Kaori Nishikida

Chandra Spectra of the Prototype Dwarf Nova U Geminorum at Quiescence

We have observed the prototypical dwarf nova U Gem at quiescence with the Chandra medium- and high-resolution gratings to accomplish the first study of resolved X-ray emission lines in a disk-accreting cataclysmic variable. Doppler tomograms constructed from optical spectra obtained close in time to the X-ray observation show a typical quiescent disk structure with an irradiated secondary but no prominent disk hot spot. The unprecedented spectral resolution of Chandra over past X-ray telescopes reveals prominent narrow emission lines of O, Ne, Mg, Si, S, and Fe. The line fluxes, ratios, and widths indicate that X-ray emission arises from a range of temperatures in a high-density (greater than 1014 cm-3) gas, moving at low (less than 300 km s-1) velocity, with a small (less than 107 cm) scale height compared to the white dwarf radius. Simple models with cooling flows, cooling flows plus isothermal zones, and thermal conduction give reasonable agreement with the low-temperature emission lines, but a good fit to the entire range of lines will require a better understanding of all the parameters that affect the boundary layer.

Molecular Hydrogen Fluorescence in the Eridanus Superbubble

The first far-ultraviolet (1350-1750 A) spectral imaging observations of the Eridanus superbubble, obtained with the SPEAR/FIMS mission, have revealed distinct fluorescent emission from molecular hydrogen. Here we compare the observed emission features with those from a photodissociation region model with assumed illuminating stellar fields. The result shows rather high line ratios I1580/I1610, which may imply the existence of high-temperature molecular clouds in the region. The H2 fluorescence intensity shows a proportional correlation with Hα emission, indicating that the fluorescence and the recombination emission have similar physical origins.

Far-Ultraviolet Observations of the North Ecliptic Pole with SPEAR

We present SPEAR/FIMS far-ultraviolet observations near the north ecliptic pole. This area, at b ~ 30° and with intermediate H I column, seems to be a fairly typical line of sight that is representative of general processes in the diffuse interstellar medium. We detect a surprising number of emission lines of many elements at various ionization states, representing gas phases from the warm neutral medium to the hot ionized medium. We also detect fluorescence bands of H2, which may be due to the ubiquitous diffuse H2 previously observed in absorption.

Optics development for the SPEAR mission

We describe the development of optics for the SPEAR space-mission to map the far ultraviolet (900-1750 Å) sky. The SPEAR spectrometers contain unusual reflective optics to optimize sensitivity to diffuse emission. We describe the manufacture, test and performance of the collecting mirrors: Pyrex parabolic cylinders with a 90 degree off-axis angle. We also describe the development of the diffraction gratings: ellipses of rotation that are holographically-ruled with constant spacing and blazed with ion-beam ablation.