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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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At least 163 records · Page 9
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NASA NTRS1979

An intense soft high-latitude X-ray source H2156-304 - A new BL Lacertae object

The discovery of an intense high-latitude soft X-ray source (designated H2156-304) detected with the low-energy detectors of the HEAO A-2 experiment is reported. The error box of the source includes a 14th mag starlike object that may be coincident with the radio source PKS 2155-304 and has been suggested as a BL Lac candidate. Intensity variations of H2156-304 on time scales of about 1 sec to 1 day are discussed, along with a probable flare from the source. The derived energy spectrum of the source is shown to be well fitted by either a two-temperature thermal model with temperatures of 16 million and 1.6 million K or a single power-law model with a spectral index of 2.4 + or - 0.3. Hydrogen column densities of approximately 2.5 x 10 to the 20th and 2.0 x 10 to the 20th H atoms/sq cm are obtained for the two-temperature and power-law models, respectively. The emission mechanism of H2156-304 is considered, and the characteristics of this source are compared with those of three known soft X-ray-emitting BL Lac objects.

Agrawal, P. C.↗
NASA NTRS1979

IUE observations of two high redshift /z more than 3/ QSO's

By means of a collaboration across the Atlantic, very long exposures (about 14 hours) have been made with IUE of two high redshift QSO's during October 1978. Target acquisition and initiation was carried out at Vilspa, and the completion of the exposure and read-out of the image was executed at Goddard. The first object Q 2204-408 is an optical QSO with m(v) = 17.5 and z = 3.18 whose spectrum had just been detected earlier with a 350 minute exposure in the short wavelength spectrograph (Wilson, et al. 1978). In this case two longer exposures were obtained in both spectrographs. The other object is the radio QSO PKS 2126-158 with m(v) = 17.3 and z = 3.27 for which a detectable spectrum was obtained in the short wavelength spectrograph.

Boggess, A.↗
NASA NTRS1983

The ultraviolet excess of luminous quasars. II - Evidence for massive accretion disks

This paper presents new infrared, optical, and ultraviolet spectra of three high-redshift quasars. They are analyzed along with three published quasar spectra. After accounting for other sources of radiation, the continua are fitted with spectra predicted for optically thick steady-state accretion disks which include the effects of general relativity. The two fitting parameters, the mass of the accreting black hole and the accretion rate, are determined by the data with a formal accuracy of 20 percent. The masses range from 0.2-0.5 billion solar mass for 3C 273 and PKS 0405-123 to 1-3 billion solar masses for the high-redshift quasars. The values depend on the assumed disk inclination and the angular momentum of the hole. The high luminosities of these six quasars are all within a factor of 2 of their Eddington limits, and this conclusion is not extremely sensitive to the modeling assumptions.

Malkan, M. A.↗
NASA NTRS1984

The spectral evolution of low-frequency variable radio sources

The dynamic spectra of several low frequency extragalactic radio sources are presented. The observations were made at 318, 430, 606, 880, and 1400 MHz at several different radio observatories around the U.S. Two outbursts were observed in AO 0235 + 16 at 1.4 GHz, followed by a diminished variation at the lower frequencies. The dynamic frequencies of NRAO 140, PKS 1117 + 14, DA 406, CTA 102, and 3C 454.3 do not fit the same pattern. These radio sources displayed the following characteristics: (1) departure from straight or curved spectra at the frequencies of variation; (2) no obvious frequency drifting; and (3) negligible variation at 1.4 GHz. Possible explanations for this behavior are briefly discussed.

Dennison, B.↗
NASA NTRS1984

Multifrequency light curves of low-frequency variable radio sources

Light curves for the low-frequency variable sources AO 0235 + 16, NRAO 140, PKS 1117 + 14, DA 406, CTA 102, and 3C 454.3, obtained in monthly observations at 318, 430, and 606 MHz using the 305-m telescope at Arecibo and in bimonthly observations at 880 MHz and 1.4 GHz using the 91-m Green Bank transit telescope during 1980-1983, are presented and analyzed. AO 0235 + 16 is found to have basically canonical variability which is attributed to relativistically moving evolving synchrotron components; but in the other sources, strong simultaneous variations at 318, 430, and 606 MHz are observed to be greatly diminished in amplitude at 880 MHz and 1.4 GHz, confirming the existence of the intermediate-frequency gap at about 1 GHz proposed by Spangler and Cotton (1981). The possibility that a second variability mechanism is active in these sources is explored.

Altschuler, D. R.↗
NASA NTRS1985

Discovery of a probable galaxy with a redshift of 3.218

The discovery of a narrow emission line object, probably a galaxy, with a redshift of 3.218, is reported. The object is a companion to the quasar PKS 1614 + 051, which is at a redshift of 3.209. This is the most distant non-QSO, nongravitationally lensed object presently known by a large margin. Its properties are consistent with those expected of a high-redshift galaxy. This object has an age of only a few percent of the present age of the universe. The object was discovered with a novel technique, which promises to push studies of distant galaxies to redshifts as high as those of the most distant quasars known, and which may eventually lead to the discovery of primeval galaxies. This discovery opens the way for studies of galaxies beyond redshift three, which should prove invaluable for observational cosmology.

