2,681 research outputs found
Scaling law in target-hunting processes
We study the hunting process for a target, in which the hunter tracks the
goal by smelling odors it emits. The odor intensity is supposed to decrease
with the distance it diffuses. The Monte Carlo experiment is carried out on a
2-dimensional square lattice. Having no idea of the location of the target, the
hunter determines its moves only by random attempts in each direction. By
sorting the searching time in each simulation and introducing a variable to
reflect the sequence of searching time, we obtain a curve with a wide plateau,
indicating a most probable time of successfully finding out the target. The
simulations reveal a scaling law for the searching time versus the distance to
the position of the target. The scaling exponent depends on the sensitivity of
the hunter. Our model may be a prototype in studying such the searching
processes as various foods-foraging behavior of the wild animals.Comment: 7 figure
The XMM-Newton Slew Survey
XMM-Newton, with the huge collecting area of its mirrors and the high quantum
efficiency of its EPIC detectors, is the most sensitive X-ray observatory ever
flown. This is strikingly evident during slew exposures, which, while yielding
only at most 14 seconds of on-source exposure time, actually constitute a 2-10
keV survey ten times deeper than all other "all-sky" surveys. The current
(April 2005) XMM archive contains 374 slew exposures which give a uniform
coverage over around 10,000 square degrees (approx. 25% of the sky). Here we
describe the results of pilot studies, the current status of the XMM-Newton
Slew Survey, up-to-date results and our progress towards constructing a
catalogue of slew detections in the full 0.2-12 keV energy band.Comment: 3 pages, 4 figures, XMM-Newton EPIC Consortium Meeting, Schloss
Ringberg, Germany, April 2005, to appear in MPE Repor
Anomalous escape governed by thermal 1/f noise
We present an analytic study for subdiffusive escape of overdamped particles
out of a cusp-shaped parabolic potential well which are driven by thermal,
fractional Gaussian noise with a power spectrum. This
long-standing challenge becomes mathematically tractable by use of a
generalized Langevin dynamics via its corresponding non-Markovian,
time-convolutionless master equation: We find that the escape is governed
asymptotically by a power law whose exponent depends exponentially on the ratio
of barrier height and temperature. This result is in distinct contrast to a
description with a corresponding subdiffusive fractional Fokker-Planck
approach; thus providing experimentalists an amenable testbed to differentiate
between the two escape scenarios
Extended sources in the XMM-Newton slew survey
The low background, good spatial resolution and great sensitivity of the
EPIC-pn camera on XMM-Newton give useful limits for the detection of extended
sources even during the short exposures made during slewing maneouvers. In this
paper we attempt to illustrate the potential of the XMM-Newton slew survey as a
tool for analysing flux-limited samples of clusters of galaxies and other
sources of spatially extended X-ray emission.Comment: 2 pages, 4 figures, to appear in the proceedings of "The X-ray
Universe 2005", San Lorenzo de El Escorial (Spain), 26-30 September 200
The XMM-Newton Slew Survey: towards the XMMSL1 catalogue
The XMM-Newton satellite is the most sensitive X-ray observatory flown to
date due to the great collecting area of its mirrors coupled with the high
quantum efficiency of the EPIC detectors. It performs slewing manoeuvers
between observation targets tracking almost circular orbits through the
ecliptic poles due to the Sun constraint. Slews are made with the EPIC cameras
open and the other instruments closed, operating with the observing mode set to
the one of the previous pointed observation and the medium filter in place.
Slew observations from the EPIC-pn camera in FF, eFF and LW modes provide
data, resulting in a maximum of 15 seconds of on-source time. These data can be
used to give a uniform survey of the X-ray sky, at great sensitivity in the
hard band compared with other X-ray all-sky surveys.Comment: 2 pages, 2 figures, to appear in the proceedings of "The X-ray
Universe 2005", San Lorenzo de El Escorial (Spain), 26-30 September 200
Oxygen isotopes implanted in the LDEF spacecraft
Secondary ion mass spectrometry was used to study oxygen implanted in the surface of copper from the Long Duration Exposure Facility (LDEF). Oxidation that occurred in orbit shows a characteristic oxygen isotope composition, depleted in O-18. The measured depletion is comparable to the predicted depletion (45 percent) based on a model of the gravitational separation of the oxygen isotopes. The anomalous oxygen was contained within 10nm of the surface. Tray E10 was calculated to have received 5.14 x 10(exp 21) atoms of oxygen cm(sup -2) during the LDEF mission and so there is sufficient anomalous implanted oxygen present in the surface to obtain a reliable isotopic profile
Aging and aerobic fitness affect the contribution of noradrenergic sympathetic nerves to the rapid cutaneous vasodilator response to local heating
Sedentary aging results in a diminished rapid cutaneous vasodilator response to local heating. We investigated whether this diminished response was due to altered contributions of noradrenergic sympathetic nerves; assessing 1) the age-related decline and, 2) the effect of aerobic fitness. We measured skin blood flow (SkBF)(laser-Doppler flowmetry) in young (24±1 yr) and older (64±1 yr) endurance-trained and sedentary men (n=7 per group) at baseline and during 35 min of local skin heating to 42 °C at three forearm sites: 1) untreated; 2) bretylium tosylate (BT), preventing neurotransmitter release from noradrenergic sympathetic nerves; and 3) yohimbine and propranolol (YP), antagonising α- and β-adrenergic receptors. SkBF was converted to cutaneous vascular conductance (CVC) (SkBF/mean arterial pressure) and normalized to maximal CVC (%CVCmax) achieved by skin heating to 44 °C. Pharmacological agents were administered using microdialysis. In the young trained, the rapid vasodilator response was reduced at the BT and YP sites (P0.05) but treatment with BT did (P>0.05). Neither BT nor YP treatments affected the rapid vasodilator response in the older sedentary group (P>0.05). These data suggest that the age-related reduction in the rapid vasodilator response is due to an impairment of sympathetic-dependent mechanisms, which can be partly attenuated with habitual aerobic exercise. Rapid vasodilation involves noradrenergic neurotransmitters in young trained men, and non-adrenergic sympathetic cotransmitters (e.g.,
neuropeptide Y) in young sedentary and older trained men, possibly as a compensatory mechanism. Finally, in older sedentary men, the rapid vasodilation appears not to involve the sympathetic system
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