The HEASARC holdings have enabled comprehensive studies of the X-ray background (XRB) spectrum and composition using joint ROSAT and ASCA data sets. Although Guest Observers have often performed long looks in the directions of previous deep surveys, the joint analysis has typically been performed by archival investigators. The joint ASCA/ROSAT spectral studies based on data in the QSF3 field (Chen et al. 1997, MNRAS 285, 449) and Lockman and Lynx fields (Miyaji et al. 1998, A & A 334, L3) are two examples. While these results are generally consistent with the results of either instrument alone, and despite a substantial overlap in energy bandpass, the joint approach is necessary to identify (relatively) simple models capable of explaining the XRB spectrum over the almost 2 orders of magnitude in energy sampled by the two instruments.

Fluctuation analysis, has been a useful tool in extending the log N-log S relationships for numerous experiments (HEAO-1 A2: Shafer 1983, UMd PhD thesis; Ginga: Butcher et al. 1997, MNRAS 291, 437; ASCA SIS: Gendreau et al. 1998, MNRAS 297, 41). Results from imaging experiments such as the ASCA SIS require the use of many independent pointings. There is no disadvantage to using suitably selected pointings from the archive (i.e. primary target relatively faint and not extended) compared to a series of dedicated pointings. Given the scheduling pressure experienced by ASCA, it is unlikely that an observational program to measure XRB fluctuations would be selected. However, the uniform and accessible archive makes it possible to do this experiment.

The local hot bubble which engulfs the solar system has a characteristic temperature near 106 degrees. While the existence of the bubble has been postulated for many years, quantitative predictions and measurements have awaited sensitive all-sky maps of both X-ray emission and tracers of X-ray absorbing material. The ROSAT all-sky survey (1/4 keV band) and the DIRBE corrected IRAS 100 micron maps (Schlegel et al. 1998, ApJ 500, 525) provide such data. Snowden et al. (2000, ApJ in press) used the now publicly available (via MPE and a HEASARC mirror) 12' diffuse XRB maps along with the Schlegel et al. data to identify and characterize almost 400 X-ray shadows, providing the most detailed information on the variation of temperature and emission measure of the local hot bubble to date.

At lower latitudes, ROSAT shadowing experiments at energies of 0.5 -- 2 keV have been used to separate emission from the Galactic bulge and to define the properties of molecular absorbers within the Galaxy (Park et al. 1997, ApJ 476, L77; 1998, ApJ 509, 203). Emission from the bulge of the Galaxy is extended over many degrees; however the positive identification of such emission with the bulge is complicated by the many other small scale Galactic phenomena. Removing the effects of stars and other X-ray sources requires imaging experiments, which necessarily limits the field of view to be too small to study the bulge. Combinations of many exposures are necessary, and again form the kind of study ideally done by mining the archive.

At higher energies still (> 2 keV) the Galaxy is largely transparent to X-rays. The HEAO-1 A2 survey provides the most precise measurements of surface brightness. Correlating fluctuations in the surface brightness with catalogs of optically selected galaxies (Jahoda et al. 1991, ApJ 378, L37; 1992, ApJ 399, L10), infra-red selected galaxies (Lahav et al. 1993, Nature 364, 693; Miyaji et al 1994, ApJ 434, 424) demonstrates that X-ray emission traces the distribution of matter. This is a confirmation that X-ray light traces mass, recently demonstrated by the alignment of the flux dipole defined by X-ray selected AGN (Miyaji and Boldt 1990, ApJ 353, L3; Miyaji et al. 1991, AIP Conf. Proc. 222, 431). The fact that X-ray emission traces mass has also made the HEAO-1 A2 surveys useful for measuring upper limits to foreground contamination of microwave maps. Boughn and Jahoda (1993, ApJ 412, L1) correlated the HEAO-1 A2 data with a balloon borne 19 GHz map, while Bennett et al. (1993, ApJ 414, L77) used the same data to correlate with the CODE DMR maps. These results both indicate that the microwave fluctuations observed by COBE carry cosmological information from the surface of last scattering rather than the imprint of astrophysical foregrounds. Boughn et al. (1998, New Astronomy, 3, 275) latter used the HEAO-1 A2/COBE cross correlation to set an upper limit on a cosmological constant.

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