1,679 research outputs found
Magnetic Fields in Quasar Cores II
Multi-frequency polarimetry with the Very Long Baseline Array (VLBA)
telescope has revealed absolute Faraday Rotation Measures (RMs) in excess of
1000 rad/m/m in the central regions of 7 out of 8 strong quasars studied (e.g.,
3C 273, 3C 279, 3C 395). Beyond a projected distance of ~20 pc, however, the
jets are found to have |RM| < 100 rad/m/m. Such sharp RM gradients cannot be
produced by cluster or galactic-scale magnetic fields, but rather must be the
result of magnetic fields organized over the central 1-100 pc. The RMs of the
sources studied to date and the polarization properties of BL Lacs, quasars and
galaxies are shown to be consistent so far with the predictions of unified
schemes. The direct detection of high RMs in these quasar cores can explain the
low fractional core polarizations usually observed in quasars at centimeter
wavelengths as the result of irregularities in the Faraday screen on scales
smaller than the telescope beam. Variability in the RM of the core is reported
for 3C 279 between observations taken 1.5 years apart, indicating that the
Faraday screen changes on that timescale, or that the projected superluminal
motion of the inner jet components samples a new location in the screen with
time. Either way, these changes in the Faraday screen may explain the dramatic
variability in core polarization properties displayed by quasars.Comment: Accepted to the ApJ. 27 pages, 9 figures including figure 6 in colo
An Absolute Flux Density Measurement of the Supernova Remnant Casseopia A at 32 GHz
We report 32 GHz absolute flux density measurements of the supernova remnant
Cas A, with an accuracy of 2.5%. The measurements were made with the 1.5-meter
telescope at the Owens Valley Radio Observatory. The antenna gain had been
measured by NIST in May 1990 to be .
Our observations of Cas A in May 1998 yield . We also report absolute flux density measurements of 3C48, 3C147, 3C286,
Jupiter, Saturn and Mars.Comment: 30 pages, 4 figures; accepted for publication by AJ. Revised
systematic error budget, corrected typos, and added reference
The Radio/Optical Catalog of the SSA13 Field
We present a 1.4-GHz catalog of 810 radio sources (560 sources in the
complete sample) with 1.8" resolution found within a 17' radius in the SSA13
field (RA=13h12m,DEC=42d38'). The radio image from the VLA has an rms noise
level of 4.82 microJy/beam at the field center, and Subaru optical images in
r-band (6300A) and z-band (9200A) have a three-sigma detection magnitude of
26.1 and 24.9, respectively. 88% of the radio sources are identified with an
optical counterpart, and there is significantly more reddening for objects
fainter than 24-mag. The radio and optical parameters are tabulated, and source
morphologies are displayed by radio contours overlaying optical false-color
images. The radio structures show a wealth of complexity and these are
classified into a small number of categories. About one-third of the radio
sources are larger than 1.2" and their orientation is often similar to that of
the associated galaxy or binary-galaxy system. The density of sources in the
SSA13 field above 75 microJy is 0.40 per square arcmin, with a slope of -2.43
in the differential counts. The radio spectral index may steepen for sources
below 75 microJy. We estimate that at most 40% of the microJansky radio sources
are dominated by AGN processes.Comment: 50 pages, 14 figures of which fig 6 contains 33 parts. In press,
Astrophysical Journal, Supp
Jet emission in NGC1052 at radio, optical, and X-ray frequencies
We present a combined radio, optical, and X-ray study of the nearby LINER
galaxy NGC 1052. Data from a short (2.3 ksec) {\it CHANDRA} observation of NGC
1052 reveal the presence of various jet-related X-ray emitting regions, a
bright compact core and unresolved knots in the jet structure as well as an
extended emitting region inside the galaxy well aligned with the radio
synchrotron jet-emission. The spectrum of the extended X-ray emission can best
be fitted with a thermal model with keV, while the compact
core exhibits a very flat spectrum, best approximated by an absorbed power-law
with . We compare the radio
structure to an optical ``structure map'' from a {\it Hubble Space Telescope}
({\it HST}) observation and find a good positional correlation between the
radio jet and the optical emission cone. Bright, compact knots in the jet
structure are visible in all three frequency bands whose spectrum is
inconsistent with synchrotron emission.Comment: 8 pages, 5 figures (figure 2 in color), image resolution degraded wrt
journal version, needs aa.cls. Accepted for publication in A&
Sub-milliarcsecond Imaging of Quasars and AGN
We have used the VLBA at 15 GHz to image the structure of 132 strong compact
AGN and quasars with a resolution better than one milliarcsecond and a dynamic
range typically exceeding 1000 to 1. These observations were made as part of a
program to investigate the sub-parsec structure of quasars and AGN and to study
the changes in their structure with time. Many of the sources included in our
study, particularly those located south of +35 degrees, have not been
previously imaged with milliarcsecond resolution. Each of the sources has been
observed at multiple epochs. In this paper we show images of each of the 132
sources which we have observed. For each source we present data at the epoch
which had the best quality data. The milliarcsecond jets generally appear
one-sided but two-sided structure is often found in lower luminosity radio
galaxies and in high luminosity quasars with gigahertz peaked spectra. Usually
the structure is unresolved along the direction perpendicular to the jet, but a
few sources have broad plumes. In some low luminosity radio galaxies, the
structure appears more symmetric at 2 cm than at long wavelengths. The apparent
long wavelength symmetry in these sources is probably due to absorption by
intervening material. A few sources contain only a single component with any
secondary feature at least a thousand times weaker. We find no obvious
correlation of radio morphology and the detection of gamma-ray emission by
