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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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NASA NTRS1992

An X-ray image of the Seyfert galaxy NGC 1068

An image of NGC 1068 with 4-5 arcsec obtained with the High Resolution Imager on the Rosat X-ray Observatory in the energy band 0.1-2.4 keV is presented and discussed. The map reveals an unresolved nuclear source, extended (about 1.5 kpc) emission around the nucleus, and extended (about 13 kpc) emission from the starburst disk. The extended circumnuclear emission aligns toward the NE, the same direction as found for the resolved emission of the active nucleus in several other wavebands. Thermal emission from a hot wind is argued to be the source of the steep-spectrum, nuclear, and circumnuclear emission. The disk of NGC 1068 has ratios of soft X-ray to B band and soft X-ray to 60-micron luminosities which are similar to those found for other starburst systems. The X-ray spectrum of the starburst disk is harder than that of the nuclear emission. By adopting a plausible spectrum and extrapolating the present measured flux, it is concluded that the starburst disk contributes most of the hard component seen in the 2-10 keV band.

Wilson, A. S.↗
NASA NTRS1992

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. III - Further observations of NGC 5548 at optical wavelengths

The results of the second year of an intensive ground-based spectroscopic and photometric study of variability in the bright Seyfert 1 galaxy NGC 5548 are reported in order to study the relationship between continuum and emission-line variability. Relative to the first year of the monitoring program, the nucleus of NGC 5548 was considerably fainter and the continuum variations slower during the second year, but the continuum H-beta cross-correlation results for the two years are nearly identical. The variations in the broad H-beta emission-line lag behind those in the continuum by somewhat less than 20 days, as concluded from the first year's data.

Peterson, B. M.↗
NASA NTRS1992

PKS 0483-436 - A high-redshift quasar with strong X-ray absorption

The first X-ray spectrum of a high-redshift (z = 2.85) quasar is reported. The Rosat PSPC spectrum of PKS 0438-436, covering 0.3-9 keV in the quasar's rest frame, reveals unexpected absorption of about 1 x 10 exp 22/sq cm, assuming it occurs at the source. Only one other high-luminosity quasar (of greater than about 50 observed by Einstein) shows significant absorption in its X-ray spectrum. Of the common line-of-sight absorbers, only highly ionized Ly-alpha forest clouds may be able to explain this amount of absorption. Candidates for an intrinsic absorber are discussed. Absorption at about 1 keV (rest frame) is due primarily to heavy elements. (O, Ne, Mg, Si, S) raising the possibility of measuring early universe abundances via X-ray absorption in this and like quasars. PKS 0438-436 may be a high-redshift member of a population of quasars which can contribute to the X-ray background above 2 keV, without being detectable by previous imaging missions.

Wilkes, Belinda J.↗
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 NTRS1992

Quasar x-ray spectra revisited

A sample of 45 quasars observed by the Imaging Proportional Counter (IPC) on the Einstein satellite is used to re-examine the relationship between the soft (0.2-3.5 keV) X-ray energy index and radio-loudness. We found the following: (1) the tendency for radio-loud quasars to have systematically flatter X-ray slopes than radio-quiet quasars (RQQ's) is confirmed with the soft X-ray excess having negligible effect; (2) there is a tendency for the flatness of the X-ray slope to correlate with radio core-dominance for radio-loud quasars, suggesting that a component of the X-ray emission is relativistically beamed; (3) for the RQQ's the soft X-ray slopes, with a mean of approximately 1.0, are consistent with the slopes found at higher energies (2-10 keV) although steeper than those observed for Seyfert 1 galaxies (also 2-10 keV) where the reflection model gives a good fit to the data; (4) the correlation of FeII emission line strength with X-ray energy index is confirmed for radio-quiet quasars using a subset of 18 quasars. The radio-loud quasars show no evidence for a correlation. This relation suggests a connection between the ionizing continuum and the line emission from the broad emission line region (BELR) of radio-quiet quasars, but in the opposite sense to that predicted by current photoionization models; and (5) the correlations of X-ray slope with radio core dominance and FeII equivalent width within the radio-loud and radio-quiet sub-classes respectively imply that the observed wide range of X-ray spectral slopes is real rather than due to the large measuring uncertainties for individual objects.

