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NASA NTRS1989

The soft X ray excess in Einstein quasar spectra

The soft X-ray excess component is studied for a signal to noise limited subsample of 14 quasars from the WE87 sample observed with the Einstein Imaging Proportional Counter (IPC). Detailed analysis of the IPC data, combined with Einstein Monitor Proportional Counter (MPC) data where possible, and use of accurate galactic N sub H values allows estimation of the strength of any excess and improvement of constraints on the spectral slope at higher X-ray energies. A significant excess in 9 of the 14 objects is found. It is confined in all but one case to below 0.6 keV and variable in the two cases where there are multiple observations. The relation of the soft excess to other continuum properties of the quasars is investigated.

Wilkes, Belinda J.↗

NASA NTRS1989

Fitting improved accretion disk models to the multiwavelength continua of quasars and active galactic nuclei

Fits to the ultraviolet-optical-infrared spectra of 60 quasars and active galactic nuclei (AGNs) are investigated, using black hole accretion disk models. The disk is assumed to be geometrically thin and optically thick. The observed spectrum from a black hole accretion disk, rotating (Kerr) or nonrotating (Schwarzschild), is transformed by the geometric inclination effect, Doppler shifts, gravitational redshifts, and gravitational focusing. These effects alter the locally emitted fluxes and produce harder spectra for an observer at higher inclination angles. The relativistic corrections are much more significant in the Kerr geometry since the inner edge of the disk is much closer to the rotating black hole. A simple inverse relation between the inferred black hole mass and the assumed inclination angle was found in the Kerr case, while the inferred accretion rate (solar masses/yr) remains independent of the angle. In the Schwarzschild case, the ratio of the inferred accretion rate to the black hole mass remains roughly constant when the viewing angle changes. In both geometries, low-redshift Seyfert galaxies have relatively low accretion rates, only a few percent of their Eddington luminosities, while the most luminous quasars are accreting near their Eddington limits.

Sun, Wei-Hsin↗

NASA NTRS1990

Optical environments of radio quasars

CCD images of 36 radio-loud quasars in the redshift range 0.1-0.75 are discussed. The objects were chosen to represent the full range of radio structure found with VLA A-configuration observations. Measured quantities are compared and correlated from both radio and optical observations. At least 70 percent of the quasars show evidence for current or recent interaction. Consideration is given to host galaxy sizes and luminosities, states of tidal interaction and numbers of companions, radio morphology, and luminosity.

Hutchings, J. B.↗

NASA NTRS1991

Evolution of the luminosity function of quasar accretion disks

Using an accretion-disk model, accretion disk luminosities are calculated for a grid of black hole masses and accretion rates. It is shown that, as the black-hole mass increases with time, the monochromatic luminosity at a given frequency first increases and then decreases rapidly as this frequency is crossed by the Wien cutoff. The upper limit on the monochromatic luminosity, which is characteristic for a given epoch, constrains the evolution of quasar luminosities and determines the evolultion of the quasar luminosity function.

Caditz, David M.↗

NASA NTRS1990

Redshifts and the SMF model for quasars and AGNs

The extended strong magnetic field model (ESMF) explains Arp's deduction of a partly nonvelocity redshift in quasars. It predicts a possible few percent variation in the redshift of blob-emitting quasars over one cycle.

Greyber, Howard D.↗

NASA NTRS1992

The snapshot survey - A search for gravitationally lensed quasars with the Hubble Space Telescope

The first results from the HST Non-Proprietary Snapshot Survey are presented in order to investigate new examples of multiply imaged quasars that are the result of gravitational lensing. None of the approximately 30 quasars observed at high spatial resolution show evidence of multiple imaging by gravitational lensing. These results are compared with lower resolution ground-based surveys and current theoretical predictions. Several engineering problems in the observatory's performance are addressed. It particular, the large telescope pointing errors and drift rates are found to be primarily the result of the lack of correction for stellar aberration when pointing and tracking are performed solely with gyroscopes. The implications for the possibly low intrinsic gyro drift rate on future observations are discussed.

Bahcall, J. N.↗

NASA NTRS1992

The cluster of galaxies associated with the low-redshift quasar H1821 + 643

Low resolution spectra have been obtained of eight R is approximately equal to 19 mag galaxies located within 1 arcmin of the low-redshift, high luminosity quasar H1821 + 643. Six of the eight galaxies belong to a cluster that has the same redshift, within the observational errors, as the quasar. Five of the six cluster members have the spectra of early type galaxies; the spectrum of the sixth cluster member contains H-alpha, semiforbidden O III, and possibly O II emission lines. The other two objects are an emission line foreground galaxy and a background galaxy. The cluster has a Galactocentric redshift of 0.299 +/- 0.002 and a radial velocity dispersion of 1050 +/- 320 km/s. The photometric and spectroscopic data suggest that H1821 + 643 is associated with a rich cluster of galaxies.

