Engineering topics

Elvis, M.

Publications and source records attributed to Elvis, M..

At least 37 records · Page 2

NASA NTRS1998

A Long-Term Space Astrophysics Research Program. The Evolution of the Quasar Continuum

The grant "The Evolution of the Quasar Continuum" resulted in over 53 published referred papers and conference proceedings. The more significant of these papers are listed below, and abstracts are attached. The papers address a wide range of issues involving the evolution of quasars, their electromagnetic emissions, and their environment, from nearby low luminosity Seyfert galaxies to quasars at the highest redshifts. Primarily observational in content the work nonetheless included theoretical studies of quasar accretion disks that attempt to explain the observed time variability of quasars, and the overall 'demographics' of the quasar population. The work carried out under this grant has laid a strong foundation for ongoing and future research with AXAF, HST and other new facilities.

Elvis, M.↗

NASA NTRS1995

The origin of the soft X-ray excess in the Seyfert 2 galaxy NGC 2110

We present and discuss an X-ray image and a medium-resolution X-ray spectrum of the Seyfert 2 galaxy NGC 2110 obtained with the high-resolution imager (HRI) on ROSAT (0.1-2.4 keV) and Goddard's Broad Band X-ray Telescope (BBXRT; 0.3-11 keV), respectively. Spatially resolved soft X-ray emission, which peaks 4 arcsecs to the north of the nucleus and near the position of the highest excitation optical emission line gas is seen in the HRI observation. The extent has a flux of approximately 3 x 10(exp -13) ergs/sq cm/s and accounts for 11% +/- 3% of the total 0.1-2.4 keV flux. To model the BBXRT spectrum, a soft excess component is required which has a flux of approximately 3.5 x 10(exp -13) ergs/sq cm/s and accounts for approximately 14% +/- 6% of the total 0.1-2.4 keV flux. In addition, BBXRT confirms the presence of an approximately 175 eV equivalent width Fe K alpha flourescence line in NGC 2110. Because of the good agreement between their fluxes, we propose that the soft excess in NGC 2110 is due to leakage of the X-ray continuum through a patchy absorber. The temperature of the gas responsible for the soft excess is too high to be accounted for by local shock heating. In order to explain the soft excess and extent as either scattered continuum X-rays or flourescence from gas photoionized by the nuclear source, the hard X-rays must be emitted anisotropically. However, the soft excess and extent can be well modeled as thermal emission from a hot, outflowing wind, which may also be responsible for confirming at least some portion of the optical narrow line-emitting clouds.

Weaver, K. A.↗

NASA NTRS1995

The spectral energy distribution of the z=3 quasar: HS 1946+7658

For the bright z = 3.02 radio-quiet quasar, HS 1946+7658, we have obtained radio to X-ray data within the past year: 5 GHz and 1415 MHz data from the Very Large Array (VLA); IR photometry at J, H, K, L prime (3.4 micrometers and N; IR spectroscopy; UBVRI photometry; optical spectrophotometry and high-resolution spectra; and an X-ray spectrum from the ROSAT Position Sensitive Proportional Counter (PSPC). The spectral energy distribution (SED) constructed from these data is compared to the mean SED for a set of low-redshift quasars, and while they appear generally similar, there are several differences. In relation to the low-redshift mean, the SED of HS 1946+7658 shows (1) only an upper limit at 10 micrometers (a rest wavelength of 2.5 micrometers, indicating that HS 1946+7658 does not have a strong near-IR excess such as hot dust would produce; (2) relatively weak Fe II and Balmer continuum emission at approximately 3000 A; and (3) a steeper turndown shortward of Ly alpha, even after correction for the Ly alpha forest absorption lines.

Kuhn, O.↗

NASA NTRS1994

X-ray continuum and iron K emission line from the radio galaxy 3C 390.3

X-ray properties of the radio galaxy 3C 390.3 were investigated using the European X-ray Observatory Satellite (EXOSAT) and Ginga satellites. Long-term, large-amplitude X-ray intensity changes were detected over a period extending from 1984 through 1991, and high-quality X-ray spectra were obtained especially with Ginga. The X-ray continuum spectra were described with power-law model with photon slope in the range 1.5-1.8, and the slope flattened as the 2-20 keV luminosity decreased by 40%. There was a first detection of the iron emission line from this source at the 90% confidence level. An upper limit was derived on the thermal X-ray component. X-ray emission mechanisms and possible origins of the long-term variation are discussed.

