681 research outputs found
COMPTEL observations of the quasar PKS 0528+134 during the first 3.5 years of the CGRO mission
The COMPTEL observations of the blazar-type quasar PKS 0528+134 in the energy
range 0.75 MeV to 30 MeV carried out between April 1991 and September 1994 have
been analyzed. During the first two years PKS 0528+134 was most significantly
detected at energies above 3 MeV. During the last year there is only evidence
for the quasar at energies below 3 MeV indicating a spectral change. The
time-averaged COMPTEL energy spectrum between 0.75 MeV and 30 MeV is well
represented by a power-law shape. Spectra collected from different
observational periods reveal different power-law shapes: a hard state during
flaring observations reported by EGRET, and a soft state otherwise. The
combined simultaneous EGRET and COMPTEL spectra indicate these two spectral
states as well. During low intensisty gamma-ray phases no spectral break is
obvious from the combined COMPTEL and EGRET measurements. For the gamma-ray
flaring phases however, the combined COMPTEL and EGRET data require a spectral
bending at MeV-energies. By fitting broken power-law functions the best-fit
values for the break in photon index range between 0.6 and 1.7, and for the
break energy between ~5 MeV and ~20 MeV. Because the flux values measured by
COMPTEL below 3 MeV in both states are roughly equal, the observations would be
consistent with an additional spectral component showing up during gamma-ray
flaring phases of PKS 0528+134. Such a component could be introduced by e.g. a
high-energy electron-positron population with a low-energy cutoff in their bulk
Lorentz factor distribution. The multiwavelength spectrum of PKS 0528+134 for
gamma-ray flaring phases shows that the major energy release across the entire
electro-magnetic spectrum is measured at MeV-energies.Comment: 10 pages, 8 postscript figures, latex, to appear in: A&A 328, 33
(1997
The Spectral Variability of Cygnus X-1 at MeV Energies
In previous work, we have used data from the first three years of the CGRO
mission to assemble a broad-band -ray spectrum of the galactic black
hole candidate Cygnus X-1. Contemporaneous data from the COMPTEL, OSSE and
BATSE experiments on CGRO were selected on the basis of the hard X-ray flux
(45--140 keV) as measured by BATSE. This provided a spectrum of Cygnus X-1 in
its canonical low X-ray state (as measured at energies below 10 keV), covering
the energy range from 50 keV to 5 MeV. Here we report on a comparison of this
spectrum to a COMPTEL-OSSE spectrum collected during a high X-ray state of
Cygnus X-1 (May, 1996). These data provide evidence for significant spectral
variability at energies above 1 MeV. In particular, whereas the hard X-ray flux
{\it decreases} during the high X-ray state, the flux at energies above 1 MeV
{\it increases}, resulting in a significantly harder high energy spectrum. This
behavior is consistent with the general picture of galactic black hole
candidates having two distinct spectral forms at soft -ray energies.
These data extend this picture, for the first time, to energies above 1 MeV.Comment: 5 pages, 4 figures, to be published in AIP Conf. Proc., "The Fifth
Compton Symposium
Images archivĂ©es, images dâarchives : fortunes terminologiques
On ne peut manquer dâĂȘtre frappĂ© de la quasi-totale absence dans lâinstitution archivistique française et dans les milieux professionnels qui lâaniment, du moins jusquâĂ une date rĂ©cente, non seulement de lâexpression « image dâarchives » mais encore du mot mĂȘme « image ». Cette constatation constitue le point de dĂ©part dâune enquĂȘte terminologique et lexicologique : dâoĂč vient cette expression « images dâarchives » ? Quelles sont les Ă©tapes et les raisons de sa diffusionÂ
Recent results from COMPTEL observations of Cygnus Xâ1
The COMPTEL experiment on the Compton GammaâRay Observatory (CGRO) has now observed Cyg Xâ1 on four separate occasions during phase 1 and phase 2 of its orbital mission (April, 1991 to August, 1993). Here we report on the results of the latest analysis of these data, which provide a spectrum extending to energies greater than 2 MeV. A spectral analysis of these data, in the context of a classical Comptonization model, indicates an electron temperature much higher than previous hard Xâray measurements would suggest (200 keV vs 60â80 keV). This implies either some limitations in the standard Comptonization model and/or the need to incorporate a reflected component in the hard Xâray spectrum. Although significant variability near 1 MeV has been observed, there is no evidence for any âMeV excess.
Images du quotidien et patrimoines sociaux : la mise en images dâarchives
Lâobjet du chapitre est de faire Ă©merger des Ă©lĂ©ments de convergence quant aux Ă©tapes de la mise en images dâarchives. Dâabord, la conservation. Que lâon se situe dans une dialectique du passĂ© Ă travers des images anciennes ou que lâon ait affaire au dĂ©sir de transmission dâimages rĂ©centes, lâimpĂ©ratif de la pĂ©rennisation joue et lâinjonction des archives est Ă lâĆuvre. Ensuite, la monstration. Que lâimage originelle soit intĂ©grĂ©e au dĂ©cor dâun restaurant ou partagĂ©e dans une communautĂ© virtuelle, il nâest guĂšre dâimage dâarchives que vue et offerte au regard. Le statut dâimage dâarchives est confĂ©rĂ© par un regard extĂ©rieur, en dehors du cercle de production et de diffusion initial â la famille par exemple. Enfin, la modification. Lâimage dâarchives ne se conçoit que mĂ©diatisĂ©e, insĂ©rĂ©e dans un discours et presque toujours modifiĂ©e : dĂ©coupĂ©e, agrandie, teintĂ©e, dĂ©tournĂ©e de son objet initial, dĂ©cryptĂ©e, câest vĂ©ritablement une autre image qui est produite Ă partir de lâimage primaire. L\u27image crĂ©Ă©e et mise en scĂšne joue sur des codes convenus, dans une allusion explicite ou plus discrĂšte, Ă ceux des patrimoines savants. Les procĂ©dĂ©s de mĂ©diation co-construisent l\u27image d\u27archives parce qu\u27ils la mettent en exposition et ainsi dĂ©clenchent son accrĂ©ditation comme objet patrimonial. Le statut d\u27image d\u27archives ne serait donc que transitoire, valable uniquement au moment et dans lâinstant de son usage, contrairement Ă celui de document dâarchives ou dâimage archivĂ©e, dĂ©fini par un lieu de conservation fixe et des modalitĂ©s dâaccĂšs stables
Implementation and evaluation of the VA DPP clinical demonstration: protocol for a multi-site non-randomized hybrid effectiveness-implementation type III trial.