Schneider, Donald P.↗

NASA NTRS1992

The evolution of the quasar continuum

We now have in hand a large data base of Roentgen Satellite (ROSAT), optical, and IR complementary data. We are in the process of obtaining a large amount of the International Ultraviolet Explorer (IUE) data for the same quasar sample. For our complementary sample at high redshifts, where the UV was redshifted into the optical, we have just had approved large amounts of observing time to cover the quasar continuum in the near-IR using the new Near-Infrared Camera and Multi-Object Spectrometer (NICMOS) array spectrographs. Ten micron, optical, and VLA radio, data also have approved time. An ISO US key program was approved to extend this work into the far-IR, and the launch of ASTRO-D (early in 1993) promises to extend it to higher energy X-rays.

Elvis, M.↗

NASA NTRS1993

Can quasars photoionize the intergalactic medium at high redshift?

The reionization of the intergalactic medium (IGM) by quasar sources at high redshift are discussed. The integrated UV background from observed QSO's, taking into account the hydrogen opacity associated with intervening Ly-alpha clouds and Lyman limit systems are computed. It is noted that the published data appear to indicate a significant underdensity of absorption systems in the Ly-alpha forest with column densities N(sub HI) greater than 10(exp 15) cm(sup -2). This deficit results in a reduction of the opacity of the universe by a factor of 1.5-3 at z = 3-5 relative to previous estimates. The QSO contribution to the metagalactic flux at the Lyman edge may be as large as J(sub 912)(z) is approximately 6((1 + z)/4.5)(sup 0.5) x 10(exp -22) erg cm(sup -2) s(sup -1) Hz(sup -1) sr(sup -1) for q(sup o) = O, and slightly lower for q(sub o) = 1/2. For a density of the diffuse component of the IGM of omega(sub D)(h(sub 50)(sup 2)) less than 0.025, QSO's could photoionize a smooth IGM sufficiently to satisfy the constraints imposed by the Gunn-Peterson effect. The epoch of reionization could be as recent as z is approximately greater than 5. As a result, neutral patches of IGM would be detectable in the spectra of high redshift quasars. The patches would appear as absorption line systems with typical column densities of 10(exp 19) - 10(exp 20) cm(sup -2), and velocity widths of 100 - 1000 km s(sup -1).

Meiksin, Avery↗

NASA NTRS1993

Quasars in rich galaxy clusters

The evolution of AGN activity in rich clusters of galaxies is found to be approximately 5 times more rapid than that in poor clusters. This rapid evolution may be driven by evolution in the dynamics of galaxy cluster cores. Results from our spectroscopic studies of galaxies associated with quasars are consistent with this scenario, in that bright AGN are preferentially found in regions of lower velocity dispersion. Alternately, the evolution may be driven by formation of a dense intra-cluster medium (ICM). Galaxies close to quasars in rich cluster cores are much bluer (presumably gas rich) than galaxies in the cores of other rich clusters, in support of this model.

Ellingson, Erica↗

NASA NTRS1993

The Lyman alpha forest of the high-z quasar 0000-263

Medium-resolution (delta(v) = 45 km/s) optical spectra of the bright, high-redshift (z = 4.1) quasar 0000-263 taken at the ESO 3.5-m NTT telescope were analyzed to determine the distribution of column densities, velocities and line widths of the Lyman-alpha forest absorption components. The values of NH, b, and z were determined by fitting Voigt profiles to the lines, and convolving with a Gaussian instrumental response function. Over 350 components with log N(sub H) greater than 13.2 were identified. An analysis of the dependence of the number of components with z reveals that the number evolution of components obeys the power law dN/dz varies as (1+z)(sup gamma), where gamma = 0.5 +/- 0.4 for the sample of 182 lines with log N(sub H) greater than 14.0. The distribution of component strengths is found to obey f(N(sub H)) varies as N(sub h)(sup -beta), where beta = -1.55 for components with log(N(sub H)) is greater than 14.7, and beta = -0.68 for the components with log(N(sub H)) greater than 13.5. A distinct break in the f(N(sub H)) histogram is also observed, at log(N(sub H)) is approximately 14.7. The results are briefly considered in the context of theoretical models of quasar Lyman alpha clouds and their evolution.

