John C. Raymond

HST-STIS Observations of the Cygnus Loop: Spatial Structure of a Non-radiative Shock

We present a spatially resolved ultraviolet spectrum of a nonradiative shock front in the Cygnus Loop, obtained with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. The spectrum covers the wavelength range 1118–1716 A with an effective spectral resolution of ~12 A. The 01 spatial resolution of these data provides a huge improvement over earlier ultraviolet spectra, allowing us to study the spatial distribution of high-ionization line emissions directly behind the shock front. We are able to isolate individual shock features in our spectrum by comparing the STIS spectrum with a Wide Field Planetary Camera 2 Hα image of the region. Isolating the brightest shock tangency, we identify lines of N V λ1240, C IV λ1549, He II λ1640, O V λ1371, O IV], Si IV λ1400, and N IV] λ1486, as well as the hydrogen two-photon continuum. The N V line peaks ~03 behind the C IV and He II emission and is spatially broader than the other emissions. Also, the observed line ratios of C IV and He II to N V are higher in our bright-shock spectrum than in previous observations of the same filament obtained through much larger apertures (and little or no spatial resolution), indicating that there must be a more widely distributed component of the N V emission. We calculate shock models and show that the observed separation between the C IV and N V emission zones and observed line intensities constrain the combinations of shock velocity and preshock density that are allowed.

ATOMIC X-RAY SPECTRA OF ACCRETION DISK ATMOSPHERES IN THE KERR METRIC

We calculate the atmospheric structure of an accretion disk around a Kerr black hole and obtain its X-ray spectrum, which exhibits prominent atomic transitions under certain circumstances. The gravitational and Doppler (red)shifts of the C V, C VI, O VII, O VIII, and Fe I-XXVI emission lines are observable in active galaxies. We quantify the line emissivities as a function of radius, to identify the effects of atmospheric structure, and to determine the usefulness of these lines for probing the disk energetics. The line emissivities do not always scale linearly with the incident radiative energy, as in the case of Fe XXV and Fe XXVI. Our model incorporates photoionization and thermal balance for the plasma, the hydrostatic approximation perpendicular to the plane of the disk, and general relativistic tidal forces. We include radiative recombination rates, fluorescence yields, Compton scattering, and photoelectric opacities for the most abundant elements.

The EUV emission of cataclysmic variables

Approximately half the luminosity of a typical cataclysmic variable may emerge as an optically thick component peaking in the EUV. Observations of this component are important for understanding the energetics and accretion rates of CVs in general, as well as for understanding the physics of the accretion process. The nature of the turbulent boundary layers and winds of disk accretors and the heating of the white dwarfs by accretion are among the problems which can be addressed by observations in the EUV. 46 refs., 2 figs.