arXiv subjects

N. S. Brickhouse

Publications and source records attributed to N. S. Brickhouse.

33 records · Page 2Linked to original sources

arXiv2006

Soft X-ray emission lines of Fe XV in solar flare observations and the Chandra spectrum of Capella

Recent calculations of atomic data for Fe XV have been used to generate theoretical line ratios involving n = 3-4 transitions in the soft X-ray spectral region (52-83 A), for a wide range of electron temperatures and densities applicable to solar and stellar coronal plasmas. A comparison of these with solar flare observations from a rocket-borne spectrograph (XSST) reveals generally good agreement between theory and experiment. In particular, the 82.76 A emission line in the XSST spectrum is identified, for the first time to our knowledge in an astrophysical source. Most of the Fe XV transitions which are blended have had the species responsible clearly identified, although there remain a few instances where this has not been possible. The line ratio calculations are also compared with a co-added spectrum of Capella obtained with the Chandra satellite, which is probably the highest signal-to-noise observation achieved for a stellar source in the 25-175 A soft X-ray region. Good agreement is found between theory and experiment, indicating that the Fe XV lines are reliably detected in Chandra spectra, and hence may be employed as diagnostics to determine the temperature and/or density of the emitting plasma. However the line blending in the Chandra data is such that individual emission lines are difficult to measure accurately, and fluxes may only be reliably determined via detailed profile fitting of the observations. The co-added Capella spectrum is made available to hopefully encourage further exploration of the soft X-ray region in astronomical sources.

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arXiv2005

The Transparency of Solar Coronal Active Regions

Resonance scattering has often been invoked to explain the disagreement between the observed and predicted line ratios of Fe XVII 15.01 A to Fe XVII 15.26 A (the ``3C/3D'' ratio). In this process photons of 15.01, with its much higher oscillator strength, are preferentially scattered out of the line of sight, thus reducing the observed line ratio. Recent laboratory measurements, however, have found significant inner-shell Fe XVI lines at 15.21 and 15.26 Angstroms, suggesting that the observed 3C/3D ratio results from blending. Given our new understanding of the fundamental spectroscopy, we have re-examined the original solar spectra, identifying the Fe XVI 15.21 line and measuring its flux to account for the contribution of Fe XVI to the 15.26 flux. Deblending brings the 3C/3D ratio into good agreement with the experimental ratio; hence, we find no need to invoke resonance scattering. Low opacity in Fe XVII 15.01 also implies low opacity for Fe XV 284.2, ruling out resonance scattering as the cause of the fuzziness of TRACE and SOHO EIT 284-Angstrom images. The images must, instead, be unresolved due to the large number of structures at this temperature. Insignificant resonance scattering implies that future instruments with higher spatial resolution could resolve the active region plasma into its component loop structures.

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arXiv2005

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

Spectroscopy of the infrared He I (10830A) line with KECK/NIRSPEC and IRTF/CSHELL, and of the ultraviolet C III (977A) and O VI (1032A) 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), hot (~300,000 K) accelerating outflow with a mass loss rate \~10^-11 - 10^-12 solar masses per year, or larger. Spectra of T Tauri 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.

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arXiv2005

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 inflowing 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.

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arXiv2005

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, EUV, and FUV line emission, formed at the peak of Capella's (alpha Aurigae) 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, and the Extreme Ultraviolet Explorer. Some flux discrepancies larger than factors of two are found between observations of Fe XVIII and Fe XIX lines and predictions by APEC and other models in common usage. 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 10^7 K plasmas currently observed with Chandra, XMM-Newton, and FUSE.

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arXiv2004

Inferring coronal structure from X-ray lightcurves and Doppler shifts: a Chandra study of AB Doradus

The Chandra X-ray observatory monitored the single cool star, AB Doradus, continuously for a period lasting 88 ksec (1.98 Prot) in 2002 December with the LETG/HRC-S. The X-ray lightcurve 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 18.97 A, indicate Doppler rotational velocities with a semi-amplitude of 30 +/- 10 km/s. By taking these diagnostics into account along with constraints on the rotational broadening of line profiles (provided by archival Chandra HETG Fe XVII and FUSE Fe XVIII profile) 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 lightcurve 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 one of the other active regions may be located at 40 degrees. 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.

