11,487 research outputs found
Improved Si:As BIBIB (Back-Illuminated Blocked-Impurity-Band) hybrid arrays
Results of a program to increase the short wavelength (less than 10 microns) detective quantum efficiency, eta/beta, of Si:As Impurity Band Conduction arrays are presented. The arrays are epitaxially grown Back-Illuminated Blocked (BIB) Impurity-Band (BIBIB) 10x50 detectors bonded to switched-FET multiplexers. It is shown that the 4.7 microns detective quantum efficiency increases proportionately with the thickness of the infrared active layer. A BIB array with a thick active layer, designed for low dark current, exhibits eta/beta = 7 to 9 percent at 4.7 microns for applied bias voltages between 3 and 5 V. The product of quantum efficiency and photoelectric gain, etaG, increases from 0.3 to 2.5 as the voltage increases from 3 to 5 V. Over this voltage range, the dark current increases from 8 to 120 e(-)s(-1) at a device temperature of 4.2 K and is under 70 e(-)s(-1) for all voltages at 2 K. Because of device gain, the effective dark current (equivalent photon rate) is less than 3 e(-)s(-1) under all operating conditions. The effective read noise (equivalent photon noise) is found to be less than 12 electrons under all operating conditions and for integration times between 0.05 and 100 seconds
Construction of coarse-grained order-parameters in non-equilibrium systems
We develop a renormalization group (RG) procedure that includes important
system-specific features. The key ingredient is to systematize the coarse
graining procedure that generates the RG flow. The coarse graining technology
comes from control and operator theoretic model reduction. The resulting
"generalized" RG is a consistent generalization of the Wilsonian RG. We derive
the form of the projection operator from the dynamics of a nonlinear wave
equation and renormalize the distribution of initial conditions. The
probability density of the initial conditions is chosen to be the Boltzmann
weight for a standard -theory. In our calculation, we find that in
contrast to conventional implementations of the RG, na\"ive power counting
breaks down. The RG-equations that we derive are different from those derived
from the conventional RG.Comment: 13 pages, 0 figure
Associated Absorption Lines in the Radio-Loud Quasar 3C 351: Far-Ultraviolet Echelle Spectroscopy from the Hubble Space Telescope
As one of the most luminous radio-loud quasars showing intrinsic ultraviolet
(UV) and X-ray absorption, 3C 351 provides a laboratory for studying the
kinematics and physical conditions of such ionized absorbers. We present an
analysis of the intrinsic absorption lines in the high-resolution ( 7
km/s) far-UV spectrum which was obtained from observations with the Space
Telescope Imaging Spectrograph (STIS) on board the Hubble Space Telescope
(HST). The spectrum spans wavelengths from 1150 \AA to 1710 \AA, and shows
strong emission lines from O VI and Ly. Associated absorption lines are
present on the blue wings of the high-ionization emission doublets O VI
1032,1038 and N V 1238,1242, as well as the
Lyman lines through Ly. These intrinsic absorption features are
resolved into several distinct kinematic components, covering rest-frame
velocities from -40 to -2800 km/s, with respect to the systemic redshift of
. For the majority of these absorption line regions, strong
evidence of partial covering of both the background continuum source and the
BELR is found, which supports the intrinsic absorption origin and rules out the
possibility that the absorption arises in some associated cluster of galaxies.
The relationship between the far-UV absorbers and X-ray `warm' absorbers are
studied with the assistance of photoionization models. Most of the UV
associated absorption components have low values of the ionization parameter
and total hydrogen column densities, which is inconsistent with previous claims
that the UV and X-ray absorption arises in the same material. Analysis of these
components supports a picture with a wide range of ionization parameters,
temperatures, and column densities in AGN outflows.Comment: 27 pages with 5 figures, accepted by Ap
Intrinsic Absorption Lines in Seyfert 1 Galaxies. I. Ultraviolet Spectra from the Hubble Space Telescope
We present a study of the intrinsic absorption lines in the ultraviolet
spectra of Seyfert 1 galaxies. We find that the fraction of Seyfert 1 galaxies
that show absorption associated with their active nuclei is more than one-half
(10/17), which is much higher than previous estimates (3 - 10%) . There is a
one-to-one correspondence between Seyferts that show intrinsic UV absorption
and X-ray ``warm absorbers''. The intrinsic UV absorption is generally
characterized by high ionization: C IV and N V are seen in all 10 Seyferts with
detected absorption (in addition to Ly-alpha), whereas Si IV is present in only
four of these Seyferts, and Mg II absorption is only detected in NGC 4151. The
absorption lines are blueshifted (or in a few cases at rest) with respect to
the narrow emission lines, indicating that the absorbing gas is undergoing net
radial outflow. At high resolution, the absorption often splits into distinct
kinematic components that show a wide range in widths (20 - 400 km/s FWHM),
indicating macroscopic motions (e.g., radial velocity subcomponents or
turbulence) within a component. The strong absorption components have cores
that are much deeper than the continuum flux levels, indicating that the
regions responsible for these components lie completely outside of the broad
emission-line regions. The covering factor of the absorbing gas in the line of
sight, relative to the total underlying emission, is C > 0.86, on average. The
global covering factor, which is the fraction of emission intercepted by the
absorber averaged over all lines of sight, is C > 0.5.Comment: 56 pages, Latex, includes 4 figures (encapsulated postscript), Fig. 1
has 2 parts and Fig. 2 has 3 parts, to appear in the Astrophysical Journa
Unresolved H-Alpha Enhancements at High Galactic Latitude in the WHAM Sky Survey Maps
We have identified 85 regions of enhanced H-Alpha emission at |b| > 10
degrees subtending approximately 1 degree or less on the Wisconsin H-Alpha
Mapper (WHAM) sky survey. These high latitude ``WHAM point sources'' have
H-Alpha fluxes of 10^{-11} to 10^{-9} erg cm^-2 s^-1, radial velocities within
about 70 km/s of the LSR, and line widths that range from less than 20 km/s to
about 80 km/s (FWHM). Twenty nine of these enhancements are not identified with
either cataloged nebulae or hot stars and appear to have kinematic properties
that differ from those observed for planetary nebulae. Another 14 enhancements
are near hot evolved low mass stars that had no previously reported detections
of associated nebulosity. The remainder of the enhancements are cataloged
planetary nebulae and small, high latitude H II regions surrounding massive O
and early B stars.Comment: 22 pages, 8 figures, 2 tables, to appear in Feb. 2005 A
First e-VLBI observations of GRS 1915+105
We present results from the first successful open call e-VLBI science run,
observing the X-ray binary GRS 1915+105. e-VLBI science allows the rapid
production of VLBI radio maps, within hours of an observation rather than
weeks, facilitating a decision for follow-up observations. A total of 6
telescopes observing at 5 GHz across the European VLBI Network (EVN) were
correlated in real time at the Joint Institute for VLBI in Europe (JIVE).
