Engineering Papers⌕ Search

SEARCH · Engineering Papers

Results for “LaC”

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.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 361 records · Page 20
All sourcesNASA NTRSDOE OSTI
NASA NTRS1987

EX56a study of extended X-ray emission around isolated galaxies EX56b identification and spectra of bright X-ray sources at high galactic latitude

The EXOSAT observations confirmed the identification and extended nature of PKS 2345-35. It gave a good 2 to 10 keV X-ray spectrum and a detailed spatial profile indicating asymmetry of the structure. In the high galactic latitidue investigation, the BL Lac object identified with the HEAO-1 source 1430+423 was detected, and the first X-ray spectrum was obtained. Several simulataneous observations of H0323+022 were obtained over a broad range of electromagnetic spectrum. Studies of luminous active galactic nuclei have given significant information on the spectrum of the quasar PKS 0558-504. In a study of Southern sky cataclysmic variables, the EXOSAT was used to determine the X-ray spectrum and search for periodicities in two objects. Studies of complete identifications have revealed that X-ray sources in two high galactic latitude fields are stars, and therefore are to be excluded from the Piccinotti extragalactic sample. Only one Piccinotti source remains to be identified.

Schwartz, Daniel A.↗
NASA NTRS1986

Seven years of IUE observations of the BL Lacertae object PKS 2155-304

Over 50 IUE spectra of the brightest known BL Lac object at ultraviolet wavelengths, PKS 2155-304, were reanalyzed using Gaussian extraction techniques that maximize the signal-to-noise. The ultraviolet continuum of PKS 2155-304 is well fitted by a power law model Fv varies as V sup-alpha, with an average spectral index for the combined SWP-LWR-P fits of less than alpha greater than 0.89 + or - 0.11 (standard deviation). While the ultraviolet intensity of PKS 2155-304 varies by a factor of 2, the spectral index changes very little and is uncorrelated with intensity. Timescales for ultraviolet variability are as short as 10 days. In one case, a well-matched pair of IUE images was obtained nearly simultaneously with a very soft X-ray measurement; the combined spectra clearly show a steepening between the ultraviolet and soft X-ray bands. The relevance of these findings to jet models and accretion models is discussed.

Urry, C. M.↗
NASA NTRS1986

Cooling flows in ellipticals and the nature of radio galaxies

Analytical arguments are presented to demonstrate that the morphology of the host galaxy will influence the type of radio signature emitted by the nucleus, which is in many cases presumed to a black hole. The types of matter inflows and consequent shapes of the accretion disks around the nuclei of spiral and elliptical galaxies are discussed. If the inflowing matter is predominantly cold and clumpy, then stars will tend to form and only the available hot gas will descend into the center. Collimated radio jets will appear only in the presence of hot interstellar gas in elliptical galaxies and not in spiral galaxies. The implications of the predictions for the interpretation of observational data on, e.g., BL Lac objects and N galaxies are discussed.

Begelman, M. C.↗
NASA NTRS1986

X-ray spectra of BL Lacertae objects

An analysis of previous X-ray spectral measurements of BL Lac objects indicates that the ultraviolet-through-X-ray spectra have a two-component, concave-up power law structure and that the soft X-ray spectrum is an extrapolation of the ultraviolet power law. The X-ray intensities of these objects were found to be highly variable. Though no significant amount of cold, low ionization gas is found to be present in these objects, evidence is presented of intrinsic excess absorption that may be due to hot highly ionized gas.

Urry, C. Megan↗
NASA NTRS1986

Polarization distributions in compact radio sources

Intensity and polarization maps of active galactic nuclei, obtained with marcsec resolution at 5 GHz using the Mark III VLBI array during 1981-1982, are presented. The techniques of high-resolution polarization synthesis are described, and maps for the superluminal quasar 3C345, the superluminal galaxy 3C120, and the BL Lac objects 0735 + 178 and OJ287 are characterized, with an emphasis on the relationship between arcsec-scale and marcsec-scale jets.

