2,181 research outputs found
A purely geometric distance to the binary star Atlas, a member of the Pleiades
We present radial velocity and new interferometric measurements of the double
star Atlas, which permit, with the addition of published interferometric data,
to precisely derive the orbital parameters of the binary system and the masses
of the components. The derived semi-major axis, compared with its measured
angular size, allows to determine a distance to Atlas of 132+-4 pc in a purely
geometrical way. Under the assumption that the location of Atlas is
representative of the average distance of the cluster, we confirm the distance
value generally obtained through main sequence fitting, in contradiction with
the early Hipparcos result (118.3+-3.5 pc).Comment: 5 pages, 3 figures, accepted for publication in A&A Letter
Guidance on personal dosimetry for occupational exposure in interventional radiology
During examinations that involve significant levels of exposure, radiology staff wear protective aprons and move towards various exposure orientations. The resulting body exposure is extremely non-uniform. Under such conditions the partially unshielded organs in the trunk together with tissues and organs in the head and neck region determine the effective dose equivalent. In principle, the same will be true for the newly introduced primary quantity effective dose. New calculations are required for quantitative assessments, because the selection of relevant organs and their weighting factors were changed. In this paper we describe our calculations and present and discuss conversion factors for the assessment of effective dose in typical exposure situations for radiology staff in interventional radiology.</p
Where and When: {S}pace-Time Attention for Audio-Visual Explanations
Explaining the decision of a multi-modal decision-maker requires to determine the evidence from both modalities. Recent advances in XAI provide explanations for models trained on still images. However, when it comes to modeling multiple sensory modalities in a dynamic world, it remains underexplored how to demystify the mysterious dynamics of a complex multi-modal model. In this work, we take a crucial step forward and explore learnable explanations for audio-visual recognition. Specifically, we propose a novel space-time attention network that uncovers the synergistic dynamics of audio and visual data over both space and time. Our model is capable of predicting the audio-visual video events, while justifying its decision by localizing where the relevant visual cues appear, and when the predicted sounds occur in videos. We benchmark our model on three audio-visual video event datasets, comparing extensively to multiple recent multi-modal representation learners and intrinsic explanation models. Experimental results demonstrate the clear superior performance of our model over the existing methods on audio-visual video event recognition. Moreover, we conduct an in-depth study to analyze the explainability of our model based on robustness analysis via perturbation tests and pointing games using human annotations
Testing Radiatively-Inefficient Accretion Flow Theory: an XMM-Newton Observation of NGC 3998
We present the results of a 10 ks XMM-Newton observation of NGC 3998, a
``type-I'' LINER galaxy. Our goal is to test the extent to which
radiatively-inefficient accretion flow (RIAF) models and/or scaled-down AGN
models are consistent with the observed properties of NGC 3998. The upper-limit
for narrow Fe-K emission derived from a combined fit of the XMM-Newton and
BeppoSAX spectra is 25 eV, which is one of the strictest limits to date for any
AGN. This significantly rules out Fe-K emission as is expected to be observed
in typical Seyfert 1 galaxies. The lack of any reflection features suggests
that any optically-thick, geometrically-thin accretion disk must be truncated,
probably at a radius of order 100-300 R_s. RIAF models fit the UV to X-ray
spectral energy distribution of NGC 3998 reasonably well. In these models the
mid-IR flux also constrains the emission from any outer thin disk component
that might be present. The UV to X-ray SED is also consistent with a
Comptonized thin disk with a very low accretion rate, in which case the lack of
Fe-K emission may be due to an ionized accretion disk. Accretion models in
general do not account for the observed radio flux of NGC 3998, and the radio
flux may be due to a jet. Recent jet models may also be consistent with the
nuclear fluxes of NGC 3998 in general, including the X-ray, optical/UV and
mid-IR bands. We also derive nuclear fluxes using archival HST WFPC2 data to
constrain the SED of NGC 3998. We discuss a possible OM U band and USNO-B
detection of the NGC 3998 ULX.Comment: Accepted for publication in ApJ. 13 pages and 5 figures formatted
with emulateapj. Version with black-and-white only plots available at
