Nancy S. Brickhouse

Collisional Plasma Models with APEC/APED: Emission-Line Diagnostics of Hydrogen-like and Helium-like Ions

New X-ray observatories (Chandra and XMM-Newton) are providing a wealth of high-resolution X-ray spectra in which hydrogen- and helium-like ions are usually strong features. We present results from a new collisional-radiative plasma code, the Astrophysical Plasma Emission Code (APEC), which uses atomic data in the companion Astrophysical Plasma Emission Database (APED) to calculate spectral models for hot plasmas. APED contains the requisite atomic data such as collisional and radiative rates, recombination cross sections, dielectronic recombination rates, and satellite line wavelengths. We compare the APEC results to other plasma codes for hydrogen- and helium-like diagnostics and test the sensitivity of our results to the number of levels included in the models. We find that dielectronic recombination with hydrogen-like ions into high (n = 6-10) principal quantum numbers affects some helium-like line ratios from low-lying (n = 2) transitions.

Toward a Self-Consistent Model of the Ionized Absorber in NGC 3783

We present a detailed model for the ionized absorbing gas evident in the 900 ks Chandra HETGS spectrum of NGC 3783. The analysis was carried out with PHASE, a new tool designed to model X-ray and UV absorption features in ionized plasmas. The 0.5-10 keV intrinsic continuum of the source is well represented by a single power law (Γ = 1.53) and a soft blackbody component (kT ~ 0.1 keV). The spectrum contains over 100 features, which are well fitted by PHASE with just six free parameters. The model consists of a simple two-phase absorber with a difference of ≈35 in the ionization parameter and a difference of ≈4 in the column density of the phases. The two absorption components turned out to be in pressure equilibrium and are consistent with a single outflow (≈750 km s-1), a single turbulent velocity (300 km s-1), and solar elemental abundances. The main features of the low-ionization phase are an Fe M-shell unresolved transition array (UTA) and the O VII lines. The O VII features, usually identified with the O VIII and a warm absorber, are instead produced in a cooler medium that also produces O VI lines. The UTA sets tight constraints on the ionization degree of the absorbers, making the model more reliable. The high-ionization phase is required by the O VIII and the Fe L-shell lines, and there is evidence for an even more ionized component in the spectrum. A continuous range of ionization parameters is disfavored by the fits, particularly to the UTA. Our model indicates a severe blending of the absorption and emission lines, as well as strong saturation of the most intense O absorption lines. This is in agreement with the O VII (τλ = 0.33) and O VIII (τλ = 0.13) absorption edges required to fit the spectrum. The low-ionization phase can be decomposed into three subcomponents on the basis of the outflow velocity, FWHM, and H column densities found for three of the four UV absorbers detected in NGC 3783. However, the ionization parameters are systematically smaller in our model than those derived from UV data, indicating a lower degree of ionization. Finally, our model predicts a Ca XVI line for the feature observed at around 21.6 A (a feature formerly identified as O VII), constraining the contribution from a zero-redshift absorber.

New model of iron spectra in the extreme ultraviolet and application to SERTS and EUV observations: A solar active region and capella

We report new predictions for the EUV spectral emission of FeIX-FeXXIV, based on data now available from the Solar EUV Rocket Telescope and Spectrograph (SERTS) and the Extreme Ultraviolet Explorer (EUVE) spectrometers. The iron spectral emission model is the first result of a larger effort to revise the Raymond & Smith model and to update the atomic rates. We present here predicted emissivities for selected densities and temperatures applicable to various astrophysical plasmas. Comparisons of our predicted spectra with two recent observations provide important tests of the atomic data. They also test to some extent some basic assumptions of coronal emission codes: optically thin spectral lines and ionization equilibrium.

The extreme ultraviolet spectrum of Alpha Aurigae (Capella)

Extreme ultraviolet spectra (λλ 70-740) of the bright spectroscopic binary system, Capella (Alpha Aurigae) obtained with the Extreme Ultraviolet Explorer satellite (EUVE), show a rich emission spectrum dominated by iron emission lines: Fe XV-XXIV. The emission measure for the system reveals a continuous distribution of plasma temperatures between 10 5 and 10 7.8 K, with a clear minimum near 10 6 K and a local maximum at 6×10 6 K. Electron density diagnostics based on Fe XXI indicate N e ≃4×10 11 -10 13 cm 3 at T e =10 7 K

