97 research outputs found
Search for Primordial Black Holes with SGARFACE
The Short GAmma Ray Front Air Cherenkov Experiment (SGARFACE) uses the
Whipple 10 m telescope to search for bursts of rays. SGARFACE is
sensitive to bursts with duration from a few ns to 20 s and with
-ray energy above 100 MeV. SGARFACE began operating in March 2003 and
has collected 2.2 million events during an exposure time of 2267 hours. A
search for bursts of rays from explosions of primordial black holes
(PBH) was carried out. A Hagedorn-type PBH explosion is predicted to be visible
within 60 pc of Earth. Background events were caused by cosmic rays and by
atmospheric phenomena and their rejection was accomplished to a large extent
using the time-resolved images. No unambiguous detection of bursts of
rays could be made as the remaining background events mimic the expected shape
and time development of bursts. Upper limits on the PBH explosion rate were
derived from the SGARFACE data and are compared to previous and future
experiments. We note that a future array of large wide-field air-Cherenkov
telescopes equipped with a SGARFACE-like trigger would be able to operate
background-free with a 20 to 30 times higher sensitivity for PBH explosions.Comment: 18 pages, 30 figures, accepted by Astroparticle Physics, corrected
author list and Section 2.
Distribution and characteristics of overdeepenings beneath the Greenland and Antarctic ice sheets: Implications for overdeepening origin and evolution
Glacier bed overdeepenings are ubiquitous in glacier systems and likely exert significant influence on ice dynamics, subglacial hydrology, and ice stability. Understanding of overdeepening formation and evolution has been hampered by an absence of quantitative empirical studies of their location and morphology, with process insights having been drawn largely from theoretical or numerical studies. To address this shortcoming, we first map the distribution of potential overdeepenings beneath the Antarctic and Greenland ice sheets using a GIS-based algorithm that identifies closed-contours in the bed topography and then describe and analyse the characteristics and metrics of a subset of overdeepenings that pass further quality control criteria. Overdeepenings are found to be widespread, but are particularly associated with areas of topographically laterally constrained ice flow, notably near the ice sheet margins where outlet systems follow deeply incised troughs. Overdeepenings also occur in regions of topographically unconstrained ice flow (for example, beneath the Siple Coast ice streams and on the Greenland continental shelf). Metrics indicate that overdeepening growth is generally allometric and that topographic confinement of ice flow in general enhances overdeepening depth. However, overdeepening depth is skewed towards shallow values – typically 200 to 300 m – indicating that the rate of deepening slows with overdeepening age. This is reflected in a decline in adverse slope steepness with increasing overdeepening planform size. Finally, overdeepening long-profiles are found to support headward quarrying as the primary factor in overdeepening development. These observations support proposed negative feedbacks related to hydrology and sediment transport that stabilise overdeepening growth through sedimentation on the adverse slope but permit continued overdeepening planform enlargement by processes of headward erosion
The First VERITAS Telescope
The first atmospheric Cherenkov telescope of VERITAS (the Very Energetic
Radiation Imaging Telescope Array System) has been in operation since February
2005. We present here a technical description of the instrument and a summary
of its performance. The calibration methods are described, along with the
results of Monte Carlo simulations of the telescope and comparisons between
real and simulated data. The analysis of TeV -ray observations of the
Crab Nebula, including the reconstructed energy spectrum, is shown to give
results consistent with earlier measurements. The telescope is operating as
expected and has met or exceeded all design specifications.Comment: Accepted by Astroparticle Physic
The back pain beliefs of health care providers: Are we fear-avoidant?
The purpose of this study was to survey the level of fear-avoidance beliefs for practicing general practitioners and physical therapists and to relate this to self-reported practice behaviors for patients with back pain. To this end, 60 general practitioners and 71 physical therapists were recruited. These participants completed a questionnaire including 11 items slightly revised from instruments designed to assess fear-avoidance beliefs in patients, and four items about treatment practices. The results indicated that these health care practitioners on the average generally held beliefs that are consistent with the current evidence, but there were also indications that some practitioners held beliefs reflecting fear-avoidance. More than two-thirds reported that they would advise a patient to avoid painful movements, more than one-third believed a reduction in pain is a prerequisite for return-to-work, while more than 25% reported that they believe sick leave is a good treatment for back pain. These beliefs were found to be related to reported practice behavior. Those with high levels of fear-avoidance beliefs were compared to those with low levels. Those with high levels of fear-avoidance belief had an increased risk for believing sick leave to be a good treatment (RR = 2.0; 90%CI = 1.02-3.92), not providing good information about activities (RR = 1.7; 90%CI = 1.19-2.45), and being uncertain about identifying patients at risk for developing persistent pain problems (RR = 1.5; 90%CI = 1.00-2.27). It is concluded that some practitioners hold beliefs reflecting fear-avoidance and that these beliefs may influence treatment practice
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