In collisional ionization equilibrium (CIE), the X-ray spectrum from a plasma depends on the distribution of emission measure over temperature (DEM). Due to the well-known ill conditioning problem, no precisely resolved DEM can be inverted directly f
rom the spectrum, so often only a gross parametrization of the DEM is used to approximate the data, in hopes that the parametrization can provide useful model-independent constraints on the heating process. However, ill conditioning also introduces ambiguity into the various different parametrizations that could approximate the data, which may spoil the perceived advantages of model independence. Thus, this paper instead suggests a single parametrization for both the heating mechanism and the X-ray sources, based on a model of impulsive heating followed by complete cooling. This approach is similar to a ``cooling flow approach, but allows injection at multiple initial temperatures, and applies even when the steady state is distribution of different shock strengths, as for a standing shock with a range of obliquities, or for embedded stochastic shocks that are only steady in a statistical sense. This produces an alternative parametrization for X-ray spectra that is especially streamlined for higher density plasmas with efficient radiative cooling, and provides internal consistency checks on the assumption of impulsive heating followed by complete cooling. The result is no longer model independent, but the results are more directly interpretable in terms of useful physical constraints on the impulsive heating distribution.
We present the first high-resolution X-ray spectrum of a putatively single Wolf-Rayet star. 400 ks observations of WR 6 by the XMM-Newton-telescope resulted in a superb quality high-resolution X-ray spectrum. Spectral analysis reveals that the X-rays
originate far out in the stellar wind, more than 30 stellar radii from the photosphere, and thus outside the wind acceleration zone where the line-driving instability could create shocks. The X-ray emitting plasma reaches temperatures up to 50,MK, and is embedded within the un-shocked, cool stellar wind as revealed by characteristic spectral signatures. We detect a fluorescent Fe line at approx 6.4 keV. The presence of fluorescence is consistent with a two-component medium, where the cool wind is permeated with the hot X-ray emitting plasma. The wind must have a very porous structure to allow the observed amount of X-rays to escape. We find that neither the line-driving instability nor any alternative binary scenario can explain the data. We suggest a scenario where X-rays are produced when the fast wind rams into slow sticky clumps that resist acceleration. Our new data show that the X-rays in single WR-star are generated by some special mechanism different from the one operating in the O-star winds.
During the past decade there have been several attempts to detect cosmogenic ultra high energy (UHE) neutrinos by searching for radio Cerenkov bursts resulting from charged impact showers in terrestrial ice or the lunar regolith. So far these radio s
earches have yielded no detections, but the inferred flux upper limits have started to constrain physical models for UHE neutrino generation. For searches which use the Moon as a target, we summarize the physics of the interaction, properties of the resulting Cerenkov radio pulse, detection statistics, effective aperture scaling laws, and derivation of upper limits for isotropic and point source models. We report on initial results from the RESUN search, which uses the Expanded Very Large Array configured in multiple sub-arrays of four antennas at 1.45 GHz pointing along the lunar limb. We detected no pulses of lunar origin during 45 observing hours. This implies upper limits to the differential neutrino flux E^2 dN/dE < 0.003 EeV km^{-2} s^{-1} sr^{-1} and < 0.0003 EeV km$^{-2} s^{-1} at 90% confidence level for isotropic and sampled point sources respectively, in the neutrino energy range 10^{21.6} < E(eV) < 10^{22.6}. The isotropic flux limit is comparable to the lowest published upper limits for lunar searches. The full RESUN search, with an additional 200 hours observing time and an improved data acquisition scheme, will be be an order of magnitude more sensitive in the energy range 10^{21} < E(eV) < 10^{22} than previous lunar-target searches, and will test Z burst models of neutrino generation.
We calculate the circularly polarized Stokes V profile for emission lines, formed in hot-star winds threaded with a weak radial magnetic field. For simplicity, the field is treated as a split monopole under the assumptions that it has been radially c
ombed by the wind, and rotation is not playing a central role. Invoking the weak-field approximation, we find that the V profile has a characteristic ``heartbeat shape exhibiting multiple sign
By considering the advection and interaction of the vector momentum flux in highly supersonic spherically diverging winds, we derive a simple analytic description of the asymptotic opening angle of a wind-collision shock cone, in the approximation th
at the shocked gas is contained in a cone streaming out along a single characteristic opening angle. Both highly radiative and highly adiabatic limits are treated, and their comparison is the novel result. Analytic closed-form expressions are obtained for the inferred wind momentum ratios as a function of the observed shock opening angle, allowing the conspicuous shape of the asymptotic bow shock to be used as a preliminary constraint on more detailed modeling of the colliding winds. In the process, we explore from a general perspective the limitations in applying to the global shock geometry the so-called Dyson approximation, which asserts a local balance in the perpendicular ram pressure across the shock.
We derive analytic expressions, and approximate them in closed form, for the effective detection aperture for Cerenkov radio emission from ultra-high-energy neutrinos striking the Moon. The resulting apertures are in good agreement with recent Monte
Carlo simulations and support the conclusion of James & Protheroe (2009)that neutrino flux upper limits derived from the GLUE search (Gorham et al.2004) were too low by an order of magnitude. We also use our analytic expressions to derive scaling laws for the aperture as a function of observational and lunar parameters. We find that at low frequencies downward-directed neutrinos always dominate, but at higher frequencies, the contribution from upward-directed neutrinos becomes increasingly important, especially at low neutrino energies. Detecting neutrinos from Earth near the GZK regime will likely require radio telescope arrays with extremely large collecting area and hundreds of hour of exposure time. Higher energy neutrinos are most easily detected using lower frequencies. Lunar surface roughness is a decisive factor for obtaining detections at higher frequencies and higher energies.
To better understand Wolf-Rayet stars as progenitors of gamma-ray bursts, an understanding of the effect metallicity has on Wolf-Rayet mass loss is needed. Using simple analytic models, we study the Mdot - Z relation of a WN star and compare the resu
lts to similar models. We find that Mdot roughly follows a power law in Z with index 0.88 from -2.5 < log Z/Z_sun < -1 and appears to flatten by log Z/Z_sun ~ -0.5.
