High-Energy Astrophysics American and Soviet Perspectives/Proceedings from the U.S.-U.S.S.R. Workshop on High-Energy Astrophysics (1991) / Chapter Skim
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ON THE ORIGIN OF THE DIFFUSE X-RAY BACKGROUND
ON THE ORIGIN OF THE DIFFUSE X-RAY BACKGROUND
Pages 174-191
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From page 174... ...
A reanalysis of the faint end of the log N-log S distribution for X-ray point sources in the 1.~3.0 keV band reveals an extragalactic source surface density of only 6 to 10 per square degree at the Einstein Deep Survey limit of 4 x 10~~4erg cm~2s-i in this band; the integrated contn~ution of all detected sources above this limit to the observed diffuse intensity is ~ 12%. We also report preliminary evidence for the association of faint radio sources with Peale in Me arcminute-scale fluctuations of the diffuse X-ray surface brightness distribution and then 174
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From page 175... ...
Their application to six deep-exposure fields at high galactic latitudes (the Deep Surveys has allowed us to measure the intensity and spectrum of the background ~ the Gl~3.5 keV band with an accuracy of ~ 5%. We have
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From page 176... ...
These techniques include editing the data to remove solar X-rays scattered into the detector by the residual atmosphere, a source excision algorithm which removes the effects of photons scattered far from a source's centroid by imperfections In the telescope mirror surface, a flat-fielding algorithm for removal of spatial irregularities in the counter response, and, most importantly, a determination of the fraction of counts which result from the interaction of cosmic rays with the detector. We present our principal scientific result in the third section: a source e spectrum of the diffuse X-ray background in the 0.1~3.5 keV band, where "source-free" implies removal of the contribution of all discrete sources for which /= > 4 x 10~~4erg cm~2s~~.
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From page 177... ...
Additional complications in studying diQuse emission with the IPC include non-uniformities in the spatial response of the instrument, detector gain changes, and the off-axis response of the telescope mirrors which introduces an energy-dependent vignetting for X-rays but does not affect cosmic ray-induced events. A few of these problems are handled adequately by the standard IPC processing routines.
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From page 178... ...
1b eliminate point-source photons, it is necessary first to identif,r the sources and then to subtract the system point response function, appropriately normalized, from the data. In the Eumein mirror/detector system, the point response profile Is energy dependent both as a result of a blur circle pulse-height dependence of the counter response (lower energy photons are spread over a greater area - Harnden et aL 19843 and as a result of scattering by imperfections on the surface of the grazing incidence mirror Higher energy photons scatter to larger distances from the source centroid - Mauche and Gorenstein 19843.
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From page 179... ...
The features are remarkably reproducible, appearing in all subsets of the data we examined: the Large Magellanic Cloud fields, the satellite-night-only Deep Survey fields, SO fields at the same galactic latitude as the LMC, and the remaining fields included in the final composite image. The fractional intensity of the features appears to be relatively independent of the ratio of X-rays to particles in the summed images: the LMC fields with their extensive diffuse emission shows structure of the same amplitude (measured as a percent of total count rate)
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From page 180... ...
count rates derived below and in utilizing our source detection algorithms, we first apply this flat-field correction to all data by multiplying the flat-field image value in a given pixel by the exposure map used to turn raw count rate maps into fluxed images. A Determination of the Cosmic Ray Particle Contamination in the IPC From the flattened, source-free, night-only images, we proceed via two independent methods to estimate the cosmic ray-induced contamination in the IPC count rate.
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From page 181... ...
1b test the robustness of thus important measurement, we have made a completetr independent estimate of the mean IPC particle event rate by taking advantage of the large number of overlapping paintings which comprise the Columbia EMC survey. The essence of the approach is the fact that, for a given sly direction, two separate IPC observations with different field centers will receive the same number of diffuse X-rays, but will record different numbers of events because 1)
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From page 182... ...
40 FIGURE 2 (a) The radial distribution of total counts in the summed VG=1 LMC data fitted to the nominal E - ein mirror vignetting function.
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From page 183... ...
In order to measure the spectrum of the X-ray background, it is necessary to extend this analysis to derive the spectrum of the particle contamination. Thus, we have repeated the radial profile fitting analysis on the summed Deep Survey fields for each pulse height bin independents.
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From page 184... ...
This is clearly in excess of the extrapolation of the power law with a slope of-0.4 observed in the ~30 keV band. The amount of the excess is unclear, however, because of a long-standing but little-known discrepancy between the normalization of the ~10 keV background spectrum.
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From page 185... ...
100.0 FIGURE 3 (a) The photon spectrum of the X-ray background as measured in the Ems~ Observatory IPC compared to an extrapolation of the Wisconsin ~9 keV background spectrum folded through the IPC response The integrated contribution of discrete sources with 1-3 keV fluxes > 6 X 10~14 erg cm~2 s-1 has been subtracted Cam the Wisconsin results to provide an app~pnate comparison with the source-free Epstein data.
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From page 186... ...
In the discussion that follows we use the 1-3 keV background flux derived from high galactic latitude fields in the longitude range 120° < e < 240° as our best estimator of the cosmic background intensity, although a residual galactic contribution cannot be excluded; the adopted value in this band is 2.5 x 10~8erg cm~2s~isr~~. THE POINT SOURCE CONTRIBUTION A number of extragalactic source classes contribute to the integrated surface brightness of the X-ray sly; the most important known sources are galaxy clusters, Seyferts, BL Lac objects, and quasars.
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From page 187... ...
A somewhat more difficult problem arises when the statistics of faint point source detections is considered in detail. Because increasing the size of the candidate region adds an amount of noise which scales linearly with the area while adding a signal contribution which drops exponentially, the optimum Size of the candidate region is considerably less than the size which includes almost all of the true source flux.
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From page 188... ...
In fact, all 11 of the optically identified sources listed in the earlier analysis are found in our survey, whereas only 6 of the 38 other source candidates found earlier are now detected. The other three Deep Survey fields show a similar surface density of sources.
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From page 189... ...
, concluding that the log N-log S of discrete X-ray emitters must extend below the Deep Survey limit by at least a factor of 3 to explain the graininess of the images, but that the entire background was too smooth to be explained completely by such a population. The upper limit to the contribution of discrete sources was set at ~ 50%, with the remainder of the emission requiring a source surface density of greater than several thousand per square degree.
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From page 190... ...
Unfortunately, it appears that the Einstein data are not quite sufficient to provide a quantitative test of this prediction. Primordial galaxies themselves are a potential background contributor and the high-source surface density our fluctuation analysis requires plus the tantalyzing evidence for associated faint radio sources makes this an attractive option to pursue.
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From page 191... ...
HIGH-ENER~ ~TROP~ICS 191 of the National Aeronautics and Space Administration under grant NAG8497, me U.S. National Academy of Sciences for making attendance at this worlo;hop possible, and the Academy of Sciences of the U.S.S.R.
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