7,026 research outputs found
Overcoming the boundary layer turbulence at Dome C: ground-layer adaptive optics versus tower
The unique atmospheric conditions present at sites such as Dome C on the Antarctic plateau are very favorable for high spatial resolution astronomy. At Dome C, the majority of the optical turbulence is confined to a 30 to 40 m thick stable boundary layer that results from the strong temperature inversion created by the heat exchange between the air and the ice-covered ground. To fully realize the potential of the exceptionally calm free atmosphere, this boundary layer must be overcome. In this article we compare the performance of two methods proposed to beat the boundary layer: mounting a telescope on a tower that physically puts it above the turbulent layer, and installing a telescope at ground level with a ground-layer adaptive optics system. A case is also made to combine these two methods to further improve the image quality
Pulse Profiles, Accretion Column Dips and a Flare in GX 1+4 During a Faint State
The Rossi X-ray Timing Explorer (RXTE) spacecraft observed the X-ray pulsar
GX 1+4 for a period of 34 hours on July 19/20 1996. The source faded from an
intensity of ~20 mCrab to a minimum of <~0.7 mCrab and then partially recovered
towards the end of the observation. This extended minimum lasted ~40,000
seconds. Phase folded light curves at a barycentric rotation period of
124.36568 +/- 0.00020 seconds show that near the center of the extended minimum
the source stopped pulsing in the traditional sense but retained a weak dip
feature at the rotation period. Away from the extended minimum the dips are
progressively narrower at higher energies and may be interpreted as
obscurations or eclipses of the hot spot by the accretion column. The pulse
profile changed from leading-edge bright before the extended minimum to
trailing-edge bright after it. Data from the Burst and Transient Source
Experiment (BATSE) show that a torque reversal occurred <10 days after our
observation. Our data indicate that the observed rotation departs from a
constant period with a Pdot/P value of ~-1.5% per year at a 4.5 sigma
significance. We infer that we may have serendipitously obtained data, with
high sensitivity and temporal resolution about the time of an accretion disk
spin reversal. We also observed a rapid flare which had some precursor
activity, close to the center of the extended minimum.Comment: 19 pages, 6 figures, accepted for publication in Astrophysical
Journal (tentatively scheduled for vol. 529 #1, 20 Jan 2000
Bistability in a simple fluid network due to viscosity contrast
We study the existence of multiple equilibrium states in a simple fluid
network using Newtonian fluids and laminar flow. We demonstrate theoretically
the presence of hysteresis and bistability, and we confirm these predictions in
an experiment using two miscible fluids of different viscosity--sucrose
solution and water. Possible applications include bloodflow, microfluidics, and
other network flows governed by similar principles
Double layer in ionic liquids: Overscreening vs. crowding
We develop a simple Landau-Ginzburg-type continuum theory of solvent-free
ionic liquids and use it to predict the structure of the electrical double
layer. The model captures overscreening from short-range correlations, dominant
at small voltages, and steric constraints of finite ion sizes, which prevail at
large voltages. Increasing the voltage gradually suppresses overscreening in
favor of the crowding of counterions in a condensed inner layer near the
electrode. The predicted ion profiles and capacitance-voltage relations are
consistent with recent computer simulations and experiments on room-temperature
ionic liquids, using a correlation length of order the ion size.Comment: 4 pages + supplementary informatio
The UV Properties of the Narrow Line Quasar I Zwicky 1
I Zw 1 is the prototype narrow line quasar. We report here the results of our
study of the UV emission of I Zw 1 using a high S/N (50-120) spectrum obtained
with the HST FOS. The following main new results are obtained: 1. The Mg II and
Al III doublets are partially/fully resolved. The measured doublet ratios
verify theoretical predictions that the lines are thermalized in the BLR. 2. A
weak associated UV absorption system is detected in N~V, and possibly also in C
IV and Lya, suggesting an outflow with a velocity of 1870 km/s and velocity
dispersion <300 km/s. 3. Lines from ions of increasing ionization level show
increasing excess blue wing flux, and an increasing line peak velocity shift,
reaching a maximum blueshift of about 2000 km/s for He II 1640. This may
indicate an out-flowing component in the BLR, where the ionization level
increases with velocity, and which is visible only in the approaching
direction. The highest velocity part of this outflow may produce the associated
UV absorption system. 4. The small C III] 1909 EW, and the small C III]
1909/Lya and C III] 1909/Si III] 1892 flux ratios indicate a typical BLR
