Other
Scientific paper
Nov 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004phdt........11h&link_type=abstract
Thesis (PhD). HARVARD UNIVERSITY, Source DAI-B 65/05, p. 2451, Nov 2004, 360 pages.
Other
1
Scientific paper
I investigate the dim (LX < 10 35 ergs s-1) X-ray sources in galactic globular clusters, constraining their nature and properties with Chandra X-ray observations. Three faint X-ray sources are identified in a short Chandra observation of the globular cluster NGC 6652, in addition to a known low-mass X-ray binary (LMXB). I study the spectral and temporal properties of two quiescent LMXBs (qLMXBs) in 47 Tuc, fitting their spectra with unmagnetized hydrogen-atmosphere neutron star models. Neither shows evidence for a power-law component, as observed in most qLMXBs in the galaxy. Spectral modeling of one (X7) requires a relatively high neutron star mass, >1.8 M&sun; , for a plausible radius range of 9 14 km. However, if accretion is continuing, it could alter the atmospheric opacity and allow a smaller mass. I detect nine faint X-ray sources in the dense, obscured globular cluster Terzan 5, in addition to the known LMXB which entered a rare bright state. The rich globular cluster M80 presents nineteen faint X-ray sources. I identify probable qLMXBs and cataclysmic variables (CVs) in Terzan 5 and M80 based on their X-ray spectra and colors. I review several X-ray studies of globular clusters to study the populations of qLMXBs and probable CVs. Most of the identified qLMXBs show little evidence for a power- law component to their X-ray spectra. Their distribution among globular clusters is consistent with formation through close encounters. The distribution of probable CVs shows a weaker dependence on cluster density than that of qLMXBs, indicating that CVs undergo additional processes of destruction or formation. I detect three hundred X-ray sources within the half-mass radius of 47 Tuc, of which seventy may be background sources. Twenty-nine sources are newly identified with chromospherically active binaries (ABs). X-ray color, spectral, and variability information indicates that fewer than sixty of the X-ray sources are millisecond pulsars. Three new qLMXB candidates are identified in 47 Tuc, one showing variable absorption and eclipses. The other two qLMXB candidates' spectra are dominated by hard power-law components, and show relatively low X-ray luminosities.
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