PhD Thesis (PDF) - Department of Astronomy - University of Virginia

these sources may be extended or multiple, and are marked with a note in Table 5.1.
Masking circular regions around the source at a radius encircling > 97% of the PSF
were created for every source.
The refined source positions for a majority of the sources came from the average
position of 0.3–6 keV photons in the source extraction regions. For Sources 142, 148,
152, and 154–158, whose average positional offset from the optical axis is more than
5 ′ , we correlated the 0.3–6 keV photons near the wavdetect coordinates against the
average X-ray PSF of each source to refine their positions.
To subtract out overlapping diffuse gas and blank-sky background we used a local
background with an area approximately three times that of each source’s extraction
region. The background region excluded photons in the masking region. In cases
where background regions overlapped or fell along node/chip boundaries, we slightly
altered these overlapping regions, preserving the ratio of source to background ar-
eas and ensuring that the source region and background region had similar mean
exposures.
The observed net count rates, their errors, and S/N were calculated by stacking
the observations, correcting for background photons, and dividing by the sum of the
mean exposures over each source region.
We list the results of all analyses for all sources in this chapter’s tables; however, we
restrict discussion of sources, except for identification of possible optical counterparts,
to the 126 that have photometric count rates determined at the ≥ 3σ level. These
significantly detected sources, hereafter the Analysis Sample, all have at least 18 net
counts.
The minimum detected count rate in the 0.3–6 keV band for our Analysis Sample
sources is 1.0 × 10 −4 counts s −1 . This count rate is 2.6 times as deep as the count
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