R. Hoogerwerf

Inferring coronal structure from X-ray light curves and doppler shifts: A Chandra study of ab doradus

The Chandra X-Ray Observatory continuously monitored the single cool star AB Dor for a period lasting 88 ks (1.98Prot) in 2002 December with the Low-Energy Transmission Grating HRC-S. The X-ray light curve shows rotational modulation with three peaks that repeat in two consecutive rotation cycles. These peaks may indicate the presence of compact emitting regions in the quiescent corona. Centroid shifts as a function of phase in the strongest line profile, O VIII λ18.97, indicate Doppler rotational velocities with a semiamplitude of 30 ± 10 km s-1. By taking these diagnostics into account along with constraints on the rotational broadening of line profiles (provided by archival Chandra High-Energy Transmission Grating Fe XVII and Far Ultraviolet Spectroscopic Explorer Fe XVIII profiles), we can construct a simple model of the X-ray corona that requires two components. One of these components is responsible for 80% of the X-ray emission and arises from the pole and/or a homogeneously distributed corona. The second component consists of two or three compact active regions that cause modulation in the light curve and contribute to the O VIII centroid shifts. These compact regions account for 16% of the emission and are located near the stellar surface with heights of less than 0.3R*. At least one of the compact active regions is located in the partially obscured hemisphere of the inclined star, while another of the active regions may be located at 40°. High-quality X-ray data such as these can test the models of the coronal magnetic field configuration as inferred from magnetic Zeeman Doppler imaging.

The Radial Velocity and Mass of the White Dwarf of EX Hydrae Measured with Chandra

We present the first detection of orbital motion in the cataclysmic variable EX Hydrae based on X-ray data from the Chandra X-Ray Observatory. The large collecting area of the telescope and the high resolution of the HETG spectrometers allow for an unprecedented velocity accuracy of {approx}km s{sup -1} in the X-ray wavelength regime. We find an emission line velocity amplitude of 58.2{+-}3.7 km s{sup -1} and infer a white dwarf mass of 0.49{+-}0.13 M{sub {circle_dot}}, in good agreement with previous studies using optical, ultraviolet, and far ultraviolet data.

An assessment of the fe XVIII and Fe XIX line ratios from the Chandra grating observations of capella

Observations of Fe XVIII and Fe XIX X-ray, extreme-UV, and far-UV line emission, formed at the peak of Capellas (α Aurigaes) emission measure distribution and ubiquitous in spectra of many cool stars and galaxies, provide a unique opportunity to test the robustness of Fe XVIII and Fe XIX spectral models. The Astrophysical Plasma Emission Code (APEC) is used to identify over 35 lines from these two ions alone, and to compare model predictions with spectra obtained with the Chandra Low Energy Transmission Grating and High Energy Transmission Grating Spectrometers, the Far Ultraviolet Spectroscopic Explorer (FUSE), and the Extreme Ultraviolet Explorer. Some flux discrepancies larger than a factor of 2 are found between observations of Fe XVIII and Fe XIX lines and predictions by APEC and other models in common use. In particular, the X-ray resonance lines for both ions are stronger than predicted by all models relative to the EUV resonance lines. The multiwavelength observations demonstrate the importance of including dielectronic recombination and proton-impact excitation, and of using accurate wavelengths in spectral codes. These ions provide important diagnostic tools for 107 K plasmas currently observed with Chandra, XMM-Newton, and FUSE.

X-ray Light Curves and Accretion Disk Structure of EX Hydrae

We present X-ray light curves for the cataclysmic variable EX Hydrae obtained with the Chandra High Energy Transmission Grating Spectrometer and the Extreme Ultraviolet Explorer Deep Survey photometer. We confirm earlier results on the shape and amplitude of the binary light curve and discuss a new feature: the phase of the minimum in the binary light curve, associated with absorption by the bulge on the accretion disk, increases with wavelength. We discuss several scenarios that could account for this trend and conclude that, most likely, the ionization state of the bulge gas is not constant, but rather decreases with binary phase. We also conclude that photoionization of the bulge by radiation originating from the white dwarf is not the main source of ionization, but that it is heated by shocks originating from the interaction between the in-flowing material from the companion and the accretion disk. The findings in this paper provide a strong test for accretion disk models in close binary systems.

