The Evolution of Cas A at Low Radio Frequencies

We have used archival 74 MHz VLA data spanning the last 15 years in combination with new data from the Long Wavelength Demonstrator Array (LWDA) and data from the literature covering the last 50 years to explore the evolution of Cas A at low radio frequencies. We find that the secular decrease of the flux density of Cas A at ~80 MHz is rather stable over five decades of time, decreasing at a rate of 0.7-0.8% yr^-1. This is entirely consistent with previous estimates at frequencies as low as 38 MHz, indicating that the secular decrease is roughly the same at low frequencies, at least between 38 and 80 MHz. We also find strong evidence for as many as four modes of flux density oscillation about the slower secular decrease with periods of 3.10+/-0.02$ yr, 5.1+/-0.3 yr, 9.0+/-0.2 yr, and 24+/-2 yr. These are also consistent with fluctuations seen previously to occur on scales of a few years. These results provide compelling motivation for a thorough low frequency monitoring campaign of Cas A to constrain the nature and physical origins of these fluctuations, and to be able to better predict the flux density of Cas A at any given epoch so that it may be used as a reliable low frequency calibrator.Comment: accepted for publication in A

A multiwavelength study of the remnant of nova GK Persei

We present new observations of the nebular remnant of the old nova GK Persei 1901, in the optical using the 2m HCT and at low radio frequencies using the GMRT. The evolution of the nova remnant indicates shock interaction with the ambient medium, especially in the southwest quadrant. Application of a simple model for the shock and its evolution to determine the time dependence of the radius of the shell in the southwest quadrant indicates that the shell is now expanding into an ambient medium that has a lower density compared to the density of the ambient medium ahead of the shock in 1987.There are indications of a recent interaction of the nova remnant with the ambient medium in the northeast quadrant also. The nova remnant of GK Per is detected at all the observed radio frequencies and is of similar extent as the optical remnant. Putting together our radio observations with VLA archival data on GK Per from 1997, we obtain three interesting results: 1. The spectrum above 1.4 GHz follows a power law with an index -0.7 and below 1.4 GHz follows a power law with an index ~ -0.85. This could be due to the presence of at least two populations of electrons dominating the global emission at different frequencies. 2. We record an annual secular decrease of 2.1% in the flux density of the nova remnant at 1.4 and 4.9 GHz between 1984 and 1997 which has left the spectral index unchanged at -0.7. No such decrease is observed in the flux densities below 1 GHz. 3. We record an increase in the flux density at 0.33 GHz compared to the previous estimate in 1987. We conclude that the remnant of nova GK Per is similar to supernova remnants and in particular, to the young supernova remnant Cas A.Comment: 10 pages; uses A&A style; figures 1, 2 & 6 are in JPEG format. Accepted for publication in Astronomy & Astrophysics. Full paper including Figures 1, 2 & 6 may be downloaded from http://www.iiap.res.in/personnel/gca/gca.htm

Evolution of the East Greenland Current from Fram Strait to Denmark Strait : synoptic measurements from summer 2012

Author Posting. © American Geophysical Union, 2017. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research: Oceans 122 (2017): 1974–1994, doi:10.1002/2016JC012228.We present measurements from two shipboard surveys conducted in summer 2012 that sampled the rim current system around the Nordic Seas from Fram Strait to Denmark Strait. The data reveal that, along a portion of the western boundary of the Nordic Seas, the East Greenland Current (EGC) has three distinct components. In addition to the well-known shelfbreak branch, there is an inshore branch on the continental shelf as well as a separate branch offshore of the shelfbreak. The inner branch contributes significantly to the overall freshwater transport of the rim current system, and the outer branch transports a substantial amount of Atlantic-origin Water equatorward. Supplementing our measurements with historical hydrographic data, we argue that the offshore branch is a direct recirculation of the western branch of the West Spitsbergen Current in Fram Strait. The total transport of the shelfbreak EGC (the only branch sampled consistently in all of the sections) decreased toward Denmark Strait. The estimated average transport of dense overflow water (rh > 27.8 kg/m3 and h>08C) in the shelfbreak EGC was 2.860.7 Sv, consistent with previous moored measurements. For the three sections that crossed the entire EGC system the freshwater flux, relative to a salinity of 34.8, ranged from 127613 to 8168 mSv. The hydrographic data reveal that, between Fram Strait and Denmark Strait, the core of the Atlantic-origin Water in the shelfbreak EGC cools and freshens but changes very little in density.Norwegian Research Council Grant Number: 231647; European Union 7th Framework Grant Number: 308299; National Science Foundation Grant Number: OCE-09593812017-09-1

