QPO frequency derivative - frequency correlation indicates non-Keplerian boundary layer with a maximum in rotation rate

The correlation between the frequency and the absolute value of the frequency derivative of the kilohertz QPOs observed for the first time from 4U 1636-53 is a simple consequence and indicator of the existence of a non-Keplerian rotation rate in the accretion disk boundary layer. This paper interprets the observed correlation, showing that the observations provide strong evidence in support of the fundamental assumption of disk accretion models around slow rotators, that the boundary layer matches the Keplerian disk to the neutron star magnetosphere.Comment: Accepted for publication in MNRAS Letter

On the enhanced X-ray emission from SGR 1900+14 after the August 27th giant flare

We show that the giant flares of soft gamma ray repeaters (E similar to 10(44) erg) can push the inner regions of a fall-back disk out to larger radii by radiation pressure, while matter remains bound to the system for plausible parameters. The subsequent relaxation of this pushed-back matter can account for the observed enhanced X-ray emission after the August 27(th) giant flare of SGR 1900+14

Microscopic Vortex Velocity in the Inner Crust and Outer Core of Neutron Stars

Treatment of the vortex motion in the superfluids of the inner crust and the outer core of neutron stars is a key ingredient in modeling a number of pulsar phenomena, including glitches and magnetic field evolution. After recalculating the microscopic vortex velocity in the inner crust, we evaluate the velocity for the vortices in the outer core for the first time. The vortex motion between pinning sites is found to be substantially faster in the inner crust than in the outer core, v_0^{\rm crust} \sim 10^{7}\mbox{\cms} \gg v_0^{\rm core} \sim 1\mbox{\cms}. One immediate result is that vortex creep is always in the nonlinear regime in the outer core in contrast to the inner crust, where both nonlinear and linear regimes of vortex creep are possible. Other implications for pulsar glitches and magnetic field evolution are also presented.Comment: Accepted for publication in MNRA

Is the lack of pulsations in low mass X-Ray binaries due to comptonizing coronae?

The spin periods of the neutron stars in most Low Mass X-ray Binary (LMXB) systems still remain undetected. One of the models to explain the absence of coherent pulsations has been the suppression of the beamed signal by Compton scattering of X-ray photons by electrons in a surrounding corona. We point out that simultaneously with wiping out the pulsation signal, such a corona will upscatter (pulsating or not) X-ray emission originating at and/or near the surface of the neutron star leading to appearance of a hard tail of Comptonized radiation in the source spectrum. We analyze the hard X-ray spectra of a selected set of LMXBs and demonstrate that the optical depth of the corona is not likely to be large enough to cause the pulsations to disappear

On the Outbursts of Soft X-ray Transients

We suggest a new scenario to explain the outburst light curves of black hole soft X-ray transients together with the secondary maximum and the bump seen on their decay phases. Our explanations are based on the disk instability models considering the effect of X-ray irradiation. The scenario is consistent with the observed X-ray delays by a few days with respect to the optical rise for both the main outburst and the later maxima. We test our scenario by numerically solving the disk diffusion equation. The obtained model curve fits well to the observed X-ray outburst photon flux curve of the black hole soft X-ray transient GS/GRS 1124-68, a typical representative of the four BH SXTs with very similar light curves.Comment: 11 pages, 8 figures, accepted for publication in A&

Vortex Creep Against Toroidal Flux Lines, Crustal Entrainment, and Pulsar Glitches

A region of toroidally oriented quantized flux lines must exist in the proton superconductor in the core of the neutron star. This region will be a site of vortex pinning and creep. Entrainment of the neutron superfluid with the crustal lattice leads to a requirement of superfluid moment of inertia associated with vortex creep in excess of the available crustal moment of inertia. This will effect constraints on the equation of state. The toroidal flux region provides the moment of inertia necessary to complement the crust superfluid with postglitch relaxation behavior fitting the observations.Comment: Published in The Astrophysical Journal Letter