We present the calibration and background model for the Proportional Counter Array on board the Rossi X-Ray Timing Explorer. The energy calibration is systematics-limited below 10 keV, with deviations from a power-law fit to the Crab Nebula plus pulsar of less than 1%. Unmodeled variations in the instrumental background amount to less than 2% of the observed background below 10 keV and less than 1% between 10 and 20 keV. Individual photon arrival times are accurate to 4.4 s at all times during the mission and to 2.5 s after 1997 April 29. The peak pointing direction of the five collimators is known to a precision of a few arcseconds.

We report the detection with the Proportional Counter Array (PCA) on board the Rossi X-Ray Timing Explorer (RXTE) of millisecond variability in the X-ray emission from the low-mass X-ray binary 4U 1728Ϫ34. Pulsations at 363 Hz with amplitudes (rms) of 2.5%-10% are present in six of the eight bursts analyzed to date. The strongest were seen in two successive bursts recorded on 1996 February 16 when the quiescent count rate was near the highest seen by PCA. The pulsations during these bursts show frequency changes of 1.5 Hz during the first few seconds but become effectively coherent as the burst decays. We interpret the 363 Hz pulsations as rotationally induced modulations of inhomogeneous burst emission. This represents the first compelling evidence for a millisecond spin period in a low-mass X-ray binary. Complex, intensity-dependent, millisecond X-ray variability is also present in all the quiescent flux intervals we examined. Most interesting was the behavior as the count rate approached its highest observed level. Two quasi-periodic oscillations (QPOs) were simultaneously observed in the 650 -1100 Hz range. Both QPOs increased in frequency together, maintaining a nearly constant frequency separation of about 363 Hz, the spin period inferred from the burst oscillations. This phenomenology is strongly suggestive of the magnetospheric beat frequency model proposed for the horizontal-branch oscillations (HBOs) seen in Z sources. We discuss this and several other possible physical interpretations for the observed X-ray variability.

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We report the discovery of a new X-ray pulsar, XTE J1810Ϫ197, that was serendipitously discovered on 2003 July 15 by the Rossi X-Ray Timing Explorer (RXTE) while observing the soft gamma repeater SGR 1806Ϫ20. The pulsar has a 5.54 s spin period, a soft X-ray spectrum (with a photon index of ≈4), and is detectable in earlier RXTE observations back to 2003 January but not before. These show that a transient outburst began between 2002 November 17 and 2003 January 23 and that the source's persistent X-ray flux has been declining since then. The pulsar exhibits a high spin-down rate s s Ϫ1 with no evidence of Doppler shifts due to Ϫ11 P ≈ 10 a binary companion. The rapid spin-down rate and slow spin period imply a supercritical characteristic magnetic field G and a young age yr. Follow-up Chandra observations provided an accurate 14 B Ӎ 3 # 10 t ≤ 7600 position of the source. Within its error radius, the 1.5 m Russian-Turkish Optical Telescope found a limiting magnitude . All such properties are strikingly similar to those of anomalous X-ray pulsars and soft R p 21.5 C gamma repeaters, providing strong evidence that the source is a new magnetar. However, archival ASCA and ROSAT observations found the source nearly 2 orders of magnitude fainter. This transient behavior and the observed long-term flux variability of the source in absence of an observed SGR-like burst activity make it the first confirmed transient magnetar and suggest that other neutron stars that share the properties of XTE J1810Ϫ197 during its inactive phase may be unidentified transient magnetars awaiting detection via a similar activity. This implies a larger population of magnetars than previously surmised and a possible evolutionary connection between magnetars and other neutron star families.

We report the results from our timing analysis of 15 Rossi X-Ray T iming Explorer observations of Cygnus X-1 throughout its 1996 spectral transitions. The entire period can be divided into three distinct phases : (1) transition from the hard state to the soft state, (2) soft state, and (3) transition from the soft state back to the hard state. The observed X-ray properties (both temporal and spectral) in Phases 1 and 3 are remarkably similar, suggesting that the same physical processes are likely involved in triggering such transitions. The power density spectrum (PDS) during the transition can be characterized by a lowfrequency red-noise (power-law) component, followed by a white-noise (Ñat) component that extends to roughly 1È3 Hz, where it is cut o †, and a steeper power law (D1/f 2) at higher frequencies. The X-ray Ñux also exhibits apparent quasi-periodic oscillations (QPOs), with the centroid frequency varying in the range of 4È12 Hz. The QPO shows no correlation with the source Ñux, but it becomes more prominent at higher energies. This type of PDS bears resemblance to that of other black hole candidates often observed in a so-called very high state, although the origin of the observed QPO may be very di †erent. The low-frequency red noise has not been observed in the hard state, and thus seems to be correlated positively with the disk mass accretion rate, which is presumably low in the hard state and high in the soft state ; in fact, it completely dominates the PDS in the soft state. In the framework of thermal Comptonization models, Cui et al. recently speculated that the di †erence in the observed spectral and timing properties between the hard and soft states is due to the presence of a "" Ñuctuating ÏÏ Comptonizing corona during the transition. Here we present the measured hard X-ray time lags and coherence functions between various energy bands, and we show that the results strongly support such a scenario.