1,854 research outputs found
The Power of Jets: New Clues from Radio Circular Polarization and X-rays
Jets are ubiquitous in accreting black holes. Often ignored, they may be a
major contributor to the emitted spectral energy distribution for sub-Eddington
black holes. For example, recent observations of radio-to-X-ray correlations
and broad band spectra of X-ray binaries in the low/hard state can be explained
by a significant synchrotron contribution from jets also to their IR-to-X-ray
spectrum as proposed by Markoff, Falcke, Fender 2001. This model can also
explain state-transitions from low/hard to high/soft states. Relativistic
beaming of the jet X-ray emission could lead to the appearance of seemingly
Super-Eddington X-rays sources in other galaxies. We show that a simple
population synthesis model of X-ray binaries with relativistic beaming can well
explain the currently found distribution of off-nucleus X-ray point sources in
nearby galaxies. Specifically we suggest that the so-called ultra-luminous
X-ray sources (ULXs, also IXOs) could well be relativistically beamed
microblazars. The same model that can be used to explain X-ray binaries also
fits Low-Luminosity AGN (LLAGN) and especially Sgr A* in the Galactic Center.
The recent detection of significant circular polarization in AGN radio cores,
ranging from bright quasars down to low-luminosity AGN like M81*, Sgr A* and
even X-ray binaries, now places additional new constraints on the matter
contents of such jets. The emerging picture are powerful jets with a mix of hot
and cold matter, a net magnetic flux, and a stable magnetic north pole.Comment: to appear in: ``Lighthouses of the Universe'', Springer Verlag, ESO
Astrophysics Symposia, Eds: R.Sunyaev, M.Gilfanov, E.Churazov, LaTex, 8
pages, 5 figures, also available at
http://www.mpifr-bonn.mpg.de/staff/hfalcke/publications.html#lighthouse
Explaining the entropy excess in clusters and groups of galaxies without additional heating
The X-ray luminosity and temperature of clusters and groups of galaxies do
not scale in a self-similar manner. This has often been interpreted as a sign
that the intracluster medium has been substantially heated by non-gravitational
sources. In this paper, we propose a simple model which, instead, uses the
properties of galaxy formation to explain the observations. Drawing on
available observations, we show that there is evidence that the efficiency of
galaxy formation was higher in groups than in clusters. If confirmed, this
would deplete the low-entropy gas in groups, increase their central entropy and
decrease their X-ray luminosity. A simple, empirical, hydrostatic model appears
to match both the luminosity-temperature relation of clusters and properties of
their internal structure as well.Comment: 5 pages, 4 figures, accepted in ApJL; added one reference, otherwise
unchange
PROGRESS IN PHOTOVOLTAIC COMPONENTS AND SYSTEMS
ABSTRACT The Photovoltaic Manufacturing Research and Development project is a governmenthidustry partnership between the U.S. Department of Energy and members of the U.S. photovoltaic (TV) industry. The purpose of the project is to work with industry to improve manufacturing processes. reduce manufacturing costs. and improve the performance of PV products. This project is conducted through phased solicitations with industry participants selected through a competitive evaluation process. Starting in 1995, the two most recent solicitations rnclude manufacturing improvements for balance-ofsystem (J30S) components, energy storage, and PV system design improvements. This paper surveys the wwrk accomplished since that time, as well as BOS work currently in progress in the PV Manufacturing R&D project to identi& areas of continued interest and product trends. Industry participants continue to work to improve inverters and to expand the features and capabilities of this key mmponent. The industry also continues to advance fully integrated systems that meet standards for performance and safety. All pruticipants included manm%cturing improvements to reduce costs and improve reliability. Accomplishments of the project's participants are summarized to illustrate tbe product and manufacturing trends
Evolution of X-ray cluster scaling relations in simulations with radiative cooling and non-gravitational heating
We investigate the redshift dependence of X-ray cluster scaling relations
drawn from three hydrodynamic simulations of the LCDM cosmology: a Radiative
model that incorporates radiative cooling of the gas, a Preheating model that
additionally heats the gas uniformly at high redshift, and a Feedback model
that self-consistently heats cold gas in proportion to its local star-formation
rate. While all three models are capable of reproducing the observed local
Lx-Tx relation, they predict substantially different results at high redshift
(to z=1.5), with the Radiative, Preheating and Feedback models predicting
strongly positive, mildly positive and mildly negative evolution, respectively.
