3,065 research outputs found
Constraining Radiatively Inefficient Accretion Flows with Polarization
The low-luminosity black hole Sgr A* provides a testbed for models of
Radiatively Inefficient Accretion Flows (RIAFs). Recent sub-millimeter linear
polarization measurements of Sgr A* have provided evidence that the electrons
in the accretion flow are relativistic over a large range of radii. Here, we
show that these high temperatures result in elliptical plasma normal modes.
Thus, polarized millimeter and sub-millimeter radiation emitted within RIAFs
will undergo generalized Faraday rotation, a cyclic conversion between linear
and circular polarization. This effect will not depolarize the radiation even
if the rotation measure is extremely high. Rather, the beam will take on the
linear and circular polarization properties of the plasma normal modes. As a
result, polarization measurements of Sgr A* in this frequency regime will
constrain the temperature, density and magnetic profiles of RIAF models.Comment: 4 pages, 3 figures, accepted by ApJ Letter
The luminous X-ray hotspot in 4C 74.26: synchrotron or inverse-Compton emission?
We report the discovery of an X-ray counterpart to the southern radio hotspot
of the largest-known radio quasar 4C 74.26 (whose redshift is z=0.104). Both
XMM-Newton and Chandra images reveal the same significant (10arcsec, i.e.
19kpc) offset between the X-ray hotspot and the radio hotspot imaged with
MERLIN. The peak of the X-ray emission may be due to synchrotron or
inverse-Compton emission. If synchrotron emission, the hotspot represents the
site of particle acceleration and the offset arises from either the jet
exhibiting Scheuer's `dentist's drill' effect or a fast spine having less
momentum than the sheath surrounding it, which creates the radio hotspot. If
the emission arises from the inverse-Compton process, it must be
inverse-Compton scattering of the CMB in a decelerating relativistic flow,
implying that the jet is relativistic (Gamma >= 2) out to a distance of at
least 800kpc. Our analysis, including optical data from the Liverpool
Telescope, rules out a background AGN for the X-ray emission and confirms its
nature as a hotspot, making it the most X-ray luminous hotspot yet detected.Comment: 9 pages, 9 figures, definitive version published by MNRA
Radio Synchrotron Emission from Secondary Leptons in the Vicinity of Sgr A*
A point-like source of ~TeV gamma-rays has recently been seen towards the
Galactic center by HESS and other air Cerenkov telescopes. In recent work
(Ballantyne et al. 2007), we demonstrated that these gamma-rays can be
attributed to high-energy protons that (i) are accelerated close to the event
horizon of the central black hole, Sgr A*, (ii) diffuse out to ~pc scales, and
(iii) finally interact to produce gamma-rays. The same hadronic collision
processes will necessarily lead to the creation of electrons and positrons.
Here we calculate the synchrotron emissivity of these secondary leptons in the
same magnetic field configuration through which the initiating protons have
been propagated in our model. We compare this emission with the observed ~GHz
radio spectrum of the inner few pc region which we have assembled from archival
data and new measurements we have made with the Australia Telescope Compact
Array. We find that our model predicts secondary synchrotron emission with a
steep slope consistent with the observations but with an overall normalization
that is too large by a factor of ~ 2. If we further constrain our theoretical
gamma-ray curve to obey the implicit EGRET upper limit on emission from this
region we predict radio emission that is consistent with observations, i.e.,
the hadronic model of gamma ray emission can, simultaneously and without
fine-tuning, also explain essentially all the diffuse radio emission detected
from the inner few pc of the Galaxy.Comment: 11 pages, 2 figures. Two references missing from published version
added and acknowledgements extende
Galactic center at very high-energies
Employing data collected during the first 25 months' observations by the
Fermi-LAT, we describe and subsequently seek to model the very high energy
(>300 MeV) emission from the central few parsecs of our Galaxy. We analyze the
morphological, spectral and temporal characteristics of the central source,
1FGL J1745.6-2900. Remarkably, the data show a clear, statistically significant
signal at energies above 10 GeV, where the Fermi-LAT has an excellent angular
resolution comparable to the angular resolution of HESS at TeV energies, which
makes meaningful the joint analysis of the Fermi and HESS data. Our analysis
does not show statistically significant variability of 1FGL J1745.6-2900. Using
the combination of Fermi data on 1FGL J1745.6-2900 and HESS data on the
coincident, TeV source HESS J1745-290, we show that the spectrum of the central
gamma-ray source is inflected with a relatively steep spectral region matching
between the flatter spectrum found at both low and high energies. We seek to
model the gamma-ray production in the inner 10 pc of the Galaxy and examine, in
particular, cosmic ray (CR) proton propagation scenarios that reproduce the
observed spectrum of the central source. We show that a model that instantiates
a transition from diffusive propagation of the CR protons at low energy to
almost rectilinear propagation at high energies (given a reasonable
energy-dependence of the assumed diffusion coefficient) can well explain the
spectral phenomenology. In general, however, we find considerable degeneracy
between different parameter choices which will only be broken with the addition
of morphological information that gamma-ray telescopes cannot deliver given
current angular resolution limits.We argue that a future analysis done in
