1,757 research outputs found
Mechanical Fasteners for Advanced Composite Materials
Advanced composite materials, which are increasingly being used to build aircraft, have different properties than the metals they replace. Fasteners intended for composite-material joints must be designed and selected to allow for these differences. For example, blind fasteners (one-sided access) used to assemble composite-to-composite joints have been redesigned to expand to larger diameters to resist pull-through and cocking failures. The fastener designs needed for composite materials are reviewed. Topics discussed are: galvanic corrosion, pull-through resistance, fastener rotation, installation damage, fastener galling and conductivity. A blind fastener recently developed by SPS Technologies is described to show how these requirements are incorporated
Radio detection of solar wind discontinuities
Geomagnetic field sudden increases or storm sudden commencements are compared with measurements of electron content of the solar wind. The latter data are obtained by a radio propagation experiment, which measures the electron content along a radio path between transmitters on the ground and several spacecraft in solar orbit. Measurements were examined during time periods that included 40 of the geomagnetic disturbances (gmd) reported between January 1, 1966, and June 30, 1969. These studies indicate that some widely reported solar wind discontinuities have been detected by the radio propagation experiment. Eleven of the 40 gmd were classified as storm sudden commencements (ssc), which usually result when a shock in the solar wind strikes the magnetosphere. The relative timings of these 11 events are consistent with conclusions drawn from comparisons of experiment geometry to prevailing shock models. Compared to the nature of these 11 events, the characteristics of the solar-wind disturbances corresponding to the remaining 29 gmd were generally found to have been less favorable for detection by the radio propagation experiment, but sharp changes in the content were clearly evident at the time of several minor gmd
The Sedentary Survey of Extreme High Energy Peaked BL Lacs III. Results from Optical Spectroscopy
The multi-frequency Sedentary Survey is a flux limited, statistically
well-defined sample of highly X-ray dominated BL Lacertae objects (HBLs) which
includes 150 sources. In this paper, the third of the series, we report the
results of a dedicated optical spectroscopy campaign that, together with
results from other independent optical follow up programs, led to the
spectroscopic identification of all sources in the sample. We carried out a
systematic spectroscopic campaign for the observation of all unidentified
objects of the sample using the ESO 3.6m, the KPNO 4m, and the TNG optical
telescopes. We present new identifications and optical spectra for 76 sources,
50 of which are new BL Lac objects, 18 are sources previously referred as BL
Lacs but for which no redshift information was available, and 8 are broad
emission lines AGNs. We find that the multi-frequency selection technique used
to build the survey is highly efficient (about 90%) in selecting BL Lacs
objects. We present positional and spectroscopic information for all confirmed
BL Lac objects. Our data allowed us to determined 36 redshifts out of the 50
new BL Lacs and 5 new redshifts for the previously known objects. The redshift
distribution of the complete sample is presented and compared with that of
other BL Lacs samples. For 26 sources without recognizable absorption features,
we calculated lower limits to the redshift using a method based on simulated
optical spectra with different ratios between jet and galaxy emission. For a
subsample of 38 object with high-quality spectra, we find a correlation between
the optical spectral slope, the 1.4 GHz radio luminosity, and the Ca H&K break
value, indicating that for powerful/beamed sources the optical light is
dominated by the non-thermal emission from the jet.Comment: 23 pages, accepted by A&
On the relationship between BL Lacertae objects and radio galaxies
We present deep radio images at 1.4 GHz of a large and complete sample of BL
Lacertae objects (BL Lacs) selected from the Deep X-ray Radio Blazar Survey
(DXRBS). We have observed 24 northern sources with the Very Large Array (VLA)
in both its A and C configurations and 15 southern sources with the Australia
Telescope Compact Array (ATCA) in its largest configuration. We find that in
the DXRBS, as in the 1-Jy survey, which has a radio flux limit roughly ten
times higher than the DXRBS, a considerable number (about a third) of BL Lacs
can be identified with the relativistically beamed counterparts of
Fanaroff-Riley type II (FR II) radio galaxies. We attribute the existence of FR
II-BL Lacs, which is not accounted for by current unified schemes, to an
inconsistency in our classification scheme for radio-loud active galactic
nuclei (AGN). Taking the extended radio power as a suitable measure of
intrinsic jet power, we find similar average values for low- (LBL) and
high-energy peaked BL Lacs (HBL), contrary to the predictions of the blazar
sequence.Comment: 21 pages, 10 figures, accepted by MNRA
The outer dusty edge of accretion disks in active galactic nuclei
Recent models for the inner structure of active galactic nuclei (AGN) aim at connecting the outer region of the accretion disk with the broad-line region and dusty torus through a radiatively accelerated, dusty outflow. Such an outflow not only requires the outer disk to be dusty and thus predicts disk sizes beyond the self-gravity limit but requires the presence of nuclear dust with favorable properties. Here, we investigate a large sample of type 1 AGN by near-infrared (near-IR) cross-dispersed spectroscopy with the aim to constrain the astrochemistry, location, and geometry of the nuclear hot dust region. Assuming a thermal equilibrium for optically thin dust, we derive the luminosity-based dust radius for different grain properties using our measurement of the temperature. We combine our results with independent dust radius measurements from reverberation mapping and interferometry, and show that large dust grains that can provide the necessary opacity for the outflow are ubiquitous in AGN. Using our estimates of the dust covering factor, we investigate the dust geometry using the effects of the accretion disk anisotropy. A flared disk-like structure for the hot dust is favored. Finally, we discuss the implication of our results for the dust radius-luminosity plane
Strong variability of the coronal line region in NGC 5548
We present the second extensive study of the coronal line variability in an
active galaxy. Our data set for the well-studied Seyfert galaxy NGC 5548
consists of five epochs of quasi-simultaneous optical and near-infrared
spectroscopy spanning a period of about five years and three epochs of X-ray
spectroscopy overlapping in time with it. Whereas the broad emission lines and
hot dust emission varied only moderately, the coronal lines varied strongly.
