4,987 research outputs found
Electromagnetic field application to underground power cable detection
Before commencing excavation or other work where power or other cables may be buried, it is important to determine the location of cables to ensure that they are not damaged. This paper describes a method of power-cable detection and location that uses measurements of the magnetic field produced by the currents in the cable, and presents the results of tests performed to evaluate the method. The cable detection and location program works by comparing the measured magnetic field signal with values predicted using a simple numerical model of the cable. Search coils are used as magnetic field sensors, and a measurement system is setup to measure the magnetic field of an underground power cable at a number of points above the ground so that it can detect the presence of an underground power cable and estimate its position. Experimental investigations were carried out using a model and under real site test conditions. The results show that the measurement system and cable location method give a reasonable prediction for the position of the target cable
Detection and Location of Underground Power Cable using Magnetic Field Technologies
The location of buried underground electricity cables is becoming a major engineering and social issue worldwide. Records of utility locations are relatively scant, and even when records are available, they almost always refer to positions relative to ground-level physical features that may no longer exist or that may have been moved or altered. The lack of accurate positioning records of existing services can cause engineering and construction delays and safety hazards when new construction, repairs, or upgrades are necessary. Hitting unknown underground obstructions has the potential to cause property damage, injuries and, even deaths. Thus, before commencing excavation or other work where power or other cables may be buried, it is important to determine the location of the cables to ensure that they are not damaged during the work. This paper describes the use of an array of passive magnetic sensors (induction coils) together with signal processing techniques to detect and locate underground power cables. The array consists of seven identical coils mounted on a support frame; one of these coils was previously tested under laboratory conditions, and relevant results have been published in [1]. A measurement system was constructed that uses a battery powered data acquisition system with two NI 9239 modules connected to the coil array, and controlled by a laptop. The system is designed to measure the magnetic field of an underground power cable at a number of points above the ground. A 3 by 3 m test area was chosen in one of our campus car parks. This area was chosen because the university’s utility map shows an isolated power cable there. Measurements were taken with the array in 16 different test positions, and compared with the values predicted for a long straight horizontal cable at various positions. Finally, error maps were plotted for different Z-coordinate values, showing the minimum fitting error for each position in this plane. One such map is shown in Figure 1; the low error values of 4-5% give a high degree of confidence that most of the measured signal is due to a cable near to these positions. This view is supported by the fact that the university’s utility map shows the cable at X = 1.4 m, and by amplitude measurements taken with a hand-held magnetic field meter
The Nature of the Giant Outbursts in the Bursting Pulsar GRO J 1744-28
We investigate the possible role of an accretion disk instability in
producing the giant outbursts seen in GRO J1744-28. Specifically, we study the
global, time dependent evolution of the Lightman-Eardley instability which can
develop near the inner edge of an accretion disk when the radiation pressure
becomes comparable to the gas pressure. Broadly speaking, our results are
compatible with earlier works by Taam \& Lin and by Lasota \& Pelat. The
uniqueness of GRO J1744-28 appears to be associated with the constraint that,
in order for outbursts to occur, the rate of accretion at the inner edge must
be within a narrow range just above the critical accretion rate at which
radiation pressure is beginning to become significant.Comment: 11 pages in .tex file, 4 Postscript figures, .tex file uses
aasms.sty; Ap. J. L. 1996, in pres
The Burst Spectra of EXO 0748-676 during a Long 2003 XMM-Newton Observation
Gravitationally redshifted absorption lines from highly ionized iron have
been previously identified in the burst spectra of the neutron star in EXO
0748-676. To repeat this detection we obtained a long, nearly 600 ks
observation of the source with XMM-Newton in 2003. The spectral features seen
in the burst spectra from the initial data are not reproduced in the burst
spectra from this new data. In this paper we present the spectra from the 2003
observations and discuss the sensitivity of the absorption structure to changes
in the photospheric conditions.Comment: 18 Pages, 3 Figures. Accepted for publication in Ap
Diffuse Gamma-ray Emission from the Galactic Center - A Multiple Energy Injection Model
We suggest that the energy source of the observed diffuse gamma-ray emission
from the direction of the Galactic center is the Galactic black hole Sgr A*,
which becomes active when a star is captured at a rate of
yr^{-1}. Subsequently the star is tidally disrupted and its matter is accreted
into the black hole. During the active phase relativistic protons with a
characteristic energy erg per capture are ejected. Over
90% of these relativistic protons disappear due to proton-proton collisions on
a timescale years in the small central bulge region with
radius pc within Sgr A*, where the density is cm^{-3}. The
gamma-ray intensity, which results from the decay of neutral pions produced by
proton-proton collisions, decreases according to , where t is
the time after last stellar capture. Less than 5% of relativistic protons
escaped from the central bulge region can survive and maintain their energy for
>10^7 years due to much lower gas density outside, where the gas density can
drop to cm. They can diffuse to a pc region before
disappearing due to proton-proton collisions. The observed diffuse GeV
gamma-rays resulting from the decay of neutral pions produced via collision
between these escaped protons and the gas in this region is expected to be
