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Results for “GRAVITATIONAL LENSES”

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NASA NTRS1984

The statistics of gravitational lenses - The distributions of image angular separations and lens redshifts

An analytical calculation is presented which determines the probability that the light from a distant quasar can be lensed by objects in a different redshift range. Three types of information are used to make the calculation: (1) the physical properties of the intervening scattering lenses; (2) the intrinsic properties of the quasars and (3) the selection procedures by which the quasar samples are acquired. A series of differential equations is given in which the probability is derived, and a number of recommendations are offered with respect to VLA and Space Telescope observations of quasars lensed in this way.

Turner, E. L.↗
NASA NTRS1985

Gravitational lensing effects of vacuum strings - Exact solutions

Exact interior and exterior solutions to Einstein's field equations are derived for vacuum strings. The exterior solution for a uniform density vacuum string corresponds to a conical space while the interior solution is that of a spherical cap. For Mu equals 0-1/4 the external metric is ds-squared = -dt-squared + dr-squared + (1-4 Mu)-squared r-squared dphi-squared + dz-squared, where Mu is the mass per unit length in the string in Planck masses per Planck length. A maximum mass per unit length for a string is 6.73 x 10 to the 27th g/cm. It is shown that strings cause temperature fluctuations in the cosmic microwave background and produce equal brightness double QSO images separated by up to several minutes of arc. Formulae for lensing probabilities, image splittings, and time delays are derived for strings in a realistic cosmological setting. String searches using ST, the VLA, and the COBE satellite are discussed.

Gott, J. R., III↗
NASA NTRS1986

Effect of gravitational lenses on the microwave background, and 1146+111B,C

A simple check of the proposal by Turner et al. (1986) that the pair of quasars 1146+111B,C are, in fact, two images of the same object lensed by an intervening object is suggested. If the lens is a cluster, either of normal galaxies or of dark matter, the microwave background seen through the cluster will be distorted by the Zeldovich-Sunyaev effect at the 0.001 level. The effects on the proposed check of the cluster not having the shape of an isothermal sphere, of containing no gas or only cold gas, and of having a low ratio of baryonic mass to total mass are considered.

Ostriker, J. P.↗
NASA NTRS1987

An analytically soluble problem in fully nonlinear statistical gravitational lensing

The amplification probability distribution p(I)dI for a point source behind a random star field which acts as the deflector exhibits a I exp-3 behavior for large amplification, as can be shown from the universality of the lens equation near critical lines. In this paper it is shown that the amplitude of the I exp-3 tail can be derived exactly for arbitrary mass distribution of the stars, surface mass density of stars and smoothly distributed matter, and large-scale shear. This is then compared with the corresponding linear result.

Schneider, P.↗
NASA NTRS1990

Gravitational lensing by a smoothly variable three-dimensional mass distribution

A smooth three-dimensional mass distribution is approximated by a model with multiple thin screens, with surface mass density varying smoothly on each screen. It is found that 16 screens are sufficient for a good approximation of the three-dimensional distribution of matter. It is also found that in this multiscreen model the distribution of amplifications of single images is dominated by the convergence due to matter within the beam. The shear caused by matter outside the beam has no significant effect. This finding considerably simplifies the modeling of lensing by a smooth three-dimensional mass distribution by effectively reducing the problem to one dimension, as it is sufficient to know the mass distribution along a straight light ray.

Lee, Man Hoi↗
NASA NTRS1991

Gravitational lensing frequencies - Galaxy cross-sections and selection effects

Four issues - (1) the best currently available data on the galaxy velocity-dispersion distribution, (2) the effects of finite core radii potential ellipticity on lensing cross sections, (3) the predicted distribution of lens image separations compared to observational angular resolutions, and (4) the preferential inclusion of lens systems in flux limited samples - are considered in order to facilitate more realistic predictions of multiple image galaxy-quasar lensing frequencies. It is found that (1) the SIS lensing parameter F equals 0.047 +/-0.019 with almost 90 percent contributed by E and S0 galaxies, (2) observed E and S0 core radii are remarkably small, yielding a factor of less than about 2 reduction in total lensing cross sections, (3) 50 percent of galaxy-quasar lenses have image separations greater than about 1.3 arcsec, and (4) amplification bias factors are large and must be carefully taken into account. It is concluded that flat universe models excessively dominated by the cosmological constant are not favored by the small observed galaxy-quasar lensing rate.