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arXiv2004

Opacity Variations in the Ionized Absorption in NGC 3783: A Compact Absorber

We show that the Fe (VII-XII) M-shell unresolved transition array (UTA) in the NGC 3783 900 ks Chandra HETGS observation clearly changes in opacity in 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 sigma. There is also evidence for variability in the O VI K edge (at 3 sigma level). The observed changes are consistent with the gas producing these absorption features (the low ionization component) being close to photoionization equilibrium. The gas responsible for the Fe (XVII-XXII) L-shell absorption (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 >1E4 cm-3, and so locates 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 by heavily clumped gas.

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arXiv2004

Inferring coronal structure using X-ray spectra: A Chandra study of AB Dor

The Chandra X-ray observatory monitored the single cool star, AB Doradus, continuously for a period lasting 88ksec (1.98 Prot) in 2002 December with the LETG/HRC-S. The X-ray lightcurve shows significant rotational modulation. It can be represented as having a flat level of emission superimposed with bright flaring regions that appear at the same phases in both rotation cycles. Phase-binned OVIII line profiles show centroid shifts that also repeat in consecutive rotation cycles. These Doppler shifts trace a roughly sinusoidal pattern with a a semi-amplitude of 30 +/-10km/s. By taking both the lightcurve and spectral diagnostics into account along with constraints on the rotational broadening of line profiles (provided by archival Chandra HETG FeXVII line profiles) we can construct a simple model of the X-ray corona. The corona can be described as having two components, one component is homogeneously distributed, extending less than 1.75R*; and the other consists of at least two compact emitting regions near the stellar surface. These compact regions account for 16% of the X-ray emission and are likely to be located less than 0.4R* above the stellar surface.

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arXiv2004

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

(Abridged) We investigate the ionized environment of the Seyfert 1 galaxy NGC 985 with a new Chandra-HETGS observation and an archival BeppoSAX observation. Two absorption components are clearly required to fit 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 (UTA) produced by charge states VII to 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 does not allow to constrain its properties. An X-ray luminosity variation by a factor 2.3 is observed between the BeppoSAX and Chandra observations (separated by almost 3 years). 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 multi-phase wind. Such scenario can explain the change in opacity of both absorption components during the observations, but requires that a third hotter component is pressure-confining the two phases. Hence, our analysis points to a 3-phase medium similar to the wind found in NGC 3783, and further suggests that such a wind might be a common characteristic in AGN.

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arXiv2004

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 ~15 km/s in the X-ray wavelength regime. We find an emission line velocity amplitude of 58.2+-3.7 km/s and infer a white dwarf mass of 0.49+-0.13 M_sun, in good agreement with previous studies using optical, ultraviolet, and far ultraviolet data.

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arXiv2003

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 ksec 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 (Gamma=1.53) and a soft black-body component (kT=10 keV). The spectrum contains over 100 features, which are well fit by PHASE with just six free parameters. The model consists of a simple two phase absorber with difference of 35 in the ionization parameter and 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 kms-1) an a single turbulent velocity (300 km s-1), and with solar elemental abundances. The main features of the low ionization phase are an Fe M-shell unresolved transition array (UTA) and the OVII lines. The OVII features, usualy identified with the OVIII and a warm absorber, are instead produced in a cooler medium also producing OVI 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 OVII 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. The low ionizaton phase can be decomposed into three subcomponents based on the outflow velocity, FWHM, and H column densities found for three out of the four UV absorbers detected in NGC 3783. However, the ionization parametes are systematically smaller in our model than derived from UV data, indicating a lower degree of ionization.

