radio pulsars with millisecond spin periods are thought to have been spun up
by transfer of matter and angular momentum from a low-mass companion star
during an X-ray-emitting phase. The spin periods of the neutron stars in
several such low-mass X-ray binary (LMXB) systems have been shown to be in the
millisecond regime, but no radio pulsations have been detected. Here we report
on detection and follow-up observations of a nearby radio millisecond pulsar
(MSP) in a circular binary orbit with an optically identified companion star.
Optical observations indicate that an accretion disk was present in this system
within the last decade. Our optical data show no evidence that one exists
today, suggesting that the radio MSP has turned on after a recent LMXB phase.Comment: published in Scienc
The Green Bank Northern Celestial Cap Pulsar Survey - I: Survey Description, Data Analysis, and Initial Results
We describe an ongoing search for pulsars and dispersed pulses of radio
emission, such as those from rotating radio transients (RRATs) and fast radio
bursts (FRBs), at 350 MHz using the Green Bank Telescope. With the Green Bank
Ultimate Pulsar Processing Instrument, we record 100 MHz of bandwidth divided
into 4,096 channels every 81.92 . This survey will cover the entire sky
visible to the Green Bank Telescope (, or 82% of the sky)
and outside of the Galactic Plane will be sensitive enough to detect slow
pulsars and low dispersion measure (30 ) millisecond
pulsars (MSPs) with a 0.08 duty cycle down to 1.1 mJy. For pulsars with a
spectral index of 1.6, we will be 2.5 times more sensitive than previous and
ongoing surveys over much of our survey region. Here we describe the survey,
the data analysis pipeline, initial discovery parameters for 62 pulsars, and
timing solutions for 5 new pulsars. PSR J02145222 is an MSP in a long-period
(512 days) orbit and has an optical counterpart identified in archival data.
PSR J06365129 is an MSP in a very short-period (96 minutes) orbit with a
very low mass companion (8 ). PSR J06455158 is an isolated MSP
with a timing residual RMS of 500 ns and has been added to pulsar timing array
experiments. PSR J14347257 is an isolated, intermediate-period pulsar that
has been partially recycled. PSR J18164510 is an eclipsing MSP in a
short-period orbit (8.7 hours) and may have recently completed its spin-up
phase.Comment: 18 pages, 10 figures, 5 tables, accepted by Ap
Long-term timing and emission behavior of the young Crab-like pulsar PSR B0540-69
We present timing solutions and spin properties of the young pulsar PSR B0540-69 from analysis of 15.8 years of data from the Rossi X-Ray Timing Explorer. We perform a partially phase-coherent timing analysis in order to mitigate the pronounced effects of timing noise in this pulsar. We also perform fully coherent timing over large subsets of the data set in order to arrive at a more precise solution. In addition to the previously reported first glitch undergone by this pulsar, we find a second glitch, which occurred at MJD 52927 +/- 4, with fractional changes in spin frequency Delta nu/nu = (1.64 +/- 0.05) x 10(-9) and spin-down rate Delta(nu) over dot/(nu) over dot = (0.930 +/- 0.011) x 10(-4) (taken from our fully coherent analysis). We measure a braking index that is consistent over the entire data span, with a mean value n = 2.129 +/- 0.012, from our partially coherent timing analysis. We also investigated the emission behavior of this pulsar, and have found no evidence for significant flux changes, flares, burst-type activity, or pulse profile shape variations. While there is strong evidence for the much-touted similarity of PSR B0540-69 to the Crab pulsar, they nevertheless differ in several aspects, including glitch activity, where PSR B0540-69 can be said to resemble certain other very young pulsars. It seems clear that the specific processes governing the formation, evolution, and interiors of this population of recently born neutron stars can vary significantly, as reflected in their observed properties
The international pulsar timing array project: using pulsars as a gravitational wave detector
The International Pulsar Timing Array project combines observations of
pulsars from both Northern and Southern hemisphere observatories with the main
aim of detecting ultra-low frequency (~10^-9 to 10^-8 Hz) gravitational waves.
Here we introduce the project, review the methods used to search for
gravitational waves emitted from coalescing supermassive binary black-hole
systems in the centres of merging galaxies and discuss the status of the
project.Comment: accepted by Classical and Quantum Gravity. Review talk for the
Amaldi8 conference serie
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