Observations of millisecond X-ray pulsars

I present an observational review of the five accretion-driven millisecond X-ray pulsars currently known, focusing on the results obtained with the Rossi X-ray Timing Explorer (RXTE) satellite. A prominent place in this review is given to the first such system discovered, SAX J1808.4-3658. Currently four outbursts have been detected from this source, three of which have been studied using RXTE. This makes this source the best studied example of all accretion-driven millisecond pulsars. Its October 2002 outburst is of particular interest because of the discovery of kilohertz quasi-periodic oscillations and burst oscillations during its thermonuclear X-ray bursts. The other four accreting millisecond pulsars were discovered within the last two years and only limited results have been published so far. A more extended review can be found at http://zon.wins.uva.nl/~rudy/admxp/Comment: To appear in the proceedings of "X-ray Timing 2003: Rossi and Beyond", eds. P. Kaaret, F. K. Lamb, & J. H. Swank (Melvill, NY; AIP). A more extended version of this review can be found at http://zon.wins.uva.nl/~rudy/admxp

The rms-flux relations in different branches in Cyg X-2

In this paper, the rms-flux (root mean square-flux) relation along the Z-track of the bright Z-Source Cyg X-2 is analyzed using the observational data of Rossi X-ray Timing Explorer (RXTE). Three types of rms-flux relations, i.e. positive, negative, and 'arch'-like correlations are found in different branches. The rms is positively correlated with flux in normal branch (NB), but anti-correlated in the vertical horizontal branch (VHB). The rms-flux relation shows an 'arch'-like shape in the horizontal branch (HB). We also try to explain this phenomenon using existing models.Comment: Accepted for publication in Astrophysics & Space Scienc

Morphological analysis on the coherence of kHz QPOs

We take the recently published data of twin kHz quasi-period oscillations (QPOs) in neutron star (NS) lowmass X-ray binaries (LMXBs) as the samples, and investigate the morphology of the samples, which focuses on the quality factor, peak frequency of kHz QPOs, and try to infer their physical mechanism. We notice that: (1) The quality factors of upper kHz QPOs are low (2 ~ 20 in general) and increase with the kHz QPO peak frequencies for both Z and Atoll sources. (2) The distribution of quality factor versus frequency for the lower kHz QPOs are quite different between Z and Atoll sources. For most Z source samples, the quality factors of lower kHz QPOs are low (usually lower than 15) and rise steadily with the peak frequencies except for Sco X-1, which drop abruptly at the frequency of about 750 Hz. While for most Atoll sources, the quality factors of lower kHz QPOs are very high (from 2 to 200) and usually have a rising part, a maximum and an abrupt drop. (3) There are three Atoll sources (4U 1728-34, 4U 1636-53 and 4U 1608-52) of displaying very high quality factors for lower kHz QPOs. These three sources have been detected with the spin frequencies and sidebands, in which the source with higher spin frequency presents higher quality factor of lower kHz QPOs and lower difference between sideband frequency and lower kHz QPO frequency.Comment: 8 pages, 8 figures, publishe