LOFAR 144-MHz follow-up observations of GW170817

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society, Volume 494, Issue 4, June 2020, Pages 5110–5117, ©: 2020 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.We present low-radio-frequency follow-up observations of AT 2017gfo, the electromagnetic counterpart of GW170817, which was the first binary neutron star merger to be detected by Advanced LIGO-Virgo. These data, with a central frequency of 144 MHz, were obtained with LOFAR, the Low-Frequency Array. The maximum elevation of the target is just 13.7 degrees when observed with LOFAR, making our observations particularly challenging to calibrate and significantly limiting the achievable sensitivity. On time-scales of 130-138 and 371-374 days after the merger event, we obtain 3$\sigma$ upper limits for the afterglow component of 6.6 and 19.5 mJy beam$^{-1}$, respectively. Using our best upper limit and previously published, contemporaneous higher-frequency radio data, we place a limit on any potential steepening of the radio spectrum between 610 and 144 MHz: the two-point spectral index $\alpha^{610}_{144} \gtrsim -2.5$. We also show that LOFAR can detect the afterglows of future binary neutron star merger events occurring at more favourable elevations.Peer reviewe

Pervasive gaps in Amazonian ecological research

Biodiversity loss is one of the main challenges of our time, and attempts to address it require a clear understanding of how ecological communities respond to environmental change across time and space. While the increasing availability of global databases on ecological communities has advanced our knowledge of biodiversity sensitivity to environmental changes, vast areas of the tropics remain understudied. In the American tropics, Amazonia stands out as the world's most diverse rainforest and the primary source of Neotropical biodiversity, but it remains among the least known forests in America and is often underrepresented in biodiversity databases. To worsen this situation, human-induced modifications may eliminate pieces of the Amazon's biodiversity puzzle before we can use them to understand how ecological communities are responding. To increase generalization and applicability of biodiversity knowledge, it is thus crucial to reduce biases in ecological research, particularly in regions projected to face the most pronounced environmental changes. We integrate ecological community metadata of 7,694 sampling sites for multiple organism groups in a machine learning model framework to map the research probability across the Brazilian Amazonia, while identifying the region's vulnerability to environmental change. 15%–18% of the most neglected areas in ecological research are expected to experience severe climate or land use changes by 2050. This means that unless we take immediate action, we will not be able to establish their current status, much less monitor how it is changing and what is being lost

Evaluation of stainless steel pipe cracking: causes and fixes

Leaks and cracks in the heat-affected zones of weldments in austenitic stainless steel piping and associated components of boiling water reactors (BWRs) have been observed since the mid-1960s. Since that time, cracking has continued to occur and indications have been found in all parts of the recirculation system, including the largest diameter lines. Proposed solutions include remedies primarily intended to produce a more favorable residual stress state, materials which are more resistant to stress corrosion cracking (SCC), and changes in the reactor environment which decrease the susceptibility to cracking. In addition to evaluating these remedies, it is also important to gain a better understanding of key variables such as residual stresses, crack growth rates, and the leak-before-break margin in flawed piping, which may impact regulatory decisions on operating plants. The main areas of effort during the past year have been (1) studies of impurity effects on susceptibility to intergranular stress corrosion cracking (IGSCC), (2) crack growth rate measurements, (3) finite-element studies of residual stress produced by induction heating stress improvement (IHSI) and the addition of weld overlays to flawed piping, (4) leak-before-break analyses of piping with 360/sup 0/ part-through cracks, and (5) parametric studies on the effect of through-wall residual stresses on IGSCC crack growth behavior in large-diameter piping weldments. 5 references, 15 figures, 2 tables