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At least 433 records · Page 24
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NASA NTRS1991

Infrared and visible photometry of the gravitational lens systems 2237 + 030

As part of a program of high spatial resolution imaging of gravitationally lensed sources in the visible and IR, images of 2237 + 030 were obtained in the Gunn r and infrared J, H, K, and 3.3 micron filters. The results of the photometry of the four bright quasar components provide evidence of extinction through the lens and a determination of the extinction law in the galaxy is made. The energy distribution shows evidence of a sharp decrease of the spectral index at wavelengths longer than a rest wavelength of 1 micron. Assuming that microlensing amplification in the IR is of similar strength as in the visible, the data constrain the suggested microlensing event of August-September 1988 to a time scale of 100 days.

Nadeau, Daniel↗
NASA NTRS1991

High-Energy Astrophysics. American and Soviet Perspectives

The proceedings of the American-Soviet high energy astrophysics workshop, which was held at the Institute for Space Research in Moscow and the Abastumani Laboratory and Observatory in the republic of Georgia from June 18 to July 1, 1989, is presented. Topics discussed at the workshop include the inflationary universe; the large scale structure of the universe, the diffuse x-ray background; gravitational lenses, quasars, and active galactic nuclei (AGNs); infrared galaxies (results from IRAS); Supernova 1987A; millisecond radio pulsars; quasi-periodic oscillations in the x-ray flux of low mass X-ray binaries; and gamma ray bursts.

Lewin, Walter H. G.↗
NASA NTRS1991

Microlensing and the structure of active galactic nucleus accretion disks

Rapid variability has been reported in two of the four gravitationally lensed images of Q2237 + 0305, and this is attributed to microlensing caused by the intervening stars. The associated constraints on the source size and properties are studied and compared with a variety of stationary accretion disk models. The reported microlensing variation in Q2237 + 0305 requires the disk size to be over 3 times smaller than a blackbody disk of similar luminosity, implying that the optical emission is either nonthermal or optically tin. An exploration of nonstationary disk models including orbiting, transient hot spots leads to a similar conclusion. Implications for models of active galactic nucleus optical continua are briefly discussed.

Rauch, Kevin P.↗
NASA NTRS1991

Cosmological gamma-ray bursts

The distribution in angle and flux of gamma-ray bursts indicates that the majority of gamma-ray bursters are at cosmological distances, i.e., at z of about 1. The rate is then about 10 exp -8/yr in a galaxy like the Milky Way, i.e., orders of magnitude lower than the estimated rate for collisions between neutron stars in close binary systems. The energy per burst is about 10 exp 51 ergs, assuming isotropic emission. The events appear to be less energetic and more frequent if their emission is strongly beamed. Some tests for the distance scale are discussed: a correlation between the burst's strength and its spectrum; the absorption by the Galactic gas below about 2 keV; the X-ray tails caused by forward scattering by the Galactic dust; about 1 month recurrence of some bursts caused by gravitational lensing by foreground galaxies; and a search for gamma-ray bursts in M31. The bursts appear to be a manifestation of something exotic, but conventional compact objects can provide an explanation. The best possibility is offered by a decay of a bindary composed of a spinning-stellar-mass black-hole primary and a neutron or a strange-quark star secondary. In the final phase the secondary is tidally disrupted, forms an accretion disk, and up to 10 exp 54 ergs are released. A very small fraction of this energy powers the gamma-ray burst.

Paczynski, Bohdan↗
NASA NTRS1992

A gravitational lens candidate discovered with the Hubble Space Telescope

Evidence is reported for gravitational lensing of the high-redshift (z = 3.8) quasar 1208 + 101, observed as part of the Snapshot survey with the HST Planetary Camera. An HST V image taken on gyroscopes resolves the quasar into three point-source components, with the two fainter images having separations of 0.1 and 0.5 arcsec from the central bright component. A radio observation of the quasar with the VLA at 2 cm shows that, like most quasars of this redhsift, 1208 + 101 is radio quiet. Based on positional information alone, the probability that the observed optical components are chance superpositions of Galactic stars is small, but not negligible. Analysis of a combined ground-based spectrum of all three components, using the relative brightnesses of the HST image, supports the lensing hypothesis. If all the components are lensed images of the quasar, the observed configuration cannot be reproduced by simple lens models.

