28,193 research outputs found
Mean and Extreme Radio Properties of Quasars and the Origin of Radio Emission
We investigate the evolution of both the radio-loud fraction (RLF) and (using
stacking analysis) the mean radio-loudness of quasars. We consider how these
values evolve as a function of redshift and luminosity, black hole (BH) mass
and accretion rate, and parameters related to the dominance of a wind in the
broad emission line region. We match the FIRST source catalog to samples of
luminous quasars (both spectroscopic and photometric), primarily from the Sloan
Digital Sky Survey. After accounting for catastrophic errors in BH mass
estimates at high-redshift, we find that both the RLF and the mean radio
luminosity increase for increasing BH mass and decreasing accretion rate.
Similarly both the RLF and mean radio loudness increase for quasars which are
argued to have weaker radiation line driven wind components of the broad
emission line region. In agreement with past work, we find that the RLF
increases with increasing luminosity and decreasing redshift while the mean
radio-loudness evolves in the exact opposite manner. This difference in
behavior between the mean radio-loudness and the RLF in L-z may indicate
selection effects that bias our understanding of the evolution of the RLF;
deeper surveys in the optical and radio are needed to resolve this discrepancy.
Finally, we argue that radio-loud (RL) and radio-quiet (RQ) quasars may be
parallel sequences but where only RQ quasars at one extreme of the distribution
are likely to become RL, possibly through slight differences in spin and/or
merger history.Comment: 55 pages, 28 figures, accepted to A
A 100 micro Kelvin bolometer system for SIRTF
Progress toward a prototype of 100 mK bolometric detection system for the Space Infrared Telescope Facility (SIRTF) is described. Two adiabatic demagnetization refrigerators (ADR's) were constructed and used to investigate the capabilities necessary for orbital operation. The first, a laboratory ADR, demonstrated a hold time at 0.1 K of over 12 hours, with temperature stability approx. 3 micro-K RMS achieved by controlling the magnetic field. A durable salt pill and an efficient support system have been demonstrated. A second ADR, the SIRTF flight prototype, has been built and will be flown on a balloon. Techniques for magnetic shielding, low heat leak current leads, and a mechanical heat switch are being developed in this ADR. Plans for construction of 100 mK bolometers are discussed. Three important cosmological investigations which will be carried out by these longest wavelength SIRTF detectors are described
AGN and starbursts at high redshift: High resolution EVN radio observations of the Hubble Deep Field
We present deep, wide-field European VLBI Network (EVN) 1.6 GHz observations
of the Hubble Deep Field (HDF) region with a resolution of 0.025 arcseconds.
Above the 210 microJy/beam (5sigma) detection level, the EVN clearly detects
two radio sources in a field that encompasses the HDF and part of the Hubble
Flanking Fields (HFF). The sources detected are: VLA J123644+621133 (a z=1.013,
low-luminosity FR-I radio source located within the HDF itself) and VLA
J123642+621331 (a dust enshrouded, optically faint, z=4.424 starburst system).
A third radio source, J123646+621404, is detected at the 4sigma level. The VLBI
detections of all three sources suggest that most of the radio emission of
these particular sources (including the dusty starburst) is generated by an
embedded AGN.Comment: 4 pages, 1 figure; Accepted by Astron. & Astrophys Letters ... See
http://www.nfra.nl/~mag/hdf_evn.htm
Charge trapping in polymer transistors probed by terahertz spectroscopy and scanning probe potentiometry
Terahertz time-domain spectroscopy and scanning probe potentiometry were used
to investigate charge trapping in polymer field-effect transistors fabricated
on a silicon gate. The hole density in the transistor channel was determined
from the reduction in the transmitted terahertz radiation under an applied gate
voltage. Prolonged device operation creates an exponential decay in the
differential terahertz transmission, compatible with an increase in the density
of trapped holes in the polymer channel. Taken in combination with scanning
probe potentionmetry measurements, these results indicate that device
degradation is largely a consequence of hole trapping, rather than of changes
to the mobility of free holes in the polymer.Comment: 4 pages, 3 figure
The Parsec-scale Structure, Kinematics, and Polarization of Radio-Loud Narrow-Line Seyfert 1 Galaxies
Several narrow-line Seyfert 1 galaxies (NLS1s) have now been detected in
gamma rays, providing firm evidence that at least some of this class of active
galactic nuclei (AGN) produce relativistic jets. The presence of jets in NLS1s
is surprising, as these sources are typified by comparatively small black hole
masses and near- or super-Eddington accretion rates. This challenges the
current understanding of the conditions necessary for jet production. Comparing
the properties of the jets in NLS1s with those in more familiar jetted systems
is thus essential to improve jet production models. We present early results
from our campaign to monitor the kinematics and polarization of the
parsec-scale jets in a sample of 15 NLS1s through multifrequency observations
with the Very Long Baseline Array. These observations are complemented by
fast-cadence 15 GHz monitoring with the Owens Valley Radio Observatory 40m
telescope and optical spectroscopic monitoring with with the 2m class telescope
at the Guillermo Haro Astrophysics Observatory in Cananea, Mexico.Comment: 4 pages, 1 figure. To appear in the Proceedings of the IAU Symposium
No. 313: "Extragalactic jets from every angle," Galapagos, Ecuador, 15-19
September 2014, F. Massaro, C. C. Cheung, E. Lopez, and A. Siemiginowska
(Eds.), Cambridge University Pres
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Rate of photosynthetic induction in fluctuating light varies widely among genotypes of wheat.
Crop photosynthesis and yield are limited by slow photosynthetic induction in sunflecks. We quantified variation in induction kinetics across diverse genotypes of wheat for the first time. Following a preliminary study that hinted at wide variation in induction kinetics across 58 genotypes, we grew 10 genotypes with contrasting responses in a controlled environment and quantified induction kinetics of carboxylation capacity (Vcmax) from dynamic A versus ci curves after a shift from low to high light (from 50 µmol m-2 s-1 to 1500 µmol m-2 s-1), in five flag leaves per genotype. Within-genotype median time for 95% induction (t95) of Vcmax varied 1.8-fold, from 5.2 min to 9.5 min. Our simulations suggest that non-instantaneous induction reduces daily net carbon gain by up to 15%, and that breeding to speed up Vcmax induction in the slowest of our 10 genotypes to match that in the fastest genotype could increase daily net carbon gain by up to 3.4%, particularly for leaves in mid-canopy positions (cumulative leaf area index ≤1.5 m2 m-2), those that experience predominantly short-duration sunflecks, and those with high photosynthetic capacities
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