Quasi-periodic Oscillations in the X-ray Light Curves from Relativistic Tori

We use a relativistic ray-tracing code to analyze the X-ray emission from a pressure-supported oscillating relativistic torus around a black hole. We show that a strong correlation exists between the {\it intrinsic} frequencies of the torus normal modes and the {\it extrinsic} frequencies seen in the observed light curve power spectrum. This correlation demonstrates the feasibility of the oscillating-torus model to explain the multiple peaks seen in black hole high-frequency quasi-periodic oscillations. Using an optically thin, monochromatic emission model, we also determine how a relativistically broadened emission line and the amplitude of the X-ray modulations are dependent on the observer's inclination angle and on the torus oscillation amplitudes. Observations of these features can provide important information about the torus as well as the black hole.Comment: 4 pages, 3 figures, submitted to ApJ

3-D SPH simulations of colliding winds in eta Carinae

We study colliding winds in the superluminous binary eta Carinae by performing three-dimensional, Smoothed Particle Hydrodynamics (SPH) simulations. For simplicity, we assume both winds to be isothermal. We also assume that wind particles coast without any net external forces. We find that the lower density, faster wind from the secondary carves out a spiral cavity in the higher density, slower wind from the primary. Because of the phase-dependent orbital motion, the cavity is very thin on the periastron side, whereas it occupies a large volume on the apastron side. The model X-ray light curve using the simulated density structure fits very well with the observed light curve for a viewing angle of i=54 degrees and phi=36 degrees, where i is the inclination angle and phi is the azimuth from apastron.Comment: 6 pages, 3 figures, To be published in Proceedings of IAU Symposium 250: Massive Stars as Cosmic Engines, held in Kauai, Hawaii, USA, Dec 2007, edited by F. Bresolin, P.A. Crowther & J. Puls (Cambridge University Press

Modeling the RXTE light curve of η\eta Carinae from a 3-D SPH simulation of its binary wind collision

The very massive star system $\eta$ Carinae exhibits regular 5.54-year (2024-day) period disruptive events in wavebands ranging from the radio to X-ray. There is a growing consensus that these events likely stem from periastron passage of an (as yet) unseen companion in a highly eccentric ($\epsilon \sim 0.9$) orbit. This paper presents three-dimensional (3-D) Smoothed Particle Hydrodynamics (SPH) simulations of the orbital variation of the binary wind-wind collision, and applies these to modeling the X-ray light curve observed by the Rossi X-ray Timing Explorer (RXTE). By providing a global 3-D model of the phase variation of the density of the interacting winds, the simulations allow computation of the associated variation in X-ray absorption, presumed here to originate from near the apex of the wind-wind interaction cone. We find that the observed RXTE light curve can be readily fit if the observer's line of sight is within this cone along the general direction of apastron. Specifically, the data are well fit by an assumed inclination $i = 45^{\circ}$ for the orbit's polar axis, which is thus consistent with orbital angular momentum being along the inferred polar axis of the Homunculus nebula. The fits also constrain the position angle $\phi$ that an orbital-plane projection makes with the apastron side of the semi-major axis, strongly excluding positions $\phi < 9^{\circ}$ along or to the retrograde side of the axis, with the best fit position given by $\phi = 27^{\circ}$. Overall the results demonstrate the utility of a fully 3-D dynamical model for constraining the geometric and physical properties of this complex colliding-wind binary system.Comment: 5 pages, 4 figures, accepted to MNRAS Letter

X-ray Modeling of \eta\ Carinae and WR140 from SPH Simulations

The colliding wind binary (CWB) systems \eta\ Carinae and WR140 provide unique laboratories for X-ray astrophysics. Their wind-wind collisions produce hard X-rays that have been monitored extensively by several X-ray telescopes, including RXTE. To interpret these RXTE X-ray light curves, we model the wind-wind collision using 3D smoothed particle hydrodynamics (SPH) simulations. Adiabatic simulations that account for the absorption of X-rays from an assumed point source at the apex of the wind-collision shock cone by the distorted winds can closely match the observed 2-10keV RXTE light curves of both \eta\ Car and WR140. This point-source model can also explain the early recovery of \eta\ Car's X-ray light curve from the 2009.0 minimum by a factor of 2-4 reduction in the mass loss rate of \eta\ Car. Our more recent models relax the point-source approximation and account for the spatially extended emission along the wind-wind interaction shock front. For WR140, the computed X-ray light curve again matches the RXTE observations quite well. But for \eta\ Car, a hot, post-periastron bubble leads to an emission level that does not match the extended X-ray minimum observed by RXTE. Initial results from incorporating radiative cooling and radiatively-driven wind acceleration via a new anti-gravity approach into the SPH code are also discussed.Comment: 5 pages, 3 figures, Proceedings of the 39th Li\'ege Astrophysical Colloquium, held in Li\`ege 12-16 July 2010, edited by G. Rauw, M. De Becker, Y. Naz\'e, J.-M. Vreux, P. William

Role of electron-electron and electron-phonon interaction effect in the optical conductivity of VO2

We have investigated the charge dynamics of VO2 by optical reflectivity measurements. Optical conductivity clearly shows a metal-insulator transition. In the metallic phase, a broad Drude-like structure is observed. On the other hand, in the insulating phase, a broad peak structure around 1.3 eV is observed. It is found that this broad structure observed in the insulating phase shows a temperature dependence. We attribute this to the electron-phonon interaction as in the photoemission spectra.Comment: 6 pages, 8 figures, accepted for publication in Phys. Rev.

The binary period and outburst behaviour of the SMC X-ray binary pulsar system SXP504

A probable binary period has been detected in the optical counterpart to the X-ray source CXOU J005455.6-724510 = RX J0054.9-7245 = AXJ0054.8-7244 = SXP504 in the Small Magellanic Cloud. This source was detected by Chandra on 04 Jul 2002 and subsequently observed by XMM-Newton on 18 Dec 2003. The source is coincident with an Optical Gravitational Lensing (OGLE) object in the lightcurves of which several optical outburst peaks are visible at ~ 268 day intervals. Timing analysis shows a period of 268.6 +/- 0.1 days at > 99% significance. Archival Rossi X-ray Timing Explorer (RXTE) data for the 504s pulse-period has revealed detections which correspond closely with predicted or actual peaks in the optical data. The relationship between this orbital period and the pulse period of 504s is within the normal variance found in the Corbet diagram.Comment: Accepted by MNRAS. 1 LATEX page. 4 figure

Magnetization Plateau of an S=1 Frustrated Spin Ladder

We study the magnetization plateau at 1/4 of the saturation magnetization of the S=1 antiferromagnetic spin ladder both analytically and numerically, with the aim of explaining recent experimental results on BIP-TENO by Goto et al. We propose two mechanisms for the plateau formation and clarify the plateau phase diagram on the plane of the coupling constants between spins