The physical explanation for these differences lies in the structure of the
intracluster medium. All three models predict significant temperature
fluctuations at any given radius due to the presence of cool subclumps and, in
the case of the Feedback simulation, reheated gas. The mean gas temperature
lies above the dynamical temperature of the halo for all models at z=0, but
differs between models at higher redshift with the Radiative model having the
lowest mean gas temperature at z=1.5.
We have not attempted to model the scaling relations in a manner that mimics
the observational selection effects, nor has a consistent observational picture
yet emerged. Nevertheless, evolution of the scaling relations promises to be a
powerful probe of the physics of entropy generation in clusters. First
indications are that early, widespread heating is favored over an extended
period of heating that is associated with galaxy formation.Comment: Accepted for publication in ApJ. Minor changes following referee's
comment
A major radio outburst in III Zw 2 with an extremely inverted, millimeter-peaked spectrum
III Zw 2 is a spiral galaxy with an optical spectrum and faint extended radio
structure typical of a Seyfert galaxy, but also with an extremely variable,
blazar-like radio core. We have now discovered a new radio flare where the
source has brightened more than twenty-fold within less than two years. A
broad-band radio spectrum between 1.4 and 666 GHz shows a textbook-like
synchrotron spectrum peaking at 43 GHz, with a self-absorbed synchrotron
spectral index +2.5 at frequencies below 43 GHz and an optically thin spectral
index -0.75 at frequencies above 43 GHz. The outburst spectrum can be well
fitted by two homogenous, spherical components with equipartition sizes of 0.1
and 0.2 pc at 43 and 15 GHz, and with magnetic fields of 0.4 and 1 Gauss. VLBA
observations at 43 GHz confirm this double structure and these sizes. Time
scale arguments suggest that the emitting regions are shocks which are
continuously accelerating particles. This could be explained by a frustrated
jet scenario with very compact hotspots. Similar millimeter-peaked spectrum
(MPS) sources could have escaped our attention because of their low flux
density at typical survey frequencies and their strong variability.Comment: ApJ Letters, in press, (AAS)LaTeX, 3 figures, available at
http://www2.mpifr-bonn.mpg.de/staff/hfalcke/publications.html#iiizw2 or in a
few weeks at
http://www.mpifr-bonn.mpg.de/staff/falcke/publications.html#iiizw
Cooling Flow Star Formation and the Apparent Stellar Ages of Elliptical Galaxies
Observational constraints and theoretical arguments indicate that cooled
interstellar gas in bright elliptical galaxies forms into a young stellar
population throughout the region within the half-light radius. The young
population has a bottom-heavy, but optically luminous IMF extending to 1 - 2
M_sun. When the colors and spectral features of this young population are
combined with those of the underlying old stellar population, the apparent ages
are significantly reduced, similar to the relatively young apparent ages
observed in many ellipticals. Galactic mergers are not required to resupply
young stars. The sensitivity of continuous star formation to L_B and L_x/L_B is
likely to account for the observed spread in apparent ages among elliptical
galaxies. Local star formation is accompanied by enhanced stellar H_beta
equivalent widths, stronger optical emission lines, enhanced thermal X-ray
emission and lower apparent temperatures in the hot gas. The young stars should
cause M/L to vary with galactic radius, perturbing the fundamental plane
occupied by the old stars.Comment: 6 pages with 2 figures; accepted by Astrophysical Journal Letter
Outcomes of carotid artery stenting versus historical surgical controls for radiation-induced carotid stenosis
