3,197 research outputs found
A multiband study of Hercules A. II. Multifrequency VLA imaging
We have mapped the powerful radio galaxy Hercules A at six frequencies
spanning 1295 to 8440 MHz using the VLA in all four configurations. Here we
discuss the structure revealed in total intensity, spectral index,
polarization, and projected magnetic field. Our observations clearly reveal the
relation between the bright jets, radio source. The jets and rings form a
coherent structure with a dramatically flatter spectrum than the surrounding
lobes and bridge, strongly suggesting that they represent a recently renewed
outburst from the active nucleus. The spectrum of the lobes is also steeper
than in typical radio sources, and steepens further towards the centre. The
compact core is optically thin and also has a remarkably steep spectrum (alpha
\~ -1.2). There is some evidence that the old lobe material has been swept up
and compressed ahead of the new outburst. We interpret the dramatic asymmetry
in the bright structure, and more subtle differences between diffuse lobe
structures, in terms of relativistic beaming combined with front-to-back
light-travel delays which mean that we view the two lobes at different stages
of the outburst. After correcting for Faraday rotation the projected magnetic
field closely follows the edge of the lobes, the jets, and the rings; the field
pattern in the two lobes is broadly similar. We confirm a strong asymmetry in
depolarization and Faraday rotation, with the jet side the less depolarized and
the flatter spectrum, consistent with general correlations between these
asymmetries. The spectral index asymmetry is clearly present in the `old' lobe
material and so, at least in this case, is not due to beaming; but it can be
understood in terms of the light-travel delay.Comment: 25 pages, 18 figures, accepted for publication in MNRAS. For
associated jpeg files, see http://www.jb.man.ac.uk/~jpl/hera
Vegetation and soils field research data base: Experiment summaries
Understanding of the relationships between the optical, spectral characteristics and important biological-physical parameters of earth-surface features can best be obtained by carefully controlled studies over fields and plots where complete data describing the condition of targets are attainable and where frequent, timely spectral measurement can be obtained. Development of a vegetation and soils field research data base was initiated in 1972 at Purdue University's Laboratory for Applications of Remote Sensing and expanded in the fall of 1974 by NASA as part of LACIE. Since then, over 250,000 truck-mounted and helicopter-borne spectrometer/multiband radiometer observations have been obtained of more than 50 soil series and 20 species of crops, grasses, and trees. These data are supplemented by an extensive set of biophysical and meteorological data acquired during each mission. The field research data form one of the most complete and best-documented data sets acquired for agricultural remote sensing research. Thus, they are well-suited to serve as a data base for research to: (1) quantiatively determine the relationships of spectral and biophysical characteristics of vegetation, (2) define future sensor systems, and (3) develop advanced data analysis techniques
A Compact Tri-band Printed Antenna for MIMO Applications
In this paper, a compact tri-band printed multi-input multi-output (MIMO) antenna with high isolation is presented to operate within WLAN and WiMAX frequency bands. By adopting a rectangular open-ended slot combined with a rectangular strip with an inverted L-shaped open-ended slot, three operating frequency bands can be obtained. The proposed compact MIMO antenna occupies an overall size of 19×33 mm2. Good port-to-port isolation is obtained. The simulated and measured results show that the presented antenna is suitable for multiband MIMO applications
Multiband polarimetric and total intensity imaging of 3C345
We monitored the superluminal QSO 3C 345 at three epochs during a one-year
period in 1995--1996, observing with the VLBA at 22, 15, 8.4, and 5 GHz. We
imaged the radio source both in total and in polarized intensity. In the images
at 5 and 8.4 GHz, the jet emission is traced up to 20 milliarcseconds (mas)
from the jet core. In the 15 and 22 GHz images, we identify several enhanced
emission regions moving at apparent speeds of 5c. Images of the linear
polarized emission show predominantly an alignment of the electric vector with
the extremely curved jet along the inner part of the high frequency jet. At 5
GHz, the jet shows remarkably strong fractional polarization (m~15%) with the
electric vector perpendicular to the jet orientation.Comment: LaTeX file, 6 pages, 2 figures, needs "elsart" style package To be
published in New Astronomy Reviews, special issue: Proceedings of the 4th
EVN/JIVE VLBI Symposium, Eds. Garrett, M.A., Campbell, R.M., & Gurvits, L.
