1,061 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
XeCl laser pumped iodine laser using t-C4F9I
An iodine photodissociation laser using t-C4F9I as the active material was pumped by an XeCl laser. An iodine laser output energy of 3 mJ with pulse duration of 25 ns was obtained when the pumping pulse energy was 80 mJ, the iodide pressure was 70 torr, and the reflectance of the output mirror was 85 percent. The high pumping efficiency and low threshold pump power achieved in this experiment are attributable to the high absorption cross section at the pump laser wavelength (308 nm) of the iodide used
Direct solar-pumped iodine laser amplifier
The optimum conditions of a solar pumped iodine laser are found in this research for the case of a continuous wave operation and a pulsed operation. The optimum product of the pressure(p) inside the laser tube and the tube diameter(d) was pd=40 approx. 50 torr-cm on the contrary to the case of a high intensity flashlamp pumped iodine laser where the optimum value of the product is known to be pd=150 torr-cm. The pressure-diameter product is less than 1/3 of that of the high power iodine laser. During the research period, various laser materials were also studied for solar pumping. Among the laser materials, Nd:YAG is found to have the lowest laser threshold pumping intensity of about 200 solar constant. The Rhodamine 6G was also tested as the solar pumped laser material. The threshold pumping power was measured to be about 20,000 solar constant. The amplification experiment for a continuously pumped iodine laser amplifier was performed using Vortek solar simulator and the amplification factors were measured for single pass amplification and triple pass amplification of the 15 cm long amplifier tube. The amplification of 5 was obtained for the triple pass amplification
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
Segmentation of Intensity Inhomogeneous Brain MR Images Using Active Contours
Segmentation of intensity inhomogeneous regions is a well-known problem in image analysis applications. This paper presents a region-based active contour method for image segmentation, which properly works in the context of intensity inhomogeneity problem. The proposed region-based active contour method embeds both region and gradient information unlike traditional methods. It contains mainly two terms, area and length, in which the area term practices a new region-based signed pressure force (SPF) function, which utilizes mean values from a certain neighborhood using the local binary fitted (LBF) energy model. In turn, the length term uses gradient information. The novelty of our method is to locally compute new SPF function, which uses local mean values and is able to detect boundaries of the homogenous regions. Finally, a truncated Gaussian kernel is used to regularize the level set function, which not only regularizes it but also removes the need of computationally expensive reinitialization. The proposed method targets the segmentation problem of intensity inhomogeneous images and reduces the time complexity among locally computed active contour methods. The experimental results show that the proposed method yields better segmentation result as well as less time complexity compared with the state-of-the-art active contour methods
Detection Probability of a Low-Mass Planet for Triple Lens Events: Implication of Properties of Binary-Lens Superposition
In view of the assumption that any planetary system is likely to be composed
of more than one planet, and the multiple planet system with a large mass
planet has more chance of detailed follow-up observations, the multiple planet
system may be an efficient way to search for sub-Jovian planets. We compare the
magnification pattern of the triple lens system with that of a best-fitted
binary system composed of a star and a Jovian mass planet, and check the
probability in detecting the low-mass secondary planet whose signature will be
superposed on that of the primary Jovian mass planet. Detection probabilities
of the low-mass planet in the triple lens system are quite similar to the
probability in detecting such a low-mass planet in a binary system with a star
and only a low-mass planet, which shows that the signature of a low-mass planet
can be effectively detected even when it is concurrent with the signature of
the more massive planet, implying that the binary superposition approximation
works over a relatively broad range of planet mass ratio and separations, and
the inaccuracies thereof do not significantly affect the detection probability
of the lower mass secondary planet. Since the signature of the Jovian mass
planet will be larger and lasts longer, thereby warranting more intensive
follow-up observations, the actual detection rate of the low-mass planet in a
triple system with a Jovian mass can be significantly higher than that in a
binary system with a low-mass planet only. We conclude that it may be
worthwhile to develop an efficient algorithm to search for `super-Earth'
planets in the paradigm of the triple lens model for high-magnification
microlensing events.Comment: 10 pages, 5 figures, accepted by MNRA
Direct solar-pumped iodine laser amplifier
A XeCl laser pumped iodine laser oscillator was developed which will be incorporated into the Master Oscillator Power Amplifier (MOPA) system. The developed XeCl laser produces output energy of about 60 mJ per pulse. The pulse duration was about 10 nsec. The kinetic model for the solar-pumped laser was refined and the algorithm for the calculation of a set of rate equations was improved to increase the accuracy and the efficiency of the calculation. The improved algorithm was applied to explain the existing experimental data taken from a flashlamp pumped iodine laser for three kinds of lasants, i-C3F7I, n-C4F9I, and t-C4F9I. Various solid laser materials were evaluated for solar-pumping. The materials studied were Nd:YAG, Nd:YLF, and Cr:Nd:GSGG crystals. The slope efficiency of 0.17 percent was measured for the Nd:YLF near the threshold pump intensity which was 211 solar constants (29W/sq cm). The threshold pump intensity of the Nd:YAG was measured to be 236 solar constants (32W/sq cm) and the near-threshold slope efficiency was 0.12 percent. True CW laser operation of Cr:Nd:GSGG was possible only at pump intensities less than or equal to 1,500 solar constants (203 W/sq cm). This fact was attributed to the high thermal focusing effect of the Cr:Nd:GSGG rod
Robust Adaptive Depth Control of Hybrid Underwater Glider in Vertical Plane
Hybrid underwater glider (HUG) is an advanced autonomous underwater vehicle with propellers capable of sustainable operations for many months. Under the underwater disturbances and parameter uncertainties, it is difficult that the HUG coordinates with the desired depth in a robust manner. In this study, a robust adaptive control algorithm for the HUG is proposed. In the descend and ascend periods, the pitch control is designed using backstepping technique and direct adaptive control. When the vehicle approaches the target depth, the surge speed control using adaptive control combined with the pitch control is used to keep the vehicle at the desired depth with a constant cruising speed in the presence of the disturbances. The stability of the proposed controller is verified by using the Lyapunov theorem. Finally, the computer simulation using the numerical method is conducted to show the effectiveness of the proposed controller for a hybrid underwater glider system
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