arXiv subjects

L. Ma

Publications and source records attributed to L. Ma.

96 records · Page 6Linked to original sources

arXiv2008

Crab cavities for linear colliders

Crab cavities have been proposed for a wide number of accelerators and interest in crab cavities has recently increased after the successful operation of a pair of crab cavities in KEK-B. In particular crab cavities are required for both the ILC and CLIC linear colliders for bunch alignment. Consideration of bunch structure and size constraints favour a 3.9 GHz superconducting, multi-cell cavity as the solution for ILC, whilst bunch structure and beam-loading considerations suggest an X-band copper travelling wave structure for CLIC. These two cavity solutions are very different in design but share complex design issues. Phase stabilisation, beam loading, wakefields and mode damping are fundamental issues for these crab cavities. Requirements and potential design solutions will be discussed for both colliders.

physics.acc-ph↗

arXiv2008

Spectral hardness evolution characteristics of tracking Gamma-ray Burst pulses

Employing a sample presented by Kaneko et al. (2006) and Kocevski et al. (2003), we select 42 individual tracking pulses (here we defined tracking as the cases in which the hardness follows the same pattern as the flux or count rate time profile) within 36 Gamma-ray Bursts (GRBs) containing 527 time-resolved spectra and investigate the spectral hardness, $E_{peak}$ (where $E_{peak}$ is the maximum of the $νF_ν$ spectrum), evolutionary characteristics. The evolution of these pulses follow soft-to-hard-to-soft (the phase of soft-to-hard and hard-to-soft are denoted by rise phase and decay phase, respectively) with time. It is found that the overall characteristics of $E_{peak}$ of our selected sample are: 1) the $E_{peak}$ evolution in the rise phase always start on the high state (the values of $E_{peak}$ are always higher than 50 keV); 2) the spectra of rise phase clearly start at higher energy (the median of $E_{peak}$ are about 300 keV), whereas the spectra of decay phase end at much lower energy (the median of $E_{peak}$ are about 200 keV); 3) the spectra of rise phase are harder than that of the decay phase and the duration of rise phase are much shorter than that of decay phase as well. In other words, for a complete pulse the initial $E_{peak}$ is higher than the final $E_{peak}$ and the duration of initial phase (rise phase) are much shorter than the final phase (decay phase). This results are in good agreement with the predictions of Lu et al. (2007) and current popular view on the production of GRBs. We argue that the spectral evolution of tracking pulses may be relate to both of kinematic and dynamic process even if we currently can not provide further evidences to distinguish which one is dominant. Moreover, our statistical results give some witnesses to constrain the current GRB model.

astro-ph↗

arXiv2008

Absorption of 10 Gev--1 Tev Gamma Rays by Radiation from Broad-Line Region in 3C 279

In this paper, we study the photon-photon pair production optical depth for 10 GeV--1 TeV gamma rays from 3C 279 due to the diffuse radiation of broad-line region (BLR). Assuming a power-law spectrum of $E_γ^{-a_2}$ for the photon intensity of very high energy (VHE) gamma rays, $a_1 \gtrsim 405$ and $a_2\gtrsim 6.4$ are inferred by the integrated photon fluxes measured by MAGIC and HESS. Based on this power-law spectrum, the pre-absorbed spectra are inferred by correcting the photon-photon absorption on the diffuse photons of the BLR (internal absorption) and the extragalactic background light (external absorption). Position of gamma-ray emitting region $R_{\rmγ}$ determines the relative contributions of this two diffuse radiation to the total absorption for 10 GeV--1 TeV gamma rays. The internal absorption could make spectral shape of gamma rays more complex than only corrected for the external absorption, and could lead to the formation of arbitrary softening and hardening gamma-ray spectra. It should be necessary for the internal absorption to be considered in studying 10 GeV--1 TeV gamma rays from powerful blazars. The energies of annihilated gamma-ray photons due to the internal absorption are likely to be mainly reradiated around GeV. Our results indicate that $R_{\rmγ}$ may be between the inner and outer radii of the BLR for 3C 279. This implies for powerful blazars that $R_{\rmγ}$ might be neither inside the BLR cavity nor outside the BLR, but be within the BLR shell. Observations of $\it GLAST$, MAGIC, HESS, and VERITAS in the near future could give more constraints on the position of the gamma-ray emitting region relative to the BLR.

astro-ph↗

arXiv2008

Tests for Standard Accretion Disk Models by Variability in Active Galactic Nuclei

In this paper, standard accretion disk models of AGNs are tested using light curves of 26 objects well observed for reverberation mapping. Time scales of variations are estimated by the most common definition of the variability time scale and the zero-crossing time of the autocorrelation function of the optical light curves for each source. The measured time scales of variations by the two methods are consistent with each other. If the typical value of the viscosity parameter $α\sim 0.1$ is adopted, the measured optical variability time scales are most close to the thermal time scales of the standard disks. If $α$ is allowed to range from $\sim 0.03$ to $\sim 0.2$, the measured time scales are consistent with the thermal time scales of the standard disks. There is a linear relation between the measured variability time scales and black hole masses, and this linear relation is qualitatively consistent with expectation of the standard accretion disk models. The time lags measured by the ZDCF between different bands are on the order of days. The measured time lags of NGC 4151 and NGC 7469 are marginally consistent with the time lags estimated in the case of continuum thermal reprocessing for the standard accretion disk models. However, the measured time lags of NGC 5548 and Fairall 9 are unlikely to be the case of continuum thermal reprocessing. Our results are unlikely to be inconsistent with or are likely to be conditionally in favor of the standard accretion disk models of AGNs.

astro-ph↗

arXiv2004

Operator-sum representation of time-dependent density operators and its applications

We show that any arbitrary time-dependent density operator of an open system can always be described in terms of an operator-sum representation regardless of its initial condition and the path of its evolution in the state space, and we provide a general expression of Kraus operators for arbitrary time-dependent density operator of an $N$-dimensional system. Moreover, applications of our result are illustrated through several examples.

quant-ph↗

arXiv2003

A Remark on Soliton Equation of Mean Curvature Flow

In this short note, we consider self-similar immersions $F: \mathbb{R}^n \to \mathbb{R}^{n+k}$ of the Graphic Mean Curvature Flow of higher co-dimension. We show that the following is true: Let $F(x) = (x,f(x)), x \in \mathbb{R}^{n}$ be a graph solution to the soliton equation $$ \bar{H}(x) + F^{\bot}(x) = 0. $$ Assume $\sup_{\mathbb{R}^{n}}|Df(x)| \le C_{0} < + \infty$. Then there exists a unique smooth function $f_{\infty}: \mathbb{R}^{n}\to \mathbb{R}^k$ such that $$ f_{\infty}(x) = \lim_{λ\to \infty}f_λ(x) $$ and $$ f_{\infty}(r x)=r f_{\infty}(x) $$ for any real number $r\not= 0$, where $$ f_λ(x) = λ^{-1}f(λx). $$

math.DG↗