Roberts, David H.↗
NASA NTRS1987

Simultaneous multifrequency observations of the BL Lacertae object Markarian 421

Simultaneous multifrequency observations of the BL Lac object Mrk 421 covering radio through X-ray wavelengths were performed on two occasions separated by 5 weeks in January and March 1984, and each observation was coordinated for about 1 week. Composite multifrequency spectra of the central nonthermal component were obtained at the two epochs after subtracting the optical and infrared light of the underlying galaxy. The spectra show the gradual steepening toward high frequency; the power law indices are about 0.1, about 0.6, and about 1.0 for radio, infrared-optical, and UV bands, respectively. The UV and optical-infrared fluxes decreased by about 20 percent in 5 weeks, while the radio flux remained stable. The X-ray flux decreased by a factor of about 2, and the change was more pronounced at hard X-rays, which suggest that X-ray emission possibly consists of two components. The degree of polarization at the optical band varied on the time scale of a few days, while the position angle remained unchanged. Physical parameters of Mrk 421 are discussed in terms of the synchrotron self-Compton model.

Makino, F.↗
NASA NTRS1987

Optical and near-infrared observations of BL Lacertae objects and active quasars

UBVRI and JHK photometric and polarimetric observations, obtained using the 1.5-m UCSD/UM telescope at Mount Lemmon, AZ during 1982-1984, are reported for a sample of 10 BL Lac objects, eight optically violent variable quasars, and the low-polarization quasar 3C 273. The data are presented in extensive tables and graphs and analyzed in detail. Most of the objects are found to have undergone large polarization variations on time scales ranging from 1 d to several months; five objects had fractional linear polarization P greater than 30 percent at least once during the period of observation, but four never had P greater than 10 percent. Of four objects which exhibited a preferred polarization position angle (theta), three had theta within 20 deg of the jet position angle determined by VLBI.

Smith, Paul S.↗
NASA NTRS1987

The IRAS view of the extragalactic sky

The IR-observable characteristics of the extragalactic sky are reviewed, summarizing the results of recent studies based on the IRAS survey, which covers over 96 percent of the sky to about 500 mJy at 12, 25, and 60 microns and to about 1.5 Jy at 100 microns. The numerical and morphological data are described; possible mechanisms for the IR emission are discussed; and the object classes are considered separately. Consideration is given to spiral and disk galaxies, barred and ring galaxies, irregular and dwarf galaxies, blue compact galaxies, elliptical and S0 galaxies, AGN observations (BL Lacs and OVV quasars, Seyfert galaxies, and quasars), highly luminous IR galaxies, and the cosmological implications of the IRAS findings. Diagrams, graphs, and tables are provided.

Soifer, B. T.↗
NASA NTRS1987

Quasar energy distributions. I - Soft X-ray spectra of quasars

As the initial stage of a study of quasar energy distributions (QEDs), Einstein IPC spectra of 24 quasars are presented. These are combined with previously reported IPC spectra to form a sample of 33 quasars with well-determined soft X-ray slopes. A correlation analysis shows that radio loudness, rather than redshift or luminosity, is fundamentally related to the X-ray slope. This correlation is not followed by higher energy spectra of active galaxies. Two components are required to explain both sets of results. The best-fit column densities are systematically smaller than the Galactic values. The same effect is not present in a sample of BL Lac objects, implying that the effect is intrinsic to the quasars and is caused by a low-energy turnup in the quasar spectra.

Wilkes, Belinda J.↗
NASA NTRS1988

Studies of BL Lacertae objects with the Einstein Observatory - The soft X-ray spectra of OJ 287 and PKS 0735+178

Accurate, soft X-ray spectra of two BL Lac objects, OJ 287 and PKS 0735+178, are presented. The X-ray spectra are well described by a power-law model with a low-energy cutoff consistent with photoelectric absorption within the Galaxy. The best-fit values of the energy spectral index in the 0.2-4.0 keV band are 0.91 and 0.76 respectively. The X-ray flux from OJ 287 is variable by a ratio of three from low to high state; PKS 0735+178 shows no indication of X-ray variability. The X-ray emission in OJ 287 is interpreted to be due to the synchrotron process from a volume common with either a beamed radio component or a stationary optical component. In PKS 0735+178, where the X-ray emission is most likely due to the Compton process operating in one of the VLBI radio components. The synchrotron self-Compton process with modest kinematic Doppler factors predicts the measured X-ray flux from PKS 0735+178 and lower than the measured flux in OJ 287.

