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

tracted the spectrum from an elliptical annulus between one and three effective radii.
We first fitted this region using the source best-fit model from the inner one effec-
tive radius region, which provided an acceptable fit (row 5). When we allowed the
power-law index to vary (row 6), we found a better fit (∆χ 2 = −2.9 with 1 less dof);
however, the two values of Γ agree their 90% confidence limits. Freeing the absorp-
tion also yielded a better fit, (∆χ 2 = −1.9 with one fewer dof); however, Galactic
absorption was allowed at the 90% confidence limit.
We also attempted to fit the collective spectrum of all of the resolved sources on
the S3 chip, which had 2660 net counts. Again, freeing the power-law index from the
best-fit value for the inner effective radii produced a better fit; however, the power-law
index is still consistent with the best-fit 90% confidence limit (rows 8 and 9). Freeing
the absorbing column did not produce a better fit (rows 9 and 10). We find no clear
trend between radius and spectral fit. This is consistent with results from a study of
the sources in 15 galaxies by Irwin et al. (2003).
NGC 4382
For NGC 4382, the lower number of sources and counts required using the inner
two effective radii to attain 921 net counts in the spectrum. For this region, the
adopted best-fit model had a power-law index of 1.52 with the Galactic absorption
column of 2.45 × 10 20 cm −2 (Table 2.4, row 3). Neither using a bremsstrahlung model
(row 1) nor freeing the absorbing column (rows 2 and 4) gave a better fit. Again, we
remark that although hardness ratios indicate that each source is probably fitted by
a more complex model than a power-law and Galactic absorption, the summation of
counts from objects with different emission properties can be well fitted with a single
power-law and Galactic absorption. The best-fit indicated no excess absorption when
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