Shastri, P.↗
NASA NTRS1992

Extragalactic counterparts to Einstein slew survey sources

The Einstein slew survey consists of 819 bright X-ray sources, of which 636 (or 78 percent) are identified with counterparts in standard catalogs. The importance of bright X-ray surveys is stressed, and the slew survey is compared to the Rosat all sky survey. Statistical techniques for minimizing confusion in arcminute error circles in digitized data are discussed. The 238 slew survey active galactic nuclei, clusters, and BL Lacertae objects identified to date and their implications for logN-logS and source evolution studies are described.

Schachter, Jonathan F.↗
NASA NTRS1992

The Einstein All-Sky Slew Survey

The First Einstein IPC Slew Survey produced a list of 819 x-ray sources, with f(sub x) approximately 10(exp -12) - 10(exp -10) erg/sq cm s and positional accuracy of approximately 1.2 feet (90 percent radius). The aim of this program was to identify these x-ray sources.

Elvis, Martin S.↗
NASA NTRS1992

Identifications of Einstein Slew Survey sources

The status of identifications of the Einstien Slew Survey, a bright soft x-ray catalog with 550 new x-ray sources, is discussed. Possible counterparts were found for greater than 95 percent of the Slew Survey based on positional coincidences and color-color diagnostics. The survey will be fully identified via upcoming radio and optical observations.

Schachter, Jonathan F.↗
NASA NTRS1992

Einstein Slew Survey: Data analysis innovations

Several new methods were needed in order to make the Einstein Slew X-ray Sky Survey. The innovations which enabled the Slew Survey to be done are summarized. These methods included experimental approach to large projects, parallel processing on a LAN, percolation source detection, minimum action identifications, and rapid dissemination of the whole data base.

Elvis, Martin S.↗
NASA NTRS1992

Thirteen new BL Lacertae objects discovered by an efficient x ray/radio/optical technique

The discovery of 13 serendipitous BL Lac objects in the Einstein IPC Slew Survey by means of x ray/radio vs. x ray/optical color-color diagrams and confirmation by optical spectroscopy are reported. These 13 BL Lacs were discovered using a technique which exploits the characteristic broad band spectra of BL Lacs. New VLA detections provide accurate fluxes (f(6 cm) is approximately 0.5 mJy) and 2 in. positions, facilitating the determination of an optical counterpart. All 13 new BL Lacs show essentially featureless optical spectra. Nine of these lie within the range of colors of known x ray selected BL Lacs. Of the remaining four, one is apparently x ray louder (by a factor of 1.5) or optically quieter (by 0.8 mags); and three are optically louder (by 1-1.3 mags) than x ray selected BL Lacs. Approximately 50 new BL Lacs in total are expected from VLA work and upcoming Australia Telescope observations, yielding a complete Slew Survey sample of approximately 90 BL Lacs.

Schachter, Jonathan F.↗
NASA NTRS1993

Finding the rarest objects in the universe: A new, efficient method for discovering BL Lacertae objects

We present a new, efficient method for discovering new BL Lac Objects based upon the results of the Einstein Extended Medium Sensitivity Survey (EMSS). We have found that all x-ray selected BL Lacs are radio emitters, and further, that in a 'color-color' diagram (radio/optical and optical/x-ray) the BL Lac Objects occupy an area distinct from both radio loud quasars and the radio quiet QSOs and Seyferts which dominate x-ray selected samples. After obtaining radio counterparts via VLA 'snapshot' observations of a large sample of unidentified x-ray sources, the list of candidates is reduced. These candidates then can be confirmed with optical spectroscopy and/or polarimetry. Since greater than 70 percent of these sources are expected to be BL Lacs, the optical observations are very efficient. We have tested this method using unidentified sources found in the Einstein Slew Survey. The 162 Slew Survey x-ray source positions were observed with the VLA in a mixed B/C configuration at 6 cm resulting in 60 detections within 1.5 position error circle radii. These x-ray/optical/radio sources were then plotted, and 40 BL Lac candidates were identified. To date, 10 candidates have been spectroscopically observed resulting in 10 new BL Lac objects! Radio flux, optical magnitude, and polarization statistics (obtained in white light with the Steward Observatory 2.3 m CCD polarimeter) for each are given.

