2,158 research outputs found
Modeling of rolling element bearing mechanics
Roller element bearings provide the primary mechanical interface between rotating and nonrotating components in the high performance turbomachinery of the Space Shuttle Main Engine (SSME). Knowledge of bearing behavior under various loading and environmental conditions is essential to predicting and understanding the overall behavior of turbopumps, including rotordynamic stability, critical speeds and bearing life. The objective is to develop mathematical models and computer programs to describe the mechanical behavior of ball and cylinder roller bearings under the loading and environmental conditions encountered in the SSME and future high performance rocket engines. This includes characteristics such as nonlinear load/motion relationships, stiffness and damping, rolling element loads for life prediction, and roller and cage stability
The structure of the accretion disk in NGC 4258 derived from observations of its water vapor masers
A wealth of new information about the structure of the maser disk in NGC 4258
has been obtained from a series of 18 VLBA observations spanning three years,
as well as from 32 additional epochs of spectral monitoring data from 1994 to
the present, acquired with the VLA, Effelsberg, and GBT. The warp of the disk
has been defined precisely. The thickness of the maser disk has been measured
to be 12 microarcseconds (FWHM), which is slightly smaller than previously
quoted upper limits. Under the assumption that the masers trace the true
vertical distribution of material in the disk, from the condition of
hydrostatic equilibrium the sound speed is 1.5 km/s, corresponding to a thermal
temperature of 600K. The accelerations of the high velocity maser components
have been accurately measured for many features on both the blue and red side
of the spectrum. The azimuthal offsets of these masers from the midline (the
line through the disk in the plane of the sky) and derived projected offsets
from the midline based on the warp model correspond well with the measured
offsets. This result suggests that the masers are well described as discrete
clumps of masing gas, which accurately trace the Keplerian motion of the disk.
However, we have continued to search for evidence of apparent motions caused by
``phase effects.'' This work provides the foundation for refining the estimate
of the distance to NGC 4258 through measurements of feature acceleration and
proper motion. The refined estimate of this distance is expected to be
announced in the near future.Comment: 8 pages, 5 figures, to appear in proceedings of IAU Symposium 242
"Astrophysical Masers and their Environments", held in Alice Springs, March
200
A 43GHz VLBI mapping of SiO maser emission associated with Orion-KL IRC-2
A milliarcsecond resolution spot map of the SiO maser emission associated with IRC-2 in Orion-KL is presented. The two dominant groups of spectral features, near V(LRS) = -6 and 16 km/s, were observed in the 43 GHz, v = 1 to 0 transition of SiO, using a Mark III VLBI system. The 74 km baseline ran from Haystack Observatory in Westford, Massachusetts to Five College Radio Astronomy Observatory (FCRAO) in New Salem, Massachusetts. Five distinct maser features were observed: -8.5 to -6.5 km/s; -5 to -1.5 km/s; 12 to 13.5 km/s; 16.5 to 19 km/s; and 20 to 21 km/s (stellar velocity = 5 km/s). The relative positions were established, from an analysis of fringe phases, to an accuracy of about 5 milliarcseconds. All the features lay within an area of radius 0.08 arcseconds or 6x10(14) cm, at a distance of 500 pc. Previous interferometric studies were only able to measure the gross separation between the red and the blue shifted groups. Our measurement of the separation between these two gropus is consistent with those of the previous studies, indicating the persistence of these two centers of activity. The absolute positions of the masers with respect to IRC-2 are only known to an accuracy of about 1 arcsecond. It is assumed that IRC-2 is centered between the red shifted and the blue shifted maser features. The relative placement of these two groups of maser features agrees with observations of thermal emission from SO, which traces the outflow on a much larger scale. The SiO masers trace the neutral outflow from IRC-2 on the smallest scale yet observed
The Megamaser Cosmology Project: I. VLBI observations of UGC 3789
The Megamaser Cosmology Project (MCP) seeks to measure the Hubble Constant
(Ho) in order to improve the extragalactic distance scale and constrain the
nature of dark energy. We are searching for sources of water maser emission
from AGN with sub-pc accretion disks, as in NGC 4258, and following up these
discoveries with Very Long Baseline Interferometric (VLBI) imaging and spectral
monitoring. Here we present a VLBI map of the water masers toward UGC 3789, a
galaxy well into the Hubble Flow. We have observed masers moving at rotational
speeds up to 800 km/s at radii as small as 0.08 pc. Our map reveals masers in a
nearly edge-on disk in Keplerian rotation about a 10^7 Msun supermassive black
hole. When combined with centripetal accelerations, obtained by observing
spectral drifts of maser features (to be presented in Paper II), the UGC 3789
masers may provide an accurate determination of Ho, independent of luminosities
and metallicity and extinction corrections.Comment: 18 pages, 3 figures, 4 table
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
The optical counterpart of SAX J1808.4-3658, the transient bursting millisecond X-ray pulsar
A set of CCD images have been obtained during the decline of the X-ray
transient SAX J1808.4-3658 during April-June 1998. The optical counterpart has
been confirmed by several pieces of evidence. The optical flux shows a
modulation on several nights which is consistent with the established X-ray
binary orbit period of 2 hours. This optical variability is roughly in
antiphase with the weak X-ray modulation. The source mean magnitude of V=16.7
on April 18 declined rapidly after April 22. From May 2 onwards the magnitude
was more constant at around V=18.45 but by June 27 was below our sensitivity
limit. The optical decline precedes the rapid second phase of the X-ray
decrease by 3 +/- 1 days. The source has been identified on a 1974 UK Schmidt
plate at an estimated magnitude of ~20. The nature of the optical companion is
discussed.Comment: 5 pages, 3 figures; published in MNRAS, March 15th 199
Engineered Migration as a Coercive Instrument: The 1994 Cuban Balseros Crisis
This paper presents a case study of the August 1994 Cuban “balseros”—i.e. rafters—crisis, commonly known as Mariel II, during which over 35,000 Cubans fled the island and headed towards Florida. This paper argues that Castro launched the crisis in an attempt to manipulate the US’s fears of another Mariel boatlift, in order to compel a shift in United States (US) policy, both on immigration and on a wider variety of issues. As the end of the crisis brought with it a radical redefinition of US immigration policy toward Cuba, the paper further contends that from Castro’s perspective, this exercise in coercion proved a qualified success—his third such successful use of the Cuban people as an asymmetric political weapon against the US
Toward a New Distance to the Active Galaxy NGC 4258: II. Centripetal Accelerations and Investigation of Spiral Structure
We report measurements of centripetal accelerations of maser spectral
components of NGC 4258 for 51 epochs spanning 1994 to 2004. This is the second
paper of a series, in which the goal is determination of a new geometric maser
distance to NGC 4258 accurate to possibly ~3%. We measure accelerations using a
formal analysis method that involves simultaneous decomposition of maser
spectra for all epochs into multiple, Gaussian components. Components are
coupled between epochs by linear drifts (accelerations) from their centroid
velocities at a reference epoch. For high-velocity emission, accelerations lie
in the range -0.7 to +0.7 km/s/yr indicating an origin within 13 degrees of the
disk midline (the perpendicular to the line-of-sight to the black hole).
