1,320 research outputs found
Deriving AGN properties from radio CP and LP
We report multi-frequency circular polarization measurements for the radio
source 0056-00 taken at the Effelsberg 100-m radiotelescope. The data reduction
is based on a new calibration procedure that allows the contemporary
measurement of the four Stokes parameters with single-dish radiotelescopesComment: 2 pages, Proceeding of "IAU Symposium No.259. Cosmic Magnetic Fields
from planets, to stars and galaxies
High-Spatial Resolution SED of NGC 1068 from Near-IR to Radio. Disentangling the thermal and non-thermal contributions
We investigate the ideas that a sizable fraction of the interferometrically
unresolved infrared emission of the nucleus of NGC 1068 might originate from
other processes than thermal dust emission from the torus. We examine the
contribution of free-free or synchrotron emissions to the central mid- and
near-IR parsec-scale emitting region of NGC 1068. Each mechanism is constrained
with parsec scale radio data available for NGC 1068 in the 10^9 - 10^11 Hz
regime, and compared to the highest-resolution interferometric data available
in the mid-infrared. It is shown that the unresolved emission in the
interferometric observation (<~1pc) is still dominatedd by dust emission and
not by contributions from synchrotron or free-free emission. As recent studies
suggest, the interferometric observations prefer a clumpy structure of the dust
distribution. Extrapolation of the radio free-free or synchrotron emission to
the IR indicates that their contribution is <20% even for the unresolved
fraction of the interferometric flux. The slope of the available radio data is
consistent with a power law exponent alpha = 0.29 +/- 0.07 which we interprete
in terms of either free-free emission or synchrotron radiation from
quasi-monochromatic electrons. We apply emission models for both mechanisms in
order to obtain physical parameters. (abridged)Comment: 8 pages, 3 figures; accepted by A&
Hybrid Thermal-Nonthermal Synchrotron Emission from Hot Accretion Flows
We investigate the effect of a hybrid electron population, consisting of both
thermal and non-thermal particles, on the synchrotron spectrum, image size, and
image shape of a hot accretion flow onto a supermassive black hole. We find two
universal features in the emitted synchrotron spectrum: (i) a prominent
shoulder at low (< 10^11 Hz) frequencies that is weakly dependent on the shape
of the electron energy distribution, and (ii) an extended tail of emission at
high (> 10^13 Hz) frequencies whose spectral slope depends on the slope of the
power-law energy distribution of the electrons. In the low-frequency shoulder,
the luminosity can be up to two orders of magnitude greater than with a purely
thermal plasma even if only a small fraction (< 1%) of the steady-state
electron energy is in the non-thermal electrons. We apply the hybrid model to
the Galactic center source, Sgr A*. The observed radio and IR spectra imply
that at most 1% of the steady-state electron energy is present in a power-law
tail in this source. This corresponds to no more than 10% of the electron
energy injected into the non-thermal electrons and hence 90% into the thermal
electrons. We show that such a hybrid distribution can be sustained in the flow
because thermalization via Coulomb collisions and synchrotron self-absorption
are both inefficient. The presence of non-thermal electrons enlarges the size
of the radio image at low frequencies and alters the frequency dependence of
the brightness temperature. A purely thermal electron distributions produces a
sharp-edged image while a hybrid distribution causes strong limb brightening.
These effects can be seen up to frequencies ~10^11 Hz and are accessible to
radio interferometers.Comment: 33 pages with figures, to appear in the Astrophysical Journa
High Proper Motion Stars in the Vicinity of Sgr A*: Evidence for a Supermassive Black Hole at the Center of Our Galaxy
Over a two year period (1995-1997), we have conducted a diffraction-limited
imaging study at 2.2 microns of the inner 6"x6" of the Galaxy's central stellar
cluster using the Keck 10-m telescope. The K band images obtained reveal a
large population of faint stars. We use an unbiased approach for identifying
and selecting stars to be included in this proper motion study, which results
in a sample of 90 stars with brightness ranging from K=9-17 and velocities as
large as 1,400+-100 km/sec. Compared to earlier work (Eckart et al. 1997;
Genzel et al. 1997), the source confusion is reduced by a factor of 9, the
number of stars with proper motion measurement in the central 25 arcsec^2 of
our galaxy is doubled, and the accuracy of the velocity measurements in the
central 1 arcsec^2 is improved by a factor of 4. The peaks of both the stellar
surface density and the velocity dispersion are consistent with the position of
the unusual radio source and blackhole candidate, Sgr A*, suggesting that Sgr
A* is coincident (+-0."1) with the dynamical center of the Galaxy. As a
function of distance from Sgr A*, the velocity dispersion displays a falloff
well fit by Keplerian motion about a central dark mass of 2.6(+-0.2)x10^6 Mo
confined to a volume of at most 10^-6 pc^3, consistent with earlier results.
