Benthic community structure and ecosystem functions in above- and below-waterfall pools in Borneo

Waterfalls are geomorphic features that often partition streams into discrete zones. Our study examined aquatic communities, litter decomposition and periphyton growth rates for above- and below-waterfall pools in Ulu Temburong National Park, Brunei. We observed higher fish densities in below-waterfall pools (0.24 fish m−2 vs. 0.02 fish m−2 in above-waterfall pools) and higher shrimp abundance in above-waterfall pools (eight shrimp/pool vs. less than one shrimp/pool in below-waterfall pools). However, macroinvertebrate densities (excluding shrimp) were similar among both pool types. Ambient periphyton was higher in below-waterfall pools in 2013 (4.3 vs. 2.8 g m−2 in above-waterfall pools) and 2014 (4.8 vs. 3.4 g m−2 in above-waterfall pools), while periphyton growth rates varied from 0.05 to 0.26 g m−2 days−1 and were significantly higher in below-waterfall pools in 2014. Leaf litter decomposition rates (0.001 to 0.024 days−1) did not differ between pool types, suggesting that neither shrimp nor fish densities had consistent impacts on this ecosystem function. Regardless, this research demonstrates the varied effects of biotic and abiotic factors on community structure and ecosystem function. Our results have highlighted the importance of discontinuities, such as waterfalls, in tropical streams.</p

Multi-messenger observations of a binary neutron star merger

On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ~1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40+8-8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 Mo. An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ~40 Mpc) less than 11 hours after the merger by the One- Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ~10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ~9 and ~16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta

The construction and analysis of a whole-sky map using underground muons

A sample of 16 million underground muons has been collected between January, 1989, and June, 1993, at a depth of 2090 m.w.e. in the SOUDAN 2 detector. This data-set has been passed through standard event reconstruction programs to give a sample of 14 million muons after run and event cuts. The trajectories of all muons in this data-set have been projected back onto the celestial sphere. Software has been developed to compute the apparent motions of the Sun and Moon as viewed from the surface of the Earth. The errors on the coordinates computed by these programs are well understood. The programs have been used to search the data-set for evidence of the cosmic ray shadow cast by the stopping of primaries in the Sun and Moon. This procedure was used to test the assumption that the detector alignment is known to ~ 0.25° and that the resolution is ~ 1°. The observed distribution of events was found to be consistent with this hypothesis. However the test performed lacked sufficient power to reject strongly the alternative hypothesis, that no shadow had been observed. A 95% lower limit of 0.46° was set on the single-muon resolution. A procedure has been developed to construct and analyse a map of the underground muon flux across the entire celestial sphere. A whole-sky background calculation has been developed and it has been verified that the background computed for the observed data-set is an acceptable description of the distribution of event coordinates. By a monte-carlo procedure, it has been demonstrated that the statistical properties of the analysis method are understood and agree with the distribution computed analytically. It has been shown that there is no evidence for drift or bias in the background computed. The procedure has been applied to the data-set, asymmetrically divided into two subsets of 10.6 and 3.3 million muons respectively. A search for anomalous muon excess in the larger data-set found the largest deviation to be an excess within 1.9° degrees of Active Galactic Nucleus 3C 273. The significance of the excess was estimated to be ~ 10% and the significance of it's proximity to 3C 273 to be ~ 2%. A 1.7 s.d. excess was found in the identical position in the smaller data set. The number of excess events for both data-sets, scaled by their relative exposures, were consistent at the level of 1 s.d. The overall level of confidence is conservatively estimated to be better than 99%. On the assumption that this excess does represent a real flux of muons, due to primary cosmic rays from 3C 273, the region in excess was examined in greater detail. A maximum likelihood fit was used to extract confidence limits for the strength, coordinates, and r.m.s. width of the excess. It was found that association with 3C 273 should be rejected, with a confidence of 95%, unless the r.m.s. source width was permitted to increase from the a priori value of 1° to 2.4°. With this modification excess coordinates were estimated to be ⍺ = (188.2 +2.1 [above] -2.8)°, δ = (2.2 +2.3 [above] –2.1)°. The background subtracted distributions of a selection of event parameters were examined and found to be consistent with the background expectation. On the assumption that the flux has the same spectral index as the background, the underground muon flux was estimated to be 10.3 +11.3 [above] -9.2 x 10⁻¹² cm⁻²s⁻¹. Such a flux is not consistent with the observed ɣ-ray flux from 3C 273, and can only be explained by unconventional physics.</p

ULTRAHIGH-ENERGY COSMIC-RAY COMPOSITION FROM SURFACE AIR SHOWER AND UNDERGROUND MUON MEASUREMENTS AT SOUDAN-2

The Soudan 2 experiment has performed time-coincident cosmic ray air shower and underground muon measurements. Comparisons to Monte Carlo predictions show that such measurements can make statistically significant tests of the primary composition in the knee region of the cosmic ray spectrum. The results do not support any significant increase in the average primary mass with energy in the range of similar to 10(4) TeV per nucleus. Some systematic uncertainties remain, however, particularly in the Monte Carlo modeling of the cosmic ray shower