4,753 research outputs found
The origin of the optical flashes: The case study of GRB 080319B and GRB 130427A
Correlations between optical flashes and gamma-ray emissions in gamma-ray
bursts have been searched in order to clarify the question whether these
emissions occur at internal and/or external shocks. Among the most powerful
gamma-ray bursts ever recorded are GRB 080319B and GRB 130427A which at early
phase presented bright optical flashes possible correlated with -ray
components. Additionally, both bursts were fortuitously located within the
field of view of the TeV -ray Milagro and HAWC observatories, and
although no statistically significant excess of counts were collected, upper
limits were placed on the GeV - TeV emission. Considering the synchrotron
self-Compton emission from internal shocks and requiring the GeV-TeV upper
limits we found that the optical flashes and the -ray components are
produced by different electron populations. Analyzing the optical flashes
together the multiwavelength afterglow observation, we found that these flashes
can be interpreted in the framework of the synchrotron reverse-shock model when
outflows have arbitrary magnetizations.Comment: 9 pages, 5 figures and 4 tables. Accepted for publication in Ap
Gamma-Ray Bursts: Temporal Scales and the Bulk Lorentz Factor
For a sample of Swift and Fermi GRBs, we show that the minimum variability
timescale and the spectral lag of the prompt emission is related to the bulk
Lorentz factor in a complex manner: For small 's, the variability
timescale exhibits a shallow (plateau) region. For large 's, the
variability timescale declines steeply as a function of (). Evidence is also presented for an intriguing
correlation between the peak times, t, of the afterglow emission and the
prompt emission variability timescale.Comment: Accepted for publication in Ap
Laser thermoelastic generation in metals above the melt threshold
An approach is presented for calculating thermoelastic generation of ultrasound in a metal plate exposed to nanosecond pulsed laser heating, sufficient to cause melting but not ablation. Detailed consideration is given to the spatial and temporal profiles of the laser pulse, penetration of the laser beam into the sample, the appearance and subsequent growth and then contraction of the melt pool, and the time dependent thermal conduction in the melt and surrounding solid throughout. The excitation of the ultrasound takes place during and shortly after the laser pulse and occurs predominantly within the thermal diffusion length of a micron or so beneath the surface. It is shown how, because of this, the output of the thermal simulations can be expressed as axially symmetric transient radial and normal surface force distributions. The epicentral displacement response to these force distributions is obtained by two methods, the one based on the elastodynamic Green’s functions for plate geometry determined by the Cagniard generalized ray method and the other using a finite
element numerical method. The two approaches are in very close agreement. Numerical simulations are reported on the epicentral displacement response of a 3.12mm thick tungsten plate irradiated with a 4 ns pulsed laser beam with Gaussian spatial profile, at intensities below and above the melt threshol
Learning of Signaling Networks: Molecular Mechanisms
Molecular processes of neuronal learning have been well described. However, learning mechanisms of non-neuronal cells are not yet fully understood at the molecular level. Here, we discuss molecular mechanisms of cellular learning, including conformational memory of intrinsically disordered proteins (IDPs) and prions, signaling cascades, protein translocation, RNAs [miRNA and long noncoding RNA (lncRNA)], and chromatin memory. We hypothesize that these processes constitute the learning of signaling networks and correspond to a generalized Hebbian learning process of single, non-neuronal cells, and we discuss how cellular learning may open novel directions in drug design and inspire new artificial intelligence methods. © 2020 The Author
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