1,606 research outputs found
On behavior strategy solutions in finite extended decision processes
Techniques for finding best behavior strategies on arbitrary information collection scheme
A view of PKS 2155-304 with XMM-Newton Reflection Grating Spectrometers
We present the high resolution X-ray spectrum of the BL Lac object PKS
2155-304 taken with the RGS units onboard XMM-Newton in November 2000. We
detect a OVII Kalpha resonant absorption line from warm/hot local gas at 21.59A
(~4.5 sigma detection). The line profile is possibly double peaked. We do not
confirm the strong 20.02 A absorption line seen with Chandra and interpreted as
z~0.05 OVIII Kalpha. A 3sigma upper limit of 14 mA on the equivalent width is
set. We also detect the ~23.5 A interstellar OI 1s-->2p line and derive a
factor <=1.5 subsolar O/H ratio in the ISM along PKS 2155-304 line of sight.Comment: 13 pages, 5 figures, 3 tables, emulateapj style. Accepted by Ap
Determining X-Ray Source Intensity and Confidence Bounds in Crowded Fields
We present a rigorous description of the general problem of aperture
photometry in high energy astrophysics photon-count images, in which the
statistical noise model is Poisson, not Gaussian. We compute the full posterior
probability density function for the expected source intensity for various
cases of interest, including the important cases in which both source and
background apertures contain contributions from the source, and when multiple
source apertures partially overlap. A Bayesian approach offers the advantages
that it allows one to (a) include explicit prior information on source
intensities, (b) propagate posterior distributions as priors for future
observations, and (c) use Poisson likelihoods, making the treatment valid in
the low counts regime. Elements of this approach have been implemented in the
Chandra Source Catalog.Comment: 32 pages, 12 figures. Fixed typos and incorrect statement at end of
4.2.3. Results unchange
On behavior strategy solutions of finite two- person constant-sum extended games
Recall-sensitivity and behavior strategy solutions in finite two-person constant-sum extended game
Deposing the Cool Corona of KPD 0005+5106
The ROSAT PSPC pulse height spectrum of the peculiar He-rich hot white dwarf
KPD 0005+5106 provided a great surprise when first analysed by Fleming, Werner
& Barstow (1993). It defied the best non-LTE modelling attempts in terms of
photospheric emission from He-dominated atmospheres including C, N and O and
was instead interpreted as the first evidence for a coronal plasma around a
white dwarf. We show here that a recent high resolution Chandra LETGS spectrum
has more structure than expected from a thermal bremsstrahlung continuum and
lacks the narrow lines of H-like and He-like C expected from a coronal plasma.
Moreover, a coronal model requires a total luminosity more than two orders of
magnitude larger than that of the star itself. Instead, the observed 20-80 AA
flux is consistent with photospheric models containing trace amounts of heavier
elements such as Fe. The soft X-ray flux is highly sensitive to the adopted
metal abundance and provides a metal abundance diagnostic. The weak X-ray
emission at 1 keV announced by O'Dwyer et al (2003) instead cannot arise from
the photosphere and requires alternative explanations. We echo earlier
speculation that such emission arises in a shocked wind. Despite the presence
of UV-optical O VIII lines from transitions between levels n=7-10, no X-ray O
VIII Ly alpha flux is detected. We show that O VIII Lyman photons can be
trapped by resonant scattering within the emitting plasma and destroyed by
photoelectric absorption.Comment: 15 Pages, 4 figures. Accepted for the Astrophysical Journa
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