Postglacial expansion of the arctic keystone copepod calanus glacialis

Calanus glacialis, a major contributor to zooplankton biomass in the Arctic shelf seas, is a key link between primary production and higher trophic levels that may be sensitive to climate warming. The aim of this study was to explore genetic variation in contemporary populations of this species to infer possible changes during the Quaternary period, and to assess its population structure in both space and time. Calanus glacialis was sampled in the fjords of Spitsbergen (Hornsund and Kongsfjorden) in 2003, 2004, 2006, 2009 and 2012. The sequence of a mitochondrial marker, belonging to the ND5 gene, selected for the study was 1249 base pairs long and distinguished 75 unique haplotypes among 140 individuals that formed three main clades. There was no detectable pattern in the distribution of haplotypes by geographic distance or over time. Interestingly, a Bayesian skyline plot suggested that a 1000-fold increase in population size occurred approximately 10,000 years before present, suggesting a species expansion after the Last Glacial Maximum.GAME from the National Science Centre, the Polish Ministry of Science and Higher Education Iuventus Plus [IP2014 050573]; FCT-PT [CCMAR/Multi/04326/2013]; [2011/03/B/NZ8/02876

The low frequency structure of Cassiopeia

The general structure of the supernova remnant Cassiopeia A is described, with particular reference to the optical jet of fast moving knots, the low frequency anomaly and previous VLBI results. The implementation of a three station interferometer system, measuring the visibility function of the nebula at 38MHz along a mean position angle of 83°, is described; both the closure results and the reconstructed visibility function are presented, from which a significant asymmetry in the source structure is evident. The results are interpreted by Fourier transforming the weighted visibility data, regu­larly gridded by interpolation of the reconstructed visibility function. The resultant, convolved brightness distribution of the nebula is compared with distributions synthesized to the same resolution, from the results of Jennison and Latham, and Matheson. A model fitting procedure is described, based on a two component, four parameter model, from which it is concluded that the results are consistent with a symmetri­cal nebula of radius 135±3 arcsec. with a jet, of indeter­minate width, containing between 4% to 6% of the total flux at a distance of 200±20 arcsec. east of the source phase centre. The remarkable coincidence of the point source solutions of the model fitting procedure with some of Hutton et. al.'s results at slightly higher frequencies is pointed out, and the likelihood of a relationship between the new asymmetrical component and the optical and X-ray jet is commented on. A brief discussion of the astrophysical consequences of the asymmetrical component follows, with reference to a possible association between the jet and the low frequency anomaly

Estimation of glacial meltwater discharge into Svalbard coastal waters. Oceanologia 39

Abstract During the summer expeditions of r/v 'Oceania' in 1995'Oceania' in -1996, oceanographic investigations comprising CTD profiling and suspension measurements were conducted in Svalbard fjords and shelf waters. The freshwater volume was estimated independently from the salinity drop as compared with the assumed background salinity and from the distribution of mineral suspension density in surface waters. Preliminary calculations of the instantaneous freshwater volume based on the distribution of suspended matter (at depths of < 150 m) yielded a figure of 80 km 3 in Svalbard coastal waters in summer. Values for Hornsund and Kongsfjord ranged from 0.4 to 0.7 km 3 of freshwater at the height of summer. This corresponds well with glaciological estimations, which give an annual discharge of 14.6 to 27.5 km 3 of freshwater for Svalbard. The glacial discharge is estimated to make up some 42% of the freshwater budget of Svalbard shelf waters, the remainder being derived from Barents Sea Arctic waters of reduced salinity

Freshwater input to the Arctic fjord Hornsund (Svalbard)

Glaciers draining to the Hornsund basin (southern Spitsbergen, Svalbard) have experienced a significant retreat and mass volume loss over the last decades, increasing the input of freshwater into the fjord. An increase in freshwater input can influence fjord hydrology, hydrodynamics, sediment flux and biota, especially in a changing climate. Here, we describe the sources of freshwater supply to the fjord based on glaciological and meteorological data from the period 2006 to 2015. The average freshwater input from land to the Hornsund bay is calculated as 2517 ± 82 Mt a−1, with main contributions from glacier meltwater runoff (986 Mt a−1; 39%) and frontal ablation of tidewater glaciers (634 Mt a−1; 25%). Tidewater glaciers in Hornsund lose ca. 40% of their mass by frontal ablation. The terminus retreat component accounts for ca. 30% of the mass loss by frontal ablation, but it can vary between 17% and 44% depending on oceanological, meteorological and geomorphological factors. The contribution of the total precipitation over land excluding winter snowfall (520 Mt a−1), total precipitation over the fjord area (180 Mt a−1) and melting of the snow cover over unglaciated areas (197 Mt a−1) to the total freshwater input appear to be small: 21%, 7% and 8%, respectively

A new ocean dataset describing Boundary Current systems states and their variability

A report on the development and implementations of new methods and dataset productions for Boundary Current systems monitorin