BackgroundThe Diabetes Prevention Program (DPP) study showed that lifestyle intervention resulted in a 58% reduction in incidence of type 2 diabetes among individuals with prediabetes. Additional large randomized controlled trials have confirmed these results, and long-term follow-up has shown sustained benefit 10-20 years after the interventions ended. Diabetes is a common and costly disease, especially among Veterans, and despite strong evidence supporting the feasibility of type 2 diabetes prevention, the DPP has not been widely implemented. The first aim of this study will evaluate implementation of the Veterans Affairs (VA) DPP in three VA medical centers. The second aim will assess weight and hemoglobin A1c (A1c) outcomes, and the third aim will determine the cost-effectiveness and budget impact of implementation of the VA DPP from a health system perspective.Methods/designThis partnered multi-site non-randomized systematic assignment study will use a highly pragmatic hybrid effectiveness-implementation type III mixed methods study design. The implementation and administration of the VA DPP will be funded by clinical operations while the evaluation of the VA DPP will be funded by research grants. Seven hundred twenty eligible Veterans will be systematically assigned to the VA DPP clinical demonstration or the usual care VA MOVE!Âź weight management program. A multi-phase formative evaluation of the VA DPP implementation will be conducted. A theoretical program change model will be used to guide the implementation process and assess applicability and feasibility of the DPP for VA. The Consolidated Framework for Implementation Research (CFIR) will be used to guide qualitative data collection, analysis, and interpretation of barriers and facilitators to implementation. The RE-AIM framework will be used to assess Reach, Effectiveness, Adoption, Implementation, and Maintenance of the VA DPP. Twelve-month weight and A1c change will be evaluated for the VA DPP compared to the VA MOVE!ProgramMediation analyses will be conducted to identify whether program design differences impact outcomes.DiscussionFindings from this pragmatic evaluation will be highly applicable to practitioners who are tasked with implementing the DPP in clinical settings. In addition, findings will determine the effectiveness and cost-effectiveness of the VA DPP in the Veteran population
Unveiling the Active Nucleus of Centaurus A
We report new HST WFPC2 and NICMOS observations of the center of the nearest
radio galaxy Centaurus A (NGC 5128) and discuss their implications for our
understanding of the active nucleus and jet. We detect the active nucleus in
the near-IR (K and H) and, for the first time, in the optical (I and V),
deriving the spectral energy distribution of the nucleus from the radio to
X-rays. The optical and part of the near-IR emission can be explained by the
extrapolation of the X-ray power law reddened by A_V~14mag, a value consistent
with other independent estimates.
The 20pc-scale nuclear disk discovered by Schreier et al. (1998) is detected
in the [FeII] 1.64mic line and presents a morphology similar to that observed
in Pa alpha with a [FeII]/Pa alpha ratio typical of low ionization Seyfert
galaxies and LINERs. NICMOS 3 Pa alpha observations in a 50"x50" circumnuclear
region suggest enhanced star formation (~0.3Msun/yr) at the edges of the
putative bar seen with ISO, perhaps due to shocks driven into the gas.
The light profile, reconstructed from V, H and K observations, shows that
Centaurus A has a core profile with a resolved break at ~4" and suggests a
black--hole mass of ~10^9 Msun. A linear blue structure aligned with the
radio/X-ray jet may indicate a channel of relatively low reddening in which
dust has been swept away by the jet.Comment: 19 pages, 13 figures, Astrophysical Journal, in press. High quality
figures available at http://www.arcetri.astro.it/~marconi/colpic.htm
The COMPTEL instrumental line background
The instrumental line background of the Compton telescope COMPTEL onboard the
Compton Gamma-Ray Observatory is due to the activation and/or decay of many
isotopes. The major components of this background can be attributed to eight
individual isotopes, namely 2D, 22Na, 24Na, 28Al, 40K, 52Mn, 57Ni, and 208Tl.
The identification of instrumental lines with specific isotopes is based on the
line energies as well as on the variation of the event rate with time,
cosmic-ray intensity, and deposited radiation dose during passages through the
South-Atlantic Anomaly. The characteristic variation of the event rate due to a
specific isotope depends on its life-time, orbital parameters such as the
altitude of the satellite above Earth, and the solar cycle. A detailed
understanding of the background contributions from instrumental lines is
crucial at MeV energies for measuring the cosmic diffuse gamma-ray background
and for observing gamma-ray line emission in the interstellar medium or from
supernovae and their remnants. Procedures to determine the event rate from each
background isotope are described, and their average activity in spacecraft
materials over the first seven years of the mission is estimated.Comment: accepted for publication in A&A, 22 pages, 21 figure
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