Parallel finite element method utilizing the mode splitting and sigma coordinate for shallow water flows

Abstract Parallel ®nite element method for the analysis of quasi-three dimensional shallow water¯ow is presented. The mode splitting technique and the sigma coordinate (generalized coordinate) are employed to use parallel computers effectively. Parallel implementation of the unstructured grid-based formulation is carried out on the Hitachi parallel-super computer SR2201. The tidal¯ow of Tokyo Bay is simulated for a numerical example. The speed-up ratio and the ef®ciency of the parallelization are investigated. The present method is shown to be a useful and powerful tool for the large scale computation of shallow water¯ows. Introduction The shallow water¯ow analysis is usefully applied to thē ows in oceans, lakes and rivers. A number of numerical methods for the shallow water¯ow based on the two dimensional model have been presented in the past. The present authors have been presented a parallel ®nite element method to solve the large scale computations of shallow water¯ows, such as the storm surges and tidal ows, using the ®ne mesh which represents the geography accurately This paper presents a parallel ®nite element modeling utilizing the mode-splitting and sigma coordinate for the quasi-three dimensional shallow water analysis. The three dimensional¯ow ®eld is divided into two ®elds; the horizontal¯ow ®eld and the vertical¯ow ®eld. The quasi-three dimensional¯ow analysis can be achieved by solving both ®eld mutually. The ®nite element method is employed for the horizontal¯ow ®eld and the ®nite difference method is employed for the vertical distribution of¯ow ®eld. Parallel implementation of the unstructured-grid-based formulations are carried out on the Hitachi parallel-super computer SR2201. The effect of parallelization on the ef®ciency of the computations are examined. Governing equations The shallow water equations can be obtained from the conservation of momentum and mass, assuming a hydrostatic pressure distribution

A Search for Late-time Radio Emission and Fast Radio Bursts from Superluminous Supernovae

We present results of a search for late-time radio emission and fast radio bursts (FRBs) from a sample of type-I superluminous supernovae (SLSNe-I). We used the Karl G. Jansky Very Large Array to observe 10 SLSN-I more than 5 yr old at a frequency of 3 GHz. We searched fast-sampled visibilities for FRBs and used the same data to perform a deep imaging search for late-time radio emission expected in models of magnetar-powered supernovae. No FRBs were found. One SLSN-I, PTF10hgi, is detected in deep imaging, corresponding to a luminosity of 1.2 × 10²⁸ erg s⁻¹. This luminosity, considered with the recent 6 GHz detection of PTF10hgi in Eftekhari et al., supports the interpretation that it is powered by a young, fast-spinning (~ms spin period) magnetar with ~15 M⊙ of partially ionized ejecta. Broadly, our observations are most consistent with SLSNe-I being powered by neutron stars with fast spin periods, although most require more free–free absorption than is inferred for PTF10hgi. We predict that radio observations at higher frequencies or in the near future will detect these systems and begin constraining properties of the young pulsars and their birth environments

Late-Time Radio and Millimeter Observations of Superluminous Supernovae and Long Gamma Ray Bursts: Implications for Obscured Star Formation, Central Engines, and Fast Radio Bursts

We present the largest and deepest late-time radio and millimeter survey to date of superluminous supernovae (SLSNe) and long duration gamma-ray bursts (LGRBs) to search for associated non-thermal synchrotron emission. Using the Karl G. Jansky Very Large Array (VLA) and the Atacama Large Millimeter/submillimeter Array (ALMA), we observed 43 sources at 6 and 100 GHz on a timescale of $\sim 1 - 19$ yr post-explosion. We do not detect radio/mm emission from any of the sources, with the exception of a 6 GHz detection of PTF10hgi (Eftekhari et al. 2019), as well as the detection of 6 GHz emission near the location of the SLSN PTF12dam, which we associate with its host galaxy. We use our data to place constraints on central engine emission due to magnetar wind nebulae and off-axis relativistic jets. We also explore non-relativistic emission from the SN ejecta, and place constraints on obscured star formation in the host galaxies. In addition, we conduct a search for fast radio bursts (FRBs) from some of the sources using VLA Phased-Array observations; no FRBs are detected to a limit of $16$ mJy ($7\sigma$; 10 ms duration) in about 40 min on source per event. A comparison to theoretical models suggests that continued radio monitoring may lead to detections of persistent radio emission on timescales of $\gtrsim {\rm decade}$.Comment: 30 pages; 12 figures; accepted to Ap

A machine learning classifier for fast radio burst detection at the VLBA

Time domain radio astronomy observing campaigns frequently generate large volumes of data. Our goal is to develop automated methods that can identify events of interest buried within the larger data stream. The V-FASTR fast transient system was designed to detect rare fast radio bursts within data collected by the Very Long Baseline Array. The resulting event candidates constitute a significant burden in terms of subsequent human reviewing time. We have trained and deployed a machine learning classifier that marks each candidate detection as a pulse from a known pulsar, an artifact due to radio frequency interference, or a potential new discovery. The classifier maintains high reliability by restricting its predictions to those with at least 90% confidence. We have also implemented several efficiency and usability improvements to the V-FASTR web-based candidate review system. Overall, we found that time spent reviewing decreased and the fraction of interesting candidates increased. The classifier now classifies (and therefore filters) 80%–90% of the candidates, with an accuracy greater than 98%, leaving only the 10%–20% most promising candidates to be reviewed by humans

An extreme magneto-ionic environment associated with the fast radio burst source FRB 121102

Fast radio bursts are millisecond-duration, extragalactic radio flashes of unknown physical origin(1-3). The only known repeating fast radio burst source(4-6)-FRB 121102-has been localized to a star-forming region in a dwarf galaxy(7-9) at redshift 0.193 and is spatially coincident with a compact, persistent radio source(7,10). The origin of the bursts, the nature of the persistent source and the properties of the local environment are still unclear. Here we report observations of FRB 121102 that show almost 100 per cent linearly polarized emission at a very high and variable Faraday rotation measure in the source frame (varying from + 1.46 x 10(5) radians per square metre to + 1.33 x 10(5) radians per square metre at epochs separated by seven months) and narrow (below 30 microseconds) temporal structure. The large and variable rotation measure demonstrates that FRB 121102 is in an extreme and dynamic magneto-ionic environment, and the short durations of the bursts suggest a neutron star origin. Such large rotation measures have hitherto been observed(11,12) only in the vicinities of massive black holes (larger than about 10,000 solar masses). Indeed, the properties of the persistent radio source are compatible with those of a low-luminosity, accreting massive black hole(10). The bursts may therefore come from a neutron star in such an environment or could be explained by other models, such as a highly magnetized wind nebula(13) or supernova remnant(14) surrounding a young neutron star.</p