595 research outputs found
Determination of Stellar Ellipticities in Future Microlensing Surveys
We propose a method that can determine the ellipticities of source stars of
microlensing events produced by binary lenses. The method is based on the fact
that the products of the caustic-crossing timescale, , and the cosine
of the caustic incidence angle of the source trajectory, , of the
individual caustic crossings are different for events involving an elliptical
source, while the products are the same for events associated with a circular
source. The product corresponds to the
caustic-crossing timescale when the incidence angle of the source trajectory is
. For the unique determination of the source ellipticity, resolutions
of at least three caustic crossings are required. Although this requirement is
difficult to achieve under the current observational setup based on
alert/follow-up mode, it will be possible with the advent of future lensing
experiments that will survey wide fields continuously at high cadence. For
typical Galactic bulge events, the difference in between
caustic crossings is of the order of minutes depending on the source
orientations and ellipticities. Considering the monitoring frequency of the
future lensing surveys of times/hr and the improved photometry
especially of the proposed space-based survey, we predict that ellipticity
determinations by the proposed method will be possible for a significant
fraction of multiple caustic-crossing binary lens events involving source stars
having non-negligible ellipticities.Comment: 6 pages, 4 figures, ApJ, submitte
Relativistic Conic Beams and Spatial Distribution of Gamma-Ray Bursts
We study the statistics of gamma-ray bursts, assuming that gamma-ray bursts
are cosmological and they are beamed in the form of a conical jet with a large
bulk Lorentz factor . In such a conic beam, the relativistic ejecta
may have a spatial variation in the bulk Lorentz factor and the density
distribution of gamma-ray emitting jet material. An apparent luminosity
function arises because the axis of the cone is randomly oriented with respect
to the observer's line of sight. The width and the shape of the luminosity
function are determined by the ratio of the beam opening angle of the conical
jet to the inverse of the bulk Lorentz factor, when the bulk Lorentz factor and
the jet material density is uniform on the photon emitting jet surface. We
calculate effects of spatial variation of the Lorentz factor and the spatial
density fluctuations within the cone on the luminosity function and the
statistics of gamma-ray bursts. In particular, we focus on the redshift
distribution of the observed gamma-ray bursts. The maximum distance to and the
average redshift of the gamma-ray bursts are strongly affected by the
beaming-induced luminosity function. The bursts with the angle-dependent
Lorentz factor which peaks at the center of the cone have substantially higher
average gamma-ray burst redshifts. When both the jet material density and the
Lorentz factor are inhomogeneous in the conical beam, the average redshift of
the bursts could be 5 times higher than that of the case in which relativistic
jet is completely homogeneous and structureless. Even the simplest models for
the gamma-ray burst jets and their apparent luminosity distributions have a
significant effect on the redshift distribution of the gamma-ray bursts.Comment: 15 pages, 4 figures, submitted to ApJ
Effects of Luminosity Functions Induced by Relativistic Beaming on Statistics of Cosmological Gamma-Ray Bursts
We study the effects of the beaming-induced luminosity function on statistics
of observed GRBs, assuming the cosmological scenario. We select and divide the
BATSE 4B data into 588 long bursts (T sec) and 149 short bursts
(T sec), and compare the statistics calculated in each subgroup. The
of the long bursts is $ 0.2901\pm 0.0113$, and that of the
short bursts is $0.4178\pm 0.0239$, which is a Euclidean value. For luminosity
function models, we consider a cylindrical-beam and a conic-beam. We take into
account the spatial distribution of GRB sources as well. A broad luminosity
function is naturally produced when one introduces beaming of GRBs. We
calculate the maximum detectable redshift of GRBs, $z_{\rm max}$. The estimated
$z_{\rm max}$ for the cylindrical-beam case is as high as $\sim 14$ for the
long bursts and $\sim 3$ for the short bursts. The large $z_{\rm max}$ value
for the short bursts is rather surprising in that the for
this subgroup is close to the so-called Euclidean value, 0.5. We calculate the
fraction of bursts whose redshifts are larger than a certain redshift ,
i.e. . When we take and apply the luminosity function
derived for the cylindrical-beam, the expected is
for long bursts. When we increase the opening angle of the conic beam to
, decreases to at . We conclude that the beaming-induced luminosity functions are
compatible with the redshift distribution of observed GRBs and that the
apparent Euclidean value of may not be due to the Euclidean
space distribution but to the luminosity distribution.Comment: Accepted for publication in the Astronomical Journal (vol. 548, Feb.
20 2001
Another Channel to detect Close-in Binary Companions via Gravitational Microlensing
Gaudi & Gould (1997) showed that close companions of remote binary systems
can be efficiently detected by using gravitational microlensing via the
deviations in the lensing light curves induced by the existence of the lens
companions. In this paper, we introduce another channel to detect faint
close-in binary companions by using microlensing. This method utilizes a
caustic-crossing binary lens event with a source also composed of binary stars,
where the companion is a faint star. Detection of the companion is possible
because the flux of the companion can be highly amplified when it crosses the
lens caustic. The detection is facilitated since the companion is more
amplified than the primary because it, in general, has a smaller size than the
primary, and thus experiences less finite source effect. The method is
extension of the previous one suggested to detect close-in giant planets by
Graff & Gaudi (2000) and Lewis & Ibata (2000) and further developed by Ashton &
Lewis (2001). From the simulations of realistic Galactic bulge events, we find
that companions of K-type main sequence or brighter can be efficiently detected
from the current type microlensing followup observations by using the proposed
method. We also find that compared to the method of detecting lens companions
for which the efficiency drops significantly for binaries with separations
of the angular Einstein ring radius, the proposed method has an
important advantage of being able to detect companions with substantially
smaller separations down to order of 0.01 \theta_E.Comment: total 7 pages, including 5 figures and 2 tables, MNRAS, submitte
Dependence of GCRs influx on the Solar North-South Asymmetry
We investigate the dependence of the amount of the observed galactic cosmic
ray (GCR) influx on the solar North-South asymmetry using the neutron count
rates obtained from four stations and sunspot data in archives spanning six
solar cycles from 1953 to 2008. We find that the observed GCR influxes at
Moscow, Kiel, Climax and Huancayo stations are more suppressed when the solar
activity in the southern hemisphere is dominant compared with when the solar
activity in the northern hemisphere is dominant. Its reduction rates at four
stations are all larger than those of the suppression due to other factors
including the solar polarity effect on the GCR influx. We perform the student's
t-test to see how significant these suppressions are. It is found that
suppressions due to the solar North-South asymmetry as well as the solar
polarity are significant and yet the suppressions associated with the former
are larger and more significant.Comment: 17 pages, 3figures, accepted to JAST
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