Constant data rates of 128 Mbps were transferred from each telescope, giving 4
TB of raw sampled data over the 12 hours of the whole experiment. Throughout
this, GRS 1915+105 was observed for a total of 5.5 hours, producing 2.8 GB of
visibilities of correlated data. A weak flare occurred during our observations,
and we detected a slightly resolved component of 2.7 x 1.2 milliarcsecond with
a position angle of 140 (+/-2) degrees. The peak brightness was 10.2 mJy per
beam, with a total integrated radio flux of 11.1 mJy.Comment: Accepted for publication in MNRAS 4 pages, 3 figure
Skyrmion Physics Beyond the Lowest Landau Level Approximation
The effects of Landau level mixing and finite thickness of the
two-dimensional electron gas on the relative stability of skyrmion and single
spin-flip excitations at Landau level filling factor have been
investigated. Landau level mixing is studied by fixed-phase diffusion Monte
Carlo and finite thickness is included by modifying the effective Coulomb
interaction. Both Landau level mixing and finite thickness lower skyrmion
excitation energies and favor skyrmions with fewer spin flips. However, the two
effects do not work `coherently'. When finite thickness is included the effect
of Landau level mixing is strongly suppressed.Comment: 4 pages, 4 figure
Chandra Observations of G11.2-0.3: Implications for Pulsar Ages
We present Chandra X-ray Observatory imaging observations of the young
Galactic supernova remnant G11.2-0.3. The image shows that the previously known
young 65-ms X-ray pulsar is at position (J2000) RA 18h 11m 29.22s, DEC -19o 25'
27.''6, with 1 sigma error radius 0.''6. This is within 8'' of the geometric
center of the shell. This provides strong confirming evidence that the system
is younger, by a factor of ~12, than the characteristic age of the pulsar. The
age discrepancy suggests that pulsar characteristic ages can be poor age
estimators for young pulsars. Assuming conventional spin down with constant
magnetic field and braking index, the most likely explanation for the age
discrepancy in G11.2-0.3 is that the pulsar was born with a spin period of ~62
ms. The Chandra image also reveals, for the first time, the morphology of the
pulsar wind nebula. The elongated hard-X-ray structure can be interpreted as
either a jet or a Crab-like torus seen edge on. This adds to the growing list
of highly aspherical pulsar wind nebulae and argues that such structures are
common around young pulsars.Comment: 16 pages, 3 figures, Accepted for publication in ApJ. For a full
resolution version of Fig 1, see
http://www.physics.mcgill.ca/~vkaspi/G11.2-0.3/f1.ep
A simple model of radiative emission in M87
We present a simple physical model of the central source emission in the M87
galaxy. It is well known that the observed X-ray luminosity from this galactic
nucleus is much lower than the predicted one, if a standard radiative
efficiency is assumed. Up to now the main model invoked to explain such a
luminosity is the ADAF (Advection-Dominated-Accretion-Flow) model. Our approach
supposes only a simple axis-symmetric adiabatic accretion with a low angular
momentum together with the bremsstrahlung emission process in the accreting
gas. With no other special hypothesis on the dynamics of the system, this model
agrees well enough with the luminosity value measured by Chandra.Comment: 11 pages, 6 figures, accepted for publication in The Astrophysical
Journa
Bias-Dependent Generation and Quenching of Defects in Pentacene
We describe a defect in pentacene single crystals that is created by bias
stress and persists at room temperature for an hour in the dark but only
seconds with 420nm illumination. The defect gives rise to a hole trap at Ev +
0.38eV and causes metastable transport effects at room temperature. Creation
and decay rates of the hole trap have a 0.67eV activation energy with a small
(108 s-1) prefactor, suggesting that atomic motion plays a key role in the
generation and quenching process.Comment: 10 pages, 3 figure
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