Fukugita, Masataka↗
NASA NTRS1992

Gravitational lensing of quasars as seen by the Hubble Space Telescope Snapshot Survey

Results from the ongoing HST Snapshot Survey are presented, with emphasis on 152 high-luminosity, z greater than 1 quasars. One quasar among those observed, 1208 + 1011, is a candidate lens system with subarcsecond image separation. Six other quasars have point sources within 6 arcsec. Ground-based observations of five of these cases show that the companion point sources are foreground Galactic stars. The predicted lensing frequency of the sample is calculated for a variety of cosmological models. The effect of uncertainties in some of the observational parameters upon the predictions is discussed. No correlation of the drift rate with time, right ascension, declination, or point error is found.

Maoz, D.↗
NASA NTRS1993

Searching gamma-ray bursts for gravitational lensing echoes - Implications for compact dark matter

The first available 44 gamma-ray bursts (GRBs) detected by the Burst and Transient Source Experiment on board the Compton Gamma-Ray Observatory have been inspected for echo signals following shortly after the main signal. No significant echoes have been found. Echoes would have been expected were the GRBs distant enough and the universe populated with a sufficient density of compact objects composing the dark matter. Constraints on dark matter abundance and GRB redshifts from the present data are presented and discussed. Based on these preliminary results, a universe filled to critical density of compact objects between 10 exp 6.5 and 10 exp 8.1 solar masses are now marginally excluded, or the most likely cosmological distance paradigm for GRBs is not correct. We expect future constraints to be able either to test currently popular cosmological dark matter paradigms or to indicate that GRBs do not lie at cosmological distances.

Nemiroff, R. J.↗
NASA NTRS1995

On the universality of microlensing in quadruple gravitational lenses

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.

Witt, Hans J.↗
NASA NTRS1995

ROSAT observations of the gravitationally lensed system 0957+561

Observations of 0957+561 with the Einstein High Resolution Imager (HRI) in 1979 May and 1979 November and with the ROSAT HRI in 1991 May and 1992 October reveal large variations in the X-ray fluxes for images A and B which significantly differ from simultaneously observed changes of the corresponding optical continuum emission. Most striking was the apparent increase by over fivefold in the flux of image B from the late 1970's to the early 1900's. Further, in the 1990's the X-ray flux for image A increased by a factor of 1.7 +/- 0.3 and that of B by a factor of 1.9 +/- 0.2 between the two ROSAT observations (separated by 540 days), whereas optical measurements showed nearly no changes for A and B between these epochs. No significant changes in X-ray emission were observed on timescales of hours to days. Of we adopt 1.5 yr as the difference between the propagation time from the quasar to us via the two images ('time delay'), then the ratio of the X-ray flux of image B for the 1992 October epoch to that of A for the 1991 May epoch is 2.7 +/- 0.4. This ratio is significantly greater than the ratio of 1.05 +/- 0.03 observed in the optical R band, 0.75 in the broad line region (BLR), and 0.76 +/- 0.03 in the radio (VLBI lambda 13 cm core) for approximately the same epoch. The wavelength dependence in the ratio of the fluxes of the two images suggests that either microlensing may be significant for the X-ray band or the time delay is substanially different from 1.5 yr (and the intrinsic variation of the quasar emission were significant within an interval comparable to the uncertainty of the time delay), or both.

Chartas, G.↗
NASA NTRS1995

HST Observations of the Luminous IRAS Source FSC10214+4724: A gravitationally Lensed Infrared Quasar

Observations of a distant object in space with the data being taken by the Hubble Space Telescope (HST) Wide Field Planetary Camera. Scientific examination and hypothesis related to this object which appears to be either an extremely luminous dust embedded quasar, or a representative of a new class of astronomical objects (a primeval galaxy).

Hubble Space Telescope Quasar Space Object Astrono↗