Stocke, John↗
NASA NTRS1992

Using color-color diagrams to test models for the 'blue bump'

Color-color diagrams are used to study the set of accretion disk plus power-law models and to determine whether they can adequately fit the optical/UV continua of a sample of 34 quasars, and the soft X-ray excesses measured for seven of the sample objects by Masnou et al. (1991). The loci of pure disk models for cos theta is greater than 0 are virtually superimposed on each other regardless of cos theta, demonstrating that a given observed spectrum cannot be described by a unique combination of S and cos theta. The distributions of model and data colors are found to overlap, showing that the disk plus power-law models do fit the distribution of data colors. For six of the seven objects, the soft X-ray excesses may be fit with an accretion disk plus power-law model without requiring the assumption of super-Eddington accretion; this shows the utility of employing Kerr disks.

Kuhn, O.↗
NASA NTRS1992

Infrared weak quasars

Examples of quasars with anomalously weak IR emission are presented, and the effects of starlight subtraction on estimates of the UV and IR component strengths are discussed. Inferred model parameters are very sensitive to the position of the peak of the UV energy distribution. In many low redshift objects the peak is not seen; even in those objects where the turnover is clear, the turnover may not be intrinsic but instead due to reddening within the quasar host galaxy. The small number of unusual quasars with weak IR emission will be of utility as a probe of the quasar phenomenon in the absence of dominant dust reprocessing.

Mcdowell, J. C.↗
NASA NTRS1993

Steps toward determination of the size and structure of the broad-line region in active galactic nuclei. IV - Intensity variations of the optical emission lines of NGC 5548

Measurements of optical emission-line flux variations based on spectra of the Seyfert galaxy NGC 5548 obtained between December 1988 and October 1989 are reported. All of the measured optical emission lines, H-alpha, H-beta, H-gamma, He I 5876, and He II 4686, exhibit the same qualitative behavior as the UV and optical continua, but with short time delays, or lags, which are different for the various lines. Cross-correlation analysis is applied to measure the lags between the various lines and the continuum. Similar lags are found with respect to the UV continuum for H-alpha and H-beta, 17 and 19 d, respectively. The lag for H-gamma is shorter (13 d), only somewhat larger than the lag measured for Ly-alpha (about 10 d). The helium lines respond to continuum variations more rapidly than the hydrogen lines, with lags of about 7 d for He II 4686 and 11 d for He I 5876.

Dietrich, M.↗
NASA NTRS1993

A sample of lobe-dominated quasars with Mg II absorption lines

We present a sample of quasars which are lobe-dominated in their radio emission and contain Mg II absorption line systems. This sample will be useful in a search for redshifted H I 21 cm absorbers which are along lines of sight different from the optical. These systems will allow the measurement of the linear size and related physical parameters of the absorption line clouds. We have defined a set of criteria for selecting these objects and obtained the necessary data to assemble a sample based on these criteria. Out of 4234 QSOs we find 31 lobe-dominated quasars which are appropriate for optical and radio absorption line study in the Northern Hemisphere. We have obtained or located optical spectra for 29 of these 31 quasars. In the optical spectra we find 17 Mg II absorption systems with redshifts between 0.3 and 1.4 and Mg II 2796 A rest equivalent widths between 0.3 and 2.0 A. Based on statistics of 21 cm absorption in Mg II selected QSOs we expect this sample of 17 systems to yield one or two new H I absorbers. In this paper we give an analysis of our criteria, discuss the sample, and present new VLA observations of 16 quasars.