Although uncertainties in the measurements mathematically allow for the matter
to be distributed over this volume as a cluster, no realistic cluster is
physically tenable. Thus, independent of the presence of Sgr A*, the large
inferred central density of at least 10^12 Mo/pc^3, which exceeds the
volume-averaged mass densities found at the center of any other galaxy, leads
us to the conclusion that our Galaxy harbors a massive central black hole.Comment: 19 pages, 8 figures, accepted for publications in the Astrophysical
Journa
Sgr A* Polarization: No ADAF, Low Accretion Rate, and Non-Thermal Synchrotron Emission
The recent detection of polarized radiation from Sgr A* requires a
non-thermal electron distribution for the emitting plasma. The Faraday rotation
measure must be small, placing strong limits on the density and magnetic field
strength. We show that these constraints rule out advection-dominated accretion
flow models. We construct a simple two-component model which can reproduce both
the radio to mm spectrum and the polarization. This model predicts that the
polarization should rise to nearly 100% at shorter wavelengths. The first
component, possibly a black-hole powered jet, is compact, low density, and
self-absorbed near 1 mm with ordered magnetic field, relativistic Alfven speed,
and a non-thermal electron distribution. The second component is poorly
constrained, but may be a convection-dominated accretion flow with dM/dt~10^-9
M_Sun/yr, in which feedback from accretion onto the black hole suppresses the
accretion rate at large radii. The black hole shadow should be detectable with
sub-mm VLBI.Comment: 4 pages, 1 figure, accepted by ApJL, several changes from submitted
versio
What is the Accretion Rate in Sgr A*?
The radio source Sgr A* at the center of our Galaxy is believed to be a 2.6 x
10^6 solar mass black hole which accretes gas from the winds of nearby stars.
We show that limits on the X-ray and infrared emission from the Galactic Center
provide an upper limit of ~ 8 x 10^{-5} solar masses per year on the mass
accretion rate in Sgr A*. The advection-dominated accretion flow (ADAF) model
favors a rate < 10^{-5} solar masses per year. In comparison, the Bondi
accretion rate onto Sgr A*, estimated using the observed spatial distribution
of mass losing stars and assuming non-interacting stellar winds, is ~ 3 x
10^{-5} solar masses per year. There is thus rough agreement between the Bondi,
the ADAF, and the X-ray inferred accretion rates for Sgr A*. We discuss
uncertainties in these estimates, emphasizing the importance of upcoming
observations by the Chandra X-ray observatory (CXO) for tightening the X-ray
derived limits.Comment: to appear in ApJ Letter
Constraining the Accretion Rate Onto Sagittarius A* Using Linear Polarization
Two possible explanations for the low luminosity of the supermassive black
hole at the center of our galaxy are (1) an accretion rate of order the
canonical Bondi value (roughly 10^{-5} solar masses per year), but a very low
radiative efficiency for the accreting gas or (2) an accretion rate much less
than the Bondi rate. Both models can explain the broad-band spectrum of the
Galactic Center. We show that they can be distinguished using the linear
polarization of synchrotron radiation. Accretion at the Bondi rate predicts no
linear polarization at any frequency due to Faraday depolarization. Low
accretion rate models, on the other hand, have much lower gas densities and
magnetic field strengths close to the black hole; polarization may therefore be
observable at high frequencies. If confirmed, a recent detection of linear
polarization from Sgr A above 150 GHz argues for an accretion rate of order
10^{-8} solar masses per year, much less than the Bondi rate. This test can be
applied to other low-luminosity galactic nuclei.Comment: final version accepted by ApJ; references added, somewhat shortene
Watershed land use and nutrient dynamics in Maryland Coasal Bays, U.S.A.
Upstream and inshore regions of the Maryland Coastal Bays exhibit degraded water quality. Six streams and three shallow bays were sampled in May and July 2006 and 2007 to compare spatial patterns in relation to land use and nutrient loading. St. Martin River, having a high percentage of crop agriculture and a low percentage of forest and wetlands, experienced the most degraded water quality of the three regions, and stream total nitrogen in its watershed was linked to feeding operations and anthropogenic land use. Despite having a much less developed watershed, Johnson Bay experienced degraded water quality, especially in inshore regions. Sinepuxent Bay had the best water quality of the three bays, but still demonstrated anthropogenic impacts. Nutrient loading from land use is directly related to the observed patterns in St. Martin River, while residence time, groundwater flows, and within-bay cycling has led to water quality degradation in Johnson Bay
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