Maoz, Dan↗
NASA NTRS1992

The x ray variability of NGC6814: Power spectra

Simulation techniques are used to obtain the X-ray variability power spectrum of unevenly sampled GINGA data from NGC6814. A simple power law is not an adequate description of the power spectrum, with the residuals showing excess power on timescales consistent with the periodicity seen in EXOSAT observations of this object. However the shape of the folded lightcurve is very different, with 3 main peaks, two of which are separated by an extremely sharp dip instead of the single peak and small harmonic structure observed by EXOSAT. Using the dip as a fiducial mark, a second GINGA observation of this source taken one year later is found to be consistent with being completely periodic and phase coherent with this first GINGA observation. Thus the period is consistent with being constant over a period of 6 years, but phase coherence is only maintained on timescales of approximately 1 year. Over 75 percent of the total source variability is due to the periodic component (r.m.s. amplitude of 36 percent). The residual variability can be described as the more usual 'flicker noise' f(exp -1.1) powerlaw. This shows no apparent high frequency break on timescales greater than 300 seconds. Subtle differences in the shape of the folded light curve with energy, and the very large amount of power in the periodic component suggest occultation as its origin, though amplification of variability from an X-ray emitting 'hot spot' at the disk inner radius through gravitational lensing is also possible. The former suffers from the very arbitrary nature of the periodic timescale, while the latter is unattractive as it cannot simply explain the lack of high frequency break in the residual power. That these models probably fail to provide an adequate explanation may be due to the added complexity of anisotropy of the X-ray emission, suggested by the discrepancy between the lack of soft photons implied by the flat spectrum and the copious source of soft photons available from reprocessing in the iron line producing material.

Done, C.↗
NASA NTRS1993

Statistics of highly magnified gravitational images in clusters of galaxies. I - Implications for the clusters. II - Implications for the sources

A study is presently conducted of the probability distribution of observing images of background galaxies that are gravitationally lensed in the form of elongated arcs by clusters of galaxies, as a function of arc width, curvature radius, and the elongation's orientation. An upper limit is obtained on the size of the cores which is smaller than cluster core radii obtained for the gas from the X-ray surface brightness. It is argued that lensing clusters tend to be elongated along the line of sight; projection effects will enhance surface density. In the second part of this work, it is shown that the number of arcs expected in lensing clusters is primarily dependent on the angular size of the sources, since lower magnification is required to generate an arc of fixed length for a larger source.

Miralda-Escude, Jordi↗
NASA NTRS1992

The cosmological constant

The cosmological constant problem is examined in the context of both astronomy and physics. Effects of a nonzero cosmological constant are discussed with reference to expansion dynamics, the age of the universe, distance measures, comoving density of objects, growth of linear perturbations, and gravitational lens probabilities. The observational status of the cosmological constant is reviewed, with attention given to the existence of high-redshift objects, age derivation from globular clusters and cosmic nuclear data, dynamical tests of Omega sub Lambda, quasar absorption line statistics, gravitational lensing, and astrophysics of distant objects. Finally, possible solutions to the physicist's cosmological constant problem are examined.

Carroll, Sean M.↗
NASA NTRS1993

Properties of ultracompact neutron stars

Some of the effects extreme gravity would have on the propagation of particles inside and around constant density ultracompact stars are examined. The possible observational characteristics due to the gravitational lensing of surface hot spots are examined. The change that would be seen in the Eddington luminosity near such an object is computed. It is shown that such stars would exhibit a 'neutrino' sphere inside the surface and a 'neutrino cloud' partially exterior to the surface.

Nemiroff, Robert J.↗
NASA NTRS1992

Extragalactic scenarios for gamma-ray bursts

The cosmological distance scale to gamma-ray bursts is indicated by the observed distribution of the bursts in angle and intensity. The redshifts of the weakest and the strongest bursts detected by the BATSE experiment are estimated to be z = 1.5 and z = 0.2, respectively, while the strongest bursts detected by the PVO in its 11 years of operation were at z = 0.05. The bursts' rate is about 10 exp -6 per year in a galaxy like ours, and a typical energy release is about 10 exp 51 ergs, assuming isotropic emission. The rate of the neutron star and black hole collisions in close binaries is somewhat higher, and those collisions release 10 exp 53 - 10 exp 54 ergs in a burstlike form, making them the primary candidates for the observed gamma-ray events. Contrary to the common belief, no observations available to date are in conflict with the cosmological distance to the bursters. A detection of gravitational lensing of some bursts, the redshift of the weakest events, and the absorption of soft X-rays by the interstellar matter in the plane of our galaxy, are the specific predictions of the cosmological scenario.

