Jet breaks at the end of the slow decline phase of Swift GRB light curves

The Swift mission has discovered an intriguing feature of gamma-ray burst (GRBs) afterglows, a phase of shallow decline of the flux in the X-ray and optical light curves. This behaviour is typically attributed to energy injection into the burst ejecta. At some point this phase ends, resulting in a break in the light curve, which is commonly interpreted as the cessation of the energy injection. In a few cases, however, while breaks in the X-ray light curve are observed, optical emission continues its slow flux decline. This behaviour suggests a more complex scenario. In this paper, we present a model that invokes a double component outflow, in which narrowly collimated ejecta are responsible for the X-ray emission while a broad outflow is responsible for the optical emission. The narrow component can produce a jet break in the X-ray light curve at relatively early times, while the optical emission does not break due to its lower degree of collimation. In our model both components are subject to energy injection for the whole duration of the follow-up observations. We apply this model to GRBs with chromatic breaks, and we show how it might change the interpretation of the GRBs canonical light curve. We also study our model from a theoretical point of view, investigating the possible configurations of frequencies and the values of GRB physical parameters allowed in our model.     

Collisional modification of the acapulcoite/lodranite parent body revealed by the iodine‐xenon system in lodranites

Abstract— The I‐Xe system of three lodranites has been investigated. Samples of Gibson yielded no isochrons, and late model ages are attributed to late addition of iodine. Two metal and one silicate separate from the transitional lodranite Graves Nunataks (GRA) 95209 gave ages that are consistent with each other and with the literature I‐Xe age of Acapulco feldspar. These yield a mean closure age 4.19 ± 0.53 Ma after the Shallowater enstatite reference age (4562.3 ± 0.4 Ma). Such identical I‐Xe ages from distinct phases imply that the parent material underwent a period of rapid cooling, the absolute age of this event being 4558.1 ± 0.7 Ma. Such rapid cooling indicates an increase in the rate at which heat could be conducted away, requiring a significant modification of the parent body. We suggest the parent body was modified by an impact at or close to the time recorded by the I‐Xe system. An age of 10.4 ± 2.3 Ma after Shallowater has been determined for one whole‐rock sample of Lewis Cliff (LEW) 88280. Since the release pattern is similar to that of GRA 95209 this hints that the larger grain size of this sample may reflect slower cooling due to deeper post impact burial.     

A statistical study of gamma-ray burst afterglows measured by the Swift Ultraviolet Optical Telescope

We present the first statistical analysis of 27 Ultraviolet Optical Telescope (UVOT) optical/ultraviolet light curves of gamma-ray burst (GRB) afterglows. We have found, through analysis of the light curves in the observer's frame, that a significant fraction rise in the first 500 s after the GRB trigger, all light curves decay after 500 s, typically as a power law with a relatively narrow distribution of decay indices, and the brightest optical afterglows tend to decay the quickest. We find that the rise could be either produced physically by the start of the forward shock, when the jet begins to plough into the external medium, or geometrically where an off-axis observer sees a rising light curve as an increasing amount of emission enters the observers line of sight, which occurs as the jet slows. We find that at 99.8 per cent confidence, there is a correlation, in the observed frame, between the apparent magnitude of the light curves at 400 s and the rate of decay after 500 s. However, in the rest frame, a Spearman rank test shows only a weak correlation of low statistical significance between luminosity and decay rate. A correlation should be expected if the afterglows were produced by off-axis jets, suggesting that the jet is viewed from within the half-opening angle θ or within a core of a uniform energy density  θ_c  . We also produced logarithmic luminosity distributions for three rest-frame epochs. We find no evidence for bimodality in any of the distributions. Finally, we compare our sample of UVOT light curves with the X-ray Telescope (XRT) light-curve canonical model. The range in decay indices seen in UVOT light curves at any epoch is most similar to the range in decay of the shallow decay segment of the XRT canonical model. However, in the XRT canonical model, there is no indication of the rising behaviour observed in the UVOT light curves.     

The effect of bathymetric filtering on nearshore process model results

Nearshore wave and flow model results are shown to exhibit a strong sensitivity to the resolution of the input bathymetry. In this analysis, bathymetric resolution was varied by applying smoothing filters to high-resolution survey data to produce a number of bathymetric grid surfaces. We demonstrate that the sensitivity of model-predicted wave height and flow to variations in bathymetric resolution had different characteristics. Wave height predictions were most sensitive to resolution of cross-shore variability associated with the structure of nearshore sandbars. Flow predictions were most sensitive to the resolution of intermediate scale alongshore variability associated with the prominent sandbar rhythmicity. Flow sensitivity increased in cases where a sandbar was closer to shore and shallower. Perhaps the most surprising implication of these results is that the interpolation and smoothing of bathymetric data could be optimized differently for the wave and flow models. We show that errors between observed and modeled flow and wave heights are well predicted by comparing model simulation results using progressively filtered bathymetry to results from the highest resolution simulation. The damage done by over smoothing or inadequate sampling can therefore be estimated using model simulations. We conclude that the ability to quantify prediction errors will be useful for supporting future data assimilation efforts that require this information.     

Zonal versus meridional velocity variance in satellite observations and realistic and idealized ocean circulation models

Global, high-quality, satellite-based observation of oceanic currents over the past 13 years has revealed ubiquitous quasi-horizontal eddies in the mesoscale (tens to hundreds of kilometers), confirming the view of a highly turbulent ocean suggested by observational programs in the 1970s. Idealized quasigeostrophic turbulence models suggest mesoscale turbulent flow can vary between isotropic, and highly anisotropic zonal jets. Here we compare the zonal and meridional velocity variance from satellite altimetry. We find that, for an unexplained reason and despite the chaotic nature of turbulence, the surface flow is organized into mesoscale patches where either zonal or meridional velocity variance dominates. The patches persist over 13 years, much longer than the turbulent timescale of a few months. Implications include potentially highly anisotropic redistribution of tracers by the mesoscale flow. Zonally averaged velocity variances reveal a slight preference for meridional over zonal velocity variance. Realistic primitive equation models succeed in reproducing both the patchy structure in local preference for either zonal or meridional velocity variance, and the zonally averaged preference for meridional variance. Idealized models of fully developed, quasigeostrophic turbulence fail in both regards.     

