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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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Last modified: Monday, 19-Jun-2006 11:24:57 EDT
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