1,271 research outputs found
Futures for farmers: hedging participation and the Mexican corn scheme
Administered commodity price schemes in developing countries have proved ineffective in raising farmers' incomes and price stabilisation through futures markets is increasingly advocated as the alternative policy objective. A potential difficulty is that farmers tend not to hedge extensively, even in developed countries where access to futures markets is long established. Explanations for this reticence are examined here with context provided by the Mexican hedging programme, which incorporates financial incentives to spur adoption. Applying representative data for corn to a well-known analysis of the hedging decision suggests that limited participation may reflect rational calculation rather than farmer 'inertia'. A policy implication is that permanent access subsidies are difficult to justify from the national perspective.
An X-ray Survey of Galaxies in Pairs
Results are reported from the first survey of X-ray emission from galaxies in
pairs. The sample consists of fifty-two pairs of galaxies from the Catalog of
Paired Galaxies Karachentsev (1972) whose coordinates overlap ROSAT Position
Sensitive Proportional Counter pointed observations. The mean observed log l_x
for early-type pairs is 41.35 +/-0.21 while the mean log l_x predicted using
the l_x-l_b relationship for isolated early-type galaxies is 42.10 +/-0.19.
With 95% confidence, the galaxies in pairs are underluminous in the X-ray,
compared to isolated galaxies, for the same l_b. A significant fraction of the
mixed pair sample also appear similarly underluminous. A spatial analysis shows
that the X-ray emission from pairs of both types typically has an extent of ~10
- 50 kpc, much smaller than group intergalactic medium and thus likely
originates from the galaxies. CPG 564, the most X-ray luminous early-type pair,
4.7x10^42 ergs/sec, is an exception. The extent of it's X-ray emission, >169
kpc, and HWHM, ~80 kpc, is comparable to that expected from an intergalactic
medium. The sample shows only a weak correlation, ~81% confidence, between l_x
and l_b, presumably due to variations in gas content within the galaxies. No
correlation between l_x and the pair velocity difference, separation, or
far-infrared luminosity is found though the detection rate is low, 22%.Comment: 40 pages, 6 jpg figures, ApJ (in press
Simulations of the Nuclear Recoil Head-Tail Signature in Gases Relevant to Directional Dark Matter Searches
We present the first detailed simulations of the head-tail effect relevant to
directional Dark Matter searches. Investigations of the location of the
majority of the ionization charge as being either at the beginning half (tail)
or at the end half (head) of the nuclear recoil track were performed for carbon
and sulphur recoils in 40 Torr negative ion carbon disulfide and for fluorine
recoils in 100 Torr carbon tetrafluoride. The SRIM simulation program was used,
together with a purpose-written Monte Carlo generator, to model production of
ionizing pairs, diffusion and basic readout geometries relevant to potential
real detector scenarios, such as under development for the DRIFT experiment.
The results clearly indicate the existence of a head-tail track asymmetry but
with a magnitude critically influenced by two competing factors: the nature of
the stopping power and details of the range straggling. The former tends to
result in the tail being greater than the head and the latter the reverse.Comment: 22 pages, 12 figures, submitted to Astroparticle Physic
Measuring Cluster Temperature Profiles with XMM/EPIC
Using the PV observation of A1795, we illustrate the capability of XMM-EPIC
to measure cluster temperature profiles, a key ingredient for the determination
of cluster mass profiles through the equation of hydrostatic equilibrium. We
develop a methodology for spatially resolved spectroscopy of extended sources,
adapted to XMM background and vignetting characteristics. The effect of the
particle induced background is discussed. A simple unbiased method is proposed
to correct for vignetting effects, in which every photon is weighted according
to its energy and location on the detector. We were able to derive the
temperature profile of A1795 up to 0.4 times the virial radius. A significant
and spatially resolved drop in temperature towards the center (r<200 kpc) is
observed, which corresponds to the cooling flow region of the cluster. Beyond
that region, the temperature is constant with no indication of a fall-off at
large radii out to 1.2 Mpc.Comment: 7 pages, 8 figures, Accepted for publication in A&A (special Letter
issue on XMM
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