Aldcroft, Thomas L.↗
NASA NTRS1993

Quasar X-ray spectra revisited

A sample of 45 quasars observed by the IPC on the Einstein satellite is used to reexamine the relationship of the soft X-ray energy index with radio properties and the optical Fe II emission. The tendency for radio-loud quasars to have systematically flatter X-ray energy indices than radio-quiet quasars is confirmed with the soft X-ray excess having negligible effect. There is a tendency for the flatness of the X-ray slope to correlate with radio core dominance for radio-loud quasars, suggesting that a component of the X-ray emission is relativistically beamed. For the radio-quiet quasars, the soft X-ray energy indices with a mean of about 1.0 are consistent with the indices found at higher energies, although steeper than those observed for Seyfert 1 galaxies where the reflection model gives a good fit to the data. The correlation of Fe II emission line strength with X-ray energy index is confirmed for radio-quiet quasars using a subset of 18 objects. The radio-loud quasars show no evidence for a correlation. This relation suggests a connection between the ionizing continuum and line emission from the broad emission-line region (BELR) of radio-quiet quasars, but in the opposite sense to that predicted by current photoionization models. The correlations of X-ray slope with radio core dominance and Fe II equivalent width within the radio-loud and radio-quiet subclasses, respectively, imply that the observed wide range of X-ray energy indices is real rather than due to the large measuring uncertainties for individual objects.

Shastri, P.↗
NASA NTRS1993

The new BL Lacertae objects discovered by an efficient X-ray/radio/optical technique

We report the recent discovery of ten BL Lac objects in the Einstein IPC Slew Survey by means of X-ray/radio versus X-ray/optical color-color diagrams and confirming optical spectroscopy. These ten BL Lac objects were discovered using a technique proposed by Stocke et al. (1989) which exploits the characteristic broad-band spectra of BL Lac objects. New VLA detections provide accurate fluxes (limiting f(6 cm) about 0.5 mJy) and about 1 arcsec positions, facilitating the determination of an optical counterpart. All ten new BL Lac objects show essentially featureless optical spectra and lie within the range of radio/X-ray/optical colors of known X-ray-selected BL Lac objects, when about 1 mag optical variability is included. We expect about 50 new BL Lac objects in total, from complete optical follow-up of our now completed VLA work and recent Australia Telescope observations, yielding a complete Slew Survey sample of about 90 BL Lac objects.

Schachter, Jonathan F.↗
NASA NTRS1993

The ROSAT spectrum of 3C 351 - A warm absorber in an X-ray-'quiet' quasar?

3C 351 is one of the most X-ray-quiet radio quasars (alpha(ox) about 1.6). We have observed 3C 351 with the ROSAT position sensitive proportional counter (PSPC) and find a complex X-ray spectrum which is not well reproduced by a power law plus low-energy cut-off model. Soft excess, partial covering, and 'warm absorber' models can all produce acceptable fits, although only the warm absorber model gives typical values for the high-energy continuum slope. The alpha(ox) measured by using quasi-simultaneous ROSAT, MMT, and HST observations is in the range 1.5-1.6, significantly above the average of 1.37 for a complete sample of 33 3CR quasars. If the soft excess or partial covering models are correct, 3C 351 appears X-ray-quiet in the PSPC band because it has an extremely steep or flat intrinsic high-energy spectral slope. However, if the warm absorber model is correct, the quasar is intrinsically X-ray-quiet; the normalization of the intrinsic (unabsorbed) X-ray emission is unusually low relative to the optical luminosity. We investigate the properties of our warm absorber model in some detail. The apparently complicated behavior of the fit parameters may be understood by considering the effects of changing absorbing column and ionization parameter on intrinsic power-law-spectra of different slopes.

Fiore, Fabrizio↗