Variations in the pulsation and spectral characteristics of OAO 1657-415

We present broad-band pulsation and spectral characteristics of the accreting X-ray pulsar OAO 1657-415 with a 2.2 d long Suzaku observation carried out covering its orbital phase range similar to 0.12-0.34, with respect to the mid-eclipse. During the last third of the observation, the X-ray count rate in both the X-ray Imaging Spectrometer (XIS) and the HXD-PIN instruments increased by a factor of more than 10. During this observation, the hardness ratio also changed by a factor of more than 5, uncorrelated with the intensity variations. In two segments of the observation, lasting for similar to 30-50 ks, the hardness ratio is very high. In these segments, the spectrum shows a large absorption column density and correspondingly large equivalent widths of the iron fluorescence lines. We found no conclusive evidence for the presence of a cyclotron line in the broad-band X-ray spectrum with Suzaku. The pulse profile, especially in the XIS energy band, shows evolution with time but not so with energy. We discuss the nature of the intensity variations, and variations of the absorption column density and emission lines during the duration of the observation as would be expected due to a clumpy stellar wind of the supergiant companion star. These results indicate that OAO 1657-415 has characteristics intermediate to the normal supergiant systems and the systems that show fast X-ray transient phenomena.

On the Communication Complexity of Secret Key Generation in the Multiterminal Source Model

Communication complexity refers to the minimum rate of public communication required for generating a maximal-rate secret key (SK) in the multiterminal source model of Csiszar and Narayan. Tyagi recently characterized this communication complexity for a two-terminal system. We extend the ideas in Tyagi's work to derive a lower bound on communication complexity in the general multiterminal setting. In the important special case of the complete graph pairwise independent network (PIN) model, our bound allows us to determine the exact linear communication complexity, i.e., the communication complexity when the communication and SK are restricted to be linear functions of the randomness available at the terminals.

Dynamics of Glass Forming Liquids with Randomly Pinned Particles

It is frequently assumed that in the limit of vanishing cooling rate, the glass transition phenomenon becomes a thermodynamic transition at a temperature T-K. However, with any finite cooling rate, the system falls out of equilibrium at temperatures near T-g(> T-K), implying that the very existence of the putative thermodynamic phase transition at T-K can be questioned. Recent studies of systems with randomly pinned particles have hinted that the thermodynamic glass transition may be observed for liquids with randomly pinned particles. This expectation is based on the results of approximate calculations that suggest that the thermodynamic glass transition temperature increases with increasing concentration of pinned particles and it may be possible to equilibrate the system at temperatures near the increased transition temperature. We test the validity of this prediction through extensive molecular dynamics simulations of two model glass-forming liquids in the presence of random pinning. We find that extrapolated thermodynamic transition temperature T-K does not show any sign of increasing with increasing pinning concentration. The main effect of pinning is found to be a rapid decrease in the kinetic fragility of the system with increasing pin concentration. Implications of these observations for current theories of the glass transition are discussed.