2,055 research outputs found
Synchrotron radio emission in radio-quiet AGNs
The basic mechanism responsible for radio emission in radio-loud active
galactic nuclei (AGNs) is assumed to be synchrotron radiation. We suggest here
that radio emission in radio-quiet objects is also due to synchrotron radiation
of particles accelerated in shocks. We consider generic shocks and study the
resulting synchrotron properties. We estimate the synchrotron radio luminosity
and compare it with the X-ray component produced by inverse Compton emission.
We obtain that the radio to X-ray luminosity ratio is much smaller than unity,
with values typical of radio-quiet sources. The predicted trends on source
parameters, black hole mass and accretion rate, may account for the
anticorrelation between radio-loudness and Eddington ratio observed in
different AGN samples.Comment: 5 pages, accepted for publication in Astronomy and Astrophysic
The spatial coherence of noise fields evoked by continuous source distributions
In this work, analytic expressions for the spatial coherence of noise fields are derived in the modal domain with the aim of providing a sparse representation. For this purpose, the sound field in a region of interest is expressed in terms of a given pressure distribution on a virtual surrounding cylindrical or spherical surface. According to the Huygens-Fresnel principle, the sound pressure on this surface is represented by a continuous distribution of elementary line or point sources, where orthogonal basis functions characterize the spatial properties. To describe spatially windowed pressure distributions with arbitrary angular extensions, orthogonal basis functions of limited angular support are proposed. As special cases, circular and spherical pressure distributions with uncorrelated source modes of equal power are investigated. It is shown that these distributions result, respectively, in cylindrically isotropic and spherically isotropic, i.e., diffuse noise fields. The analytic expressions derived in this work allow for a prediction of the spatial coherence between arbitrary positions within the region of interest, such that no microphones need to be placed at the actual points of interest. Simulation results are presented to validate the derived relations.This work was supported by the Australian Research
Council (ARC) Discovery Projects funding scheme under
Project No. DP140103412
Estradiol Modulates Functional Brain Organization during the Menstrual Cycle: An Analysis of Interhemispheric Inhibition
According to the hypothesis of progesterone-mediated interhemispheric decoupling (Hausmann and Güntürkün, 2000), functional cerebral asymmetries (FCAs), which are stable in men and change during the menstrual cycle in women, are generated by interhemispheric inhibition of the dominant on the nondominant hemisphere. The change of lateralization during the menstrual cycle in women might indicate that sex hormones play an important role in modulating FCAs. We used functional magnetic resonance imaging to examine the role of estradiol in determining cyclic changes of interhemispheric inhibition. Women performed a word-matching task, while they were scanned twice during the cycle, once during the menstrual and once during the follicular phase. By use of a connectivity analysis we found that the inhibitory influence of left-hemispheric language areas on homotopic areas of the right hemisphere is strongest during the menses, resulting in a pronounced lateralization. During the follicular phase, due to rising estradiol levels, inhibition and thus functional cerebral asymmetries are reduced. These results reveal a powerful neuromodulatory action of estradiol on the dynamics of functional brain organization in the female brain. They may further contribute to the ongoing discussion of sex differences in brain function in that they help explain the dynamic part of functional brain organization in which the female differs from the male brain
A new solid-phase system for immunoassays
The development of a new solid-phase separation system based on silane polymers is described. A T3 radioimmunoassay (RIA) was optimized using coated tubes with polymer coatings containing hydrophilic surface aldehyde groups for antibody coupling and a T4 RIA developed on the basis of surface anilino group containing particles using a suspension method. Both RIAs offer very good performances and show the variable usability of the new separation system
A Multi-Epoch VLBI Survey of the Kinematics of CJF Sources; Part I: Model-Fit Parameters and Maps
Context: This is the first of a series of papers presenting VLBI observations
of the 293 Caltech-Jodrell Bank Flat-Spectrum (hereafter CJF) sources and their
analysis. Aims: One of the major goals of the CJF is to make a statistical
study of the apparent velocities of the sources. Methods: We have conducted
global VLBI and VLBA observations at 5 GHz since 1990, accumulating thirteen
separate observing campaigns. Results: We present here an overview of the
observations, give details of the data reduction and present the source
parameters resulting from a model-fitting procedure. For every source at every
observing epoch, an image is shown, built up by restoring the model-fitted
components, convolved with the clean beam, into the residual image, which was
made by Fourier transforming the visibility data after first subtracting the
model-fitted components in the uv-plane. Overplotted we show symbols to
represent the model components. Conclusions: We have produced VLBI images of
all but 5 of the 293 sources in the complete CJF sample at several epochs and
investigated the kinematics of 266 AGN.Comment: Figure 1 and Table 2 are only available in electronic form at the CDS
and soon at http://www.mpifr-bonn.mpg.de/staff/sbritzen/cjf.htm
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
Relativistic Beaming and the Intrinsic Properties of Extragalactic Radio Jets
Relations between the observed quantities for a beamed radio jet, apparent
transverse speed and apparent luminosity (beta_app,L), and the intrinsic
quantities, Lorentz factor and intrinsic luminosity (gamma,L_o), are
investigated. The inversion from measured to intrinsic values is not unique,
but approximate limits to gamma and L_o can be found using probability
arguments. Roughly half the sources in a flux density--limited, beamed sample
have a value of gamma close to the measured beta_app. The methods are applied
to observations of 119 AGN jets made with the VLBA at 15 GHz during 1994-2002.
The results strongly support the common relativistic beam model for an
extragalactic radio jet. The (beta_app,L) data are closely bounded by a
theoretical envelope, an aspect curve for gamma=32, L_o= 10^25 W/Hz. This gives
limits to the maximum values of gamma and L_o in the sample: gamma_max about
32, and L_o,max ~ 10^26 W/Hz. No sources with both high beta_app and low L are
observed. This is not the result of selection effects due to the observing
limits, which are flux density S>0.5 Jy, and angular velocity mu<4 mas/yr. Many
of the fastest quasars have a pattern Lorentz factor gamma_p close to that of
the beam, gamma_b, but some of the slow quasars must have gamma_p<<gamma_b.
Three of the 10 galaxies in the sample have a superluminal feature, with speeds
up to beta_app about 6. The others are at most mildly relativistic. The
galaxies are not off-axis versions of the powerful quasars, but Cygnus A might
be an exception.Comment: 12 pages, 9 figures, 1 table, accepted for publication in the
Astrophysical Journa
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