Constraining GRB Emission Physics with Extensive Early-Time, Multiband Follow-up
Understanding the origin and diversity of emission processes responsible for
Gamma-ray Bursts (GRBs) remains a pressing challenge. While prompt and
contemporaneous panchromatic observations have the potential to test
predictions of the internal-external shock model, extensive multiband imaging
has been conducted for only a few GRBs. We present rich, early-time, multiband
datasets for two \swift\ events, GRB 110205A and GRB 110213A. The former shows
optical emission since the early stages of the prompt phase, followed by the
steep rising in flux up to ~1000s after the burst ( with
). We discuss this feature in the context of the
reverse-shock scenario and interpret the following single power-law decay as
being forward-shock dominated. Polarization measurements, obtained with the
RINGO2 instrument mounted on the Liverpool Telescope, also provide hints on the
nature of the emitting ejecta. The latter event, instead, displays a very
peculiar optical to near-infrared lightcurve, with two achromatic peaks. In
this case, while the first peak is probably due to the onset of the afterglow,
we interpret the second peak to be produced by newly injected material,
signifying a late-time activity of the central engine.Comment: 48 pages,11 figures, 24 tables. Accepted to The Astrophysical Journa
Microvariability in BL Lacertae : "zooming'' into the innermost blazar regions
In this work, we present the results of our multi-band microvariability study of the famous blazar BL Lac. We performed microvariablity observations of the source in the optical VRI bands for four nights in 2016. We studied the intranight flux and spectral variability of the source in detail with an objective to characterize microvariability in the blazars, a frequently observed phenomenon in blazars. The results show that the source often displays a fast flux variability with an amplitude as large as ~0.2 magnitude within a few hours, and that the color variability in the similar time scales can be characterized as “bluer-when-brighter” trend. We also observed markedly curved optical spectrum during one of the nights. Furthermore, the correlation between multi-band emission shows that in general the emission in all the bands are highly correlated; and in one of the nights V band emission was found to lead the I band emission by ~13 min. The search for characteristic timescale using z-transformed auto-correlation function and the structure function analyses reveals characteristic timescale of ~50 min in one of the R band observations. We try to explain the observed results in the context of the passage of shock waves through the relativistic outflows in blazars
Wideband and UWB antennas for wireless applications. A comprehensive review
A comprehensive review concerning the geometry, the manufacturing technologies, the materials, and the numerical techniques, adopted for the analysis and design of wideband and ultrawideband (UWB) antennas for wireless applications, is presented. Planar, printed, dielectric, and wearable antennas, achievable on laminate (rigid and flexible), and textile dielectric substrates are taken into account. The performances of small, low-profile, and dielectric resonator antennas are illustrated paying particular attention to the application areas concerning portable devices (mobile phones, tablets, glasses, laptops, wearable computers, etc.) and radio base stations. This information provides a guidance to the selection of the different antenna geometries in terms of bandwidth, gain, field polarization, time-domain response, dimensions, and materials useful for their realization and integration in modern communication systems
Multispectral linear array multiband selection device
An apparatus for detecting multiple spectral bands, individually or concurrently, using linear detector arrays is described. The system employs a beamsplitter to divide the optical source into two or more optical beams which are directed at the linear detector arrays. Filter trays are positioned in the focal planes of the optical beams so that the beams pass through the filter trays prior to impinging upon the detector arrays. Multiple filters are placed on the filter trays. Linear actuators positioned adjacent the filter trays translate the trays across the focal planes of the optical beams so that individual filters are positioned in the path of beams such that those frequencies of the beams that fall within the spectral ranges of the individual bandpass filter through which it passes may be detected by the detector arrays for further examination and analysis
The source structure of 0642+449 detected from the CONT14 observations
The CONT14 campaign with state-of-the-art VLBI data has observed the source
0642+449 with about one thousand observables each day during a continuous
observing period of fifteen days, providing tens of thousands of closure
delays---the sum of the delays around a closed loop of baselines. The closure
delay is independent of the instrumental and propagation delays and provides
valuable additional information about the source structure. We demonstrate the
use of this new "observable" for the determination of the structure in the
radio source 0642+449. This source, as one of the defining sources in the
second realization of the International Celestial Reference Frame (ICRF2), is
found to have two point-like components with a relative position offset of -426
microarcseconds in right ascension and -66 microarcseconds in declination. The
two components are almost equally bright with a flux-density ratio of 0.92. The
standard deviation of closure delays for source 0642+449 was reduced from 139
ps to 90 ps by using this two-component model. Closure delays larger than one
nanosecond are found to be related to the source structure, demonstrating that
structure effects for a source with this simple structure could be up to tens
of nanoseconds. The method described in this paper does not rely on a priori
source structure information, such as knowledge of source structure determined
from direct (Fourier) imaging of the same observations or observations at other
epochs. We anticipate our study to be a starting point for more effective
determination of the structure effect in VLBI observations.Comment: 14 pages, 13 figures, Accepted by Astronomical Journal on 12 Jul,
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