Stereoscopic observations of gamma rays at the Whipple observatory

Observations of photons at E ≥ 550 GeV from the Crab Nebula are presented and used to assess the potential of multi-telescope systems for γ-ray astronomy.

The Whipple observatory 10 m and 8 m imaging atmospheric erenkov telescopes have been used to provide a stereoscopic view of air showers to make a more complete measurement of air shower parameters. Here we present a measurement of the spread in the arrival direction of primary γ-rays originating from a point source. The data show that the shower arrival direction can be reconstructed with an accuracy of σ = 0.°14.     

Repeating of gamma-ray bursts in light of the BATSE 2B catalog

We examine the evidence for repeating of -ray bursts in light of the BATSE 2B catalog. The temporal coverage of the 2B-1B (non-MAXBC) data set is 1/3 smaller than that of the 1B catalog. We find evidence that the distributions of the burst statistical positional errors and of the burst fluences also differ. Assuming that the bursts in the 1B catalog form a fair sample and taking these observational selection effects into account, we simulate the 2B-1B (less MAXBC) data set. We find that a repeating signal like that seen in the nearest neighbor distribution of the bursts in the 1B catalog would not be detectable in the 2B-1B (non-MAXBC) and 2B (non-MAXBC) data sets, because of the differences in the temporal coverage and in the distribution of positional errors.     

Low energy γ-ray events in imaging Čerenkov telescope arrays

One of the major goals in VHE--ray astronomy is to open the energy range below 100 GeV with earthbound detectors. This paper demonstrates a new method for analyzing erenkov light of a shower in a erenkov telescope array. This method is successful for showers in this low energy regime where previous techniques (e.g. alpha analysis) are not applicable. A Monte Carlo simulation is applied to a system of 19 Whipple type [3, Cawley 1990] Imaging Atmospheric erenkov telescopes (IAT), each built as a 10 m diameter reflector and equipped with a 109 photomultiplier tube camera. The energy threshold for a single detector of this type is given [5, Kerrick et al. 1995] as 250 GeV. Analysis of simulated coincident events of the system for those events not having enough light to apply a standard imaging analysis [4, Hillas 1985], leads to a considerably lower threshold of 85 GeV. With a new analysis method of these events it is shown that it should be possible to distinguish between -ray induced and proton induced showers. The improvement of sensitivity (Q = figure of merit) of this analysis method is found to be Q=2.9.     

The network K‐function in context: examining the effects of network structure on the network K‐function

The flaws of using traditional planar point‐pattern analysis techniques with network constrained points have been thoroughly explored in the literature. Because of this, new network‐based measures have been introduced for their planar analogues, including the network based K‐function. These new measures involve the calculation of network distances between point events rather than traditional Euclidean distances. Some have suggested that the underlying structure of a network, such as whether it includes directional constraints or speed limits, may be considered when applying these methods. How different network structures might affect the results of the network spatial statistics is not well understood. This article examines the results of network K‐functions when taking into consideration network distances for three different types of networks: the original road network, topologically correct networks, and directionally constrained networks. For this aim, four scenarios using road networks from Tampa, Florida and New York City, New York were used to test how network constraints affected the network K‐function. Depending on which network is under consideration, the underlying network structure could impact the interpretation. In particular, directional constraints showed reduced clustering across the different scenarios. Caution should be used when selecting the road network, and constraints, for a network K‐function analysis.     

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ABSTRACT

A stochastic model is proposed to describe flood-induced changes in cross sections of ephemeral streams with coarse bed-material. The model is based on the assumption that the real process can be described as a marked point process. The cumulative distribution functions of both the total change in a fixed interval of time and the maximum scour in the same period were examined. The model was run, using Monte Carlo techniques, with field data obtained from a small stream in southern Italy.     

