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  • 1
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    A new observational approach to investigate the heliospheric interstellar wind interface - The study of extreme and far ultraviolet resonantly scattered solar radiation from neon, oxygen, carbon and nitrogen (1990)
    In:  Other Sources
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    Publication Date: 2019-07-27
    Description: One of the outstanding uncertainties in the understanding of the heliosphere concerns the character of the interaction between the outflowing solar wind and the interstellar medium. A new possibility for obtaining information on this topic is suggested. The cosmically abundant elements neon, oxygen, carbon, and nitrogen will be affected differently at their interface passage depending upon the character of this region. Consequently, the distribution of these atoms and their ions will vary within the inner heliosphere. The study of resonantly scattered solar radiation from these species will then provide information on the nature of the interface. A preliminary evaluation of this approach has been carried out, and the results are encouraging. The relevant lines to be studied are in the extreme and far ulraviolet. The existing data in these bands are reviewed; unfortunately, past instrumentation has had insufficient resolution and sensitivity to provide useful information. The capabilities of future approved missions with capabilities in this area are evaluated.
    Keywords: ASTROPHYSICS
    Type: COSPAR Colloquium; Sept. 19-22, 1989; Warsaw; Poland
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  • 2
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    Can lensed stars be regarded as pointlike for microlensing by MACHOs? (1994)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: We derive an analytical formula for the maginification of an extended source with constant surface brightness lensed by a point mass. We find that the magnification for an extended source is significantly different from that for a point source when the impact parameter is comparable to the source size. In particular, the peak magnification for an extended source can be larger or smaller than the peak magnification for a point source. The results are relevant to the short-duration (approximately day) and high-magnification events in the search of massive compact halo objects in the Galactic halo by microlensing.
    Keywords: ASTROPHYSICS
    Type: The Astrophysical Journal (ISSN 0004-637X); 430; 2 pt; p. 505-510
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  • 3
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    The velocity dispersion of the caustic network due to random motion of individual stars in the lensing galaxy (1993)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: We present a method of computing the velocity distribution of the caustic network due to the random motion of stars in the lensing galaxy. This method is illustrated on the example of the two-point mass lens and then applied to a large sample of stars. We conclude that the proper motion of the stars increases significantly the frequency of the high magnification events in comparison with a static lens configuration with constant stream or constant bulk velocity. The stream velocity is the velocity of the star field relative to the global bulk velocity of the galaxy. We show that the global bulk and the stream velocity of the star field have to be considered separately for any microlensing situation. The higher the surface mass density of the stars in the lensing galaxy, the higher the influence of proper motion of stars on the statistics of high magnification events. The influence of a Gaussian velocity distribution of the stars in the lensing galaxy compared with a constant stream velocity of the stars increases the number of high magnification events by a factor 1.30 +/- 0.06 for a normalized surface density of the stars cr = 0.1 and by a factor 1.7 +/- 0.1 for sigma = 0.5. This means that for some microlensing situations the proper motion of the stars in a lensing galaxy has to be considered for exact microlensing predictions.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 409; 2; p. 537-547.
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  • 4
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    On the universality of microlensing in quadruple gravitational lenses (1995)
    In:  Other Sources
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    Publication Date: 2019-08-28
    Description: It is well known that multiply imaged quasars are likely to be affected by microlensing. Quadruply imaged systems are especially useful laboratories for studying microlensing because their macrolens models are relatively well constrained. We begin with analytical results for a simple family of galaxy models. These results can be used to estimate the magnifications and time delays for the quadruple systems. We compute expected brightness fluctuations due to microlensing in several such systems for a range of source sizes. Among these we treat for the first time the limiting case of a completely unresolved source. For the case of MG 0414+0534 we interpret the discrepant optical-to-radio flux ratios for the different components as the result of microlensing. On the assumption that the lensing galaxy is roughly isothermal this gives a 95% confidence upper limit on the optical source size of 10(exp 16) cm x (mean value of M)/0.1 solar mass(exp 1/2) where (M) is the average stellar mass of the lensing galaxy. For more centrally concentrated macromodels the upper limit is greater. Sufficiently long strings of photometric observations can in principle constrain the degree of concentration.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 443; 1; p. 18-28
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  • 5
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    Interpretation of microlensing events in Q2237 + 0305 (1994)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: We point out the uncertainties in the potential models of the foreground galaxy in the gravitational lens Q2237 + 0305. The surface densities and external shears are uncertain by a factor of appproximately less than 2. Very long light curves are calculated for different models using the method developed by Witt (1993) and Lewis et al. (1993). We find that the probability of images being in a quiescent state depends strongly on the potential models and the direction of the source motion relative to the alignment of the (gravitational lens) shear term of the macroimages. For example, the probabilities of finding variations smaller than 0.1 mag and for a period of longer than 0.5 Einstein radii (approximately equals 3-5 yr) in image C and D are approximately 30%. This offers an explanation of why image C and D showed relatively small variations. The quiescent images are more likely to be demagnified, consistent with the observations. Further, the higher the magnification of the macroimages, the weaker are the microlensing events. In addition, we conclude that a simple Gaussian profile has some difficulties in explaining the asymmetric shape and the small amplitude of the observed high-magnification events if they are typical. Source models in which the emission is produced by a brightt compact core and a more extended halo can occur better explain the observed double peak events. More data are needed to clearly distinguish different galaxy potential models.
    Keywords: ASTROPHYSICS
    Type: The Astrophysical Journal, Part 1 (ISSN 0004-637X); 429; 1; p. 66-76
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  • 6
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    An efficient method to compute microlensed light curves for point sources (1993)
    In:  Other Sources
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    Publication Date: 2019-08-27
    Description: We present a method to compute microlensed light curves for point sources. This method has the general advantage that all microimages contributing to the light curve are found. While a source moves along a straight line, all micro images are located either on the primary image track or on the secondary image tracks (loops). The primary image track extends from - infinity to + infinity and is made of many sequents which are continuously connected. All the secondary image tracks (loops) begin and end on the lensing point masses. The method can be applied to any microlensing situation with point masses in the deflector plane, even for the overcritical case and surface densities close to the critical. Furthermore, we present general rules to evaluate the light curve for a straight track arbitrary placed in the caustic network of a sample of many point masses.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 403; 2; p. 530-541.
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