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  • 1
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    Active extragalactic sources - Nearly simultaneous observations from 20 centimeters to 1400 A (1986)
    In:  Other Sources
    Details
    Publication Date: 2019-07-12
    Description: IRAS, IUE, and ground-based optical, NIR, mm and submm, and radio observations obtained mainly on Apr. 9-23, 1983, are reported for 19 active extragalactic sources and eight control sources. The overall spectra of the compact active sources are shown to be well represented by continuous-curvature functions such as parabolas. The spectra are found to be consistent with models involving continuous particle injection (with synchrotron losses) or first-order Fermi acceleration (with escape and synchrotron losses), but not with models using relativistic Maxwellian electron distributions.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 308; 78-92
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  • 2
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    The origin of infrared emission from the nucleus of NGC 1068 (1975)
    In:  Other Sources
    Details
    Publication Date: 2011-08-16
    Description: Recent infrared observational results for the nucleus of the Seyfert galaxy NGC 1068 are reviewed and analyzed in terms consistent with information available at other wavelengths. It is concluded that the infrared and optical data imply that more than 85 percent of the infrared emission at 10 microns is radiation from dust grains in the nucleus. Observed reddening of spectral lines implies geometrical optical depths at visual wavelengths of about 7-15 if the nuclear dust cloud is approximately spherically symmetric. The dust grains emitting the infrared radiation could be silicates with a 10-micron optical depth near unity, but this identification is not uniquely established. The grains are heated radiatively by an underlying source or sources of radiation also responsible for ionizing the emission-line-producing gas. The underlying source could be nonthermal, or it could be a hot plasma. Physical constraints on each of these models are derived.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; 197; Apr. 15
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  • 3
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    An infrared study of Orion Molecular Cloud-2 (OMC-2) (1990)
    In:  Other Sources
    Details
    Publication Date: 2011-08-19
    Description: This paper reports 1.2-23 micron photometry for 11 discrete sources in Orion Molecular Cloud-2 (OMC-2). These data, combined with H and K photometric and K polarimetric images, are used to model the cluster sources. Most appear to be young stars of roughly solar mass. Some have circumstellar dust reradiation or reflection nebulosity. A model based on single scattering of light from an exciting star explains some features of the IRS 1 nebula, the largest reflection nebula in OMC-2. However, the red colors and high surface brightness of the IRS 1 nebula require a cool excitation source that is more luminous than far-infrared observations would indicate.
    Keywords: ASTROPHYSICS
    Type: Astronomical Journal (ISSN 0004-6256); 100; 518-529
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  • 4
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    Far-infrared observations of young clusters embedded in the R Coronae Australis and Rho Ophiuchi dark clouds (1985)
    In:  Other Sources
    Details
    Publication Date: 2019-06-28
    Description: Multicolor far-infrared maps in two nearby dark clouds, R Coronae Australis and Rho Ophiuchi, have been made in order to investigate the individual contribution of low-mass stars to the energetics and dynamics of the surrounding gas and dust. Emission from cool dust associated with five low-mass stars has been detected in CrA and four in Rho Oph; their far-infrared luminosities range from 2 solar luminosities to 40 solar luminosities. When an estimate of the bolometric luminosity was possible, it was found that typically more than 50 percent of the star's energy was radiated longward of 20 microns. Meaningful limits to the far-infrared luminosities of an additional 11 association members in CrA and two in Rho Oph were also obtained. The dust optical depth surrounding the star R CrA appears to be asymmetric and may control the dynamics of the surrounding molecular gas. The implications of these results for the cloud energetics and star formation efficiency in these two clouds are discussed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal, Part 1 (ISSN 0004-637X); 293; 165-177
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  • 5
