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

eak at greater than 5 × 10 38 ergs s −1 could not be excluded.
In Sarazin et al. (2000), we argued that a break luminosity similar to the Ed-
dington luminosity of a 1.4M⊙ spherically accreting neutron star indicated that the
LMXBs above the break contain accreting black holes. The large luminosities of the
cutoff or broken power-laws for NGC 1316, NGC 4365, and NGC 4382 seem less
consistent with this interpretation, barring a large error in the distance.
In the luminosity function of NGC 4382, the best-fit clearly misses the brightest
source, source 38. Since source 38 is a bright background AGN, this is not indicative
of a poor fit.
2.4.4 Hardness Ratios
We determined X-ray hardness ratios for the sources, using the same techniques
and definitions we used previously (Sarazin et al. 2000, 2001; Blanton et al. 2001; Irwin
et al. 2002). Hardness ratios or X-ray colors are useful for crudely characterizing the
spectral properties of sources, and can be applied to sources that are too faint for
detailed spectral analysis. We define two hardness ratios as H21 ≡ (M − S)/(M + S)
and H31 ≡ (H −S)/(H +S), where S, M, and H are the total counts in the soft (0.3–1
keV), medium (1–2 keV), and hard (2–10 keV) bands, respectively. Since the hardness
ratios measure observed counts, Galactic absorption, as well as QE degradation, must
be taken into account when interpreting the ratios. The hardness ratios and their 1 σ
errors are listed in columns (9) and (10) of Tables 2.1 and 2.2. Figure 2.4 plots H31
versus H21 for the 48 and 39 sources of NGC 4365 and NGC 4382, respectively, with
at least 20 net counts. The hardness ratios for the sum of those sources are (H21,
H31) = (0.04, −0.25) for NGC 4365 and (0.02, −0.25) for NGC 4382. Sources with
∼40 net counts had errors similar to the median of the uncertainties, ∼0.26. The
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