Madejski, Greg M.↗
NASA NTRS1989

The radiative deceleration of ultrarelativistic jets in active galactic nuclei

A detailed study of the dynamical interaction between a highly relativistic jet and the thermal radiation field from an AGN accretion disk is reported, and the Comptonized spectrum arising from this interaction is self-consistently determined. A simple model that captures the essential radiative and geometrical features of realistic disk configurations is presented, and the disk radiation field is calculated. The results confirm Phinney's (1987) suggestion that the thermal radiation field produced by accretion in an AGN could be very effective in decelerating ultrarelativistic jets that are accreted by electromagnetic or hydromagnetic forces closer to the central black hole. Terminal Lorentz factors are consistent with the values inferred in superluminal radio sources are readily produced in this model for plausible disk and jet parameters without additional acceleration in the interaction zone. A new interpretation of the hard X-ray component detected in BL Lac spectra is proposed.

Melia, Fulvio↗
NASA NTRS1989

AVHRR for monitoring global tropical deforestation

Advanced Very High Resolution Radiometer (AVHRR) data have been used to assess the dynamics of forest trnsformations in three parts of the tropical belt. A large portion of the Amazon Basin has been systematically covered by Local Area Coverage (LAC) data in the 1985-1987 period. The analysis of the vegetation index and thermal data led to the identification and measurement of large areas of active deforestation. The Kalimantan/Borneo forest fires were monitored and their impact was evaluated using the Global Area Coverage (GAC) 4 km resolution data. Finally, High Resolution Picture Transmission (HRPT) data have provided preliminary information on current activities taking place at the boundary between the savanna and the forest in the Southern part of West Africa. The AVHRR approach is found to be a highly valuable means for carrying out deforestation assessments in regional and global perspectives.

Malingreau, J. P.↗
NASA NTRS1989

Rotational modulation and flares on RS Canum Venaticorum and BY Draconis stars. XI - Ultraviolet spectral images of AR Lacertae in September 1985

High-resolution IUE SWP and LWP spectra of the bright eclipsing RS CVn system AR Lac, obtained during 40 h of continuous observations on September 18-19, 1985, are compiled and used to derive chromosphere images. The data and results are presented graphically and discussed in detail. The K 0 IV star is found to have a globally brighter trailing hemisphere and at least three distinct plage regions; the G 2 IV star has a flaring region with significant redshift and line-emission broadening and exhibits factor-of-3 variability in total emission, attributed to the presence of a large chromospherically inactive region.

Neff, J. E.↗
NASA NTRS1989

Extent of warm haloes around medium-redshift galaxies

The properties of low-to-medium ionization gaseous haloes around galaxies are briefly reviewed. New observations concerning such haloes are presented. For the galaxy-QSO pair in the field of the radio source 3C303, the higher-redshift QSO has been found to show Mg II absorption at the lower redshift of the faint nearby galaxy. Secondly, new data are presented on one of the galaxies in the environment of the well-known BL Lac object AO 0235 + 164.

Burbidge, E. M.↗
NASA NTRS1989

X ray emission from relativistic jets in AGNs and statistical implications

Calculations of the Compton scattering interaction between an ultrarelativistic jet and a thermal radiation field, in an Active Galactic Nuclei (AGN), are presented. This process can be effective in decelerating ultrarelativistic jets that are accelerated by electromagnetic or hydromagnetic forces closer in to the central black hole. A narrow distribution of terminal Lorentz factors gamma sub infinity, consistent with the values inferred in superluminal radio sources, arises naturally in this model. The hard X-ray component detected in the spectra of 3C273 and several BL Lac objects may be due to the inverse Compton radiation produced in the course of the initial deceleration of their relativistic jets. The requirement that the luminosity of the hard X-ray component must exceed the total power in the associated jet is considered.

Melia, Fulvio↗
NASA NTRS1988

Spatially resolved flares in RS CVn systems

Mg II k emission-line profiles arising solely from the flaring region during flares on AR Lac and V 711 Tau have been isolated. From several high-resolution spectra obtained during the lifetime of the flare, the size and position of the flaring regions have been determined, and the decay of the emission line width, radial velocity, and integrated line flux has been studied.

Neff, James E.↗
NASA NTRS1990

A catalog of X-ray measurements of selected samples of active galaxies and nuclei

X-ray fluxes or upper limits in the 2-10 keV energy band from the HEAO 1 A-2 experiment are given for well-defined samples of QSOs and of Seyfert galaxies of types 1, 1.5 and 2, and for a list of BL Lac objects (334 sources in total). All on-orbit X-ray measurements of these objects published between 1976 and 1986 from other experiments have also been cataloged.

Della Ceca, R.↗