Paczynski, Bohdan↗
NASA NTRS1993

Features in the spectra of gamma-ray bursts

Gravitational lensing of cosmological gamma-ray bursts by objects in the mass range about 10 exp 17 to 10 exp 20 g (femtolensing) may introduce complicated interference patterns that might be interpreted as absorption or emission lines in the bursts' spectra. This phenomenon, if detected, may be used as a unique probe of dark matter in the universe. The BATSE spectral data should allow one to detect such spectral features or to put significant upper limits on the cosmic density of a dark matter component that may be in the femtolensing range. Software to generate theoretical spectra has been developed, and it is accessible over the computer network with anonymous ftp.

Stanek, Krzysztof Z.↗
NASA NTRS1993

Statistics of arcs in clusters of galaxies

Samples of gravitational lens events in clusters show many large arcs compared to arclets, relative to what can be obtained by idealized singular lens models. We describe the probability of image magnification for point sources and for simple but more realistic gravitational lensing models that include a finite core size and an ellipticity. In addition, we explore the changes in the probability distribution of image magnifications, distortions, and angular extents for sources of different sizes as the parameters of the lenses are varied. A finite core in spherically symmetric lens models introduces a discontinuity in the probability distribution at which the relative number of highly magnified images is increased. In elliptical lenses, this discontinuity and its effect are replaced by a continuous increase in the probability of obtaining high-magnification images relative to singular spherically symmetric models. We also find that the finite size of the source causes a further increase in the expected number of images just below the maximum possible magnification.

Bergmann, Anton G.↗
NASA NTRS1992

The universe at moderate redshift

The Final Report on the universe at moderate redshift covering the period from 1 Mar. 1988 to 28 Feb. 1991 is presented. Areas of research included: galaxy formation and large-scale structure; intergalactic medium and background radiation fields; quasar statistics and evolution; and gravitational lenses.

Ostriker, Jeremiah P.↗
NASA NTRS1994

Statistical association of QSO's with foreground galaxy clusters

We report a statistically significant overdensity of high redshift quasi-stellar objects (QSO's) in the directions of foreground galaxy clusters. QSO's are taken from the Large Bright QSO Survey (LBQS) between 1.4 less than or equal z less than or equal 2.2 with a limiting magnitude of m(sub B) = 18.5. Foreground clusters are regions within 6 Zwicky radii of small Zwicky clusters at a characteristic redshift of about z approximately = 0.2, covering about 40% of the total area surveyed (304 sq. deg). The overdensity, defined as the ratio of the number density of QSO's in the directions of clusters ('association QSO's) to that in the remainder of the fields ('background QSO's), is equal to 1.7, and formally differs from unity at 4.7 sigma significance. The observed overdensity probably is not due to statistical variation in QSO density, intrinsic QSO-QSO and/or cluster-cluster autocorrelations, or patchy Galactic obscuration. We thus interpret this observation as being due to statistical gravitational lensing of background QSO's by galaxy clusters. However, this amplitude of overdensity behind clusters cannot be accounted for in any cluster lensing model if the background QSO number-magnitude counts are similar to the intrinsic (unlensed) counts, and is implausible in any conventional model of cosmic mass distribution.