http://www.pha.jhu.edu/~ptak/paper
Distribution of Faraday Rotation Measure in Jets from Active Galactic Nuclei II. Prediction from our Sweeping Magnetic Twist Model for the Wiggled Parts of AGN Jets and Tails
Distributions of Faraday rotation measure (FRM) and the projected magnetic
field derived by a 3-dimensional simulation of MHD jets are investigated based
on our "sweeping magnetic twist model". FRM and Stokes parameters were
calculated to be compared with radio observations of large scale wiggled AGN
jets on kpc scales. We propose that the FRM distribution can be used to discuss
the 3-dimensional structure of magnetic field around jets and the validity of
existing theoretical models, together with the projected magnetic field derived
from Stokes parameters. In the previous paper, we investigated the basic
straight part of AGN jets by using the result of a 2-dimensional axisymmetric
simulation. The derived FRM distribution has a general tendency to have a
gradient across the jet axis, which is due to the toroidal component of the
magnetic field generated by the rotation of the accretion disk. In this paper,
we consider the wiggled structure of the AGN jets by using the result of a
3-dimensional simulation. Our numerical results show that the distributions of
FRM and the projected magnetic field have a clear correlation with the large
scale structure of the jet itself, namely, 3-dimensional helix. Distributions,
seeing the jet from a certain direction, show a good matching with those in a
part of 3C449 jet. This suggests that the jet has a helical structure and that
the magnetic field (especially the toroidal component) plays an important role
in the dynamics of the wiggle formation because it is due to a current-driven
helical kink instability in our model.Comment: Accepted for publication in Ap
Anatomy of helical relativistic jets: The case of S5 0836+710
Helical structures are common in extragalactic jets. They are usually
attributed in the literature to periodical phenomena in the source (e.g.,
precession). In this work, we use VLBI data of the radio-jet in the quasar S5
0836+710 and hypothesize that the ridge-line of helical jets like this
corresponds to a pressure maximum in the jet and assume that the helically
twisted pressure maximum is the result of a helical wave pattern. For our
study, we use observations of the jet in S5 0836+710 at different frequencies
and epochs. The results show that the structures observed are physical and not
generated artificially by the observing arrays. Our hypothesis that the
observed intensity ridge-line can correspond to a helically twisted pressure
maximum is confirmed by our observational tests. This interpretation allows us
to explain jet misalignment between parsec and kiloparsec scales when the
viewing angle is small, and also brings us to the conclusion that
high-frequency observations may show only a small region of the jet flow
concentrated around the maximum pressure ridge-line observed at low
frequencies. Our work provides a potential explanation for the apparent
transversal superluminal speeds observed in several extragalactic jets by means
of transversal shift of an apparent core position with time.Comment: Accepted for publication in the Astrophysical Journa
Radio Sources in Low-Luminosity Active Galactic Nuclei. I. VLA Detections of Compact, Flat-Spectrum Cores
We report a 0.2" resolution, 15 GHz survey of a sample of 48 low-luminosity
active galactic nuclei with the Very Large Array. Compact radio emission has
been detected in 57% (17 of 30) of LINERs and low-luminosity Seyferts, at least
15 of which have a flat to inverted radio spectrum (alpha > -0.3). The compact
radio cores are found in both type 1 (i.e. with broad Halpha) and type 2
(without broad Halpha) nuclei. The 2 cm radio power is significantly correlated
with the emission-line ([OI] lambda6300) luminosity. While the present
observations are consistent with the radio emission originating in star-forming
regions, higher resolution radio observations of 10 of the detected sources,
reported in a companion paper (Falcke et al. 2000), show that the cores are
very compact (= 10^8K) and
probably synchrotron self-absorbed, ruling out a starburst origin. Thus, our
results suggest that at least 50% of low-luminosity Seyferts and LINERs in the
sample are accretion powered, with the radio emission presumably coming from
jets or advection-dominated accretion flows. We have detected only 1 of 18
`transition' (i.e. LINER + HII) nuclei observed, indicating their radio cores
are significantly weaker than those of `pure' LINERs.Comment: To appear in the Astrophysical Journal, October 20, 200
Chandra Observations and the Nature of the Anomalous Arms of NGC 4258 (M 106)
This paper presents high resolution X-ray observations with Chandra of NGC
4258 and infers the nature of the so called ``anomalous arms'' in this galaxy.
