The accretion-ejection coupling in Galactic accreting black holes

During outburst, black-hole X-ray binaries (BHXRBs) exhibit time variability over a wide range of timescales from subseconds to years and wavelengths from radio to X-rays, offering a unique insight into the accretion-ejection processes and the underlying physics of these sources. Even if still being in its infancy, the modelling of the so-called quasi- periodic oscillations and the broadband noises, holds the promise to constrain the geometry of the accretion flow around BHs, especially the accreting corona that is believed to connect the accretion and the ejection zones. In the past decades, tremendous progress has been made in understanding the nature of the high-energy corona. However, many questions persist: What is the nature of the corona and how does its geometry evolve during the outburst? What is the role of the corona during the accretion-ejection process? Is the change of the time variability and spectral state determined by the appearance and disappearance of the corona? Using data from NICER and Insight-HXMT, I found that the geometry of the corona in MAXI J1535-571 evolved during its 2017 outburst. The comparison with the evolution of radio flux density further indicates a morphological coupling between the corona and the jet. Using data from RXTE, I investigated the high-frequency (~ 70 Hz) broadband variability component in GRS 1915+105 and GX 339-4. This component, as a proxy, shows a radiative coupling between the corona and the jet

Cross-layer key establishment protocols for wireless devices

There are some problems in existing key establishment protocols. To alleviate these problems, in our thesis, we designed a few cross-layer key establishment protocols by cooperatively using the characteristics of higher layers and physical layer. Additionally, the security and performance analyses show that our protocols perform better than others.<br /

Iron line spectroscopy of black holes in asymptotically safe gravity

We study the iron line shape expected in the reflection spectrum of accretion disks around black holes in asymptotically safe gravity. We compare the results of our simulations with the iron line shapes expected in the reflection spectrum of accretion disks around Kerr black holes to see if the technique of iron line spectroscopy can be used as a tool to test asymptotically safe gravity. Our analysis shows that current X-ray facilities are surely unable to distinguish black holes in asymptotically safe gravity from black holes in Einstein's gravity. In the case of the next generation of X-ray missions, which promise to provide unprecedented high quality data, the question remains open because it cannot be addressed within our simplified model.Comment: 9 pages, 6 figures. v2: refereed versio

Polynomial based key predistribution scheme in wireless mesh networks

Wireless mesh networks (WMNs) have the ability to integrate with other networks while providing a fast and cost-saving deployment. The network security is one of important challenge problems in this kind of networks. This paper is focused on key management between mesh and sensor networks. We propose an efficient key pre-distribution scheme based on two polynomials in wireless mesh networks by employing the nature of heterogeneity. Our scheme realizes the property of bloom filters, i.e., neighbor nodes can discover their shared keys but have no knowledge on the different keys possessed by the other node, without the probability of false positive. The analysis presented in this paper shows that our scheme has the ability to establish three different security level keys and achieves the property of self adaptive security for sensor networks with acceptable computation and communication consumption

Strong variational sufficiency of nonsmooth optimization problems on Riemannian manifolds

The Riemannian augmented Lagrangian method (RALM) is proposed to solve the nonsmooth optimization problems on Riemannian manifolds. However, the local convergence rate of this algorithm still remains unknown without imposing any constraint qualifications. In this paper, we introduce the manifold variational sufficient condition and show that its strong version is equivalent to the manifold strong second-order sufficient condition (M-SSOSC) in some cases. More importantly, we formulate a local dual problem based on this condition, consequently establishing the R-linear convergence rate of RALM. Furthermore, the validity of the semismooth Newton method for solving the RALM subproblem is demonstrated under the M-SSOSC