EGRET.Comment: 19 pages, 3 tables, 3 figures. Figure 2 (132 contour diagrams) is
long and is omitted here. Figure 2 may be viewed at
http://www.cv.nrao.edu/2cmsurvey/ In press, Astronomical Journal, April 199
1ES 1927+654: Persistent and rapid X-ray variability in an AGN with low intrinsic neutral X-ray absorption and narrow optical emission lines
We present X-ray and optical observations of the X-ray bright AGN 1ES
1927+654. The X-ray observations obtained with ROSAT and Chandra reveal
persistent, rapid and large scale variations, as well as steep 0.1-2.4 keV
(Gamma = 2.6 +/- 0.3) and 0.3-7.0 keV (Gamma = 2.7 +/- 0.2) spectra. The
measured intrinsic neutral X-ray column density is approximately 7e20cm^-2. The
X-ray timing properties indicate that the strong variations originate from a
region, a few hundred light seconds from the central black hole, typical for
type 1 AGN. High quality optical spectroscopy reveals a typical Seyfert 2
spectrum with some host galaxy contamination and no evidence of Fe II
multiplets or broad hydrogen Balmer wings. The intrinsic optical extinction
derived from the BLR and NLR are A_V >= 3.7 and A_V=1.7, respectively. The
X-ray observations give an A_V value of less than 0.58, in contrast to the
optical extinction values. We discuss several ideas to explain this apparent
difference in classification including partial covering, an underluminous BLR
or a high dust to gas ratio.Comment: 8 pages including 10 figures. Accepted for publication in Astronomy
and Astrophysic
The VLA Survey of the Chandra Deep Field South: I. Overview of the Radio Data
We report 20 and 6 cm VLA deep observations of the CDF-S including the
Extended CDF-S. We discuss the radio properties of 266 cataloged radio sources,
of which 198 are above a 20 cm completeness level reaching down to 43 microJy
at the center of the field. Survey observations made at 6 cm over a more
limited region covers the original CDF-S to a comparable level of sensitivity
as the 20 cm observations.
Of 266 cataloged radio sources, 52 have X-ray counterparts in the CDF-S and a
further 37 in the E-CDF-S area not covered by the 1 Megasecond exposure. Using
a wide range of material, we have found optical or infrared counterparts for
254 radio sources, of which 186 have either spectroscopic or photometric
redshifts (Paper II). Three radio sources have no apparent counterpart at any
other wavelength. Measurements of the 20 cm radio flux density at the position
of each CDF-S X-ray source detected a further 30 radio sources above a
conservative 3-sigma detection limit.
X-ray and sub-mm observations have been traditionally used as a measure of
AGN and star formation activity, respectively. These new observations probe the
faint end of both the star formation and radio galaxy/AGN population, as well
as the connection between the formation and evolution of stars and SMBHs. Both
of the corresponding gravitational and nuclear fusion driven energy sources can
lead to radio synchrotron emission. AGN and radio galaxies dominate at high
flux densities. Although emission from star formation becomes more prominent at
the microjansky levels reached by deep radio surveys, even for the weakest
sources, we still find an apparent significant contribution from low luminosity
AGN as well as from star formation.Comment: Accpted for publication in the Astrophysical Journal supplements with
3 tables and 18 figure
Helical motion and the origin of QPO in blazar-type sources
Recent observations and analysis of blazar sources provide strong evidence
for (i) the presence of significant periodicities in their lightcurves and (ii)
the occurrence of helical trajectories in their radio jets. In scenarios, where
the periodicity is caused by differential Doppler boosting effects along a
helical jet path, both of these facts may be naturally tied together. Here we
discuss four possible driving mechanisms for the occurrence of helical
trajectories: orbital motion in a binary system, Newtonian-driven jet
precession, internal jet rotation and motion along a global helical magnetic
field. We point out that for non-ballistic helical motion the observed period
may appear strongly shortened due to classical travel time effects. Finally,
the possible relevance of the above mentioned driving mechanisms is discussed
for Mkn~501, OJ 287 and AO 0235+16.Comment: 6 pages, 1 figure; presented at the 5th Microquasar Workshop,
Beijing, June 2004. Accepted for publication in the Chinese Journal of
Astronomy and Astrophysic
Resolving the Radio Source Background: Deeper Understanding Through Confusion
We used the Karl G. Jansky Very Large Array (VLA) to image one primary beam
area at 3 GHz with 8 arcsec FWHM resolution and 1.0 microJy/beam rms noise near
the pointing center. The P(D) distribution from the central 10 arcmin of this
confusion-limited image constrains the count of discrete sources in the 1 <
S(microJy/beam) < 10 range. At this level the brightness-weighted differential
count S^2 n(S) is converging rapidly, as predicted by evolutionary models in
which the faintest radio sources are star-forming galaxies; and ~96$% of the
background originating in galaxies has been resolved into discrete sources.
About 63% of the radio background is produced by AGNs, and the remaining 37%
comes from star-forming galaxies that obey the far-infrared (FIR) / radio
correlation and account for most of the FIR background at lambda = 160 microns.
Our new data confirm that radio sources powered by AGNs and star formation
evolve at about the same rate, a result consistent with AGN feedback and the
rough correlation of black hole and bulge stellar masses. The confusion at
centimeter wavelengths is low enough that neither the planned SKA nor its
pathfinder ASKAP EMU survey should be confusion limited, and the ultimate
source detection limit imposed by "natural" confusion is < 0.01 microJy at 1.4
GHz. If discrete sources dominate the bright extragalactic background reported
by ARCADE2 at 3.3 GHz, they cannot be located in or near galaxies and most are
< 0.03 microJy at 1.4 GHz.Comment: 28 pages including 16 figures. ApJ accepted for publicatio
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