Wilton T. Sanders

A High Spectral Resolution Observation of the Soft X-Ray Diffuse Background with Thermal Detectors

A high spectral resolution observation of the diffuse X-ray background in the 60–1000 eV energy range has been made using an array of 36 1 mm 2 microcalorimeters flown on a sounding rocket. Detector energy resolution ranged from 5 to 12 eV FWHM, and a composite spectrum of � 1 sr of the background centered at l ¼ 90 � , b ¼þ 60 � was obtained with a net resolution of � 9 eV. The target area includes bright 1 keV regions but avoids Loop I and the North Polar Spur. Lines of C vi ,O vii, and O viii are clearly detected with intensities of 5:4 � 2:3, 4:8 � 0:8, and 1:6 � 0:4 photons cm � 2 s � 1 sr � 1 , respectively. The oxygen lines alone account for a majority of the diffuse background observed in the ROSAT R4 band that is not due to resolved extragalactic discrete sources. We also have a positive detection of the Fe-M line complex near 70 eV at an intensity consistent with previous upper limits that indicate substantial gas-phase depletion of iron. We include a detailed description of the instrument and its detectors. Subject headings: instrumentation: detectors — instrumentation: spectrographs — intergalactic medium — space vehicles: instruments — X-rays: diffuse background — X-rays: ISM

The soft X-ray diffuse background

Maps of the diffuse X-ray background intensity covering essentially the entire sky with approx.7/sup 0/ spatial resolution are presented for seven energy bands: 130--188 eV (B band), 160--284 eV (Cband), 440--930 eV (M/sub 1/ band), 600--1100 eV (M/sub 2/ band), 770--1500 eV (I band), 1100--2200 eV (J band), and 1800--6300 eV (2--6 keV band). The data were obtained on a series of 10 sounding rocket flights conducted over a 7-year period. We have attempted to make the absolute intensities in these maps quantitatively accessible while still giving a clear qualitative display of the spatial features. The different nature of the spatial distributions in different bands implies at least three distinct origins for the diffuse X-rays, none of which is well understood. At energies > or approx. =2000 eV, an isotropic and presumably extragalactic spectrum of uncertain origin dominates. Between 500 and 1000 eV, an origin which is at least partially galactic seems called for. At energies <284 eV, the observed intensity is anticorrelated with neutral hydrogen column density, but we find it unlikely that this anticorrelation is simply due to absorption of an extragalactic or halo source.

A model for the distribution of material generating the soft X-ray background

The observational evidence relating to the soft X-ray diffuse background is discussed, and a simple model for its source and spatial structure is presented. In this simple model with one free parameter, the observed 1/4 keV X-ray intensity originates as thermal emission from a uniform hot plasma filling a cavity in the neutral material of the Galactic disk which contains the sun. Variations in the observed X-ray intensity are due to variations in the extent of the emission volume and therefore the emission measure of the plasma. The model reproduces the observed negative correlation between X-ray intensity and H I column density and predicts reasonable values for interstellar medium parameters. 64 refs.

Spectra of the ¼ keV X-Ray Diffuse Background from the Diffuse X-Ray Spectrometer Experiment

The Diffuse X-ray Spectrometer (DXS) flew as an attached payload on the STS-54 mission of the space shuttle Endeavour in 1993 January and obtained spectra of the soft X-ray diffuse background in the 148-284 eV (84-44 ?) band using a Bragg-crystal spectrometer. The spectra show strong emission lines, indicating that the emission is primarily thermal. Since the observations were made at low Galactic latitude, this thermal emission must arise from a nearby hot component of the interstellar medium, most likely the Local Hot Bubble, a region within ~100 pc of the Sun characterized by an absence of dense neutral gas. The DXS spectrum of the hot interstellar medium is not consistent with either collisional equilibrium models or with nonequilibrium ionization models of the X-ray emission from astrophysical plasmas. Models of X-ray emission processes appear not yet adequate for detailed interpretation of these data. The DXS data are most nearly consistent with models of thermal emission from a plasma with a temperature of 106.1 K and depletions of refractory elements magnesium, silicon, and iron to levels ~30% of solar.

Observations of the soft X-ray diffuse background at 0.1 keV

The results of a sounding rocket flight that observed the very soft X-ray diffuse background in a section of the northern Galactic hemisphere are presented. The ratio of the measured Be band count rates to the Wisconsin sky survey B band count rates is nearly constant over a 120 deg arc on the sky, strongly suggesting a common emission mechanism for both Be and B band X-rays. The mean free path is about seven times smaller for the Be band than for the B band. The present results imply that the neutral hydrogen column density between the emitting material and the earth varies by less than 2 x 10 to the 18th H I/sq cm over the observed region. It is concluded that such variations are unlikely unless the total intervening column density is less than a few times this value.

