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

to 2–6 keV. Since the 6–10 keV range is dominated by background photons for most
sources, this should increase the S/N of the hardness ratio techniques. The hardness
ratios measure observed counts, which are affected by Galactic absorption and QE
degradation in the Chandra ACIS detectors. In order to compare with other galaxies,
it is useful to correct the hardness ratios for these two soft X-ray absorption effects.
Therefore, we have calculated the intrinsic hardness ratios, denoted by a superscript
0, using a correction factor in each band appropriate to the best-fit spectrum of
the resolved sources. The intrinsic hardness ratios and their 1 σ errors are listed in
columns (10)–(12) of Table 3.1.
Although we have plotted H31 versus H21 in the past, plots of H32 versus H21
allow for better separation of simple spectral models. In Figure 3.4 we plot both
H31 0 versus H21 0 and H32 0 versus H21 0 for the 20 sources in the analysis sample.
The hardness ratios for the sum of those sources are (H21 0 , H32 0 , H31 0 ) = (−0.26,
−0.39, −0.59); the uncorrected hardness ratios are (H21, H32, H31) = (+0.11,
−0.33, −0.24). Sources with ∼40 net counts had errors similar to the median of the
uncertainties, ∼0.2. The errors scale roughly with the inverse square root of the net
counts.
In previously studied galaxies, most of the sources lie along a broad diagonal
swath extending roughly from (H21, H31) ∼ (−0.3, −0.7) to (0.4, 0.5). Usually,
these hardness ratios were not corrected for Galactic absorption and QE degradation;
the latter effect was not known at the time of some of the previous studies. Since there
are fewer sources and the absorption/degradation corrections tend to push sources
to the lower left part of the diagram, this swath is less evident in NGC 1600. In
Figure 3.4 the solid line corresponds to hardness ratios for power-law source spectra
with Γ =0–3.2. In calculating these model hardness ratios, Galactic absorption and
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