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

This will allow us to better probe the diffuse gas now that we know where the bright
sources are. Additionally, the optical monitor on XMM will be useful in testing
whether the candidate ULXs are in fact AGNs.
We believe that one of the crucial lessons from our work is the value of multi-
epoch strategies for X-ray observations of early-type galaxies. Such studies combine
increased sensitivity and the ability to examine variability. To that end, we have
undertaken a multi-epoch study of NGC 4365, similar in design to that of NGC 4697.
Although NGC 4365 is farther away than NGC 4697, it has a much larger number of
sources. Preliminary analysis of four out of five observation has already detected 284
sources. Other groups have also adopted similar observing strategies. Observations
(of about twice the total exposure of NGC 4365 and NGC 4697) of two other early-
type galaxies, NGC 3379 and NGC 4278, are being made publicly available as the
observations occur.
We are also involved in Chandra proposals for multi-epoch observations of addi-
tional galaxies. Deep (∼ 1 Ms) observations of Cen A will extend the study of LMXBs
down to ∼ 10 36 ergs s −1 and better sample variability timescales. Multiple, deep ob-
servations (e.g., 2 × 100 ks) of NGC 4697 would give us the opportunity to better
probe its intriguing sources, and provide the strongest variability timescale coverage.
It is important that we continue to observe the same galaxy over multiple years to
understand the long-term variability of sources.
7.2.3 Optical Observation of Early-type Galaxies
The combination of optical observations of GCs and X-ray observations of LMXBs
were indispensable in probing the GC-LMXB connection. Extending these studies to
more systems will better constrain our understanding, and, perhaps, resolve the cause
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