Inda, M.↗

NASA NTRS1994

The Cambridge-Cambridge X-ray Serendipity Survey: I X-ray luminous galaxies

We report on the first results obtained from a new optical identification program of 123 faint X-ray sources with S(0.5-2 keV) greater than 2 x 10(exp -14) erg/s/sq cm serendipitously detected in ROSAT PSPC pointed observations. We have spectroscopically identified the optical counterparts to more than 100 sources in this survey. Although the majority of the sample (68 objects) are QSO's, we have also identified 12 narrow emission line galaxies which have extreme X-ray luminosities (10(exp 42) less than L(sub X) less than 10(exp 43.5) erg/s). Subsequent spectroscopy reveals them to be a mixture of star-burst galaxies and Seyfert 2 galaxies in approximately equal numbers. Combined with potentially similar objects identified in the Einstein Extended Medium Sensitivity Survey, these X-ray luminous galaxies exhibit a rate of cosmological evolution, L(sub X) varies as (1 + z)(exp 2.5 +/- 1.0), consistent with that derived for X-ray QSO's. This evolution, coupled with the steep slope determined for the faint end of the X-ray luminosity function (Phi(L(sub X)) varies as L(sub X)(exp -1.9)), implies that such objects could comprise 15-35% of the soft (1-2 keV) X-ray background.

Boyle, B. J.↗

NASA NTRS1993

A long-term space astrophysics research program: The evolution of the quasar continuum

The research program supported by this grant now has great momentum. Numerous papers are in progress, and a strong multi-wavelength observing program is rapidly accumulating data on samples of high redshift quasars across the spectrum. ROSAT spectra of quasars continue to yield surprises. Of four z = 3 quasars with X-ray spectra, three show strong absorption. This contrasts strongly with the situation for luminous AGN at low redshifts where fewer than 1 in 20 show X-ray absorption. A new site for this absorption is probably needed, either around the quasar (e.g. in a cluster cooling flow) or along the line of sight (e.g. in a Damped Lyman-alpha system). The unabsorbed quasar allows limits on the physical conditions in a damped Lyman-alpha cloud to be calculated, and will allow a X-ray Gunn-Peterson test to be applied that will limit the fraction of the closure mass in an intergalactic medium. The X-ray spectral indices of these z = 3 quasars show no change from those of similar objects at low z, suggesting that 'short-lifetime' models apply. Eight other z = 3-4 quasars have been detected and their energy distributions from X-rays to Infrared (using new infrared spectrographs) have been compiled. These are now being compared with the low z continua from the 'Atlas of Quasar Energy Distributions' to search for evolutionary changes. The discovery of a likely warm absorber in 3C351 made recognition of another example simple. Also, modeling of the conditions in the absorber in 3C351 using the OVI absorption line from HST and the high ionization emission lines, suggests that the broad line region is indeed the origin of the warm absorber in this quasar, and by extension, others. Warm absorbers can now be used as a new diagnostic of this region. The X-ray spectrum of a 'Red Quasar', 3C212, has a cut-off spectrum, which could be fitted by an absorbed power-law, or more remarkably, by an unabsorbed black body. Using our quasi-simultaneous optical data and photoionization modeling we are able to rule out this latter possibility. The long-sought key to understanding red quasars has now been found. (Reddening was earlier ruled out because no 2175A absorption feature was seen. This is now seen to have been misleading.)

Elvis, M.↗

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

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 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

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

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

The soft X-ray excess in Einstein quasar spectra

An SNR-limited subsample of 14 quasars from the Wilkes and Elvis (1987) sample is presently investigated for low-energy excess above a high-energy power law in the X-ray spectra obtained by the Einstein Imaging Proportional Counter. A significant excess that is 1-6 times as strong as the high-energy component at 0.2 keV is noted in eight of the 14 objects. In the case of 3C273, multiple observations show the excess to be variable.

Masnou, J. L.↗

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 NTRS1987

Radio observations of a hard X-ray selected sample of active galaxies

Radio observations of a hard X-ray selected sample of active galaxies obtained with the VLA and Parkes radio telescopes are discussed, and the ratio of the radio to X-ray flux density is used to determine the degree of radio-loudness of the galaxies. A continuous distribution of the degree of radio loudness is found amongst the sample galaxies, and no evidence for distinct radio-quiet and radio-loud populations is noted. The X-ray and radio luminosity is shown to be nonlinearly correlated, with the radio-loud objects all having high X-ray luminosity.

Unger, S. W.↗

NASA NTRS1986

Combined ultraviolet studies of astronomical sources

Topics addressed include: Cygnus Loop; P Cygni profiles in dwarf novae; YY Gem; nova shells; HZ Herculis; activity cycles in cluster giants; Alpha Ori; metal deficient giant stars; ultraviolet spectra of symbiotic stars detected by the Very Large Array; time variability in symbiotic stars; blue galaxies; and quasistellar objects with X-ray spectra.

Baliunas, S. L.↗

NASA NTRS1986

The highly obscured nucleus of 3C 219

The detection of a strong, and possibly broad, Paschen-alpha line from the narrow-line radio galaxy 3C 219 is reported. The detected flux is larger than predicted from the H-alpha line and the case B recombination. This implies the presence of a highly reddened line-emitting region in the nucleus.

Fabbiano, G.↗