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

The Role of White Dwarfs in Cataclysmic Variable Spin-down

We study the effect of a white dwarf on the spin-down of a cataclysmic variable system using a three-dimensional magnetohydrodynamic numerical model. The model includes the stellar corona, the stellar wind, and the WD mass and magnetic field. The existence of the WD modifies the system spin-down by physically blocking the stellar wind, restructuring the wind, channeling the wind towards the WD surface, and by modifying the shape and size of the Alfv\'en surface. The combination of these processes differs among a set of simple test cases, and the resulting angular momentum loss rates vary by factors of 2-3, and by factors of two relative to a test model with a single M dwarf. While the model employs some simplifications, the results suggest angular momentum loss schemes currently employed in cataclysmic variable studies do not require drastic revision. Insights are also gained on wind accretion. We find that efficient accretion switches on quite rapidly with decreasing orbital separation. Accretion rates depend on magnetic field alignment and should be modulated by magnetic cycles on the M dwarf. For particular values of white dwarf magnetic field strength, an efficient syphoning of coronal plasma from the inward facing M dwarf hemisphere occurs. Wind accretion rates are expected to vary by factors of 10 or more between fairly similar close binaries, depending on magnetic field strengths and orbital separation.Comment: 7 pages, 3 figures, 1 table, accepted to ApJ Letter

Coronal Structure and Abundances in Young Fast Rotators

AB Dor, Speedy Mic and Rst137B are in their early post-T Tauri evolutionary phase (<100Myr), at the age of fastest rotation in the life of late-type stars. They straddle the coronal saturation-supersaturation boundary first defined by young stars in open clusters. High resolution Chandra X-ray spectra have been analysed to study their coronal properties as a function of coronal activity parameters Rossby number, $L_X/L_{bol}$ and a coronal temperature index. Plasma emission measure distributions as a function of temperature show broad peaks at T~10e7K. Differences between stars suggest that as supersaturation is reached the DEM slope below the temperature of peak DEM becomes shallower, while the DEM drop-off above this temperature becomes more pronounced. A larger sample comprising our three targets and 22 active stars studied in the recent literature reveals a general increase of plasma at T>10e7 toward the saturated-supersaturated boundary but a decline beyond this among supersaturated stars. All three of the stars studied in detail here show lower coronal abundances of the low FIP elements Mg, Si and Fe, relative to the high FIP elements S, O and Ne, as compared to the solar mixture. The coronal Fe abundances of the stellar sample are inversely correlated with Lx/Lbol, declining slowly with rising Lx/Lbol, but with a much more sharp decline at Lx/Lbol>3x10e-4. For dwarfs the Fe abundance is also well-correlated with Rossby number. The coronal O/Fe ratios for dwarfs show a clear increase with decreasing Rossby number, apparently reaching saturation at [O/Fe]=0.5 at the coronal supersaturation boundary. Similar increases in O/Fe with increasing coronal temperature and $L_X/L_{bol}$ are seen.Comment: 22 pages, 8 figures, 6 tables. Accepted by Ap

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

RHESSI Observations of the Solar Flare Iron-line Feature at 6.7 keV

Analysis of RHESSI 3--10 keV spectra for 27 solar flares is reported. This energy range includes thermal free--free and free--bound continuum and two line features, at 6.7keV and 8keV, principally due to highly ionized iron (Fe). We used the continuum and the flux in the so-called Fe-line feature at 6.7keV to derive the electron temperature T_e, the emission measure, and the Fe-line equivalent width as functions of time in each flare. The Fe/H abundance ratio in each flare is derived from the Fe-line equivalent width as a function of T_e. To minimize instrumental problems with high count rates and effects associated with multi-temperature and nonthermal spectral components, spectra are presented mostly during the flare decay phase, when the emission measure and temperature were smoothly varying. We found flare Fe/H abundance ratios that are consistent with the coronal abundance of Fe (i.e. 4 times the photospheric abundance) to within 20% for at least 17 of the 27 flares; for 7 flares, the Fe/H abundance ratio is possibly higher by up to a factor of 2. We find evidence that the Fe XXV ion fractions are less than the theoretically predicted values by up to 60% at T_e=25 MK appear to be displaced from the most recent theoretical values by between 1 and 3 MK.Comment: To be published, Ap