PurposeTo evaluate the outcomes of carotid artery stenting (CAS) and open surgical repair (OR) for treatment of radiation-induced carotid stenosis (RICS).MethodsWe retrospectively reviewed 60 patients treated for 73 RICSs from a group of 5,824 patients who had carotid interventions between 1992 and 2009. Thirty-three patients (37 arteries) were treated with CAS and 27 patients (36 arteries) with OR. CAS was performed using embolic protection as part of a prospective institutional registry since 2003. End-points included mortality, stroke, myocardial infarction (MI), cranial nerve injury (CNI), wound complication, restenosis, and reintervention.ResultsDemographics and cardiovascular risk factors were similar in both groups, with the exception of higher rates (P < .05) of hyperlipidemia (81% vs 56%) and coronary artery disease (63% vs 33%) in OR patients. There were more patients with tracheostomy (31% vs 4%) and time interval from irradiation to intervention was longer in the CAS group. There were no early deaths. At 30 days, OR was associated with one (3%) stroke, two (5.5%) MIs, six (17%) CNIs, and three (8%) wound complications. OR patients with prior radical neck dissections had more wound complications (14% vs 5%) and CNIs (28% vs 9%) compared with those without neck dissections. In the CAS group, there were two (6%) strokes and no MIs, CNIs, or wound complications. Mean length of hospital stay was longer after OR than CAS (4.1 ± 3.7 days vs 2.4 ± 2.1 days; P = .02). Median follow-up was 58 months. At 7 years, OR was associated with higher patient survival (75% ± 15% vs 29% ± 13%, P = .008) and freedom from neurological events (100% vs 57% ± 9.5%, P = .058), but similar freedom from restenosis (80% ± 10% vs 72% ± 9%) and reinterventions (87% ± 10% vs 86% ± 9%) compared with CAS.ConclusionCarotid artery stenting for radiation-induced stenosis has the advantages of no CNI or wound complications with similar early stroke rate compared with open carotid repair. However, the lower freedom from neurological events may offset the early benefits of carotid stenting in patients who are considered good candidates for open surgery
Systematic techniques for assisting recruitment to trials (START): study protocol for embedded, randomized controlled trials
BACKGROUND: Randomized controlled trials play a central role in evidence-based practice, but recruitment of participants, and retention of them once in the trial, is challenging. Moreover, there is a dearth of evidence that research teams can use to inform the development of their recruitment and retention strategies. As with other healthcare initiatives, the fairest test of the effectiveness of a recruitment strategy is a trial comparing alternatives, which for recruitment would mean embedding a recruitment trial within an ongoing host trial. Systematic reviews indicate that such studies are rare. Embedded trials are largely delivered in an ad hoc way, with interventions almost always developed in isolation and tested in the context of a single host trial, limiting their ability to contribute to a body of evidence with regard to a single recruitment intervention and to researchers working in different contexts. METHODS/DESIGN: The Systematic Techniques for Assisting Recruitment to Trials (START) program is funded by the United Kingdom Medical Research Council (MRC) Methodology Research Programme to support the routine adoption of embedded trials to test standardized recruitment interventions across ongoing host trials. To achieve this aim, the program involves three interrelated work packages: (1) methodology - to develop guidelines for the design, analysis and reporting of embedded recruitment studies; (2) interventions - to develop effective and useful recruitment interventions; and (3) implementation - to recruit host trials and test interventions through embedded studies. DISCUSSION: Successful completion of the START program will provide a model for a platform for the wider trials community to use to evaluate recruitment interventions or, potentially, other types of intervention linked to trial conduct. It will also increase the evidence base for two types of recruitment intervention. TRIAL REGISTRATION: The START protocol covers the methodology for embedded trials. Each embedded trial is registered separately or as a substudy of the host trial
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