A fracture mapping and extended finite element scheme for coupled deformation and fluid flow in fractured porous media

This paper presents a fracture mapping (FM) approach combined with the extended finite element method (XFEM) to simulate coupled deformation and fluid flow in fractured porous media. Specifically, the method accurately represents the impact of discrete fractures on flow and deformation, although the individual fractures are not part of the finite element mesh. A key feature of FM‐XFEM is its ability to model discontinuities in the domain independently of the computational mesh. The proposed FM approach is a continuum‐based approach that is used to model the flow interaction between the porous matrix and existing fractures via a transfer function. Fracture geometry is defined using the level set method. Therefore, in contrast to the discrete fracture flow model, the fracture representation is not meshed along with the computational domain. Consequently, the method is able to determine the influence of fractures on fluid flow within a fractured domain without the complexity of meshing the fractures within the domain. The XFEM component of the scheme addresses the discontinuous displacement field within elements that are intersected by existing fractures. In XFEM, enrichment functions are added to the standard finite element approximation to adequately resolve discontinuous fields within the simulation domain. Numerical tests illustrate the ability of the method to adequately describe the displacement and fluid pressure fields within a fractured domain at significantly less computational expense than explicitly resolving the fracture within the finite element mesh. Copyright © 2013 John Wiley & Sons, Ltd.     

Characterization of a karstic aquifer using magnetic resonance sounding and electrical resistivity tomography: a case-study of Esta?a Lakes (northern Spain)

The geophysical characterization of a previously unstudied endorheic karstic system is presented. The studied area, known as the Esta?a Lakes, is located in the Pyrenean Marginal Sierras, northern Spain. The Esta?a Lakes are a set of natural water ponds on a bedrock of Triassic evaporites, lutites and carbonates. This wetland is included in the Natura 2000 European network of nature protection areas as a “Site of Community Importance”. Two geophysical techniques were used, magnetic resonance sounding (MRS) and electrical resistivity tomography (ERT), to map the subsurface geology and characterize the aquifer layers and the hydraulic links between the aquifers and lakes. The geophysical data were integrated with the surface geology and data from six boreholes. Ten electrical profiles were performed to identify the thickness of the units and lithological changes, whereas the MRS was used to determine the top of the saturated zone. As result, the aquifer in the Esta?a Lakes system and surrounding area has been identified as Middle Triassic carbonates, which does not correspond with the regional aquifer in the area (Upper Cretaceous and Eocene). This work shows the power of geophysical methods in poorly understood and tectonically complex areas in addition to the standard aquifer tests to evaluate hydraulic properties.     

Autonomous video camera system for monitoring impacts to benthic habitats from demersal fishing gear, including longlines

Studies of the interactions of demersal fishing gear with the benthic environment are needed in order to manage conservation of benthic habitats. There has been limited direct assessment of these interactions through deployment of cameras on commercial fishing gear especially on demersal longlines. A compact, autonomous deep-sea video system was designed and constructed by the Australian Antarctic Division (AAD) for deployment on commercial fishing gear to observe interactions with benthos in the Southern Ocean finfish fisheries (targeting toothfish, Dissostichus spp). The Benthic Impacts Camera System (BICS) is capable of withstanding depths to 2500 m, has been successfully fitted to both longline and demersal trawl fishing gear, and is suitable for routine deployment by non-experts such as fisheries observers or crew. The system is entirely autonomous, robust, compact, easy to operate, and has minimal effect on the performance of the fishing gear it is attached to. To date, the system has successfully captured footage that demonstrates the interactions between demersal fishing gear and the benthos during routine commercial operations. It provides the first footage demonstrating the nature of the interaction between demersal longlines and benthic habitats in the Southern Ocean, as well as showing potential as a tool for rapidly assessing habitat types and presence of mobile biota such as krill (Euphausia superba).     

Climatic changes and variations in the atmospheric and ocean circulations

Until quite recent years meteorology was in no position to make an effective contribution to the understanding of paleoclimates. Lacking were both a theory of the general circulation of the atmosphere and the upper wind observations which have provided the essential foundation of such theoretical understanding of the circulation as we now possess. The most notable early contributions from meteorology wereSimpson's (1934) schematic construction of a presumed mean circulation at the time of the Quaternary ice age maximum andWillett's (1949) proposal of two alternative modes of the general circulation, summarized as expansions and contractions of the circumpolar vortex of upper westerly winds. Both theories lacked an observational basis, especially for their more sweeping generalizations (e.g. simultaneity of ice-ages and lowlatitude pluvials, limitation to only two main atmospheric circulation types etc.) as well as for their author's further proposals about variations of the solar radiation. Nevertheless, they marked a valuable step forward, because they stimulated workers in many fields to interpret their evidence of climatic changes as part of a world phenomenon in which the general wind circulation must be involved. A strictly empirical study of the broadest possible scope of the climatic changes observable within the last 100–1000 years was badly needed. Examination reveals that significant changes have taken place within this period. Study of them is instructive. Whatever changes (if any) of effective radiation available at the Earth's surface occur, the atmospheric and ocean circulations are the mechanism whereby the heat and moisture are distributed. As in synoptic meteorology, circulation maps have been found the most useful means of obtaining a unified view of the climatic changes of recent times. The first successful studies of this kind were byDefant (1924), who showed that the mean circulation over the North Atlantic had intensified materially from the 1880s to the present century, i.e. in part of the period of Arctic warming and increasing mildness of winters in Europe. The present paper shows how our knowledge of changes such asDefant found has been extended over more of the world and a longer period of time. This leads to some indications about the probable nature and possible origins of the phenomenon, which is revealed as rather simple and world-wide in scope. We learn some of the types of variation that actually occur.Examination of the charts indicates inter alia progressive changes of intensity and position of the mainstreams of the atmospheric circulation, and of the ocean currents and climatic zones that correspond to them, within the last 100–250 years. Consideration of the nature of the Little Ice Age epoch of 1550–1850, and comparisons with what is known of the great ice ages of the Pleistocene, appears to throw some new light on the ice age problem.

