Hawking radiation via tachyon condensation and its implications to tachyon cosmology

Hawking radiation can be derived from the collapsing process of matter to form a black hole. In this work, we show in more detail that the freely infalling process of a probe (D-)particle (or point-like object) in a non-extreme black hole background is essentially a tachyon condensation process. That is, a probe D-particle will behave as an unstable D-particle in the near-horizon region of a non-extreme black hole. From this point of view, Hawking radiation can be viewed as the thermal radiation from rolling tachyon on an unstable D-particle (i.e., the infalling probe) at the Hagedorn temperature. The result has interesting implications to tachyon cosmology: the uniform tachyon rolling in cosmology can automatically create particle pairs at late times, via a mechanism just like the Hawking radiation process near a black hole. So this particle creation process can naturally give rise to a hot universe with thermal perturbations beyond tachyon inflation, providing an alternative reheating mechanism.Comment: 22 page

Mechanism of very high energy radiation in BL Lacertae object 3C 66A

Our goal is to understand the nature of blazars and the mechanisms for the generation of high-energy $\gamma$-rays, through the investigation of the blazar 3C 66A. We model the high energy spectrum of 3C 66A, which has been observed recently with the Fermi-LAT and VERITAS telescope. The spectrum has a hard change from the energy range of 0.2-100 GeV to 200-500 GeV in recent almost contemporaneous observations of two telescopes. The de-absorbed VERITAS spectrum greatly depends on the redshift, which is highly uncertain. If z=0.444 is adopted, we are able to use the SSC model to produce the Fermi-LAT component and the EC model to the VERITAS component. However, if z=0.1, the intrinsic VERITAS spectrum will be softer, there will be a smooth link between the Fermi-LAT and VERITAS spectra which can be explained using a SSC model.Comment: 5 pages, 3 figures. accepted for publication in A&

Particle Acceleration And Emission Processes In Mrk 421

We investigate the X-ray and $\gamma$-ray flares of Mrk 421 on 2008 June 6-15 using the synchrotron self- Compton(SSC) model with electron acceleration, in which an evident correlation between the X-ray and $\gamma$-ray bands appears, while no significant correlation between the optical and X-ray band is observed. We argue that the emission from Mrk 421 may originate from two different components. One is the steady component from the outer region that is mainly attributed to the optical band, in which the electrons are accelerated by first-order Fermi acceleration mechanism. We use a steady electron spectrum to produce the synchrotron self-Compton emission. The other is the variable component from the inner region, in which the electrons are accelerated by the stochastic acceleration process. We use the time-dependent SSC model to produce the emission from the variable component. We suggest that the flares are due to the hardening of the electron spectrum under the process of the stochastic acceleration, which leads to the hardening of the observed spectrum in the X-ray and $\gamma$-ray bands. Furthermore, we find that the energy densities of electrons and magnetic fields are near equipartition in both jet regions.Comment: 6 pages, 2 figures, accepted for publication in PAS

Regularization for Cox's proportional hazards model with NP-dimensionality

High throughput genetic sequencing arrays with thousands of measurements per sample and a great amount of related censored clinical data have increased demanding need for better measurement specific model selection. In this paper we establish strong oracle properties of nonconcave penalized methods for nonpolynomial (NP) dimensional data with censoring in the framework of Cox's proportional hazards model. A class of folded-concave penalties are employed and both LASSO and SCAD are discussed specifically. We unveil the question under which dimensionality and correlation restrictions can an oracle estimator be constructed and grasped. It is demonstrated that nonconcave penalties lead to significant reduction of the "irrepresentable condition" needed for LASSO model selection consistency. The large deviation result for martingales, bearing interests of its own, is developed for characterizing the strong oracle property. Moreover, the nonconcave regularized estimator, is shown to achieve asymptotically the information bound of the oracle estimator. A coordinate-wise algorithm is developed for finding the grid of solution paths for penalized hazard regression problems, and its performance is evaluated on simulated and gene association study examples.Comment: Published in at http://dx.doi.org/10.1214/11-AOS911 the Annals of Statistics (http://www.imstat.org/aos/) by the Institute of Mathematical Statistics (http://www.imstat.org