4,847 research outputs found
Vibration characteristics of 1/8-scale dynamic models of the space-shuttle solid-rocket boosters
Vibration tests and analyses of six 1/8 scale models of the space shuttle solid rocket boosters are reported. Natural vibration frequencies and mode shapes were obtained for these aluminum shell models having internal solid fuel configurations corresponding to launch, midburn (maximum dynamic pressure), and near endburn (burnout) flight conditions. Test results for longitudinal, torsional, bending, and shell vibration frequencies are compared with analytical predictions derived from thin shell theory and from finite element plate and beam theory. The lowest analytical longitudinal, torsional, bending, and shell vibration frequencies were within + or - 10 percent of experimental values. The effects of damping and asymmetric end skirts on natural vibration frequency were also considered. The analytical frequencies of an idealized full scale space shuttle solid rocket boosted structure are computed with and without internal pressure and are compared with the 1/8 scale model results
RXTE Studies of X-ray Spectral Variations with Accretion Rate in 4U 1915-05
We present the results of detailed spectral studies of the ultra-compact low
mass X-ray binary (LMXB) 4U 1915-05 carried out with the Rossi X-ray Timing
Explorer (RXTE) during 1996. 4U 1915-05 is an X-ray burster (XRB) known to
exhibit a ~199-day modulation in its 2--12 keV flux. Observations were
performed with the PCA and HEXTE instruments on RXTE at roughly one-month
intervals to sample this long-term period and study accretion rate-related
spectral changes. We obtain good fits with a model consisting of a blackbody
and an exponentially cut-off power law. The spectral parameters are strongly
correlated with both the broad-band (2--50 keV) luminosity and the position in
the color-color diagram, with the source moving from a low hard state to a high
soft state as the accretion rate increases. The blackbody component appears to
drive the spectral evolution. Our results are consistent with a geometry in
which the soft component arises from an optically thick boundary layer and the
hard component from an extended Comptonizing corona. Comparing our results with
those of a similar study of the brighter source 4U 1820-30 (Bloser et al.
2000), we find that the two ultra-compact LMXBs occupy similar spectral states
even though the transitions occur at very different total luminosities.Comment: 27 pages LaTeX, 8 figures, accepted to the Astrophysical Journa
X-ray sources and their optical counterparts in the globular cluster M 22
Using XMM-Newton EPIC imaging data, we have detected 50 low-luminosity X-ray
sources in the field of view of M 22, where 5 +/- 3 of these sources are likely
to be related to the cluster. Using differential optical photometry, we have
identified probable counterparts to those sources belonging to the cluster.
Using X-ray spectroscopic and timing studies, supported by the optical colours,
we propose that the most central X-ray sources in the cluster are cataclysmic
variables, millisecond pulsars, active binaries and a blue straggler. We also
identify a cluster of galaxies behind this globular cluster.Comment: 11 pages, 7 figures, accepted for publication in A&
Optical variability of the accretion disk around the intermediate mass black hole ESO 243-49 HLX-1 during the 2012 outburst
We present dedicated quasi-simultaneous X-ray (Swift) and optical (Very Large
Telescope (VLT), V- and R-band) observations of the intermediate mass black
hole candidate ESO 243-49 HLX-1 before and during the 2012 outburst. We show
that the V-band magnitudes vary with time, thus proving that a portion of the
observed emission originates in the accretion disk. Using the first quiescent
optical observations of HLX-1, we show that the stellar population surrounding
HLX-1 is fainter than V~25.1 and R~24.2. We show that the optical emission may
increase before the X-ray emission consistent with the scenario proposed by
Lasota et al. (2011) in which the regular outbursts could be related to the
passage at periastron of a star circling the intermediate mass black hole in an
eccentric orbit, which triggers mass transfer into a quasi-permanent accretion
disk around the black hole. Further, if there is indeed a delay in the X-ray
emission we estimate the mass-transfer delivery radius to be ~1e11 cm.Comment: 9 pages, 2 figures, accepted for publication in ApJ
ASCA Observations of GX 354-0 and KS 1731-260
We report on ASCA observations of the low mass X-ray binaries GX 354-0 and KS
1731-260. The spectrum of GX 354-0 is best described as a power-law or a
Comptonized spectrum with tau ~ 5 and kT ~ 8 keV and a residual at ~6.5 keV.
The residual may be a disk reflection or a Compton broadened Gaussian line from
the hot inner ADAF-like coronal region. The absorption column density to the
source is 2.9e22 cm^-2. No soft thermal component was detected. The spectrum
from KS 1731-260 is softer and it is best fit with a two component model with a
column density of 1.1e22 cm^-2. The likely interpretation is emission from a
Comptonizing cloud with an optical depth tau>12 and either a neutron star or a
disk blackbody emission. We discuss the likely location of the Comptonizing
cloud for both sources within the context of several proposed emission models.Comment: Accepted for publication in the Astrophysical Journa
Constraining the equation of state of supra-nuclear dense matter from XMM-Newton observations of neutron stars in globular clusters
We report on the detailed modelling of the X-ray spectra of three likely
neutron stars. The neutron stars, observed with XMM-Newton are found in three
quiescent X-ray binaries in the globular clusters: omega Cen, M 13 and NGC
2808. Whether they are accreting at very low rates or radiating energy from an
accretion heated core, their X-ray spectra are expected to be those of a
hydrogen atmosphere. We use and compare publicly available hydrogen atmosphere
models, with constant and varying surface gravities to constrain the masses and
radii of the neutron stars. Thanks to the high XMM-Newton throughput, and the
accurate distances available for these clusters, using the latest science
analysis software release and calibration of the XMM-Newton EPIC cameras, we
derive the most stringent constraints on the masses and radii of the neutron
stars obtained to date from these systems. A comparison of the models indicate
that previously used hydrogen atmosphere models (assuming constant surface
gravity) tend to underestimate the mass and overestimate the radius of neutron
stars. Our data constrain the allowed equations of state to those which concern
normal nucleonic matter and one possible strange quark matter model, thus
constraining radii to be from 8 km and masses up to 2.4 M.Comment: 10 pages, 8 figures, accepted to be published in The Astrophysical
Journa
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