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Zusammenfassung Bis vor kurzem war die Meteorologie nicht in der Lage, einen wesentlichen Beitrag zum Verständnis der Paläoklimate zu liefern. Es fehlten sowohl eine Theorie der allgemeinen Zirkulation der Atmosphäre als auch die Beobachtungen über die höheren Winde, die die wesentliche Grundlage für unser jetziges theoretisches Verständnis der Zirkulation geliefert haben. Die wichtigsten früheren Beiträge von meteorologischer Seite warenSimpson's schematische Rekonstruktion (1934) einer angenommenen mittleren Zirkulation zur Zeit des quartären Eiszeitmaximums undWillett's Annahme (1949) zweier verschiedener Arten der allgemeinen Zirkulation (Expansion und Kontraktion des zirkumpolaren Wirbels der oberen Westwinde). Beiden Theorien fehlte die Beobachtungsgrundlage, vor allem für ihre weitgehenden Verallgemeinerungen (z. B. Gleichzeitigkeit von Eiszeiten und Pluvialen der niederen Breiten, Begrenzung auf nur zwei Haupttypen der atmosphärischen Zirkulation usw.) wie auch für die weiteren Annahmen der Autoren über änderungen der Sonnenstrahlung. Trotzdem waren sie ein wertvoller Schritt vorwärts, weil sie Forscher verschiedenster Gebiete anregten, die Tatsachen von Klimaänderungen als Teil eines weltweiten Phänomens zu interpretieren, das mit der allgemeinen Windzirkulation zusammenhängt. Eine rein empirische, möglichst breit angelegte Studie über die beobachteten Klimaänderungen in den letzten 100 bis 1000 Jahren war dringend notwendig. Es zeigt sich, da\ bedeutungsvolle änderungen in dieser Zeit stattgefunden haben. Falls überhaupt irgendwelche Schwankungen der von au\en kommenden Strahlung erfolgten — immer sind die atmosphärischen und ozeanischen Strömungen der Mechanismus, der Wärme und Niederschlag verteilt. Wie in der synoptischen Meteorologie haben sich Karten der Zirkulation als wertvoll erwiesen, ein einheitliches Bild der klimatischen änderungen in jüngster Zeit zu erhalten. Die ersten erfolgreichen Versuche dieser Art stammen vonDefant (1924); er zeigte, da\ sich die mittlere Zirkulation über dem Nordatlantik seit 1880 bis zum jetzigen Jahrhundert intensiviert hatte, d. h. in der Zeit, in der die Arktis und die europäischen Winter wärmer wurden. Der nachfolgende Aufsatz zeigt, da\ wir heute ähnliche änderungen auch in weiteren Teilen der Erde und von längeren Zeiträumen kennen. Das führt zu Hinweisen über die Natur und die möglichen Ursachen des Phänomens.Das Studium der Karten zeigt für die letzten 100–250 Jahre u. a. progressive änderungen der Intensität und Lage der Hauptströme der atmosphärischen Zirkulation sowie der ozeanischen Strömungen und Klimazonen, die ihnen entsprechen. Betrachtungen über die Kleine Eiszeit 1550–1850 und Vergleiche mit den gro\en Eiszeiten des Pleistozäns werfen einiges neue Licht auf das Eiszeitproblem überhaupt.