A New Model for the Hard Time Lags in Black Hole X-Ray Binaries

The time-dependent Comptonized output of a cool soft X-ray source drifting inward through an inhomogeneous hot inner disk or corona is numerically simulated. We propose that this scenario can explain from first principles the observed trends in the hard time lags and power spectra of the rapid aperiodic variability of the X-ray emission of Galactic black-hole candidates.Comment: 10 pages, including 2 figures; uses epsf.sty, rotate.sty; accepted for ApJ Letter

J06587-5558 -- A Very Unusual Polarised Radio Source

We have found a peculiar radio source in the field of one of the hottest known clusters of galaxies 1E0657-56. It is slightly extended, highly polarised (54% at 8.8GHz) and has a very steep spectrum, with alpha ~ -1 at 1.3 GHz, steepening to ~ -1.5 at 8.8GHz (S \propto nu^alpha). No extragalactic sources are known with such high integrated polarisation, and sources with spectra as steep as this are rare. In this paper, we report the unusual properties of the source J06587-5558 and speculate on its origin and optical identification.Comment: 5 pages, 6 figures, accepted by MNRAS letter

Tunable Quantum Fluctuation-Controlled Coherent Spin Dynamics

Temporal evolution of a macroscopic condensate of ultra cold atoms is usually driven by mean field potentials, either due to scattering between atoms or due to coupling to external fields; and coherent quantum dynamics have been observed in various cold-atom experiments. In this article, we report results of studies of a class of quantum spin dynamics which are purely driven by zero point quantum fluctuations of spin collective coordinates. Unlike the usual mean-field coherent dynamics, quantum fluctuation-controlled spin dynamics or QFCSD studied here are very sensitive to variation of quantum fluctuations and can be tuned by four to five order of magnitude using optical lattices. They have unique dependence on optical lattice potential depths and quadratic Zeeman fields. QFCSD can be potentially used to calibrate quantum fluctuations and investigate correlated fluctuations and various universal scaling properties near quantum critical points.Comment: 14 pages, 12 figures included; including detailed discussions on thermal effects, trapping potentials and spin exchange losses. (To appear in PRA

A Thermal-Nonthermal Inverse Compton Model for Cyg X-1

Using Monte Carlo methods to simulate the inverse Compton scattering of soft photons, we model the spectrum of the Galactic black hole candidate Cyg X-1, which shows evidence of a nonthermal tail extending beyond a few hundred keV. We assume an ad hoc sphere of leptons, whose energy distribution consists of a Maxwellian plus a high energy power-law tail, and inject 0.5 keV blackbody photons. The spectral data is used to constrain the nonthermal plasma fraction and the power-law index assuming a reasonable Maxwellian temperature and Thomson depth. A small but non-negligible fraction of nonthermal leptons is needed to explain the power-law tail.Comment: 5 pages, 2 PostScript figure, uses aipproc.sty, to appear in Proceedings of Fourth Compton Symposiu

Comptonization signatures in the rapid aperiodic variability of Galactic black-hole candidates

We investigate the effect of inverse-Compton scattering of flares of soft radiation in different geometries of a hot, Comptonizing region and a colder accretion disk around a solar-mass black hole. The photon-energy dependent light curves, their Fourier transforms, power spectra and Fourier-period dependent time lags of hard photons with respect to softer photons are discussed. On the basis of a comparison with existing data we find arguments against Comptonization of external soft radiation as well as Comptonization in a homogeneous medium as dominant mechanisms for the rapid aperiodic variability in Galactic black-hole candidates. Possible further observational tests for the influence of Comptonization on the rapid aperiodic variability of Galactic black-hole candidates are suggested.Comment: 32 pages, including 10 figures and 2 tables; uses epsf.sty, rotate.sty; submitted to Ap