The anomalous arms dominate the X-ray image; diffuse X-ray emission from the
``plateaux'' regions, seen in radio and H imaging, is also found. X-ray
spectra have been obtained at various locations along the anomalous arms and
are well described by thermal (mekal) models with kT in the range 0.37 - 0.6
keV. The previously known kpc-scale radio jets are surrounded by cocoons of hot
X-ray emitting gas for the first 350 pc of their length. The radio jets, seen
in previous VLBA and VLA observations, propagate perpendicular to the compact
nuclear gas disk (imaged in water vapor maser emission). The angle between the
jets and the rotation axis of the galactic disk is 60. The jets shock
the normal interstellar gas along the first 350 pc of their length, causing the
hot, X-ray emitting cocoons noted above. At a height of z = 175 pc from the
disk plane, the jets exit the normal gas disk and then propagate though the low
density halo until they reach ``hot spots'' (at 870 pc and 1.7 kpc from the
nucleus), which are seen in radio, optical line and X-ray emission. These jets
must drive mass motions into the low density halo gas. This high velocity halo
gas impacts on the dense galactic gas disk and shock heats it along and around
a ``line of damage'', which is the projection of the jets onto the galactic gas
disk as viewed down the galaxy disk rotation axis. However, because NGC 4258 is
highly inclined ( = 64), the ``line of damage'' projects on the
sky in a different direction to the jets themselves. We calculate the expected
p.a. of the ``line of damage'' on the sky and find that it coincides with the
anomalous arms to within 2. (Abstract truncated).Comment: 12 pages plus 9 figures, to be published in the Astrophysical
Journal, v560, nr 1, pt 1 (Oct 10, 2001 issue
Influence of major mergers on the radio emission of elliptical galaxies
We investigate the influence of major mergers on the radio emission of
elliptical galaxies. We use a complete sample of close pairs, which contains
475 merging and 1828 non-merging paired elliptical galaxies of M_r<-21.5
selected from the Sloan Digital Sky Survey. In addition, a control sample of
2000 isolated field galaxies is used for comparison. We cross-identify the
optical galaxies with the radio surveys of FIRST and NVSS. We find that the
radio fraction of merging paired galaxies is about 6%, which is slightly higher
than the 5% obtained for non-merging paired galaxies, although these values are
consistent with each other owing to the large uncertainty caused by the limited
sample. The radio fraction is twice as that of isolated galaxies, which is less
than 3%. Radio emission of elliptical galaxies is only slightly affected by
major mergers, but predominantly depends on their optical luminosities.
Therefore, merging is not important in triggering the radio emission of
elliptical galaxies.Comment: 5 pages, 5 figures, 1 table, accepted for publication in A&A, minor
change
Carnets de bord en sciences humaines : genèse, dynamique et fin d'une revue
Ce dernier numéro est l'occasion de faire un bilan et d'analyser les raisons qui nous ont conduits à décider de mettre un terme à l'existence d'une revue que nous avons animée pendant près de dix ans. Fidèles à la ligne éditoriale, nous dévoilons les coulisses de la production de Carnets de bord. Chemin faisant, nous nous livrons à un exercice d'auto-analyse susceptible d'éclairer la fragilité de
ce genre d'initiatives intellectuelles dans un contexte universitaire où les critères managériaux d'évaluation ont largement gagné du terrain
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