198 research outputs found
High Precision Astrometric Millimeter VLBI Using a New Method for Atmospheric Calibration
We describe a new method which achieves high precision Very Long Baseline
Interferometry (VLBI) astrometry in observations at millimeter wavelengths. It
combines fast frequency-switching observations, to correct for the dominant
non-dispersive tropospheric fluctuations, with slow source-switching
observations, for the remaining ionospheric dispersive terms. We call this
method Source-Frequency Phase Referencing. Provided that the switching cycles
match the properties of the propagation media, one can recover the source
astrometry. We present an analytic description of the two-step calibration
strategy, along with an error analysis to characterize its performance. Also,
we provide observational demonstrations of a successful application with
observations using the Very Long Baseline Array at 86 GHz of the pairs of
sources 3C274 & 3C273 and 1308+326 & 1308+328, under various conditions. We
conclude that this method is widely applicable to millimeter VLBI observations
of many target sources, and unique in providing bona-fide astrometrically
registered images and high precision relative astrometric measurements in
mm-VLBI using existing and newly built instruments.Comment: Astronomical Journal, accepted for publicatio
Tropospheric Phase Calibration in Millimeter Interferometry
We review millimeter interferometric phase variations caused by variations in
the precipitable water vapor content of the troposphere, and we discuss
techniques proposed to correct for these variations. We present observations
with the Very Large Array at 22 GHz and 43 GHz designed to test these
techniques. We find that both the Fast Switching and Paired Array calibration
techniques are effective at reducing tropospheric phase noise for radio
interferometers. In both cases, the residual rms phase fluctuations after
correction are independent of baseline length for b > b_{eff}. These techniques
allow for diffraction limited imaging of faint sources on arbitrarily long
baselines at mm wavelengths. We consider the technique of tropospheric phase
correction using a measurement of the precipitable water vapor content of the
troposphere via a radiometric measurement of the brightness temperature of the
atmosphere. Required sensitivities range from 20 mK at 90 GHz to 1 K at 185 GHz
for the MMA, and 120 mK for the VLA at 22 GHz. The minimum gain stability
requirement is 200 at 185 GHz at the MMA assuming that the astronomical
receivers are used for radiometry. This increases to 2000 for an uncooled
system. The stability requirement is 450 for the cooled system at the VLA at 22
GHz. To perform absolute radiometric phase corrections also requires knowledge
of the tropospheric parameters and models to an accuracy of a few percent. It
may be possible to perform an `empirically calibrated' radiometric phase
correction, in which the relationship between fluctuations in brightness
temperature differences with fluctuations in interferometric phases is
calibrated by observing a celestial calibrator at regular intervals.Comment: AAS LATEX preprint format. to appear in Radio Science 199
Astrometry of OH/IR stars using 1612 MHz hydroxyl masers. I. Annual parallaxes of WX Psc and OH138.0+7.2
We report on the measurement of the trigonometric parallaxes of 1612 MHz
hydroxyl masers around two asymptotic giant branch stars, WX Psc and
OH138.0+7.2, using the NRAO Very Long Baseline Array with in-beam phase
referencing calibration. We obtained a 3-sigma upper limit of <=5.3 mas on the
parallax of WX Psc, corresponding to a lower limit distance estimate of >~190
pc. The obtained parallax of OH138.0+7.2 is 0.52+/-0.09 mas (+/-18%),
corresponding to a distance of 1.9(+0.4,-0.3) kpc, making this the first
hydroxyl maser parallax below one milliarcsecond. We also introduce a new
method of error analysis for detecting systematic errors in the astrometry.
