Showing papers by "Steven L. Snowden published in 1996"
TL;DR: In this article, the authors derived upper limits on the diffuse emission in terms of upper limits to the average intensity of a putative hot halo, which can also be used to derived limits to total energy radiated by hot gas in the observed galaxies as a function of its temperature for various assumed absorbing geometries.
Abstract: We observed several nearby face-on spiral galaxies with the ROSAT PSPC to study their 0.1-2.0 keV diffuse emission. After the exclusion of resolved discrete sources, there is unresolved X-ray emission in all the galaxies observed. Since this emission is a combination of diffuse emission and a contribution from unresolved point sources, it represents an upper limit to the truly diffuse soft X-ray emission.
The derived upper limits on the diffuse emission can be interpreted in terms of upper limits to the average intensity of a putative hot halo. They can also be used to derived limits to the total energy radiated by hot gas in the observed galaxies as a function of its temperature for various assumed absorbing geometries. Beyond the equivalent solar radius (the radius at which the Sun would be in the observed galaxies), the temperature of hot gas radiating more than 30% of the total supernova power in the galaxies must be less than $10^{6.1} K$ if it is located within the disk with an assumed absorbing overburden of $3\times 10^{20} cm^{-2}$, or less than $10^{5.9} K$ if it lies in an unabsorbed halo.
18 citations
TL;DR: In this paper, the authors derived upper limits on the diffuse emission in terms of upper limits to the average intensity of a putative hot halo, which can also be used to derived limits to total energy radiated by hot gas in the observed galaxies as a function of its temperature for various assumed absorbing geometries.
Abstract: We observed several nearby face-on spiral galaxies with the ROSAT PSPC to study their 0.1-2.0 keV diffuse emission. After the exclusion of resolved discrete sources, there is unresolved X-ray emission in all the galaxies observed. Since this emission is a combination of diffuse emission and a contribution from unresolved point sources, it represents an upper limit to the truly diffuse soft X-ray emission.
The derived upper limits on the diffuse emission can be interpreted in terms of upper limits to the average intensity of a putative hot halo. They can also be used to derived limits to the total energy radiated by hot gas in the observed galaxies as a function of its temperature for various assumed absorbing geometries. Beyond the equivalent solar radius (the radius at which the Sun would be in the observed galaxies), the temperature of hot gas radiating more than 30% of the total supernova power in the galaxies must be less than $10^{6.1} K$ if it is located within the disk with an assumed absorbing overburden of $3\times 10^{20} cm^{-2}$, or less than $10^{5.9} K$ if it lies in an unabsorbed halo.
14 citations
TL;DR: In this article, the authors derived a lower and upper bound on the intensity of the 1/4 keV extragalactic X-ray background from the depth of the shadow observed in NGC 3184, assuming that there is no unresolved 1 4 keV emission from the outer region of the galaxy which may otherwise partially fill in the shadow.
Abstract: We observed several nearby face-on spiral galaxies with the ROSAT PSPC. The apparent deficiency in soft X-ray surface brightness observed at the outer portion of their disks is consistent with the absorption of the extragalactic soft X-ray background by material associated with these galaxies, and allows us to place a lower limit on the intensity of this cosmologically important background. From the depth of the soft X-ray shadow observed in NGC 3184, a 95% confidence lower limit was derived to be $32 keV cm^{-2} s^{-1} keV^{-1}$ at 1/4 keV. This was obtained by assuming that there is no unresolved 1/4 keV X-ray emission from the outer region of the galaxy which may otherwise partially fill in the shadow: any such emission, or any unresolved structure in the absorbing gas, would imply a larger value. In the deepest exposure to date in this energy range, Hasinger et al. (1993) resolved about $30 keV cm^{-2} s^{-1} keV^{-1}$ at 1/4 keV into discrete sources; our current limit is therefore consistent with an extragalactic origin for all of these sources. Our results can also be directly compared with the corresponding upper limit derived from the ROSAT PSPC detection of soft X-ray shadows cast by high-latitude clouds in Ursa Major, $\simeq 65 keV cm^{-2} s^{-1} keV^{-1}$ at 1/4 keV. The lower and upper limits are only a factor of 2 apart, and begin to provide a reasonable measurement of the intensity of the 1/4 keV extragalactic X-ray background.
14 citations
TL;DR: In this article, a mosaic of 7 ROSAT PSPC pointed observations in the direction of (l,b ~ 10,0 deg) reveals deep X-ray shadows in the 0.5-2.0 keV band cast by dense molecular gas.
Abstract: A mosaic of 7 ROSAT PSPC pointed observations in the direction of (l,b ~ 10,0 deg) reveals deep X-ray shadows in the 0.5-2.0 keV band cast by dense molecular gas. The comparison between the observed on-cloud and off-cloud X-ray fluxes indicates that ~43% of the diffuse X-ray background in this direction in both the 3/4 keV and 1.5 keV bands originates behind the molecular gas, which is located at 2-4 kpc from the Sun. Given the short mean free path of X-rays in the 3/4 keV band in the Galactic plane (~1 kpc assuming an average space density of 1 cm^-3), this large percentage of the observed flux which originates beyond the molecular gas most likely indicates a strong enhancement in the distribution of X-ray emitting gas in the Galactic center region, possibly associated with a Galactic X-ray bulge.