The High-Excitation Planetary Nebula NGC 246. II. FUSE and Chandra Observations

This paper presents Far-Ultraviolet Spectroscopic Explorer (FUSE) medium-resolution slit and Chandra X-Ray Observatory ACIS-S observations of the high-excitation planetary nebula NGC 246. The Ne V λ1146 and [Ne V] λ3426 emission lines, the latter obtained in a previous investigation, indicate that the nebula is photoionized by the intense UV radiation of the central object and has a temperature of ~20,000 K. The O VI λ1032 and λ1037 emission lines are moderately optically thick and probably contain contributions from both collisional excitation and radiative scattering of the UV continuum of the central star. The Chandra observation did not detect any X-ray emission from the nebula, but did detect the central star. The absence of nebular X-ray emission and the photoionized nature of the nebula support the hypothesis that the nebula contains a biconical cavity, which extends to the edge of the nebula. In this geometry the stellar wind does not shock the nebular material, resulting only in a cool photoionized plasma. The X-ray spectrum of the central object is consistent with NLTE models for PG 1159-type stars.

Photoionized Features in the X-ray Spectrum of EX Hydrae

We present the first results from a long (496 ks) Chandra High Energy Transmission Grating observation of the intermediate polar EX Hydrae. In addition to the narrow emission lines from the cooling post-shock gas, for the first time we have detected a broad component in some of the X-ray emission lines, namely O VIII 18.97, Mg XII 8.42, Si XIV 6.18, and Fe XVII 16.78. The broad and narrow components have widths of ~ 1600 km s^-1 and ~ 150 km s^-1, respectively. We propose a scenario where the broad component is formed in the pre-shock accretion flow, photoionized by radiation from the post-shock flow. Because the photoionized region has to be close to the radiation source in order to produce strong photoionized emission lines from ions like O VIII, Fe XVII, Mg XII, and Si XIV, our photoionization model constrains the height of the standing shock above the white dwarf surface. Thus, the X-ray spectrum from EX Hya manifests features of both magnetic and non-magnetic cataclysmic variables.

Accretion Column Structure of Magnetic Cataclysmic Variables from X-Ray Spectroscopy

Using Chandra HETG data, we present light curves for individual spectral lines of Mg XI and Mg XII for EX Hydrae, an intermediate polar-type cataclysmic variable. The Mg XI light curve, folded on the white dwarf spin period, shows two spikes that are not seen in the Mg XII or broadband light curves. Occultation of the accretion column by the body of the white dwarf would produce such spikes for an angle between the rotation axis and the accretion columns of α = 18° and a height of the Mg XI emission above the white dwarf surface of 0.0004 white dwarf radii, or 4 km. The absence of spikes in the Mg XII and broadband light curves could then be explained if the bulk of its emission forms at much larger height, more than 0.004 white dwarf radii or over 40 km, above the white dwarf surface, although this is not consistent with the predictions of the standard Aizu model of the accretion column.

Progress and Plans for the Astrophysical Plasma Emission Code (APEC)

APEC models for collisional ionization equilibrium are now available. These basic models have been benchmarked against astrophysical coronal spectra and show reasonable agreement; however, several atomic data issues need to be addressed. We plan to conduct an atomic data needs assessment and set priorities over the next year. New tools are under development to broaden the applicability of APEC models. We discuss the status of this development.

Comparison of Fe XVIII and Fe XIX Line Emissions with Spectral Models

We discuss here the observations of Fe XVIII and XIX emission lines and compare the X‐ray, EUV and FUV lines with the spectral codes widely used today (e.g. FAC and APEC). We assess the relative accuracy of these spectral models and try to identify the critical atomic data and processes. Capella with a narrow enhancement in its emission measure distribution at 6 MK provides a unique opportunity to test the Fe XVIII and Fe XIX model emissivities which peak from 6 to 8 MK. We use the summed spectra from Chandra HETG/ACIS‐S and LETG/HRC‐S, as well as contemporaneous EUVE and FUSE observations, to measure line ratios for comparison with predictions.