combination with higher-resolution radio continuum data holds out the promise
of breaking this degeneracy.Comment: submitted to Ap
A growth-rate indicator for Compton-thick active galactic nuclei
Due to their heavily obscured central engines, the growth rate of
Compton-thick (CT) active galactic nuclei (AGN) is difficult to measure. A
statistically significant correlation between the Eddington ratio,
{\lambda}, and the X-ray power-law index, {\Gamma}, observed in
unobscured AGN offers an estimate of their growth rate from X-ray spectroscopy
(albeit with large scatter). However, since X-rays undergo reprocessing by
Compton scattering and photoelectric absorption when the line-of-sight to the
central engine is heavily obscured, the recovery of the intrinsic {\Gamma} is
challenging. Here we study a sample of local, predominantly Compton-thick
megamaser AGN, where the black hole mass, and thus Eddington luminosity, are
well known. We compile results on X-ray spectral fitting of these sources with
sensitive high-energy (E> 10 keV) NuSTAR data, where X-ray torus models which
take into account the reprocessing effects have been used to recover the
intrinsic {\Gamma} values and X-ray luminosities, L. With a simple
bolometric correction to L to calculate {\lambda}, we find a
statistically significant correlation between {\Gamma} and {\lambda} (p
= 0.007). A linear fit to the data yields {\Gamma} =
(0.410.18)log{\lambda}+(2.38 0.20), which is
statistically consistent with results for unobscured AGN. This result implies
that torus modeling successfully recovers the intrinsic AGN parameters. Since
the megamasers have low-mass black holes (M M)
and are highly inclined, our results extend the {\Gamma}-{\lambda}
relationship to lower masses and argue against strong orientation effects in
the corona, in support of AGN unification. Finally this result supports the use
of {\Gamma} as a growth-rate indicator for accreting black holes, even for
Compton-thick AGN.Comment: Accepted for publication in Ap
Advanced photovoltaic power systems using tandem GaAs/GaSb concentrator modules
In 1989, Boeing announced the fabrication of a tandem gallium concentrator solar cell with an energy conversion efficiency of 30 percent. This research breakthrough has now led to panels which are significantly smaller, lighter, more radiation resistant, and potentially less expensive than the traditional silicon flat plate electric power supply. The new Boeing tandem concentrator (BTC) module uses an array of lightweight silicone Fresnel lenses mounted on the front side of a light weight aluminum honeycomb structure to focus sunlight onto small area solar cells mounted on a thin back plane. This module design is shown schematically. The tandem solar cell in this new module consists of a gallium arsenide light sensitive cell with a 24 percent energy conversion efficiency stacked on top of a gallium antimonide infrared sensitive cell with a conversion efficiency of 6 percent. This gives a total efficiency 30 percent for the cell-stack. The lens optical efficiency is typically 85 percent. Discounting for efficiency losses associated with lens packing, cell wiring, and cell operating temperature still allows for a module efficiency of 22 percent which leads to a module power density of 300 Watts/sq. m. This performance provides more than twice the power density available from a single crystal silicon flat plate module and at least four times the power density available from amorphous silicon modules. The fact that the lenses are only 0.010 ft. thick and the aluminum foil back plane is only 0.003 ft. thick leads to a very lightweight module. Although the cells are an easy to handle thickness of 0.020 ft., the fact that they are small, occupying one-twenty-fifth of the module area, means that they add little to the module weight. After summing all the module weights and given the high module power, we find that we are able to fabricate BTC modules with specific power of 100 watts/kg
X-ray bolometric corrections for Compton-thick active galactic nuclei
We present X-ray bolometric correction factors, (), for Compton-thick (CT) active galactic nuclei (AGN) with the aim
of testing AGN torus models, probing orientation effects, and estimating the
bolometric output of the most obscured AGN. We adopt bolometric luminosities,
, from literature infrared (IR) torus modeling and compile published
intrinsic 2--10 keV X-ray luminosities, , from X-ray torus modeling of
NuSTAR data. Our sample consists of 10 local CT AGN where both of these
estimates are available. We test for systematic differences in
values produced when using two widely used IR torus models and two widely used
X-ray torus models, finding consistency within the uncertainties. We find that
the mean of our sample in the range
erg/s is log
with an intrinsic scatter of dex, and that our derived
values are consistent with previously established relationships between
and and and Eddington ratio. We
investigate if is dependent on by comparing our results on
CT AGN to published results on less-obscured AGN, finding no significant
dependence. Since many of our sample are megamaser AGN, known to be viewed
edge-on, and furthermore under the assumptions of AGN unification whereby
unobscured AGN are viewed face-on, our result implies that the X-ray emitting
corona is not strongly anisotropic. Finally, we present values
for CT AGN identified in X-ray surveys as a function of their observed ,
where an estimate of their intrinsic is not available, and redshift,
useful for estimating the bolometric output of the most obscured AGN across
cosmic time.Comment: Accepted for publication in Ap
Roel of Morning versus Evening Chronotype on Insulin Sensitivity and Central Hemodynamics in Adults with Metabolic Syndrome
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