Dependence of the Frequency of the Kilohertz Quasi-Periodic Oscillations on X-ray Count Rate and Colors in 4U 1608-52

We present new results based on observations carried out with the Rossi X-ray Timing Explorer during the decay of an outburst of the low-mass X-ray binary (LMXB) and atoll source 4U 1608-52. Our results appear to resolve, at least in 4U 1608-52, one of the long-standing issues about the phenomenology of the kilohertz quasi-periodic oscillations (kHz QPOs), namely, the lack of a unique relation between the frequency of the kHz QPOs and the X-ray flux. We show that despite its complex dependence on the X-ray flux, the frequency of the kHz QPOs is monotonically related to the position of the source in the color-color diagram. Our findings strengthen the idea that, as in the case of Z sources, in the atoll sources the X-ray flux is not a good indicator of $\dot M$, and that the observed changes in the frequency of the kHz QPOs in LMXBs are driven by changes in $\dot M$. These results raise some concern about the recently reported detection of the orbital frequency at the innermost stable orbit in 4U 1820-30.Comment: Accepted for publication in The Astrophysical Journal Letters. Uses AAS LaTex v4.0 (5 pages plus 4 postscript figures

Spin-dependent transport in metal/semiconductor tunnel junctions

This paper describes a model as well as experiments on spin-polarized tunnelling with the aid of optical spin orientation. This involves tunnel junctions between a magnetic material and gallium arsenide (GaAs), where the latter is optically excited with circularly polarized light in order to generate spin-polarized carriers. A transport model is presented that takes account of carrier capture in the semiconductor surface states, and describes the semiconductor surface in terms of a spin-dependent energy distribution function. The so-called surface spin-splitting can be calculated from the balance of the polarized electron and hole flow in the semiconductor subsurface region, the polarized tunnelling current across the tunnel barrier between the magnetic material and the semiconductor surface, and the spin relaxation at the semiconductor surface. Measurements are presented of the circular-polarization-dependent photocurrent (the so-called helicity asymmetry) in thin-film tunnel junctions of Co/Al2O3/GaAs. In the absence of a tunnel barrier, the helicity asymmetry is caused by magneto-optical effects (magnetic circular dichroism). In the case where a tunnel barrier is present, the data cannot be explained by magneto-optical effects alone; the deviations provide evidence that spin-polarized tunnelling due to optical spin orientation occurs. In Co/τ-MnAl/AlAs/GaAs junctions no deviations from the magneto-optical effects are observed, most probably due to the weak spin polarization of τ-MnAl along the tunnelling direction; the latter is corroborated by bandstructure calculations. Finally, the application of photoexcited GaAs for spin-polarized tunnelling in a scanning tunnelling microscope is discussed.

Mass ratio from Doppler beaming and R{\o}mer delay versus ellipsoidal modulation in the Kepler data of KOI-74

We present a light curve analysis and radial velocity study of KOI-74, an eclipsing A star + white dwarf binary with a 5.2 day orbit. Aside from new spectroscopy covering the orbit of the system, we used 212 days of publicly available Kepler observations and present the first complete light curve fitting to these data, modelling the eclipses and transits, ellipsoidal modulation, reflection, and Doppler beaming. Markov Chain Monte Carlo simulations are used to determine the system parameters and uncertainty estimates. Our results are in agreement with earlier studies, except that we find an inclination of 87.0 \pm 0.4\degree, which is significantly lower than the previously published value. We find that the mass ratio derived from the radial velocity amplitude (q=0.104 \pm 0.004) disagrees with that derived from the ellipsoidal modulation (q=0.052 \pm 0.004} assuming corotation). This was found before, but with our smaller inclination, the discrepancy is even larger than previously reported. Accounting for the rapid rotation of the A-star is found to increase the discrepancy even further by lowering the mass ratio to q=0.047 \pm 0.004. These results indicate that one has to be extremely careful in using the amplitude of an ellipsoidal modulation signal in a close binary to determine the mass ratio, when a proof of corotation is not firmly established. The radial velocities that can be inferred from the detected Doppler beaming in the light curve are found to be in agreement with our spectroscopic radial velocity determination. We also report the first measurement of R{\o}mer delay in a light curve of a compact binary. This delay amounts to -56 \pm 17 s and is consistent with the mass ratio derived from the radial velocity amplitude. The firm establishment of this mass ratio at q=0.104 \pm 0.004 leaves little doubt that the companion of KOI-74 is a low mass white dwarf.Comment: 9 pages, 7 figures, 2 tables; accepted for publication in MNRA

Local structural disorder and superconductivity in KxFe2-ySe2

We report significantly enhanced magnetic moment in K0.69(2)Fe1.45(1)Se2.00(1) single crystals with sharp Tc and bulk superconductivity obtained by postannealing and quenching process. There are two Fe sites in the K0.69(2)Fe1.45(1)Se2.00(1) unit cell: Fe1, which has higher symmetry with longer average Fe-Se bond length, and Fe2, which has lower symmetry with shorter average Fe-Se bond length. Temperature-dependent x-ray absorption fine-structure (XAFS) analysis results on quenched and as-grown K0.69(2)Fe1.45(1)Se2.00(1) crystals show that quenched K0.69(2)Fe1.45(1)Se2.00(1) have increased average Fe-Se bond length and decreased static disorder. Our results indicate that nonzero population of Fe1 sites is the key structural parameter that governs the bulk superconductivity. We also show clear evidence that Fe1 sites carry higher magnetic moment than Fe2 sites.Comment: 3 figures, 4 page