2,519 research outputs found
On behavior strategy solutions in finite extended decision processes
Techniques for finding best behavior strategies on arbitrary information collection scheme
On the Detectability of Oxygen X-ray Fluorescence and its Use as a Solar Photospheric Abundance Diagnostic
Monte Carlo calculations of the O Kalpha line fluoresced by coronal X-rays
and emitted just above the temperature minimum region of the solar atmosphere
have been employed to investigate the use of this feature as an abundance
diagnostic. While quite weak, we estimate line equivalent widths in the range
0.02-0.2 AA, depending on the X-ray plasma temperature. The line remains
essentially uncontaminated by blends for coronal temperatures T =< 3e6 K and
should be quite observable, with a flux >~ 2 ph/s/arcmin^2. Model calculations
for solar chemical mixtures with an O abundance adjusted up and down by a
factor of 2 indicate 35-60% changes in O Kalpha line equivalent width,
providing a potentially useful O abundance diagnostic. Sensitivity of
equivalent width to differences between recently recommended chemical
compositions with ``high'' and ``low'' complements of the CNO trio important
for interpreting helioseismological observations is less accute, amounting to
20-26% at coronal temperatures T ~< 2e6 K. While still feasible for
discriminating between these two mixtures, uncertainties in measured line
equivalent widths and in the models used for interpretation would need to be
significantly less than 20%. Provided a sensitive X-ray spectrometer with
resolving power >= 1000 and suitably well-behaved instrumental profile can be
built, X-ray fluorescence presents a viable means for resolving the solar
``oxygen crisis''.Comment: To appear in the Astrophysical Journa
X-raying the coronae of HD~155555
We present an analysis of the high-resolution Chandra observation of the
multiple system, HD 155555 (an RS CVn type binary system, HD 155555 AB, and its
spatially resolved low-mass companion HD 155555 C). This is an intriguing
system which shows properties of both an active pre-main sequence star and a
synchronised (main sequence) binary. We obtain the emission measure
distribution, temperature structures, plasma densities, and abundances of this
system and compare them with the coronal properties of other young/active
stars. HD 155555 AB and HD 155555 C produce copious X-ray emission with log Lx
of 30.54 and 29.30, respectively, in the 0.3-6.0 keV energy band. The light
curves of individual stars show variability on timescales of few minutes to
hours. We analyse the dispersed spectra and reconstruct the emission measure
distribution using spectral line analysis. The resulting elemental abundances
exhibit inverse first ionisation potential effect in both cases. An analysis of
He-like triplets yields a range of coronal electron densities ~10^10-10^13
cm-3. Since HD 155555 AB is classified both as an RS CVn and a PMS star, we
compare our results with those of other slightly older active main-sequence
stars and T Tauri stars, which indicates that the coronal properties of HD
155555 AB closely resemble that of an older RS CVn binary rather than a younger
PMS star. Our results also suggests that the properties of HD 155555 C is very
similar to those of other active M dwarfs.Comment: 17 pages, 23 figues, Accepted in Ap
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
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