We discuss the observational consequences of black hole spin in X-ray binaries within the framework of the standard thin accretion disk model. When compared with theoretical flux distribution from the surface of a thin disk surrounding a Kerr black hole, the observed X-ray properties of the Galactic superluminal jet sources, GRO J1655Ϫ40 and GRS 1915ϩ105, strongly suggest that each contains a black hole spinning rapidly in the same direction as the accretion disk. We show, however, that some other black hole binaries with an ultrasoft X-ray component probably harbor only non-or slowly spinning black holes, and we argue that those with no detectable ultrasoft component above 1-2 keV in their high luminosity state may contain a fast-spinning black hole but with a retrograde disk. Therefore, all classes of known black hole binaries are united within one scheme. Furthermore, we explore the possibility that spectral state transitions in Cyg X-1 are simply due to temporary disk reversal, which can occur in a wind accretion system.

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.

We present extensive photometry at ultraviolet (UV), optical, and near-infrared (NIR) wavelengths, as well as dense sampling of optical spectra, for the normal Type Ia supernova (SN Ia) 2005cf. The optical photometry, performed at eight different telescopes, shows a 1σ scatter of 0.03 mag after proper corrections for the instrument responses. From the well-sampled light curves, we find that SN 2005cf reached a B-band maximum at 13.63 ± 0.02 mag, with an observed luminosity decline rate Δm 15 (B) = 1.05 ± 0.03 mag. The correlations between the decline rate and various color indexes, recalibrated on the basis of an expanded SN Ia sample, yield a consistent estimate for the host-galaxy reddening of SN 2005cf, E(B − V ) host = 0.10 ± 0.03 mag. The UV photometry was obtained with the Hubble Space Telescope and the Swift Ultraviolet/Optical Telescope, and the results match each other to within 0.1-0.2 mag. The UV light curves show similar evolution to the broadband U, with an exception in the 2000-2500 Å spectral range (corresponding to the F220W/uvm2 filters), where the light curve appears broader and much fainter than that on either side (likely owing to the intrinsic spectral evolution). Combining the UV data with the ground-based optical and NIR data, we establish the generic UVoptical-NIR bolometric light curve for SN 2005cf and derive the bolometric corrections in the absence of UV and/or NIR data. The overall spectral evolution of SN 2005cf is similar to that of a normal SN Ia, but with variety in the strength and profile of the main feature lines. The spectra at early times displayed strong, high-velocity (HV) features in the Ca ii H&K doublet and NIR triplet, which were distinctly detached from the photosphere (v ≈ 10,000 km s −1 ) at a velocity ranging from 20,000 to 25,000 km s −1 . One interesting feature is the flatbottomed absorption observed near 6000 Å in the earliest spectrum, which rapidly evolved into a triangular shape and then became a normal Si ii λ6355 absorption profile at about one week before maximum brightness. This premaximum spectral evolution is perhaps due to the blending of the Si iiλ6355 at photospheric velocity and another HV absorption component (e.g., an Si ii shell at a velocity ∼18,000 km s −1 ) in the outer ejecta, and may be common in other normal SNe Ia. The possible origin of the HV absorption features is briefly discussed.

Aquila X-1 is a soft X-ray transient source and emits type I X-ray bursts. A spectral state transition was observed with RXTE during its outburst decay in 1997 February and March. Its 10-30 keV and 5-10 keV count rate ratio increased suddenly when its luminosity was between 4 and ergs s Ϫ1 , assuming a 2.5 kpc 35 12 # 10 distance. Spectral fitting with a model composed of a blackbody and a power-law component showed that its blackbody component decreased and that the power-law component became much harder significantly and simultaneously. We interpret this transition to be caused by the centrifugal barrier, more commonly known as the "propeller" effect. We thus infer that the magnetic field strength of the neutron star is around G, if the 8 1 # 10 neutron star spin period is 1.8 ms. Similarly, we infer the neutron star magnetic field strength in another soft Xray transient Cen X-4 is about G. We also propose a unified scheme for spectral state transitions in soft 9 2 # 10 X-ray transients, from soft high state to hard low state and further to quiescent state. With this scheme accretion onto neutron star may take place even during the propeller regime.