Ultraviolet Interstellar Polarization of Galactic Starlight.I.Observations by the Wisconsin Ultraviolet Photo Polarimeter Experiment

The Wisconsin Ultraviolet Photo Polarimeter Experiment (WUPPE) ew twice as part of NASAs Astro Spacelab missions in 1990 December and 1995 March. A systematic survey of the interstellar polarization in the ultraviolet was one of the main projects on both ights. The program was carefully crafted to 1) sample the galactic plane as uniformly as practicable, 2) explore sight lines of diverse chemical composition and morphology, and 3) measure the shape of the UV polarization through the full range of known wavelengths of peak polarization in the optical. We present here Astro-2 data for 20 previously unobserved sight lines and combine these with previously published UV data and with optical observations from the University of Wisconsins ground based facilities and elsewhere. We thus have spectropolarimetry from 1500 A to 10,000 A for 35 galactic objects in which the polarization appears to be dominated by the interstellar component. The extrapolation of the empirical Serkowski formula based only on visual data does not provide a reliable representation of the UV polarization. We nd that there are substantial diierences in the amount of UV polarization relative to that in the visual. This may indicate that the small and large aligned grain populations are somewhat independent and this may in turn provide a new diagnostic of varying conditions in the interstellar medium. Finally there are several cases in which the UV polarization is enhanced in the 2000 A to 3000 A range.

The Ultraviolet Spectrum of a Face-on Shock Wave in the Vela Supernova Remnant

Ultraviolet emission lines in supernova remnants (SNRs) provide important clues to the shock velocities, densities, grain destruction, and thermal structure of the cooling regions in these objects. However, several of the brightest ultraviolet emission lines seen in SNRs, including C II ?1335, C IV ?1550, and O VI ?1034, are resonance lines whose brightnesses will be reduced by scattering if the column density along the line of sight is sufficiently large. Scattering is particularly important in the bright filaments of SNRs because these shock waves are seen nearly edge-on. To assess the importance of resonant scattering in the UV, we have taken advantage of the enormous diffuse source sensitivity of the Hopkins Ultraviolet Telescope to observe the UV emission from a face-on shock in the Vela supernova remnant. The position chosen is seen in projection near the center of the remnant and corresponds to a bright soft X-ray region. A 165 km s-1 shock with a 30% carbon depletion matches most of the line intensities, but the weakness of the N V ?1240 line may suggest a departure from the simple shock models. We compare the spectrum of the face-on shock to that of a nearby bright filament and find some differences in shock velocity as well as in resonance-line scattering. We also derive the ram pressure of the shock from Fabry-Perot observations, and find that it greatly exceeds the thermal pressure derived from the [S II] doublet. This result strongly suggests pressure support by magnetic fields or cosmic rays.

Limits on the density of neutral gas within 100 parsecs from observations of the soft X-ray background

Observations are presented in two soft X-ray bands, the Be band (0.077-0.111 keV) and the B-prime (0.105-0.188 keV), for nine directions in the sky. The ratio of count rates in these two bands remains constant as the rates vary by a factor of three, even though the effective interstellar absorption cross sections in the bands differ by a factor of about 3.5. For a model in which the bulk of the observed soft X-ray emission originates in a uniform low-density region surrounding the sun, the constant ratio between the band rates places an upper limit on the amount of neutral material that can be homogeneously mixed with the X-ray-emitting gas. The 2 sigma upper limit on the H I column density over an average path through the local emitting region is 6.6 x 10 to the 18th/sq cm. If the average path length is about 100 pc, then clouds similar to the one in which the sun is embedded could still have a filling factor as large as 25 percent. 24 refs.

Simulation of soft X-ray emission lines from the missing baryons

We study the soft X-ray emission (0.1-1 keV) from the warm-hot intergalactic medium (WHIM) in a hydrodynamic simulation of a cold dark matter universe. Our main goal is to investigate how such emission can be explored with a combination of imaging and spectroscopy and to motivate future X-ray missions. We first present high-resolution images of the X-ray emission in several energy bands in which emission from different ion species dominates. We pick three different areas to study the high-resolution spectra of X-rays from the WHIM: (1) a galaxy group, (2) a filament, and (3) an underluminous region. By taking into account the background X-ray emission from AGNs and foreground emission from the Galaxy, we compute composite X-ray spectra of the selected regions. We briefly investigate angular clustering of the soft X-ray emission, finding a strong signal. Most interestingly, the combination of high spectral resolution and angular information allows us to map the emission from the WHIM in three dimensions. We cross-correlate the positions of galaxies in the simulation with this redshift map of emission and detect the presence of six different ion species (Ne IX, Fe XVII, O VII, O VIII, N VII, and C VI) in the large-scale structure traced by the galaxies. Finally, we show how such emission can be detected and studied with future X-ray satellites, with particular attention to a proposed mission, the Missing Baryon Explorer (MBE). We present simulated observations of the WHIM gas with MBE.

1/f Noise and Hot Electron Effects in Variable Range Hopping Conduction

In the course of developing microcalorimeters as detectors for astronomical X-ray spectroscopy, we have undertaken an empirical characterization of non-ideal effects in the doped semiconductor thermometers used with these detectors, which operate at temperatures near 50 mK. We have found three apparently independent categories of such behavior that are apparently intrinsic properties of the variable-range hopping conduction mechanism in these devices: 1/f fluctuations in the resistance, which seems to be a 2D effect; a departure from the ideal coulomb-gap temperature dependence of the resistance at temperatures below T 0 /24; and an electrical nonlinearity that has the time dependence and extra noise that are quantitatively predicted by a simple hot electron model. This work has been done largely with ion-implanted Si:P:B, but similar behaviors have been observed in transmutation doped germanium.