2,569 research outputs found
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
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
Experiential Learning through Community Co-design in Interior Design Pedagogy
The profit‐driven tendency of interior design trends and styles today has developed in line with the decrease of social awareness in design. The majority of interior design students also decide to pursue interior design education for its marketable and profitable purposes rather than seeing interior design as a field of opportunity to contribute to the social welfare of their communities. Hence, the objective of this research is to implement community service through co‐design in interior design pedagogy. The article describes the learning and design methods used based on human‐centred design approaches of co‐design and analyses the resulting benefits from this approach. Findings reveal that the process of collective creativity and collaborative development with the community enables a direct experience of learning and fosters a deeper connection and understanding of users. They also promote novel multidisciplinary design innovations, accommodate the communitys potentials in the society and stimulate a reflexive impact, allowing students to reflect on their future role as interior designers in bringing positive changes to their community against the profit‐driven tendency of contemporary designers today
Extremely Weak Reflection Features in the X-ray Spectrum of XTE J1118+480: Possible Evidence for X-ray-Emitting Jets?
We have simultaneously fit Chandra and RXTE spectra of the Galactic black
hole XTE J1118+480 with three models for X-ray reflection. We explored a range
of accretion disc ionizations (log(xi)=1-4; xi=L_X/nR^{2}) and iron abundances
(0.10-1.00). Our fits with the constant density ionized disc models of Ross &
Fabian indicate that less than 0.5 per cent (90 per cent confidence
upper-limit) of the observed flux is reflected. Fits with the "pexrav" of model
Magdziarz & Zdziarski indicate that the two-dimensional solid angle (Omega/2pi)
subtended by the disc relative to a central source of incident hard X-rays is
0.01 +0.06 -0.01. A combination of the high inclination (i=81 degrees),
Comptonization, and bulk velocities may each contribute to the low reflection
fractions we have measured. The results are also consistent with extended jets
being the source of the hard X-ray flux, as the disc would then represent a
small solid angle as seen from the emission region.Comment: Accepted for publication in MNRA
High Energy Cosmic-ray Diffusion in Molecular Clouds: A Numerical Approach
The propagation of high-energy cosmic rays through giant molecular clouds
constitutes a fundamental process in astronomy and astrophysics. The diffusion
of cosmic-rays through these magnetically turbulent environments is often
studied through the use of energy-dependent diffusion coefficients, although
these are not always well motivated theoretically. Now, however, it is feasible
to perform detailed numerical simulations of the diffusion process
computationally. While the general problem depends upon both the field
structure and particle energy, the analysis may be greatly simplified by
dimensionless analysis. That is, for a specified purely turbulent field, the
analysis depends almost exclusively on a single parameter -- the ratio of the
maximum wavelength of the turbulent field cells to the particle gyration
radius. For turbulent magnetic fluctuations superimposed over an underlying
uniform magnetic field, particle diffusion depends on a second dimensionless
parameter that characterizes the ratio of the turbulent to uniform magnetic
field energy densities. We consider both of these possibilities and parametrize
our results to provide simple quantitative expressions that suitably
characterize the diffusion process within molecular cloud environments. Doing
so, we find that the simple scaling laws often invoked by the high-energy
astrophysics community to model cosmic-ray diffusion through such regions
appear to be fairly robust for the case of a uniform magnetic field with a
strong turbulent component, but are only valid up to TeV particle
energies for a purely turbulent field. These results have important
consequences for the analysis of cosmic-ray processes based on TeV emission
spectra associated with dense molecular clouds.Comment: Accepted for publication in The Astrophysical Journa
Computer Components and Systems
Contains reports on three research projects.United States Navy, Bureau of Ships (Contract NObsr 77603
- …