Djorgovski, S.↗
NASA NTRS1986

Computer simulation of immobilized pH gradients at acidic and alkaline extremes - A quest for extended pH intervals

Computer simulations of the concentration profiles of simple biprotic ampholytes with Delta pKs 1, 2, and 3, on immobilized pH gradients (IPG) at extreme pH values (pH 3-4 and pH 10-11) show markedly skewed steady-state profiles with increasing kurtosis at higher Delta pK values. Across neutrality, all the peaks are symmetric irrespective of their Delta pK values, but they show very high contribution to the conductivity of the background gel and significant alteration of the local buffering capacity. The problems of skewness, due to the exponential conductivity profiles at low and high pHs, and of gel burning due to a strong electroosmotic flow generated by the net charges in the gel matrix, also at low and high pHs, are solved by incorporating in the IPG gel a strong viscosity gradient. This is generated by a gradient of linear polyacrylamide which is trapped in the gel by the polymerization process.

Mosher, Richard A.↗
NASA NTRS1986

Computer simulation of two electrophoretic columns coupled for isoelectric focusing in simple buffers

Computer simulation is used to analyze a system of two electrophoretic columns coupled by mixing the anolyte of one with the catholyte of the other. A mathematical model is presented which is used to predict the pH gradients formed by monovalent buffers in this system, when the currents in the columns are unequal. In the column with the higher current a pH gradient is created which increases from anode to cathode and is potentially useful for isoelectric focusing. The breadth of this gradient is dependent upon the ratio of the currents. The function of the second column is the compensation of buffer migration which occurs in the first column, thereby maintaining constant electrolyte composition. The effects of buffer pKs and mobilities are evaluated.

Tsai, Amos↗
NASA NTRS1987

Occultation of compact radio sources by the ion tail of Halley's Comet

Enhancements of scintillations of the compact radio sources PKS 2314+03 and 1827-360 were observed at 103 MHz and 408 MHz during 18-21 December 1985 and on 29 March 1986, respectively, when the plasma tail of Halley's Comet swept across them. At 103 MHz the RMS plasma density variation along the tail was 10 and 3.3/cu cm at 0.12 AU and 0.18 AU, respectively, as measured from the comet's position. At 408 MHz it was 1.9/cu cm at 0.036 AU. Comparison of results of these two sets of observations is presented.

Alurkar, S. K.↗
NASA NTRS1988

Energy distribution and variability of BL Lac objects - Recent observations and a theoretical scheme

Spectral properties of X-ray selected and radio-selected BL Lacs are discussed. The results provide a direct indication that the X-ray emission is more isotropic than the radio emission. Recent results of multifrequency observations of the brightest X-ray selected BL Lac object, PKS 2155-304, are reviewed, showing how the fast X-ray variability is smeared out at low frequencies. A theoretical jet model is presented which can quantitatively account for the different spectral properties of X-ray selected and radio selected BL Lacs in terms of different line-of-sight angles to the jet.

Maraschi, L.↗
NASA NTRS1988

Ultraviolet and optical spectra of broadline radio galaxies

Near-simultaneous ultraviolet and optical spectra of three broadline radio galaxies (3C 382, 3C 445, and PKS 2349-014) have been obtained, and the emission lines of Ly-alpha, H-beta, and H-alpha have been deconvolved into narrow and broad components; published fluxes for 3C 390.3 are also included in this study. Although the broad Ly-alpha/H-beta ratios in these objects cover a large range (1.2-22.3), there is no evidence that these ratios are intrinsically different from those of Seyfert 1 galaxies or quasars. Thus, in general, the higher H-alpha/H-beta ratios in these broadline radio galaxies cannot entirely be due to additional reddening of the broadline region. However, in the specific case of 3C 445, there is evidence that the nonstellar continuum and broadline region are highly reddened.

Crenshaw, D. Michael↗
NASA NTRS1989

Element abundances at high redshift

Abundances of Si(+), S(+), Cr(+), Mn(+), Fe(_), and Zn(+) are considered for two absorption-line systems in the spectrum of the QSO PKS 0528 - 250. Zinc and sulfur are underabundant, relative to H, by a factor of 10 compared to their solar and Galactic interstellar abundances. The silicon-, chromium-, iron-, and nickel-to-hydrogen ratios are less than the solar values and comparable to the local interstellar ratios. A straightforward interpretation is that nucleosynthesis in these high-redshift systems has led to only about one-tenth as much heavy production as in the gas clouds around the sun, and that the amount of the observed underabundances attributable to grain depletion is small. The dust-to-gas ratio in these clouds is less than 8 percent of the Galactic value.