Comparison of high-velocity emission projected positions in VLBI images, with
those derived from acceleration data, provides evidence that masers trace real
gas dynamics. High-velocity emission accelerations do not support a model of
trailing shocks associated with spiral arms in the disk. However, we find
strengthened evidence for spatial periodicity in high-velocity emission, of
wavelength 0.75 mas. This supports suggestions of spiral structure due to
density waves in the nuclear accretion disk of an active galaxy. Accelerations
of low-velocity (systemic) emission lie in the range 7.7 to 8.9 km/s/yr,
consistent with emission originating from a concavity where the thin, warped
disk is tangent to the line-of-sight. A trend in accelerations of low-velocity
emission as a function of Doppler velocity may be associated with disk geometry
and orientation, or with the presence of spiral structure.Comment: Accepted to ApJ, 48 pages and 20 figure
Discovery of Candidate HO Disk Masers in AGN and Estimations of Centripetal Accelerations
Based on spectroscopic signatures, about one-third of known HO maser
sources in active galactic nuclei (AGN) are believed to arise in highly
inclined accretion disks around central engines. These "disk maser candidates"
are of interest primarily because angular structure and rotation curves can be
resolved with interferometers, enabling dynamical study. We identify five new
disk maser candidates in studies with the Green Bank Telescope, bringing the
total number published to 30. We discovered two (NGC1320, NGC17) in a survey of
40 inclined active galaxies (v_{sys}< 20000 kms^{-1}). The remaining three disk
maser candidates were identified in monitoring of known sources: NGC449,
NGC2979, NGC3735. We also confirm a previously marginal case in UGC4203. For
the disk maser candidates reported here, inferred rotation speeds are 130-500
kms^{-1}. Monitoring of three more rapidly rotating candidate disks (CG211,
NGC6264, VV340A) has enabled measurement of likely orbital centripetal
acceleration, and estimation of central masses (2-7x10^7 M_\odot) and mean disk
radii (0.2-0.4pc). Accelerations may ultimately permit estimation of distances
when combined with interferometer data. This is notable because the three AGN
are relatively distant (10000<v_{sys}<15000 kms^{-1}). As signposts of highly
inclined geometries at galactocentric radii of \sim0.1-1pc, disk masers also
provide robust orientation references that allow analysis of (mis)alignment
between AGN and surrounding galactic stellar disks, even without
interferometric mapping. We find no preference among published disk maser
candidates to lie in high-inclination galaxies, providing independent support
for conclusions that central engines and galactic plane orientations are not
correlated. (ABRIDGED)Comment: 7 figures, accepted for publication in ApJ, Dec. 10, 200
Towards Proper Motions in the Local Group
Key and still largely missing parameters for measuring the mass content and
distribution of the Local Group are the proper motion vectors of its member
galaxies. The problem when trying to derive the gravitational potential of the
Local Group is that usually only radial velocities are known, and hence
statistical approaches have to be used. The expected proper motions for
galaxies within the Local Group, ranging from 20 to 100 as/yr, are
detectable with VLBI using the phase-referencing technique. We present
phase-referencing observations of bright masers in IC~10 and M33 with respect
to background quasars. We observed the HO masers in IC10 three times over a
period of two months to check the accuracy of the relative positions. The
relative positions were obtained by modeling the interferometer phase data for
the maser sources referenced to the background quasars. The model allowed for a
relative position shift for the source and a single vertical atmospheric delay
error in the correlator model for each antenna. The rms of the relative
positions for the three observations is only 0.01 mas, which is approximately
the expected position error due to thermal noise. Also, we present a method to
measure the geometric distance to M33. This will allow re-calibration of the
extragalactic distance scale based on Cepheids. The method is to measure the
relative proper motions of two HO maser sources on opposite sides of M33.
The measured angular rotation rate, coupled with other measurements of the
inclination and rotation speed of the galaxy, yields a direct distance
measurement.Comment: 4 pages, Proceedings of the 6th European VLBI Network Symposium, Ros,
E., Porcas, R.W., Zensus, J.A. (eds.), MPIfR, Bonn, Germany (2002); Also
availabe http://www.mpifr-bonn.mpg.de/staff/abrunthaler/brunthal01.p
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