A toy model for X-ray spectral variability of active galactic nuclei

The long term X-ray spectral variability of ten active galactic nuclei (AGN) shows a positive spectral index-flux correlation for each object (Sobolewska & Papadakis 2009). An inner advection dominated accretion flow (ADAF) may connect to a thin disc/corona at a certain transition radius, which are responsible for hard X-ray emission in AGN. The ADAF is hot and its X-ray spectrum is hard, while the corona above the disc is relatively cold and its X-ray spectrum is therefore soft. The radiation efficiency of the ADAF is usually much lower than that of the thin disc. The increase of the transition radius may lead to decreases of the spectral index (i.e., a hard spectrum) and the X-ray luminosity even if the accretion rate is fixed, and vice versa. We propose that such X-ray variability is caused by the change of the transition radius. Our model calculations can reproduce the observed index-flux correlations, if the transition radius fluctuates around an equilibrium position, and the radiation efficiency of ADAFs is {\guillemotright} 5 per cent of that for a thin disc. The average spectral index-Eddington ratio correlation in the AGN sample can also be reproduced by our model calculations, if the equilibrium transition radius increases with decreasing mass accretion rate.Comment: 5 pages, accepted by MNRAS Letter

Examining the Financial Payoffs to Prior Entrepreneurial Experience

My dissertation includes three essays with an attempt to understand the financial payoffs to prior entrepreneurial experience. In essay one, we re-visit the important relationship between prior entrepreneurial experience and the probability of switching into entrepreneurship from wage employment using two different models: logistic regression – a standard statistical model commonly used by management scholars and random forests – a powerful machine learning tool for analyzing big data. Through comparing the findings of these two models, essay one reconciles the theoretical and empirical uncertainty on this relationship. It also illuminates the benefits of using contemporary approaches to handle big data in re-visiting fundamental questions in entrepreneurship. In essay two, we examine the relative financial payoffs to prior entrepreneurial experience inside versus outside the entrepreneurial context. Testing from a sample of entrepreneurs who were at risk of making a career choice between serial entrepreneurship and wage employment, our study shows that the financial payoffs to prior entrepreneurial experience can be extended, and much higher, in serial entrepreneurship than in wage employment. By doing so, it holds a host of novel implications for understanding the motivation of entrepreneurship and also adds to the research of serial entrepreneurship. In essay three, we argue that the matching model, which suggests individuals self-sect themselves into a career with relative advantages, may help explain serial entrepreneurship. Through decomposing prior entrepreneurial experience into venture success experience, venture managerial experience and venture industry experience, we find partial supports for this argument. Essay three then enhances our understanding for the motivation of serial entrepreneurship

High Sensitivity Stress Sensor Based on Hybrid Materials

A sensing device is used to detect the spatial distributions of stresses applied by physical contact with the surface of the sensor or induced by pressure, temperature gradients, and surface absorption. The sensor comprises a hybrid active layer that includes luminophores doped in a polymeric or organic host, altogether embedded in a matrix. Under an electrical bias, the sensor simultaneously converts stresses into electrical and optical signals. Among many applications, the device may be used for tactile sensing and biometric imaging

Multiplicative semigroup automorphisms of upper triangular matrices over rings

AbstractSuppose R is a ring with 1 and C a central subring of R. Let Tn(R) be the C-algebra of upper triangular n × n matrices over R. Recently several authors have shown that if R is sufficiently well behaved, then every C-automorphism of Tn(R) is the composites of an inner automorphism and an automorphism induced from a C-automorphism of R (see [1–5]). To generalize these results, in this paper we prove that if n ⩾ 2 and R is a semiprime ring or a ring in which all idempotents are central, then f : Tn(R) → Tn(R) (Tn(R) is only regarded as a multiplicative semigroup) is a multiplicative semigroup automorphism if and only if there exist a nonsingular matrix P in Tn(R) and a ring automorphism τ of R such that f(A) = P−1AτP ∀A = (aij)n×n ∈ τn(R),where Aτ = (τ(aij))n×n