Core lithologies and their constraints on gas-hydrate occurrence in the East Sea, offshore Korea: Results from the site UBGH1-9

Drilling at the site UBGH1-9, offshore Korea in 2007, revealed varied gas-hydrate saturation with depth and a wide variety of core litholgies, demonstrating how the variations in the lithology are linked with those in gas-hydrate saturation and morphology. Discrete excursions to low chlorinity values from in situ background chlorinity level occur between 63 and 151 mbsf. In this occurrence zone, gas-hydrate saturations estimated from the low chlorinity anomalies range up to 63.5% of pore volume with an average of 9.9% and do not show a clear depth-dependent trend. Sedimentary facies analysis based on grain-size distribution and sedimentary structures revealed nine sediment facies which mainly represent hemipelagic muds and fine- to medium-grained turbidites. According to the sedimentary facies distribution, the core sediments are divided into three facies associations (FA): FA I (0–98 mbsf) consisting mainly of alternating thin- to medium-bedded hemipelagic mud and turbidite sand or mud beds, FA II (98–126 mbsf) dominated by medium- to very thick-bedded turbidite sand or sandy debris flow beds, and FA III (126–178 mbsf) characterized by thick hemipelagic mud without intervening discrete turbidite sand layers. Thermal anomalies from IR scan, mousse-like and soupy structures on split-core surfaces, non-destructive measurements of pressure cores, and comparison of gas-hydrate saturations with sand contents of corresponding pore-water squeeze cakes, collectively suggest that the gas hydrate at the site UBGH1-9 generally occurs in two different types: “pore-filling” type preferentially associated with thin- to medium-turbidite sand beds in the FA I and “fracture-filling” type which occurs as hydrate veins or nodules in hemipelagic mud of the FA III. Gas-hydrate saturation in the FA II is generally anomalously low despite the dominance of turbidite sand or sandy debris flow beds, suggesting insufficient methane supply.     

“You know you can do this,right?”: developing geospatial technological pedagogical content knowledge and enhancing teachers’ cartographic practices with socio-environmental science investigations

Many barriers exist to K–12 classroom teachers’ adoption and implementation of geospatial technologies with their students. To address this circumstance, we have developed and implemented a geospatial curriculum approach to promote teachers’ professional growth with curriculum-linked professional development (PD) to support the adoption of socio-environmental science investigations (SESI) in an urban school environment that includes reluctant learners. SESI focus on social issues related to environmental science. The pedagogy is inquiry-driven, with students engaged in map-based mobile data collection and subsequent analysis with Web-based dynamic mapping software to answer open-ended questions. Working with four science and social studies teachers, we designed and implemented a sequence of three locally oriented, geospatial inquiry projects that were implemented with 140 9th grade students. We investigated how the geospatial curriculum approach impacted the teachers’ geospatial pedagogical content knowledge (PCK), their cartographic practices, and promoted geospatial thinking and analysis skills with their students. Findings revealed strong growth in teachers’ geospatial PCK, increased map use by teachers, use of maps as media for inquiry and not didactic instruction, and modeling to guide students’ geospatial analysis using GIS. Implications for PD to promote teachers’ geospatial PCK and in-class cartographic practices are discussed.     

Yttrium geochemistry applied to petrogenesis utilizing calcium-yttrium relationships in minerals and rocks

A survey of Y data from all sources shows that Y behaves systematically in igneous, metamorphic and sedimentary rock series, due to its incorporation in a predictable and uniform manner in Ca minerals. Compared with average calc-alkali basalt melts, plagioclase, kaersutite, augitic clinopyroxene and calcite have low Y for their CaO contents; whereas hornblende, garnet, orthopyroxene, apatite, sphene, zircon and most K, Na rich minerals other than plagioclase have high Y for their Ca contents. In sedimentary processes, Ca/Y becomes lower in shales and sandstones, but higher in limestones than their source. In metamorphic processes Y appears to be inert. In all igneous series for which Y data has been assembled Ca/Y falls as Ca falls. These series can be classified into three categories: (i) a standard calc-alkali trend, used as a reference (ii) J-type trends which become progressively impoverished in Y as Ca falls when compared with the standard trend, and (iii) L-trends which become progressively enriched in Y as Ca falls when compared with the standard trend. Despite little knowledge of partition coefficients, the J-type trends appear to have a significant component of hornblende control in their fractionation processes, whereas plagioclase and augitic clinopyroxene dominate the L-type trend. Alternative names for these series might therefore be the standard series, the hornblendic and the pyroxenic series respectively. Modern ocean floor basalts appear to be significantly richer in Y than modern calc-alkali basalts, and these in turn may be richer in Y than their Archaean counterparts.     

Conjoint analysis of small‐boat recreational fishers in New Zealand

Conjoint analysis is a useful technique to assist recreational managers formulate policy, and develop strategies to efficiently and effectively distribute resources. Accurate estimates of the value of natural resources remain challenging, despite the development of a number of decision analysis techniques. This recreation geography study uses conjoint analysis to examine the relative importance of attributes associated with small‐boat recreational fishing experiences and demonstrates the value of conjoint analysis as a non‐market valuation tool. We found that the choices associated with companionship are the most important factors of recreational fishing and more tangible attributes such as the quality of launch facilities and catch probability are the least important factors.