1,941 research outputs found
Nonthermal Emission from the Arches Cluster (G0.121+0.017) and the Origin of -ray Emission from 3EG J1746-2851
High resolution VLA observations of the Arches cluster near the Galactic
center show evidence of continuum emission at 3.6, 6, 20 and 90cm. The
continuum emission at 90cm is particularly striking because thermal
sources generally become optically thick at longer wavelengths and fall off in
brightness whereas non-thermal sources increase in brightness. It is argued
that the radio emission from this unique source has compact and diffuse
components produced by thermal and nonthermal processes, respectively. Compact
sources within the cluster arise from stellar winds of mass-losing stars (Lang,
Goss & Rodriguez 2001a) whereas diffuse emission is likely to be due to
colliding wind shocks of the cluster flow generating relativistic particles due
to diffuse shock acceleration. We also discuss the possibility that
-ray emission from 3EG J1746--2851, located within 3.3 of the Arches
cluster, results from the inverse Compton scattering of the radiation field of
the cluster.Comment: 15 pages, four figures, ApJL (in press
Evidence for a Weak Galactic Center Magnetic Field from Diffuse Low Frequency Nonthermal Radio Emission
New low-frequency 74 and 330 MHz observations of the Galactic center (GC)
region reveal the presence of a large-scale (6\arcdeg\times 2\arcdeg) diffuse
source of nonthermal synchrotron emission. A minimum energy analysis of this
emission yields a total energy of ergs
and a magnetic field strength of \muG (where is
the proton to electron energy ratio and is the filling factor of the
synchrotron emitting gas). The equipartition particle energy density is
\evcm, a value consistent with cosmic-ray data. However,
the derived magnetic field is several orders of magnitude below the 1 mG field
commonly invoked for the GC. With this field the source can be maintained with
the SN rate inferred from the GC star formation. Furthermore, a strong magnetic
field implies an abnormally low GC cosmic-ray energy density. We conclude that
the mean magnetic field in the GC region must be weak, of order 10 \muG (at
least on size scales \ga 125\arcsec).Comment: 12 pages, 1 JPEG figure, uses aastex.sty; Accepted for publication,
ApJL (2005, published
Ancient coins: cluster analysis applied to find a correlation between corrosion process and burial soil characteristics
Although it is well known that any material degrades faster when exposed to an aggressive environment as well as that "aggressive" cannot be univocally defined as depending also on the chemical-physical characteristics of material, few researches on the identification of the most significant parameters influencing the corrosion of metallic object are available
The magnetic environment in the central region of nearby galaxies
The central regions of galaxies harbor some of the most extreme physical
phenomena, including dense stellar clusters, non-circular motions of molecular
clouds and strong and pervasive magnetic field structures. In particular, radio
observations have shown that the central few hundred parsecs of our Galaxy has
a striking magnetic field configuration. It is not yet clear whether these
magnetic structures are unique to our Milky Way or a common feature of all
similar galaxies. Therefore, we report on (a) a new radio polarimetric survey
of the central 200 pc of the Galaxy to better characterize the magnetic field
structure and (b) a search for large-scale and organized magnetized structure
in the nuclear regions of nearby galaxies using data from the Very Large Array
(VLA) archive. The high angular resolution of the VLA allows us to study the
central 1 kpc of the nearest galaxies to search for magnetized nuclear features
similar to what is detected in our own Galactic center. Such magnetic features
play a important role in the nuclear regions of galaxies in terms of gas
transport and the physical conditions of the interstellar medium in this
unusual region of galaxies.Comment: 8 pages; Proceedings for "The Universe under the Microscope" (AHAR
2008), held in Bad Honnef (Germany) in April 2008, to be published in Journal
of Physics: Conference Series by Institute of Physics Publishing, R.
Schoedel, A. Eckart, S. Pfalzner, and E. Ros (eds.
Impact of van der Waals forces on the classical shuttle instability
The effects of including the van der Waals interaction in the modelling of
the single electron shuttle have been investigated numerically. It is
demonstrated that the relative strength of the vdW-forces and the elastic
restoring forces determine the characteristics of the shuttle instability. In
the case of weak elastic forces and low voltages the grain is trapped close to
one lead, and this trapping can be overcome by Coulomb forces by applying a
bias voltage larger than a threshold voltage . This allows for
grain motion leading to an increase in current by several orders of magnitude
above the transition voltage . Associated with the process is also
hysteresis in the I-V characteristics.Comment: minor revisions, updated references, Article published in Phys. Rev.