Enhanced Noble Gases in the Coronae of Active Stars

We have analyzed Chandra High-Energy Transmission Grating spectra of the active RS CVn‐type binary V711 Tauri (HR 1099; HD 22468) in order to examine the chemical composition of its coronae. Observed fluxes and flux upper limits for spectral lines from a range of charge states of iron, covering species Fe xvi‐Fe xxv, have been used to determine the emission measure distribution as a function of temperature, while the observed Fe line-to-continuum ratio has been used to examine the absolute iron abundance, Fe/H. Abundances of elements O, Ne, Mg, Si, S, and Ar relative to both Fe and H have been estimated by comparison of observed line fluxes with predictions based on the emission measure distribution. We confirm results of earlier studies finding the coronae of V711 Tau to be metal-poor and derive an iron abundance of . We find the noble Fe/H p 7.0 5 0.1 gas elements Ne and Ar to be enhanced relative to the local cosmic value and enhanced by an order of magnitude relative to Fe. Very mild enhancements of O and Mg relative to Fe are also discerned. By examination of coronal abundances of Ne relative to Fe culled from the literature, in addition to Ne lines seen in hitherto unpublished Chandra spectra, we conclude that large Ne abundance enhancements are a common feature of active stellar coronae.

A Hot Wind from the Classical T Tauri Stars: TW Hydrae and T Tauri

Spectroscopy of the infrared He I (10830 A) line with NIRSPEC on Keck and CSHELL at the IRTF, and of the ultraviolet C III (977 A) and O VI (1032 A) emission with FUSE, reveals that the classical T Tauri star TW Hydrae exhibits P Cygni profiles, line asymmetries, and absorption indicative of a continuous, fast (~400 km s-1), hot (~300,000 K) accelerating outflow with a mass-loss rate ~10-11 to 10-12 M☉ yr-1 or larger. Spectra of T Tau N appear consistent with such a wind. The source of the emission and outflow seems restricted to the stars themselves. Although the mass accretion rate is an order of magnitude less for TW Hya than for T Tau, the outflow reaches higher velocities at chromospheric temperatures in TW Hya. Winds from young stellar objects may be substantially hotter and faster than previously thought.

Modeling the Ne IX Triplet Spectral Region of Capella with the Chandra and XMM-Newton Gratings

High-resolution X-ray spectroscopy with the diffraction gratings of Chandra and XMM-Newton offers new chances to study a large variety of stellar coronal phenomena. A popular X-ray calibration target is Capella, which has been observed with all gratings with significant exposure times. We gathered together all available data of the High Energy Transmission Grating Spectrometer (HETGS; 155 ks), Low Energy Transmission Grating Spectrometer (LETGS; 219 ks), and Reflection Grating Spectrometer (RGS; 53 ks) for comparative analysis, focusing on the Ne IX triplet at around 13.5 A, a region that is severely blended by strong iron lines. We identify 18 emission lines in this region of the High-Energy Grating (HEG) spectrum, including many from Fe XIX, and find good agreement with predictions from a theoretical model constructed using the Astrophysical Plasma Emission Code. The model uses an emission measure distribution derived from Fe XV to Fe XXIV lines. The success of the model is due in part to the inclusion of accurate wavelengths from laboratory measurements. While these 18 emission lines cannot be isolated in the LETGS or RGS spectra, their wavelengths and fluxes as measured with HEG are consistent with the lower resolution spectra. In the Capella model for HEG, the weak intercombination line of Ne IX is significantly blended by iron lines, which contribute about half the flux. After accounting for blending in the He-like diagnostic lines, we find the density to be consistent with the low-density limit (ne < 2 × 1010 cm-3); however, the electron temperature indicated by the Ne IX G-ratio is surprisingly low (~2 MK) compared to the peak of the emission measure distribution (~6 MK). Models show that the Ne IX triplet is less blended in cooler plasmas and in plasmas with an enhanced neon-to-iron abundance ratio.