Rodrigues-Williams, Liliya L.↗
NASA NTRS1994

Radio continuum polarimetric imaging of high redshift radio galaxies

Multifrequency images of total and polarized radio continuum emission from the two high redshift radio galaxies 0902+343 (z = 3.40) and 0647+415 (4C 41.17, z = 3.80) are presented. These images represent the most sensitive polarimetric study of high redshift ratio galaxies to date. The emission from both galaxies is substantially polarized, up to 30% in some regions, and both sources sit behind deep 'Faraday screens,' producing large rotation measures, over 10(exp 3) rad/sq. m in magnitude, and large rotation measure gradients across the sources. Such large rotation measures provide further evidence that high redshift radio galaxies are situated in very dense environments. Drawing the analogy to a class of low redshift powerful radio galaxies with similarly large rotation measures, we suggest that 0902+343 and 0647+415 are situated at the centers of dense, x-ray 'colling flow' clusters, and that the cluster gas is substantially magnetized. The remarkable similarity between the optical and radio morphologies of 0647+415 on scales as small as 0.1 sec is presented. We consider, and reject, both synchrotron and inverse Compton radiation as possible sources of the optical emission. We also consider both scattering of light out of a 'cone' of radiation from an obscured nucleus, and jet-induced star formation, and find that both models encounter difficulties in explaining this remarkably close radio-optical alignment. High resolution spectral index images reveal compact, flat spectrum components in both sources. We suggest that these components are the active nuclei of the galaxies. Lastly, high resolution images of 0902+343 show that the southernmost component forms a 'ring' of 0.2 sec radius. We discuss the possibility that this ring is the result of gravitational lensing, along the lines proposed by Kochanek & Lawrence (1990).

Carilli, C. L.↗
NASA NTRS1994

The EMSS catalog of X-ray-selected clusters of galaxies. 1: An atlas of CCD images of 41 distant clusters

An atlas of deep, wide-field R-band charge coupled device (CCD) images of a complete sample of distant, X-ray-selected clusters of galaxies is presented. These clusters are the 41 most distant (z is greater than or equal to 0.15) and most X-ray-luminous (L(sub x) is greater than or equal to 2 x 10(exp 44) ergs/s) clusters in the Einstein Observatory Extended Medium Sensitivity Survey (EMSS) catalog that are observable from Mauna Kea (delta is greater than -40 deg). The sample spans a redshift range of 0.15 is less than or equal to z is less than or equal to 0.81 and includes at least two and possibly as many as six rich clusters with z is greater than 0.5. For the most part, the data are of superior quality, with a median seeing of 0.8 sec full width half-maximum (FWHM) and coverage of at least 1 Mpc x 1 Mpc in the cluster frame (H(sub 0) = 50; q(sub 0) = 1/2). In addition, we update the available optical, X-ray, and radio data on the entire EMSS sample of 104 clusters. We outline the cluster selection criteria in detail and emphasize that X-ray-selected cluster samples may prove to be more useful for cosmological studies than optically selected samples. The EMSS cluster sample in particular can be exploited for diverse cosmological investigations, as demonstrated by the detection of evolution in the X-ray luminosity function previously reported, and more recently by the discovery of a large number of gravitationally lensed images in these clusters.

Gioia, I. M.↗
NASA NTRS1994

Toward estimation of H(sub 0) from VLBI observations of the gravitational lens system 0957+561

We have analyzed two epochs of 6 cm very long base interferometry (VLBI) observations of the two brightest images, A and B, of the gravitationally lensed quasar 0957+561 in a search for correlated structural evolution from which to estimate delta tau(sub BA), the difference in propagation time for light traveling between the quasar and Earth along the paths corresponding to these two images. However, we detected only marginal increases in the separation between the core and the innermost jet component in each image over the 2 year interval 1987-1989. Because monitoring image brightness variations has since led to apparently reliable estimates of delta tau(sub BA), we intend to use spatial variations in the relative magnification field across the extent of the images to constrain further the mass distribution of the lens, which is now the principal contributor to the uncertainty in estimates of H(sub 0) from this lens system.

Campbell, R. M.↗
NASA NTRS1994

The faint end of the galaxy luminosity function

The evolution of the B- and K-band luminosity functions of galaxies is inferred in a relatively model-independent way from deep spectroscopic and photometric surveys. We confirm earlier evidence by Eales for an increase in the amplitude of the B-band galaxy luminosity function at modest redshift (z less than or approx. 0.2). We find in addition that the slope of the faint end of the luminosity function must systematically steepen and progress toward more luminous galaxies with increasing lookback time, assuming that the galaxy redshift distribution may be smoothly extrapolated 2 mag fainter than observed, as suggested by recent gravitational lensing studies. This evolution is shown to be color-dependent, and we predict the near-infrared color distribution of faint galaxies. The luminosity function of blue (B - K less than or approx. 4) galaxies in the range 0.2 less than or approx. z less than or approx. 1 can be represented by a Schechter function with characteristic light density phi(sup *) L(sup *) comparable to that of present-day late-type galaxies, but with a steeper faint end slope alpha approx. 1.4.

Treyer, Marie A.↗