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At least 127 records · Page 7
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NASA NTRS1978

On the origin of the absorption spectra of quasi-stellar and BL Lacertae objects

The possibility is reexamined that multiple absorption spectra of QSOs are due to gas in the disks, coronae, or halos of intervening galaxies. Using the same catalog and parameters as Burbidge et al. (1977) but assuming that the cosmological constant is approximately equal to the critical value instead of equal to zero, it is found that the discrepancy of several orders of magnitude between the predicted and observed multiplicities reported by Burbidge et al. (1977) is drastically reduced and that the observed multiplicity distribution is theoretically obtainable. The intervening-galaxy hypothesis cannot be rejected on the basis of this test.

Canuto, V.↗
NASA NTRS1979

The X-ray spectrum of the BL Lacertae object PKS 0548-322

X-ray observations of H0548-322 = PKS 0548-322 at energies from 0.15 keV to 24 keV with the HEAO A-2 experiment are reported. The spectrum shows a distinct break in slope near 2 keV, with the indices of power-law energy spectra of 2.2 below and 0.3 above this break. A synchrotron self-Compton model is consistent with radio, optical, and all X-ray data, although a modified synchrotron self-Compton model with added thermal origin for the low-energy X-ray spectrum is also plausible.

Riegler, G. R.↗
NASA NTRS1982

Two multifrequency observations of the BL Lacertae object OJ 287

Results are reported for various sets of X-ray, UV, visual, IR, millimeter, and radio observations of OJ 287. The spectral information is used to constrain nonthermal energy production models for the source, and three possible models are considered. It is found that, in order to describe all the observed flux in the radio to X-ray bands with a synchrotron self-Compton model, a high degree of relativistic beaming is required such that the equivalent Doppler factor ranges from about 20 to 165. It is concluded that submillimeter flux and X-ray spectral measurements could distinguish between the two possible models that fit the present observations.

Worrall, D. M.↗
NASA NTRS1983

X-ray studies of BL Lacertae objects with the Einstein Observatory - Confrontation with the synchrotron self-Compton predictions

X-ray flux densities measured with the Einstein Observatory are presented for a set of LB Lac objects with available VLBI angular sizes. The static, single-component synchrotron self-Compton formalism is used to predict the X-ray flux densities. Ratios of 10 to the -6th to the 10 to the -8th are found for the observed to predicted flux densities and it is concluded that such a model, at least in its simplest form, does not explain the X-ray emission from these objects as a class. The hypothesis that the compact source is in relativistic motion allows a simple resolution of the discrepancy. The results agree qualitatively with a broad distribution of bulk Lorentz factors Gamma with a fairly low typical value of Gamma approximately equal to and with a quasi-isotropic distribution of the direction of ejection relative to our line of sight.

Madejski, G. M.↗
NASA NTRS1984

Multifrequency observations of the BL Lacertae object 0735+178

In each of the present four simultaneous spectra covering the radio-through-X-ray regimes, the IR-UV synchrotron continuum dominates the total observed power and presumably becomes opague between 10 to the 11th and 10 to the 13th Hz. Nonsimultaneous observations were also conducted, over a longer time period, in order to study long- and short-term variability at X-ray, optical, and radio frequencies. These data indicate that the rapid and dramatic variations evident at IR and optical wavelengths are absent at radio and X-ray frequencies, supporting a view of IR-UV flux emanation from a small region, while the X-rays are produced by the inverse Compton process in the radio-emitting region. Particles, photons and magnetic field may not be far from equipartition in this region. Theoretical suggestions are developed regarding the radial behavior of the electron density and magnetic field.

Bregman, J. N.↗
NASA NTRS1984

A sharp X-ray absorption feature in the BL Lacertae object PKS 2155-304

The present absorption feature from PKS 2155-304 appears at 600 eV and extends to higher energies by 50-100 eV, in a 0.38-1.6 keV spectrum that can be fitted by a single absorption edge or through plus two power laws, with slopes that are consistent with simultaneous measurements of the 2-10 keV flux. Ionized oxygen absorption is the most likely cause of this sharp feature, in the form of a K edge of O VIII, or an O VIII Lyman-alpha resonance absorption trough from high velocity material, or an O VIII Lyman-alpha absorption trough from a hot, O-rich intergalactic medium with near critical density.

Kruper, J.↗
NASA NTRS1985

Multifrequency observations of the BL Lacertae object PKS 0537 - 441

PKS 0537 - 441 was repeatedly observed in the UV band with the International Ultraviolet Explorer and in the X-ray with the Einstein Observatory. On September 27, 1980, simultaneous observations in the two bands were obtained. Near-infrared photometry preceding and following the simultaneous observations by about one month is available from the literature, as is radio monitoring at 408 and 5000 MHz. Comparison of the observed X-ray flux with that predicted by the standard synchrotron self-Compton formalism, with a source dimension deduced from radio variability at 5 GHz, indicates that this component of the radio emission must be moving at relativistic speed with an effective projected Doppler beaming factor of about 10.

Maraschi, L.↗
NASA NTRS1986

Coordinated observations of X-ray bright BL Lacertae objects

No new International Ultraviolet Explorer (IUE) observations were scheduled. The work on the spectral extraction routine was continued. The computer code was adapted from the RDAF PDP 11/44 computer to a VAX 11/750, in order that it run more quickly. A chapter on ultraviolet observations of blazars for a book celebrating the scientific accomplishment of IUE was written in colloboration. A copy of the final draft is attached.

Urry, C. M.↗
NASA NTRS1993

The X-ray spectral variability of the BL Lacertae type object PKS 2155-304

We present a detailed study of the hard X-ray properties of the BL Lacertae object PKS 2155-304 based on measurements made in 1988 and 1989 with the Large Area Counter (LAC) on board the Ginga satellite. The source exhibited a high degree of variability with a dynamic range of a factor 7 in the 2-6 keV band. The fastest amplitude variation was a factor 2 decline in the intensity in this band within 4 hours. The spectrum is characterized by a break which occurs at about 4 keV. Spectral fits to the data integrated in 6400 s time bins reveal that, in common with previous observations of BL Lacertae objects, the spectral slope is generally anticorrelated with intensity in the sense that the spectrum hardens as the intensity increases. However, the tracks of sequential points in the index-intensity plane are occasionally seen to differ during the rise and decay stages of individual flares. Furthermore, during one, or possibly two, flaring episodes the spectral index is observed to correlate with intensity variations.

Sembay, S.↗
NASA NTRS1979

Some properties of the knots in the M87 jet

Photographs taken in 1956 by Baade show the knots in the jet of M87 to be polarized. In 1978, these plates were repeated under conditions as identical as possible. A careful comparison indicates that the polarization and intensity of the knots have changed in the 22 year interval. Emission lines in the nucleus of M87 not previously seen have been revealed by extensive 5 m SIT spectroscopy. The same spectroscopy with galaxy subtracted, however, has failed to reveal any features in the jet. The known optical and radio properties of the jet are reviewed. A coincidence is found in almost all of the defining properties of BL Lacertae objects with the properties of the knots in the jet. Statistical studies of the distribution of BL Lacertae objects near bright galaxies reveals a marginally significant excess of such associations.

Sulentic, J. W.↗
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.↗