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Flares, broadening of the pulse-frequency peak and quasi-periodic oscillations in the transient X-ray pulsar 4U 1901+03
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)After a long quiescence of three decades, the transient X-ray pulsar 4U 1901+03 became highly active in 2003 February. From the analysis of a large number of Rossi X-ray Timing Explorer/Proportional Counter Array (RXTE/PCA) observations of this source, we report here the detection of X-ray flares, a broadening of the pulse-frequency feature and quasi-periodic oscillations (QPOs). The X-ray flares showed spectral changes, had a duration of 100-300 s, and were more frequent and stronger during the peak of the outburst. In most of the observations during the outburst we also detected a broadening of the pulse-frequency peak. We have also found intensity-dependent changes in the pulse profile at very short time-scales. This reveals a coupling between the periodic and the low-frequency aperiodic variabilities. In addition, near the end of the outburst we have detected a strong QPO feature centred at ˜0.135 Hz. The QPO feature is broad with a quality factor of 3.3 and with an rms value of 18.5 ± 3.1 per cent. Using the QPO frequency and the X-ray luminosity during the QPO detection period we estimated the magnetic field strength of the neutron star as 0.31 × 1012 G which is consistent with the value inferred earlier under the assumption of spin equilibrium
Linear-response formula for finite-frequency thermal conductance of open systems
No full textOpen Access.An exact linear-response expression is obtained for the heat current in a classical Hamiltonian system coupled to heat baths with time-dependent temperatures. The expression is equally valid at zero and finite frequencies. We present numerical results on the frequency dependence of the response function for three different one-dimensional models of coupled oscillators connected to Langevin baths with oscillating temperatures. For momentum conserving systems, a low-frequency peak is seen that is higher than the zero-frequency response for large systems. For momentum nonconserving systems, there is no low-frequency peak. The momentum nonconserving system is expected to satisfy Fourier’s law; however, at the single bond level, we do not see any clear agreement with the predictions of the diffusion equation even at low frequencies. We also derive an exact analytical expression for the response of a chain of harmonic oscillators to a (not necessarily small) temperature difference; the agreement with the linear-response simulation results for the same system is excellent
Atom interferometry and the gravitational redshift
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)From the principle of equivalence, Einstein predicted that clocks slow down
in a gravitational field. Since the general theory of relativity is based on the
principle of equivalence, it is essential to test this prediction accurately. M¨uller,
Peters and Chu claim that a reinterpretation of decade old experiments with
atom interferometers leads to a sensitive test of this gravitational redshift effect
at the Compton frequency. Wolf et al dispute this claim and adduce arguments
against it. In this paper, we distill these arguments to a single fundamental
objection: an atom is not a clock ticking at the Compton frequency. We
conclude that atom interferometry experiments conducted to date do not yield
such sensitive tests of the gravitational redshift. Finally, we suggest a new
interferometric experiment to measure the gravitational redshift, which realizes
a quantum version of the classical clock ‘paradox’.
PACS numbers: 04.80.−y, 03.75.D
Sunyaev-Zeldovich effect
No full textOpen AccessSoon after the discovery of the cosmic background radiation, Zeldovich and Sunyaev proposed that hot gas in galaxy clusters should cast a faint shadow because of the interaction between energetic electrons and the radiation photons. Sunayev-Zeldovich effect is now routinely observed, and it has become an important tool for studying the history of the universe
The nonmodular topological phase and phase singularities
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)Generalizing an earlier definition of the noncyclic geometric phase [R. Bhandari, Phys. Lett. A 157 (1991)221], a nonmodular topological phase is defined with reference to a generic time-dependent two-slit interference experiment involving particles with N internal states in which the internal state of both the beams undergoes unitary evolution. A simple proof of the shorter geodesic rule for closure of the open
path is presented and several useful new insights into the behavior of the dynamical and geometrical components of the phase shift presented. An effective Hamiltonian interpretation of the observable phase
shifts is also presented
Spatiotemporal regulation of chemical reactions by active cytoskeletal remodeling
No full textRestricted Access.Efficient and reproducible construction of signaling and sorting
complexes, both on the surface and within the living cell, is contingent
on local regulation of biochemical reactions by the cellular
milieu. We propose that in many cases this spatiotemporal regulation
can be mediated by interaction with components of the
dynamic cytoskeleton.We show how the interplay between active
contractility and remodeling of the cytoskeleton can result in transient
focusing of passive molecules to form clusters, leading to a
dramatic increase in the reaction efficiency and output levels.
The dynamic cytoskeletal elements that drive focusing behave as
quasienzymes catalyzing the chemical reaction. These ideas are
directly applicable to the cortical actin-dependent clustering of cell
surface proteins such as lipid-tethered GPI-anchored proteins, Ras
proteins, as well as many proteins that have domains that confer
the ability to interact with the actin cytoskeleton. In general such
cytoskeletal driven clustering of proteins could be a cellular mechanism
to spatiotemporally regulate and amplify local chemical
reaction rates in a variety of contexts such as signaling, transcription,
sorting, and endocytosis
Otpical and dielectric properties of liquid crystals and liquid crystal nanocomposites
No full textRestricted Acces
Heat conduction in disordered harmonic lattices with energy-conserving noise
No full textOpen AccessWe study heat conduction in a harmonic crystal whose bulk dynamics is supplemented by random reversals (flips) of the velocity of each particle at a rate . The system is maintained in a nonequilibrium stationary state(NESS) by contacts with Langevin reservoirs at different temperatures. We show that the one-body and pair correlations in this system are the same (after an appropriate mapping of parameters) as those obtained for a model with self-consistent reservoirs. This is true both for the case of equal and random(quenched) masses. While the heat conductivity in the NESS of the ordered system is known explicitly, much less is known about the random mass case. Here we investigate the random system, with velocity flips. We improve the bounds on the Green-Kubo conductivity obtained by C.Bernardin. The conductivity of the 1D system is then studied both numerically and analytically. This sheds some light on the effect of noise on the transport properties of systems with localized states caused by quenched disorder
Stochastic pump of interacting particles
No full textRestricted Access. An open-access version is available at arXiv.org (one of the alternative locations)We consider the overdamped motion of Brownian particles, interacting via particle exclusion, in an external potential that varies with time and space. We show that periodic potentials that maintain specific position-dependent phase relations generate time-averaged directed current of particles. We obtain analytic results for a lattice version of the model using a recently developed perturbative approach. Many interesting features like particle-hole symmetry, current reversal with changing density, and system size dependence of current are obtained. We propose possible experiments to test our predictions