A Comprehensive Analysis of Fermi Gamma-ray Burst Data: III. Energy-Dependent T90 Distributions of GBM GRBs and Instrumental Selection Effect on Duration Classification

The durations (T90) of 315 GRBs detected with Fermi/GBM (8-1000 keV) by 2011 September are calculated using the Bayesian Block method. We compare the T90 distributions between this sample and those derived from previous/current GRB missions. We show that the T90 distribution of this GRB sample is bimodal, with a statistical significance level being comparable to those derived from the BeppoSAX/GRBM sample and the Swift/BAT sample, but lower than that derived from the CGRO/BATSE sample. The short-to-long GRB number ratio is also much lower than that derived from the BATSE sample, i.e., 1:6.5 vs 1:3. We measure T90 in several bands, i.e., 8-15, 15-25, 25-50, 50-100, 100-350, and 350-1000 keV, to investigate the energy-dependence effect of the bimodal T90 distribution. It is found that the bimodal feature is well observed in the 50-100 and 100-350 keV bands, but is only marginally acceptable in the 25-50 keV and 350-1000 keV bands. The hypothesis of the bimodality is confidently rejected in the 8-15 and 15-25 keV bands. The T90 distributions in these bands are roughly consistent with those observed by missions with similar energy bands. The parameter T90 as a function of energy follows \bar T90 \propto E^{-0.20\pm 0.02} for long GRBs. Considering the erratic X-ray and optical flares, the duration of a burst would be even much longer for most GRBs. Our results, together with the observed extended emission of some short GRBs, indicate that the central engine activity time scale would be much longer than T90} for both long and short GRBs and the observed bimodal T90 distribution may be due to an instrumental selection effect.Comment: 29 pages, 2 tables, 9 figures, accepted for publication in Ap

A Comprehensive Study of Gamma-Ray Burst Optical Emission: I. Flares and Early Shallow Decay Component

Well-sampled optical lightcurves of 146 gamma-ray bursts (GRBs) are compiled from the literature. By empirical fitting we identify eight possible emission components and summarize the results in a "synthetic" lightcurve. Both optical flare and early shallow-decay components are likely related to long-term central engine activities. We focus on their statistical properties in this paper. Twenty-four optical flares are obtained from 19 GRBs. The isotropic R-band energy is smaller than 1% of $E_{\gamma, \rm iso}$. The relation between isotropic luminosities of the flares and gamma-rays follows $L^{\rm F}_{\rm R, iso}\propto L_{{\gamma}, \rm iso}^{1.11\pm 0.27}$. Later flares tend to be wider and dimmer, i.e., $w^{\rm F}\sim t^{\rm F}_{\rm p}/2$ and $L^{\rm F}_{\rm R, iso}\propto [t^{\rm F}_{\rm p}/(1+z)]^{-1.15\pm0.15}$. The detection probability of the optical flares is much smaller than that of X-ray flares. An optical shallow decay segment is observed in 39 GRBs. The relation between the break time and break luminosity is a power-law, with an index of $-0.78\pm 0.08$, similar to that derived from X-ray flares. The X-ray and optical breaks are usually chromatic, but a tentative correlation is found. We suggest that similar to the prompt optical emission that tracks $\gamma$-rays, the optical flares are also related to the erratic behavior of the central engine. The shallow decay component is likely related to a long-lasting spinning-down central engine or piling up of flare materials onto the blastwave. Mixing of different emission components may be the reason of the diverse chromatic afterglow behaviors.Comment: 43 pages, 13 figures, 3 tables, accepted for publication in Ap

The formation of superdense plasma sheet in association with the IMF turning from northward to southward

International audienceWe have studied two events in which relatively superdense plasma appeared near geosynchronous orbit after the interplanetary magnetic field (IMF) changed from a long period of northward condition to southward condition. We used the magnetosphere MHD model Open Geospace General Circulation Model to simulate these two events and compared the simulation results with observations. The observations and the simulation results show that an extended period of northward IMF caused a cold dense plasma sheet (CDPS) to form in the near tail, and a subsequent southward IMF caused a highly stretched near-tail dipole field, which enhanced the earthward convection and eventually set off reconnection. Some of the cold dense plasma was thus consequently transported to geosynchronous orbit and formed a superdense plasma sheet (SDPS). CDPS is the main source of SDPS in its early stage after the IMF southward turning

The formation of superdense plasma sheet in association with the IMF turning from northward to southward

International audienceWe have studied two events in which relatively superdense plasma appeared near geosynchronous orbit after the interplanetary magnetic field (IMF) changed from a long period of northward condition to southward condition. We used the magnetosphere MHD model Open Geospace General Circulation Model to simulate these two events and compared the simulation results with observations. The observations and the simulation results show that an extended period of northward IMF caused a cold dense plasma sheet (CDPS) to form in the near tail, and a subsequent southward IMF caused a highly stretched near-tail dipole field, which enhanced the earthward convection and eventually set off reconnection. Some of the cold dense plasma was thus consequently transported to geosynchronous orbit and formed a superdense plasma sheet (SDPS). CDPS is the main source of SDPS in its early stage after the IMF southward turning

国家传染病自动预警系统运行状况分析

To analyze the results of application on China Infectious Diseases Automated-alert and Response System (CIDARS) and for further improving the system. Amount of signal, proportion of signal responded, time to signal response, manner of signal verification and the outcome of each signal in CIDARS were descriptively analyzed from July 1, 2008 to June 30, 2010. A total of 533 829 signals were generated nationwide on 28 kinds of infectious diseases in the system. 97.13% of the signals had been responded and the median time to response was 1.1 hours. Among them, 2472 signals were generated by the fixed-value detection method which involved 9 kinds of diseases after the preliminary verification, field investigation and laboratory tests. 2202 signals were excluded, and finally 246 cholera cases, 15 plague cases and 9 H5N1 cases as well as 39 outbreaks of cholera were confirmed. 531 357 signals were generated by the other method - the 'moving percentile method' which involved 19 kinds of diseases. The average amount of signal per county per week was 1.65, with 6603 signals (1.24%) preliminarily verified as suspected outbreaks and 1594 outbreaks were finally confirmed by further field investigation. For diseases in CIDARS, the proportion of signals related to suspected outbreaks to all triggered signals showed a positive correlation with the proportion of cases related to outbreaks of all the reported cases (r = 0.963, P &lt; 0.01). The signals of CIDARS were responded timely, and the signal could act as a clue for potential outbreaks, which helped enhancing the ability on outbreaks detection for local public health departments.</p