B 69, 035309 (2004
Submillimeter galaxies behind the Bullet Cluster (1E 0657-56)
Clusters of galaxies are effective gravitational lenses able to magnify
background galaxies and making it possible to probe the fainter part of the
galaxy population. Submillimeter galaxies, which are believed to be
star-forming galaxies at typical redshifts of 2 to 3, are a major contaminant
to the extended Sunyaev-Zeldovich (SZ) signal of galaxy clusters. For a proper
quantification of the SZ signal the contribution of submillimeter galaxies
needs to be quantified. The aims of this study are to identify submillimeter
sources in the field of the Bullet Cluster (1E 0657-56), a massive cluster of
galaxies at z~0.3, measure their flux densities at 870 micron, and search for
counterparts at other wavelengths to constrain their properties. We carried out
deep observations of the submillimeter continuum emission at 870 micron using
the Large APEX BOlometer CAmera (LABOCA) on the Atacama Pathfinder EXperiment
(APEX) telescope. Several numerical techniques were used to quantify the noise
properties of the data and extract sources. In total, seventeen sources were
found. Thirteen of them lie in the central 10 arcminutes of the map, which has
a pixel sensitivity of 1.2 mJy per 22 arcsec beam. After correction for flux
boosting and gravitational lensing, the number counts are consistent with
published submm measurements. Nine of the sources have infrared counterparts in
Spitzer maps. The strongest submm detection coincides with a source previously
reported at other wavelengths, at an estimated redshift z~2.7. If the submm
flux arises from two images of a galaxy magnified by a total factor of 75, as
models have suggested, its intrinsic flux would be around 0.6 mJy, consistent
with an intrinsic luminosity below 10^12 L_sun.Comment: Accepted by A&A, 15 pages, 11 figure
Expediting DECam multimessenger counterpart searches with convolutional neural networks
Searches for counterparts to multimessenger events with optical imagers use difference imaging to detect new transient sources. However, even with existing artifact-detection algorithms, this process simultaneously returns several classes of false positives: false detections from poor-quality image subtractions, false detections from low signal-to-noise images, and detections of preexisting variable sources. Currently, human visual inspection to remove the false positives is a central part of multimessenger follow-up observations, but when next generation gravitational wave and neutrino detectors come online and increase the rate of multimessenger events, the visual inspection process will be prohibitively expensive. We approach this problem with two convolutional neural networks operating on the difference imaging outputs. The first network focuses on removing false detections and demonstrates an accuracy of 92% on our data set. The second network focuses on sorting all real detections by the probability of being a transient source within a host galaxy and distinguishes between various classes of images that previously required additional human inspection. We find the number of images requiring human inspection will decrease by a factor of 1.5 using our approach alone and a factor of 3.6 using our approach in combination with existing algorithms, facilitating rapid multimessenger counterpart identification by the astronomical communit
High-Resolution, Wide-Field Imaging of the Galactic Center Region at 330 MHz
We present a wide field, sub-arcminute resolution VLA image of the Galactic
Center region at 330 MHz. With a resolution of ~ 7" X 12" and an RMS noise of
1.6 mJy/beam, this image represents a significant increase in resolution and
sensitivity over the previously published VLA image at this frequency. The
improved sensitivity has more than tripled the census of small diameter sources
in the region, has resulted in the detection of two new Non Thermal Filaments
(NTFs), 18 NTF candidates, 30 pulsar candidates, reveals previously known
extended sources in greater detail, and has resulted in the first detection of
Sagittarius A* in this frequency range.
A version of this paper containing full resolution images may be found at
http://lwa.nrl.navy.mil/nord/AAAB.pdf.Comment: Astronomical Journal, Accepted 62 Pages, 21 Figure
Measurement of Angular Distributions of Drell-Yan Dimuons in Interactions at 800 GeV/c
We report a measurement of the angular distributions of Drell-Yan dimuons
produced using an 800 GeV/c proton beam on a hydrogen target. The polar and
azimuthal angular distribution parameters have been extracted over the
kinematic range GeV/c (excluding the
resonance region), GeV/c, and . The angular
distributions are similar to those of , and both data sets are compared
with models which attribute the distribution either to the
presence of the transverse-momentum-dependent Boer-Mulders structure function
or to QCD effects. The data indicate the presence of both
mechanisms. The validity of the Lam-Tung relation in Drell-Yan is also
tested.Comment: 4 pages, 3 figure
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