Meyer, David M.↗
NASA NTRS1990

No evidence for radio-quiet BL Lacertae objects

Using a large, flux-limited sample of faint X-ray sources, a search has been conducted for radio-quiet BL Lacertae objects. None has been found. Thirty-two X-ray-selected BL Lac objects and BL Lac candidates have been found within the sources of the Einstein Medium Sensitivity Survey (EMSS). Thirty-one of these have been observed with the VLA and all have been detected at 5 GHz. While the optical magnitudes of the EMSS BL Lac objects range from 17 to 20.8, their radio-to-optical spectral indices occupy a very small range. The very bright X-ray-selected BL Lac objects like PKS 2155-304 and Markarian 501 have similar range values. Therefore, unlike the clear dichotomy between radio-loud quasars and radio-quiet QSOs, there is no evidence for two populations of Lacertids distinguished by radio loudness.

Stocke, John T.↗
NASA NTRS1990

High-resolution X-ray spectroscopy of four active galaxies - Probing the intercloud medium

The focal plane crystal spectrometer (FPCS) on the Einstein Observatory has been used to perform a high-resolution spectroscopic search for oxygen X-ray line emission from four active galaxies: Fairall 9, Mrk 421, Mrk 501, and PKS 0548 - 322. Specifically, O VIII Ly-alpha and Ly-beta, whose unredshifted energies are 653 and 775 eV, respectively, were sought. No narrow-line emission was detected within the energy bands searched. Upper limits are calculated on the line flux from these sources of 30 eV equivalent width and use a photoionization model to place corresponding upper limits on the densities of diffuse gas surrounding the active nuclei. The upper limits on gas density range from about 0.02-50/cu cm and probe various radial distances from the central source. This is the first time high-resolution X-ray spectroscopy has been used to place constraints on the intercloud medium in active galaxies.

Lum, Kenneth S. K.↗
NASA NTRS1990

Accretion disk emission from a BL Lacertae object

The accretion disk is an attractive model for BL Lac objects because of its preferred axis and high efficiency. While the smooth continuum spectra of BL Lacs do not show large UV bumps, in marked contrast to quasars, high quality simultaneous data do reveal deviations from smoothness. Using detailed calculations of cool accretion disk spectra, the best measured ultraviolet and soft x ray spectra of the BL Lac object PKS 2155-304 are fitted. The mass and accretion rate required are determined. A hot disk or corona could comptonize soft photons from the cool disk and produce the observed power law spectrum in the 1 to 10 keV range. The dynamic time scales in the disk regions that contribute most of the observed ultraviolet and soft x ray photons are consistent with the respective time scales for intensity variations. The mass derived from fitting the continuum spectrum is consistent with the limit derived from the fastest hard x ray variability.

Urry, C. Megan↗
NASA NTRS1991

Accretion disk emission from a BL Lacertae object

It is suggested here that the UV and X-ray emission of BL Lac objects may originate in an accretion disk. Using detailed calculations of accretion disk spectra, the best-measured ultraviolet and soft X-ray spectra of the BL Lac object PKS 2155-304 are fitted, and the mass and accretion rate required is determined. The ultraviolet through soft X-ray continuum is well fitted by the spectrum of an accretion disk, but near-Eddington accretion rates are required to produce the soft X-ray excess. A hot disk or corona could Comptonize soft photons from the cool disk and produce the observed power-law spectrum in the 1-10 keV range. The dynamic time scale in the disk regions that contribute most of the observed ultraviolet and soft X-ray photons are consistent with the respective time scales for intensity variations observed in these two wave bands; the mass derived from fitting the continuum spectrum is consistent with the limit derived from the fastest hard X-ray variability.

Wandel, Amri↗
NASA NTRS1991

A ubiquitous absorption feature in the X-ray spectra of BL Lacertae objects

The paper presents the broadband (0.5-20-keV) X-ray spectra of five X-ray bright BL Lac objects observed with the Einstein Observatory Solid State Spectrometer (SSS) and Monitor Proportional Counter (MPC) detectors. The combination of moderate energy resolution and broad spectral coverage makes it possible to confirm the presence of an absorption feature at an energy of 650 eV in the BL Lac object PKS 2155-304, originally reported by Canizares and Kruper (1984) based on higher resolution Einstein Objective Grating Spectrometer (OGS) data.

Madejski, Greg M.↗
NASA NTRS1991

Discovery of an apparent companion galaxy to Q1548 + 0917 with z = 2.758

The discovery of a resolved companion to the QSO Q1548 + 0917 (redshift 2.749) is reported. The possible nature of the companion, designated C1548 + 0917, is discussed, and new spectroscopic observations of the prototypical case of a Lyman alpha emitting companion to a high-redshift QSO, PKS 1614 + 0508 A, are presented for comparison. C1548 + 0917 appears to be the first case of a radio-quiet continuum object discovered in the vicinity of what must be considered a radio-quiet QSO at high redshift. Evidence is also found for extended Lyman alpha emission associated with the QSO itself.

Steidel, Charles C.↗