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    X-ray Spectral States and High-Frequency QPOs in Black Hole Binaries

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    The high-frequency quasi-periodic oscillations in the X-ray emission of black hole binaries and candidates are briefly examined with respect to the question of X-ray states. We use the prescription of McClintock and Remillard (2005), in which X-ray spectra and power density spectra are used to differentiate three main states of active accretion: "thermal", "hard", and "steep power-law". For the sources that exhibit pairs of HFQPOs with frequencies that scale in a 3:2 ratio, the QPO detections are associated with the steep power-law state. Furthermore, there is a clear trend in which the oscillation at twice the fundamental frequency is seen when the source has high X-ray luminosity, while the QPO at three times the fundamental frequency is seen at lower luminosity.Comment: 4 pages, 2 figures, Nordita Proceedings, Astron. Nachr. vol. 32

    EXPLICIT STRONG SOLUTIONS OF MULTIDIMENSIONAL STOCHASTIC DIFFERENTIAL EQUATIONS

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    Herein, we characterize strong solutions of multidimensional stochastic differential equations (formula) that can be represented locally as (formula) where W is an multidimensional Brownian motion and U, (symbole) are continuous functions. Assuming that (symbole) is continuously differentiable, we find that (symbole) must satisfy a commutation relation for such explicit solutions to exist and we identify all drift terms b as well as U and (symbole) that will allow X to be represented in this manner. Our method is based on the existence of a local change of coordinates in terms of a diffeomorphism between the solutions X and the strong solutions to a simpler Ito integral equation.Diffeomorphism, Ito processes, explicit solutions.

    Common Patterns in the Evolution between the Luminous Neutron Star Low-Mass X-ray Binary Subclasses

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    The X-ray transient XTE J1701-462 was the first source observed to evolve through all known subclasses of low-magnetic-field neutron star low-mass X-ray binaries (NS-LMXBs), as a result of large changes in its mass accretion rate. To investigate to what extent similar evolution is seen in other NS-LMXBs we have performed a detailed study of the color-color and hardness-intensity diagrams (CDs and HIDs) of Cyg X-2, Cir X-1, and GX 13+1 -- three luminous X-ray binaries, containing weakly magnetized neutron stars, known to exhibit strong secular changes in their CD/HID tracks. Using the full set of Rossi X-ray Timing Explorer Proportional Counter Array data collected for the sources over the 16 year duration of the mission, we show that Cyg X-2 and Cir X-1 display CD/HID evolution with close similarities to XTE J1701-462. Although GX 13+1 shows behavior that is in some ways unique, it also exhibits similarities to XTE J1701-462, and we conclude that its overall CD/HID properties strongly indicate that it should be classified as a Z source, rather than as an atoll source. We conjecture that the secular evolution of Cyg X-2, Cir X-1, and GX 13+1 -- illustrated by sequences of CD/HID tracks we construct -- arises from changes in the mass accretion rate. Our results strengthen previous suggestions that within single sources Cyg-like Z source behavior takes place at higher luminosities and mass accretion rates than Sco-like Z behavior, and lend support to the notion that the mass accretion rate is the primary physical parameter distinguishing the various NS-LMXB subclasses.Comment: 20 pages, 14 figures, 5 tables -- matches published version in Ap
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