Time lag between prompt optical emission and gamma-rays in GRBs

The prompt optical emission contemporaneous with the $\gamma$-rays from $\gamma$-ray bursts (GRBs) carries important information on the central engine and explosion mechanism. We study the time lag between prompt optical emission and $\gamma$-rays in GRB 990123 and GRB 041219a, which are the only two GRBs had been detected at optical wavelengths during the ascending burst phase. Assuming profiles of prompt optical light curves are the same as the prompt $\gamma$-rays, we simulate optical light curves with different time lags and compare them with the observed optical flux. Then the best fit time lag and its error are determined by chi-squared values. We find that time lags between prompt optical emission and $\gamma$-rays in GRB host galaxy rest-frames are consistent in the two GRBs, which is $5\sim7$ s for GRB 990123 and $1\sim5$ s for GRB 041219a. This result is consistent with a common origin of prompt optical and $\gamma$-ray emissions in the two GRBs. Based on synchrotron cooling model, we also derive the parameters for the two GRBs.Comment: 4 pages, 3 figures; accepted for publication in A&

No-reference analysis of decoded MPEG images for PSNR estimation and post-processing

We propose no-reference analysis and processing of DCT (Discrete Cosine Transform) coded images based on estimation of selected MPEG parameters from the decoded video. The goal is to assess MPEG video quality and perform post-processing without access to neither the original stream nor the code stream. Solutions are presented for MPEG-2 video. A method to estimate the quantization parameters of DCT coded images and MPEG I-frames at the macro-block level is presented. The results of this analysis is used for deblocking and deringing artifact reduction and no-reference PSNR estimation without code stream access. An adaptive deringing method using texture classification is presented. On the test set, the quantization parameters in MPEG-2 I-frames are estimated with an overall accuracy of 99.9% and the PSNR is estimated with an overall average error of 0.3dB. The deringing and deblocking algorithms yield improvements of 0.3dB on the MPEG-2 decoded test sequences

Comment on "Mean First Passage Time for Anomalous Diffusion"

We correct a previously erroneous calculation [Phys. Rev. E 62, 6065 (2000)] of the mean first passage time of a subdiffusive process to reach either end of a finite interval in one dimension. The mean first passage time is in fact infinite.Comment: To appear in Phys. Rev.

Low latency via redundancy

Low latency is critical for interactive networked applications. But while we know how to scale systems to increase capacity, reducing latency --- especially the tail of the latency distribution --- can be much more difficult. In this paper, we argue that the use of redundancy is an effective way to convert extra capacity into reduced latency. By initiating redundant operations across diverse resources and using the first result which completes, redundancy improves a system's latency even under exceptional conditions. We study the tradeoff with added system utilization, characterizing the situations in which replicating all tasks reduces mean latency. We then demonstrate empirically that replicating all operations can result in significant mean and tail latency reduction in real-world systems including DNS queries, database servers, and packet forwarding within networks