density of 10^11, i.e. about an order of magnitude larger than implied by C
III] 1909 in most quasars. A BLR component of a higher density is implied by
the EW and doublet ratio of the Al III 1857 doublet. 5. Prominent Fe II UV 191
emission is seen, together with weaker line emission at 1294 and 1871 A. These
three features have been proposed as evidence for significant Lya pumping of
the 8-10 eV levels of Fe II. 6. Significant Fe III emission is present. The Fe
III UV 34 and UV 48 multiplets are clearly resolved, and Fe III UV 1, UV 47, UV
50, and UV 68 may also be present. (Shortened version)Comment: 28 pages, 1 table and 7 figures included. Uses aas2pp4.sty. Scheduled
for the Astrophysical Journal November 10, 1997 issue, Vol. 48
Prevention of stroke in patients with chronic coronary syndromes or peripheral arterial disease
Stroke is a common and devastating condition caused by atherothrombosis, thromboembolism, or haemorrhage. Patients with chronic coronary syndromes (CCS) or peripheral artery disease (PAD) are at increased risk of stroke because of shared pathophysiological mechanisms and risk-factor profiles. A range of pharmacological and non-pharmacological strategies can help to reduce stroke risk in these groups. Antithrombotic therapy reduces the risk of major adverse cardiovascular events, including ischaemic stroke, but increases the incidence of haemorrhagic stroke. Nevertheless, the net clinical benefits mean antithrombotic therapy is recommended in those with CCS or symptomatic PAD. Whilst single antiplatelet therapy is recommended as chronic treatment, dual antiplatelet therapy should be considered for those with CCS with prior myocardial infarction at high ischaemic but low bleeding risk. Similarly, dual antithrombotic therapy with aspirin and very-low-dose rivaroxaban is an alternative in CCS, as well as in symptomatic PAD. Full-dose anticoagulation should always be considered in those with CCS/PAD and atrial fibrillation. Unless ischaemic risk is particularly high, antiplatelet therapy should not generally be added to full-dose anticoagulation. Optimization of blood pressure, low-density lipoprotein levels, glycaemic control, and lifestyle characteristics may also reduce stroke risk. Overall, a multifaceted approach is essential to best prevent stroke in patients with CCS/PAD
The Gattini cameras for optical sky brightness measurements at Dome C, Antarctica
The Gattini cameras are two site testing instruments for
the measurement of optical sky brightness, large area cloud cover and auroral detection of the night sky above the high altitude Dome C site in Antarctica. The cameras have been operating since installation in January 2006 and are currently at the end of the first Antarctic winter season. The cameras are transit in nature and are virtually identical both adopting Apogee Alta CCD detectors. By taking frequent images of the night sky we obtain long term cloud cover statistics, measure the sky background intensity as a function of solar and lunar altitude and
phase and directly measure the spatial extent of bright aurora if present and when they occur. The full data set will return in December 2006 however a limited amount of data has been transferred via the Iridium network enabling preliminary data reduction and system evaluation.
An update of the project is presented together with preliminary results from data taken since commencement of the winter season
A Systematic Analysis of Fe II Emission in Quasars: Evidence for Inflow to the Central Black Hole
Broad Fe II emission is a prominent feature of the optical and ultraviolet
spectra of quasars. We report on a systematical investigation of optical Fe II
emission in a large sample of 4037 z < 0.8 quasars selected from the Sloan
Digital Sky Survey. We have developed and tested a detailed line-fitting
technique, taking into account the complex continuum and narrow and broad
emission-line spectrum. Our primary goal is to quantify the velocity broadening
and velocity shift of the Fe II spectrum in order to constrain the location of
the Fe II-emitting region and its relation to the broad-line region. We find
that the majority of quasars show Fe II emission that is redshifted, typically
by ~ 400 km/s but up to 2000 km/s, with respect to the systemic velocity of the
narrow-line region or of the conventional broad-line region as traced by the
Hbeta line. Moreover, the line width of Fe II is significantly narrower than
that of the broad component of Hbeta. We show that the magnitude of the Fe II
redshift correlates inversely with the Eddington ratio, and that there is a
tendency for sources with redshifted Fe II emission to show red asymmetry in
the Hbeta line. These characteristics strongly suggest that Fe II originates
from a location different from, and most likely exterior to, the region that
produces most of Hbeta. The Fe II-emitting zone traces a portion of the
broad-line region of intermediate velocities whose dynamics may be dominated by
infall.Comment: 20 pages, 14 figures, accepted for publication in Ap
- âŠ