Finally, we compare our trigonometric distances to published phase-lag
distances toward these stars and find a good agreement between the two methods.Comment: Preprint, accepted for publication in The Astronomical Journal
(January 17, 2017
Ionization dynamics in intense pulsed laser radiation. Effects of frequency chirping
Via a non-perturbative method we study the population dynamics and
photoelectron spectra of Cs atoms subject to intense chirped laser pulses, with
gaussian beams. We include above threshold ionization spectral peaks. The
frequency of the laser is near resonance with the 6s-7p transition. Dominant
couplings are included exactly, weaker ones accounted for perturbatively. We
calculate the relevant transition matrix elements, including spin-orbit
coupling. The pulse is taken to be a hyperbolic secant in time and the chirping
a hyperbolic tangent. This choice allows the equations of motions for the
probability amplitudes to be solved analytically as a series expansion in the
variable u=(tanh(pi t/tau)+1)/2, where tau is a measure of the pulse length. We
find that the chirping changes the ionization dynamics and the photoelectron
spectra noticeably, especially for longer pulses of the order of 10^4 a.u. The
peaks shift and change in height, and interference effects between the 7p
levels are enhanced or diminished according to the amount of chirping and its
sign. The integrated ionization probability is not strongly affected.Comment: Accepted by J. Phys. B; 18 pages, 17 figures. Latex, uses
ioplppt.sty, iopl10.sty and psfig.st
Functional Analysis of the Chemokine Receptor CCR3 on Airway Epithelial Cells
The function of chemokine receptors on structural cells is only partially known. We previously reported the expression of a functional CCR3 receptor on airway epithelial cells (EC). We speculated that CCR3 might drive wound repair and expression of inflammatory genes in epithelium. The human airway EC lines BEAS-2B, 16-HBE, and primary bronchial EC were used to test the effect of in vitro challenge with the CCR3 ligands CCL11/eotaxin, CCL24/eotaxin-2, or CCL26/eotaxin-3 on 1) wound repair, using an established wound model; 2) cell proliferation and chemotaxis, using specific fluorometric assays; and 3) gene expression, using pathway-specific arrays for inflammatory and profibrotic cytokines, chemokines, and chemokine receptor genes. Agonist specificity was tested by cell pretreatment with an AstraZeneca CCR3 antagonist (10(-8) - 10(-6) M). CCL24 challenge significantly accelerated epithelial wound closure, with similar effects exerted by CCL11 and CCL26. This effect was time dependent, submaximal at 1 nM, and comparable in potency to epidermal growth factor. CCL24 induced a concentration-dependent increase in EC proliferation and chemotaxis, with significant effects observed at 10 nM. The AstraZeneca compound selectively inhibited these CCL24-mediated responses. CCL11 induced the up-regulation of several profibrogenic molecules such as fibroblast growth factor 1 and 5 and of several CC and CXC chemokines. Epithelial immunostaining for CCR3 was stronger in bronchial biopsies of asthmatics displaying marked inflammatory changes than in nondiseased samples. Epithelial CCR3 participates in key functions for wound repair, amplifies the expression of profibrogenic and chemokine transcripts, and appears up-regulated in inflamed asthmatic airways
The Distance and Size of the Red Hypergiant NML Cyg from VLBA and VLA Astrometry
We have measured the annual parallax and proper motion of NML Cyg from
multiple epoch VLBA observations of the circulstellar H2O and SiO masers. The
measured parallax of NML Cyg is 0.620+/-0.047 mas, corresponding to a distance
of 1.61+/-0.12 kpc. The measured proper motion of NML Cyg is mu_x =
-1.55+/-0.42 mas/yr eastward and mu_y= -4.59+/-0.41 mas/yr northward. Both Both
the distance and proper motion are consistent with that of Cyg OB2, within
their joint uncertainty, confirming their association. Taking into
consideration molecular absorption signatures seen toward NML Cyg, we suggest
that NML Cyg lies on the far side of the Cyg OB2 association. The stellar
luminosity revised with our distance brings NML Cyg significantly below the
empirical luminosity limit for a red supergiant. Using the VLA observation the
radio photosphere and the SiO maser as a phase reference, we partially resolve
the radio photosphere of NML Cyg at 43 GHz and find its diameter is about 44
mas, suggesting an optical stellar diameter of 22 mas, if the size of radio
photosphere is 2 times the optical photosphere. Based on the position of
circumstellar SiO masers relative to the radio photosphere, we estimate the
absolute position of NML Cyg at epoch 2008.868 to be R.A. = 20h46m25.5382s +/-
0.0010s, Decl. = 40d06'59.379" +/- 0.015". The peculiar motions of NML Cyg, the
average of stars in Cyg OB2, and four other star-forming regions rules out that
an expanding "Stroemgren sphere" centered on Cyg OB2 is responsible for the
kinematics of the Cygnus X region.Comment: 15 pages, 11 figures, accepted by A&
ALMA High-frequency Long Baseline Campaign in 2017:Band-to-band Phase Referencing in Submillimeter Waves
In 2017, an Atacama Large Millimeter/submillimeter Array (ALMA)
high-frequency long baseline campaign was organized to test image capabilities
with baselines up to 16 km at submillimeter (submm) wavelengths. We
investigated image qualities using ALMA receiver Bands 7, 8, 9, and 10 (285-875
GHz) by adopting band-to-band (B2B) phase referencing in which a phase
calibrator is tracked at a lower frequency. For B2B phase referencing, it is
expected that a closer phase calibrator to a target can be used, comparing to
standard in-band phase referencing. In the first step, it is ensured that an
instrumental phase offset difference between low- and high-frequency Bands can
be removed using a differential gain calibration in which a phase calibrator is
certainly detected while frequency switching. In the next step, comparative
experiments are arranged to investigate the image quality between B2B and
in-band phase referencing with phase calibrators at various separation angles.
In the final step, we conducted long baseline imaging tests for a quasar at 289
GHz in Band 7 and 405 GHz in Band 8 and complex structure sources of HL Tau and
VY CMa at ~670 GHz in Band 9. The B2B phase referencing was successfully
applied, allowing us to achieve an angular resolution of 14x11 and 10x8 mas for
HL Tau and VY CMa, respectively. There is a high probability of finding a
low-frequency calibrator within 5.4 deg in B2B phase referencing, bright enough
to use an 8 s scan length combined with a 7.5 GHz bandwidth.Comment: 61 pages, 17 figures, 8 table
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