However, the observed high variability is mainly due to a flux decrease. Using
the optical [FeVII] and X-ray OVII emission lines we estimate that the coronal
line gas has a relatively low density of n~10^3/cm^3 and a relatively high
ionisation parameter of log U~1. The resultant distance of the coronal line gas
from the ionising source of about eight light years places this region well
beyond the hot inner face of the dusty torus. These results imply that the
coronal line region is an independent entity. We find again support for the
X-ray heated wind scenario of Pier & Voit; the increased ionising radiation
that heats the dusty torus also increases the cooling efficiency of the coronal
line gas, most likely due to a stronger adiabatic expansion. The much stronger
coronal line variability of NGC 5548 relative to that of NGC 4151 can also be
explained within this picture. NGC 5548 has much stronger coronal lines
relative to the low ionisation lines than NGC 4151 indicating a stronger wind,
in which case a stronger adiabatic expansion of the gas and so fading of the
line emission is expected.Comment: 10 pages, 6 figures; accepted for publication in MNRAS. arXiv admin
note: substantial text overlap with arXiv:1501.0292
Variability of the coronal line region in NGC 4151
We present the first extensive study of the coronal line variability in an
active galaxy. Our data set for the nearby source NGC 4151 consists of six
epochs of quasi-simultaneous optical and near-infrared spectroscopy spanning a
period of about eight years and five epochs of X-ray spectroscopy overlapping
in time with it. None of the coronal lines showed the variability behaviour
observed for the broad emission lines and hot dust emission. In general, the
coronal lines varied only weakly, if at all. Using the optical [Fe VII] and
X-ray O VII emission lines we estimate that the coronal line gas has a
relatively low density of n~10^3 cm^-3 and a relatively high ionisation
parameter of log U~1. The resultant distance of the coronal line gas from the
ionising source is about two light years, which puts this region well beyond
the hot inner face of the obscuring dusty torus. The high ionisation parameter
implies that the coronal line region is an independent entity rather than part
of a continuous gas distribution connecting the broad and narrow emission line
regions. We present tentative evidence for the X-ray heated wind scenario of
Pier & Voit. We find that the increased ionising radiation that heats the dusty
torus also increases the cooling efficiency of the coronal line gas, most
likely due to a stronger adiabatic expansion.Comment: 12 pages, 9 figures; accepted by MNRA
What Types of Jets does Nature Make: A New Population of Radio Quasars
We use statistical results from a large sample of about 500 blazars, based on
two surveys, the Deep X-ray Radio Blazar Survey (DXRBS), nearly complete, and
the RASS-Green Bank survey (RGB), to provide new constraints on the spectral
energy distribution of blazars, particularly flat-spectrum radio quasars
(FSRQ). This reassessment is prompted by the discovery of a population of FSRQ
with spectral energy distribution similar to that of high-energy peaked BL
Lacs. The fraction of these sources is sample dependent, being ~ 10% in DXRBS
and ~ 30% in RGB (and reaching ~ 80% for the Einstein Medium Sensitivity
Survey). We show that these ``X-ray strong'' radio quasars, which had gone
undetected or unnoticed in previous surveys, indeed are the strong-lined
counterparts of high-energy peaked BL Lacs and have synchrotron peak
frequencies, nu_peak, much higher than ``classical'' FSRQ, typically in the UV
band for DXRBS. Some of these objects may be 100 GeV - TeV emitters, as are
several known BL Lacs with similar broadband spectra. Our large, deep, and
homogeneous DXRBS sample does not show anti-correlations between nu_peak and
radio, broad line region, or jet power, as expected in the so-called ``blazar
sequence'' scenario. However, the fact that FSRQ do not reach X-ray-to-radio
flux ratios and nu_peak values as extreme as BL Lacs and the elusiveness of
high nu_peak - high-power blazars suggest that there might be an intrinsic,
physical limit to the synchrotron peak frequency that can be reached by
strong-lined, powerful blazars. Our findings have important implications for
the study of jet formation and physics and its relationship to other properties
of active galactic nuclei.Comment: 15 pages, 12 figures. Accepted for publication in The Astrophysical
Journal (May 1 2003 issue). Postscript file also available at
http://www.stsci.edu/~padovani/unif_papers.htm
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