The Ionized Nuclear Environment in NGC 985 as seen by Chandra and BeppoSAX

We investigate the ionized environment of the Seyfert 1 galaxy NGC 985 with a new Chandra HETGS observation and an archival BeppoSAX observation. Both spectra exhibit strong residuals to a single-power-law model, indicating the presence of an ionized absorber and a soft excess. A detailed model over the Chandra data shows that the 0.6-8 keV intrinsic continuum can be well represented by a power law (Γ ≈ 1.6) plus a blackbody component (kT = 0.1 keV). Two absorption components are clearly required to fit the absorption features observed in the Chandra spectrum. The components have a difference of 29 in ionization parameter and 3 in column density. The presence of the low-ionization component is evidenced by an Fe M-shell unresolved transition array produced by charge states VII-XIII. The high-ionization phase is required by the presence of broad absorption features arising from several blends of Fe L-shell transitions (Fe XVII-XXII). A third highly ionized component might also be present, but the data do not allow us to constrain its properties. Although poorly constrained, the outflow velocities of the components (581 ± 206 km s-1 for the high-ionization phase and 197 ± 184 km s-1 for the low-ionization one) are consistent with each other and with the outflow velocities of the absorption components observed in the UV. In addition, the low-ionization component produces significant amounts of O VI, N V, and C IV, which suggests that a single outflow produces the UV and X-ray features. The broadband (0.1-100 keV) continuum in the BeppoSAX data can be parameterized by a power law (Γ ≈ 1.4), a blackbody (kT = 0.1 keV), and a high-energy cutoff (Ec ≈ 70 keV). An X-ray luminosity variation by a factor of 2.3 is observed between the BeppoSAX and Chandra observations (separated by almost 3 yr). Variability in the opacity of the absorbers is detected in response to the continuum variation, but while the colder component is consistent with a simple picture of photoionization equilibrium, the ionization state of the hotter component seems to increase, while the continuum flux drops. The most striking result in our analysis is that during both the Chandra and the BeppoSAX observations, the two absorbing components appear to have the same pressure. Thus, we suggest that the absorption arises from a multiphase wind. Such a scenario can explain the change in the opacity of both absorption components during the observations, but it requires that a third, hotter component be pressure-confining the two phases. Hence, our analysis points to a three-phase medium similar to the wind found in NGC 3783, and it further suggests that such a wind might be a common characteristic in active galactic nuclei. The pressure-confining scenario requires fragmentation of the confined phases into a large number of clouds.

The coronal structure of AB Doradus determined from contemporaneous Doppler imaging and X-ray spectroscopy

We obtain contemporaneous observations of the surface and corona of AB Doradus (AB Dor), a young single cool star, using ground-based circularly polarized spectra from the AngloAustralian Telescope and X-ray light curves and spectra from the Chandra satellite. The ground-based data are used to construct surface magnetic field maps, which are extrapolated to produce detailed models of the quiescent corona. The X-ray data serve as a new test for the validity of these coronal models. We find that AB Dor’s X-ray corona must be concentrated close to its surface, with a height, H ∼ 0.3‐0.4R∗; this height is determined by the high coronal density and complex multipolar magnetic field from the surface maps. There is also significant correlation between the positions of surface and coronal active longitudes as determined from the surface spot and magnetic field maps and the X-ray light curve. At this epoch (2002 December) AB Dor appears to possess one very large active longitude region, covering almost half the star; displaying enhanced activity in the form of large dark spots, strong magnetic fields and chromospheric emission. This is unusual as previous surface maps of AB Dor typically display more active regions that span a wider range of longitudes. Finally, the level of rotational modulation and shape of the X-ray light curve depend on the distribution of magnetic field in the obscured hemisphere (AB Dor is inclined by 60 ◦ ). The models that best reproduce the rotational modulation observed in the contemporaneous Chandra X-ray light curve and spectra require the magnetic field in the obscured hemisphere to be of the same polarity as that in the observed hemisphere. The Sun shows different behaviour, with the leading polarity reversed in the opposite hemisphere. The X-ray observations provide a unique constraint on the magnetic structure in the obscured hemisphere.

OPACITY VARIATIONS IN THE IONIZED ABSORPTION IN NGC 3783: A COMPACT ABSORBER

We show that the Fe VII-Fe XII M-shell unresolved transition array (UTA) in the Chandra HETGS observation of NGC 3783 (900 ks) clearly changes in opacity on a timescale of 31 days, responding to a factor of ~2 change in the ionizing continuum. The opacity variation is observed at a level >10 σ. There is also evidence for variability in the O VI K edge (at ~3 σ). The observed changes are consistent with the gas producing these absorption features (i.e., the low-ionization component) being close to photoionization equilibrium. The gas responsible for the Fe XVII-Fe XXII L-shell absorption (i.e., the high-ionization component) does not seem to be responding as expected in photoionization equilibrium. The observed change in opacity for the UTA implies a density >1 × 104 cm-3, thus locating the gas within 6 pc of the X-ray source. The scenario in which the gas is composed of a continuous radial range of ionization structures is ruled out, as in such scenario, no opacity variations are expected. Rather, the structure of the absorber is likely composed of heavily clumped gas.