Updated Atomic Data and Calculations for X-ray Spectroscopy

We describe the latest release of AtomDB, version 2.0.2, a database of atomic data and a plasma modeling code with a focus on X-ray astronomy. This release includes several major updates to the fundamental atomic structure and process data held within AtomDB, incorporating new ionization balance data, state-selective recombination data, and updated collisional excitation data for many ions, including the iron L-shell ions from Fe$^{+16}$ to Fe$^{+23}$ and all of the hydrogen- and helium-like sequences. We also describe some of the effects that these changes have on calculated emission and diagnostic line ratios, such as changes in the temperature implied by the He-like G-ratios of up to a factor of 2.Comment: Submitted to ApJ, 12 pages, 9 figure

Flares from small to large: X-ray spectroscopy of Proxima Centauri with XMM-Newton

(Abridged) We report results from a comprehensive study of the nearby M dwarf Proxima Centauri with the XMM-Newton satellite. We find strongly variable coronal X-ray emission, with flares ranging over a factor of 100 in peak flux. The low-level emission is found to be continuously variable. Several weak flares are characteristically preceded by an optical burst, compatible with predictions from standard solar flare models. We propose that the U band bursts are proxies for the elusive stellar non-thermal hard X-ray bursts suggested from solar observations. A very large X-ray flare was observed in its entirety, with a peak luminosity of 3.9E28 erg/s [0.15-10 keV] and a total X-ray energy of 1.5E32 erg. This flare has allowed to measure significant density variations from X-ray spectroscopy of the OVII He-like triplet; we find peak densities reaching up to 4E11 cm^-3 for plasma of about 1-5 MK. Abundance ratios show little variability in time, with a tendency of elements with a high first ionization potential to be overabundant relative to solar photospheric values. We do not find significant effects due to opacity during the flare, indicating that large opacity increases are not the rule even in extreme flares. We model the large flare in terms of an analytic 2-Ribbon flare model and find that the flaring loop system should have large characteristic sizes (~1R*). These results are supported by full hydrodynamic simulations. Comparing the large flare to flares of similar size occurring much more frequently on more active stars, we propose that the X-ray properties of active stars are a consequence of superimposed flares such as the example analyzed in this paper. Such a model also explains why, during episodes of low-level emission, more active stars show hotter plasma than less active stars.Comment: Accepted for A&A, 20 pages, 11 figures, 2 tables, preprint also at http://www.astro.phys.ethz.ch/papers/guedel/guedel_p_nf.htm

X-Ray Spectroscopy of Stars

(abridged) Non-degenerate stars of essentially all spectral classes are soft X-ray sources. Low-mass stars on the cooler part of the main sequence and their pre-main sequence predecessors define the dominant stellar population in the galaxy by number. Their X-ray spectra are reminiscent, in the broadest sense, of X-ray spectra from the solar corona. X-ray emission from cool stars is indeed ascribed to magnetically trapped hot gas analogous to the solar coronal plasma. Coronal structure, its thermal stratification and geometric extent can be interpreted based on various spectral diagnostics. New features have been identified in pre-main sequence stars; some of these may be related to accretion shocks on the stellar surface, fluorescence on circumstellar disks due to X-ray irradiation, or shock heating in stellar outflows. Massive, hot stars clearly dominate the interaction with the galactic interstellar medium: they are the main sources of ionizing radiation, mechanical energy and chemical enrichment in galaxies. High-energy emission permits to probe some of the most important processes at work in these stars, and put constraints on their most peculiar feature: the stellar wind. Here, we review recent advances in our understanding of cool and hot stars through the study of X-ray spectra, in particular high-resolution spectra now available from XMM-Newton and Chandra. We address issues related to coronal structure, flares, the composition of coronal plasma, X-ray production in accretion streams and outflows, X-rays from single OB-type stars, massive binaries, magnetic hot objects and evolved WR stars.Comment: accepted for Astron. Astrophys. Rev., 98 journal pages, 30 figures (partly multiple); some corrections made after proof stag

Spectroscopic Laboratory Astrophysics Experiments Conducted at the LLNL EBIT Facility in Support of NASA's X-ray Astronomy Flight Program

International audienceThe electron beam ion trap (EBIT) facility located at the Lawrence Livermore National Laboratory has been used for laboratory astrophysics experiments for over 15 years. During this time, several unique spectrometers and operating modes have been developed and implemented, including high resolution grating and crystal spectrometers, a high-resolution, high-efficiency NASA/GSFC microcalorimeter array, and the ability to operate and record datawith the electron beam turned off, i.e., in the so-called magnetic trapping mode. Targeted experiments conducted at this facility have addressed specific problems faced by the X-ray astrophysics community and have provided accurate, complete sets of atomic data such as relative and absolute excitation cross sections, transition wavelengths, line polarization, and X-ray signatures of charge exchange recombination. Here we will present a brief overview of our facility and some of the more recent results including 1/4 keV band X- ray emission produced by charge exchange between L-shell sulfur ions and neutral gas, wavelengths and relative intensities of satellite X- ray lines from Na-like Fe XVI and their contribution to the Fe XVII line emission, and the relative intensities of the 3s-2p/3d-2p lines in F-like Fe XVIII and Ni XX. Part of this work was performed under the auspices of the U. S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344 and is also supported by NASA grants to LLNL, GSFC, and Stanford University

Emission Line Spectra From Low-Density Laboratory Plasmas

Using spectroscopic equipment optimized for laboratory astrophysics, we are performing systematic measurements of the line emission from astrophysically relevant ions in the wavelength band between 1 and 400 A important to X-ray missions such as Chandra, XMM, Astro-E, and EUVE. Obtained in a controlled laboratory setting at electron densities similar to those found in stellar coronae, the data are used to test spectral modeling codes for accuracy and completeness. Our e#ort includes the compilation of the iron L-shell emission lines from 6--18 A and the iron M-shell emission lines from 50--200 A. Many lines have been identified for the first time, and the fluxes from lines missing in the spectral modeling codes are assessed. Our measurements also assess the accuracy of line excitation calculations, including direct electron-impact excitation, dielectronic recombination, and resonance excitation. These measurements yield a calibration of specific diagnostic line ratios. Examples of our current measurements are given