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arXiv2002

The Structure of Stellar Coronae in Active Binary Systems

A survey of 28 stars using EUV spectra has been conducted to establish the structure of stellar coronae in active binary systems from the EMD, electron densities, and scale sizes. Observations obtained by the EUVE during 9 years of operation are included for the stars in the sample. EUVE data allow a continuous EMD to be constructed in the range log T~5.6-7.4, using iron emission lines. These data are complemented with IUE observations to model the lower temperature range. Inspection of the EMD shows an outstanding narrow enhancement, or ``bump'' peaking around log T~6.9 in 25 of the stars, defining a fundamental coronal structure. The emission measure per unit stellar area decreases with increasing orbital (or photometric) periods of the target stars; stars in binaries generally have more material at coronal temperatures than slowly rotating single stars. High electron densities (Ne>10^12 cm^-3) are derived at ~10 MK for some targets, implying small emitting volumes. The observations suggest the magnetic stellar coronae of these stars are consistent with two basic classes of magnetic loops: solar-like loops with maximum temperature around log T~6.3 and lower electron densities (Ne>10^9-10.5), and hotter loops peaking around log T~6.9 with higher electron densities (Ne>10^12). For the most active stars, material exists at much higher temperatures (log T>6.9) as well. However, current ab initio stellar loop models cannot reproduce such a configuration. Analysis of the light curves of these systems reveals signatures of rotation of coronal material, as well as apparent seasonal changes in the activity levels.

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arXiv2001

X-ray Doppler Imaging of 44i Boo with Chandra

Chandra High-Energy Transmission Grating observations of the bright eclipsing contact binary 44i Boo show X-ray line profiles which are Doppler-shifted by orbital motions. The X-ray emission spectrum contains a multitude of lines superimposed on a weak continuum, with strong lines of O VIII, Ne X, Fe XVII, and Mg XII. The profiles of these lines from the total observed spectrum show Doppler-broadened widths of ~ 550 km s^{-1}. Line centroids vary with orbital phase, indicating velocity changes of > 180 km s^{-1}. The first-order light curve shows significant variability, but no clear evidence for either primary or secondary eclipses. Flares are observed for all spectral ranges; additionally, the light curve constructed near the peak of the emission measure distribution (T_e = 5 to 8 X 10^6 K) shows quiescent variability as well as flares. The phase-dependences of line profiles and light curves together imply that at least half of the emission is localized at high latitude. A simple model with two regions on the primary star at relatively high latitude reproduces the observed line profile shifts and quiescent light curve. These first clear X-ray Doppler shifts of stellar coronal material illustrate the power of Chandra.

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arXiv2000

High Resolution X-Ray Spectra of Capella: Initial Results from the Chandra High Energy Transmission Grating Spectrometer

High resolution spectra of the active binary Capella (G8 III + G1 III) covering the energy range 0.4-8.0 keV (1.5-30 Angstroms) show a large number of emission lines, demonstrating the performance of the HETGS. A preliminary application of plasma diagnostics provides information on coronal temperatures and densities. Lines arising from different elements in a range of ionization states indicate that Capella has plasma with a broad range of temperatures, from log T = 6.3 to 7.2, generally consistent with recent results from observations with the Extreme Ultraviolet Explorer (EUVE) and the Advanced Satellite for Cosmology and Astrophysics (ASCA). The electron density is determined from He-like O VII lines, giving the value N_e=10^10 cm^-3 at T_e=2*10^6 K; He-like lines formed at higher temperatures give only upper limits to the electron density. The density and emission measure from O VII lines together indicate that the coronal loops are significantly smaller than the stellar radius.

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arXiv1997

A SAX/LECS X-ray observation of the active binary Capella

We present a SAX/LECS X-ray observation of the active binary Capella, the first coronal source observed in the SAX Guest Investigator program. The analysis of this observation, performed using the mekal plasma emission code, shows that the LECS spectrum is well fit by a two-component optically-thin plasma model. A differential emission measure (DEM) obtained by direct inversion of the X-ray spectrum shows no additional features in addition to the double-peaked structure implied by the direct two-temperature analysis. Such a simple temperature stratification is however not compatible with the EUVE emission from the same object, which is well represented by a more complex DEM, with a power-law-like tail toward the low temperatures. At the same time, the EUVE-derived DEM predicts well the softer part of the Capella LECS spectrum, but it fails to correctly reproduce the higher energy part of the Capella LECS spectrum. Possible causes for this discrepancy are discussed. The coronal metallicity derived from the SAX observation is compatible both with the EUVE-derived metallicity as well as with the photospheric metallicity of Capella, thus showing no evidence for coronal under-abundances.

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