Penprase, Bryan↗

NASA NTRS1993

Detection of gamma-ray emission from the quasar PKS 0208-512

High-energy gamma-ray emission has been detected from the quasar PKS 0208-512 in the energy range above 30 MeV by the Energetic Gamma Ray Experiment Telescope (EGRET) on the Compton Gamma Ray Observatory. This region of sky was observed in five different viewing periods, and evidence of time variability in the gamma-ray emission by more than a factor of 3 was found. At the maximum intensity between 1991 September 19 and October 3, the flux density above 100 MeV was (9.1 +/- 0.4) x 10 exp -7 gamma/sq cm per sec. The photon spectrum during this period may be expressed as a power law with an exponent of - 1.69 +/- 0.05 between 30 MeV and 4 GeV. This is the hardest quasar spectrum observed by EGRET up to the present time.

Bertsch, D. L.↗

NASA NTRS1991

Quasars and galaxy formation

Observations of high redshift quasars provide direct information on the physical state of the universe at early epochs. The available data suggest well advanced structure formation even at the highest known quasar redshifts and are most easily understood in scenarios in which galaxies primarily formed at redshifts well above 5. Ten separate lines of argument supporting this general conclusion are summarized.

Turner, Edwin L.↗

NASA NTRS1992

A search for microvariability in five OVV quasars

We present results of monitoring a sample of five OVV qausars in an effort to determine if these types of AGNs exhibit microvariability similar to that detected for BL Lacertae objects. Three of the five quasars investigated clearly exhibited microvariability. The timescales for the variations for both BL Lacertae objects and OVV quasars are similar which suggests that the radiation is generated in similar spatial volumes. These variations are consistent with the results of models which explain the observed microvariability as the result of excess emission produced by flares or hot spots randomly appearing and disappearing on the accretion disks around supermassive black holes.

Miller, H. R.↗

NASA NTRS1993

Detection of high-energy gamma rays from quasar PKS 0528 + 134 by EGRET on the Compton Gamma Ray Observatory

The first several pointing directions of the Compton Gamma Ray Observatory, launched on 1991 April 5, were toward the Galactic anticenter. In addition to the known gamma-ray sources, Crab and Geminga, high-energy gamma-ray emission was observed from the quasar PKS 0528 + 134 by the Energetic Gamma Ray Experiment Telescope (EGRET). A redshift measurement, reported here, of 2.07 confirms the identification of this object as a quasar. The differential photon spectrum is well represented by a power law with an exponent of 2.56 +/- 0.09 and a photon intensity above 100 MeV of (8.4 +/- 1.0) x 10 exp -7 photons sq cm/s. There is evidence for time variability on a time scale of a few days.

Hunter, S. D.↗

NASA NTRS1993

The EGRET detection of quasar 1633 + 382

In the course of a full-sky survey, the EGRET instrument aboard the Compton Observatory detected an intense, high Galactic latitude source, which is identified as the OVV (optically violent variable) quasar 1633 + 382. The spectrum is represented by a power law with an energy spectral index of 0.9 between 30 MeV and 30 GeV. The gamma-ray power per decade is about 100 times larger than typical values at any longer wavelength. Significant variation of the gamma-ray flux density on a time scale as short as two days was observed, limiting the size of the region in which the gamma-rays are produced. Assuming the X-rays detected by the Einstein Observatory from this quasar are produced in the same region as the gamma-rays, and a similar X-ray flux density was extant at the time of the EGRET observation, the emission must be beamed to avoid a pair-production optical depth of 10 exp 4 which would exist otherwise. If beaming arises from bulk relativistic motion, the minimum Doppler factor required is 7.6.

Mattox, J. R.↗

NASA NTRS1993

Is 1308+326 A BL Lacertae object or a quasar?

The active galaxy 1308+326 (z = 0.996) has been classified as a BL Lacertae object because of its nearly featureless optical continuum and high and variable polarization. New VLBI polarization images of 1308+326, however, show both VLBI polarization structure and tentative superluminal motions that are characteristic of quasars rather than BL Lacertae objects. These data strongly suggest that this source is simply a quasar with emission lines of small equivalent width. We discuss the possibility that the optical continuum is enhanced by microlensing in a foreground galaxy, as suggested by Stickel et al.

Gabuzda, D. C.↗

NASA NTRS1993

Low-ionization broad absorption lines in quasars

The absorption line properties of six Mg II broad absorption line quasars (BALQs) are examined using a BALQ sample of Weymann et al. (1991). It is found that Al III absorption is always stronger than Al II absorption; C IV absorption tends to be very deep and broad; and low ionization absorption tends to be narrower and lie at the low velocity end of the C IV trough. The results indicate that the outflows in Mg II BALQs are not accelerating monotonically. It is shown that a hot cloud-confining outflow can be decelerated by adding thick absorbing clouds provided the ionization parameters of these clouds are high. It is argued that the MG II BALQ phenomenon might be a manifestation of a quasar's efforts to expel a thick shroud of gas and dust.

Voit, G. M.↗