20,866 research outputs found
Application of the Contouring Method to Extended Microlensed Sources
The method devised by Lewis et al. (1993) for calculating the light curve of
a microlensed point source is expanded to two dimensions to enable the
calculation of light curves of extended sources. This method is significantly
faster than the ray shooting method that has been used in the past. The
increased efficiency is used to obtain much higher resolution light curves over
increased timescales. We investigate the signatures arising from different
source geometries in a realistic microlensing model. We show that a large
fraction of high magnification events (HMEs) in image A of Q2237+0305 involve
only one caustic, and could therefore yield information on the structure of the
quasar continuum through the recognition of a characteristic event shape. In
addition, the cataloguing of HMEs into morphological type will, in theory,
enable the direction of the transverse motion, as well as the source size to be
obtained from long term monitoring.Comment: 10 pages including 4 figures. Accepted for publication in M.N.R.A.
Describing results of 4000 hours of multi environment model verification test Final report
Investigating categorization and formulation of stress and strength factors for semiconductor diodes to provide improved failure rate prediction from mathematical model
Cereal Leaf Beetle (Coleoptera: Chrysomelidae) Influence of Seeding Rate of Oats on Populations
In field and greenhouse studies, more cereal leaf beetle [Oulema melanopus (Lin- naeus)] eggs and larvae were found per unit area on spring oats, Avena sativa L., planted either at intermediate (54 kg/ha) or high (136 kg/ha) seeding rates, than when planted at a lower seeding rate (14 kg/ha). However, there were fewer eggs and larvae per stem in plantings of the high or intermediate rates than in those of the lower rate. Oats should not be planted at less than the recommended rates in beetle-infested areas
A measurement of the transverse velocity of Q2237+0305
Determination of microlensing parameters in the gravitationally lensed quasar
Q2237+0305 from the statistics of high magnification events will require
monitoring for more than 100 years (Wambsganss, Paczynski & Schneider 1990).
However we show that the effective transverse velocity of the lensing galaxy
can be determined on a more realistic time-scale through consideration of the
distribution of light-curve derivatives. The 10 years of existing monitoring
data for Q2237+0305 are analysed. These data display strong evidence for
microlensing that is not associated with a high magnification event. An upper
limit of v < 500 km/sec is obtained for the galactic transverse velocity which
is smaller than previously assumed values. The analysis suggests that the
observed microlensing variation may be predominantly due to stellar proper
motions. The statistical significance of the results obtained from our method
will be increased by the addition of data points from current and future
monitoring campaigns. However reduced photometric errors will be more valuable
than an increased sampling rate.Comment: 16 pages, including 17 figures. Accepted for publication in M.N.R.A.
Interpretation of the OGLE Q2237+0305 microlensing light-curve
The four bright images of the gravitationally lensed quasar Q2237+0305 are
being monitored from the ground (eg. OGLE collaboration, Apache Point
Observatory) in the hope of observing a high magnification event (HME). Over
the past three seasons (1997-1999) the OGLE collaboration has produced
microlensing light-curves with unprecedented coverage. These demonstrate
smooth, independent (therefore microlensing) variability between the images
(Wozniak et al. 2000a,b; OGLE web page). We have retrospectively compared
probability functions for high-magnification event parameters with several
observed light-curve features. We conclude that the 1999 image C peak was due
to the source having passed outside of a cusp rather than to a caustic
crossing. In addition, we find that the image C light-curve shows evidence for
a caustic crossing between the 1997 and 1998 observing seasons involving the
appearance of new critical images. Our models predict that the next image C
event is most likely to arrive 500 days following the 1999 peak, but with a
large uncertainty (100-2000 days). Finally, given the image A light-curve
derivative at the end of the 1999 observing season, our modelling suggests that
a caustic crossing will occur between the 1999 and 2000 observing seasons,
implying a minimum for the image A light-curve ~1-1.5 magnitudes fainter than
the November 1999 level.Comment: 11 pages, 15 figures. Accepted for publication in M.N.R.A.
Limits on the microlens mass function of Q2237+0305
Gravitational microlensing at cosmological distances is potentially a
powerful tool for probing the mass functions of stars and compact objects in
other galaxies. In the case of multiply-imaged quasars, microlensing data has
been used to determine the average microlens mass. However the measurements
have relied on an assumed transverse velocity for the lensing galaxy. Since the
measured mass scales with the square of the transverse velocity, published mass
limits are quite uncertain. In the case of Q2237+0305 we have properly
constrained this uncertainty. The distribution of light curve derivatives
allows quantitative treatment of the relative rates of microlensing due to
proper motions of microlenses, the orbital stream motion of microlenses and the
bulk galactic transverse velocity. By demanding that the microlensing rate due
to the motions of microlenses is the minimum that should be observed we
determine lower limits for the average mass of stars and compact objects in the
bulge of Q2237+0305. If microlenses are assumed to move in an orbital stream
the lower limit ranges between 0.005 and 0.023 solar masses where the the
systematic dependence is due to the fraction of smooth matter and the size of
photometric error assumed for published monitoring data. However, if the
microlenses are assumed to move according to an isotropic velocity dispersion
then a larger lower limit of 0.019-0.11 solar masses is obtained. A significant
contribution of Jupiter mass compact objects to the mass distribution of the
galactic bulge of Q2237+0305 is therefore unambiguously ruled out.Comment: 10 pages, 5 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Society. New version has improved presentatio
Nonzero macroscopic magnetization in half-metallic antiferromagnets at finite temperatures
Combining density-functional theory calculations with many-body
Green's-function technique, we reveal that the macroscopic magnetization in
half-metallic antiferromagnets does not vanish at finite temperature as for the
T=0 limit. This anomalous behavior stems from the inequivalent magnetic
sublattices which lead to different intrasublattice exchange interactions. As a
consequence, the spin fluctuations suppress the magnetic order of the
sublattices in a different way leading to a ferrimagnetic state at finite
temperatures. Computational results are presented for the half-metallic
antiferromagnetic CrMnZ (Z=P,As,Sb) semi-Heusler compounds.Comment: 4 pages, 2 figure
Role of the exchange and correlation potential into calculating the x-ray absorption spectra of half-metallic alloys: the case of Mn and Cu K-edge XANES in CuMnM (M = Al, Sn, In) Heusler alloys
This work reports a theoretical study of the x-ray absorption near-edge
structure spectra at both the Cu and the Mn K-edge in several CuMnM (M= Al,
Sn and In) Heusler alloys. Our results show that {\it ab-initio} single-channel
multiple-scattering calculations are able of reproducing the experimental
spectra. Moreover, an extensive discussion is presented concerning the role of
the final state potential needed to reproduce the experimental data of these
half-metallic alloys. In particular, the effects of the cluster-size and of the
exchange and correlation potential needed in reproducing all the experimental
XANES features are discussed.Comment: 15 pages, 5 figure
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