insensitive to time in the multi-injection model with the characteristic
injection rate of 10^{-5} yr^{-1}. Our model calculated GeV and 511 keV
gamma-ray intensities are consistent with the observed results of EGRET and
INTEGRAL, however, our calculated inflight annihilation rate cannot produce
sufficient intensity to explain the COMPTEL data.Comment: 8 pages, 3 figures, accepted by A&
Cataclysmic Variables and Other Compact Binaries in the Globular Cluster NGC 362: Candidates from Chandra and HST
Highly sensitive and precise X-ray imaging from Chandra, combined with the
superb spatial resolution of HST optical images, dramatically enhances our
empirical understanding of compact binaries such as cataclysmic variables and
low mass X-ray binaries, their progeny, and other stellar X-ray source
populations deep into the cores of globular clusters. Our Chandra X-ray images
of the globular cluster NGC 362 reveal 100 X-ray sources, the bulk of which are
likely cluster members. Using HST color-magnitude and color-color diagrams, we
quantitatively consider the optical content of the NGC 362 Chandra X-ray error
circles, especially to assess and identify the compact binary population in
this condensed-core globular cluster. Despite residual significant crowding in
both X-rays and optical, we identify an excess population of H{\alpha}-emitting
objects that is statistically associated with the Chandra X-ray sources. The
X-ray and optical characteristics suggest that these are mainly cataclysmic
variables, but we also identify a candidate quiescent low mass X-ray binary. A
potentially interesting and largely unanticipated use of observations such as
these may be to help constrain the macroscopic dynamic state of globular
clusters.Comment: 6 pages, 6 figures, to appear in the proceedings of the conference
"Binary Star Evolution: Mass Loss, Accretion, and Mergers," Mykonos, Greece,
June 22-25, 201
Beam Profile Measurements and Simulations of the PETRA Laser-Wire
The Laser-wire will be an essential diagnostic tool at the International
Linear Collider. It uses a finely focussed laser beam to measure the transverse
profile of electron bunches by detecting the Compton-scattered photons (or
degraded electrons) downstream of where the laser beam intersects the electron
beam. Such a system has been installed at the PETRA storage ring at DESY, which
uses a piezo-driven mirror to scan the laser-light across the electron beam.
Lat- est results of experimental data taking are presented and compared to
detailed simulations using the Geant4 based program BDSIM.Comment: 3 pagesm 4 figures. Submitted as a conference paper for the Particle
Accelerator Conference 2005 (PAC05
Dynamics of Social Balance on Networks
We study the evolution of social networks that contain both friendly and
unfriendly pairwise links between individual nodes. The network is endowed with
dynamics in which the sense of a link in an imbalanced triad--a triangular loop
with 1 or 3 unfriendly links--is reversed to make the triad balanced. With this
dynamics, an infinite network undergoes a dynamic phase transition from a
steady state to "paradise"--all links are friendly--as the propensity p for
friendly links in an update event passes through 1/2. A finite network always
falls into a socially-balanced absorbing state where no imbalanced triads
remain. If the additional constraint that the number of imbalanced triads in
the network does not increase in an update is imposed, then the network quickly
reaches a balanced final state.Comment: 10 pages, 7 figures, 2-column revtex4 forma
A Chandra observation of the long-duration X-ray transient KS 1731-260 in quiescence: too cold a neutron star?
After more than a decade of actively accreting at about a tenth of the
Eddington critical mass accretion rate, the neutron-star X-ray transient KS
1731-260 returned to quiescence in early 2001. We present a Chandra/ACIS-S
observation taken several months after this transition. We detected the source
at an unabsorbed flux of ~2 x 10^{-13} erg/cm^2/s (0.5-10 keV). For a distance
of 7 kpc, this results in a 0.5-10 keV luminosity of ~1 x 10^{33} erg/s and a
bolometric luminosity approximately twice that. This quiescent luminosity is
very similar to that of the other quiescent neutron star systems. However, if
this luminosity is due to the cooling of the neutron star, this low luminosity
may indicate that the source spends at least several hundreds of years in
quiescence in between outbursts for the neutron star to cool. If true, then it
might be the first such X-ray transient to be identified and a class of
hundreds of similar systems may be present in the Galaxy. Alternatively,
enhanced neutrino cooling could occur in the core of the neutron star which
would cool the star more rapidly. However, in that case the neutron star in KS
1731-260 would be more massive than those in the prototypical neutron star
transients (e.g., Aql X-1 or 4U 1608-52).Comment: Accepted for publicaton in ApJ letters, 13 September 200
Identification of the optical and quiescent counterparts to the bright X-ray transient in NGC 6440
After 3 years of quiescence, the globular cluster NGC 6440 exhibited a bright
transient X-ray source turning on in August 2001, as noted with the RXTE
All-Sky Monitor. We carried out a short target of opportunity observation with
the Chandra X-ray Observatory and are able to associate the transient with the
brightest of 24 X-ray sources detected during quiescence in July 2000 with
Chandra. Furthermore, we securely identify the optical counterpart and
determine that the 1998 X-ray outburst in NGC 6440 was from the same object.
This is the first time that an optical counterpart to a transient in a globular
cluster is securely identified. Since the transient is a type I X-ray burster,
it is established that the compact accretor is a neutron star. Thus, this
transient provides an ideal case to study the quiescent emission in the optical
and X-ray of a transiently accreting neutron star while knowing the distance
and reddening accurately. One model that fits the quiescent spectrum is an
absorbed power law plus neutron star hydrogen atmosphere model. We find an
intrinsic neutron star radius of 17_{-12}^{+31} km and an unabsorbed bolometric
luminosity for the neutron star atmosphere of (2.1+/-0.8)E33 erg/s which is
consistent with predictions for a cooling neutron star.Comment: Accepted for publication in ApJ Letter
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