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    Transfer of polarized radiation in self-absorbed synchrotron sources. I - Results for a homogeneous source (1977)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The solution to the equation of transfer of polarized radiation in a steady-state homogeneous rarefied medium is applied to self-absorbed synchrotron sources. Relativistic electrons (independent of the presence of any cold plasma) can quite easily produce in such sources significant Faraday rotation and/or conversion of linear to circular polarization. Structural inhomogeneities do not obviate the importance of these phenomena in compact nonthermal sources. Contrary to the calculation of Pacholczyk and Swihart (1975), the circular polarization for a homogeneous source changes sign just below the self-absorption turnover as the source becomes opaque, even when polarization conversion dominates; however, for a physically realistic source, structural inhomogeneity may alter this behavior. The observational evidence bearing upon these effects is reviewed.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 214
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  • 6
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    Hard X-ray observations of NGC 4151 (1975)
    In:  Other Sources
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    Publication Date: 2019-06-27
    Description: The X-ray source 3U 1207 + 39, associated with the Seyfert galaxy NGC 4151, has been scanned repeatedly by the UCSD X-ray telescope on board the OSO-7 satellite. A weak source with a hard spectrum has been unambiguously detected in this region in October 1972, and with lesser statistical significance at other times between November 1971 and May 1973. The photon spectrum in the 7-110 keV range is best fitted by a power law of the form dN/dE = 0.012 E to the exponent negative 1.1 photons per sq cm per second per keV, consistent with the Uhuru measurement in the 2-6 keV range. The hard spectrum observed and very short electron lifetimes implied severely constrained nonthermal models for the emission. An extremely hot gas is also consistent with the observations.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; 199; July 1
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  • 7
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    Physics of compact nonthermal sources. III - Energetic considerations (1974)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The energy content of the compact incoherent electron-synchrotron sources 3C 84, 3C 120, 3C 273, 3C 279, 3C 454.3, CTA 102, 3C 446, PKS 2134+004, VRO 42.22.01 and OJ 287 is calculated on the assumption that the low-frequency turnovers in the radio spectrum are due to self-absorption and that the electron distribution is isotropic. The dependence of the source parameters on various modifications of the standard assumptions is determined. These involve relativistic motions, alternate explanations for the low-frequency turnover, proton-synchrotron radiation, and distance to the source. The canonical interpretation is found to be accurate in many respects; some of the difficulties and ways of dealing with them are discussed in detail.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; 193; Oct. 1
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  • 8
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    Transfer of polarized radiation in self-absorbed synchrotron sources. II - Treatment of inhomogeneous media and calculation of emergent polarization (1977)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: As an extension of previous treatments of polarized radiation transfer in homogeneous synchrotron sources, a transfer procedure is derived, based on the coupled-wave formulation, which allows consideration of inhomogeneous sources. It is shown that plasma characteristic wave eccentricities in a source must be fairly large unless Faraday rotation per absorption depth is very large, and that coupling between characteristic waves can be strong in inhomogeneous self-absorbed sources. Calculations are made of the polarized radiation emergent from a source with a boundary of finite thickness. It is found that neglect of propagation effects in the source boundary introduces no significant errors at optically thin frequencies, but that below the self-absorption turnover the emergent polarization may depend significantly upon such effects. In particular, the sign of the circular polarization and the linear polarization position angle depend upon the absorption depth in the boundary region.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 215
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  • 9
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    Radiative transfer in dust and the spectral flux distribution of NGC 1068 (1977)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: The continuum spectral flux distribution of the Seyfert galaxy NGC 1068 is analyzed by detailed models of radiative transfer in an optically thick cloud of dust grains. For wavelengths short of 30 microns, models invoking a spherical dust cloud with visual optical depth near 10 in the nucleus of the galaxy can reproduce the observed spectrum in a way consistent with information derived from spectral lines. The far-infrared emission cannot be explained easily by dust in the nucleus, but it is hypothesized that this radiation is emitted by dust associated with the observed molecular clouds, and that these clouds lie outside the nucleus. This far-infrared emission, therefore, should be extended to the same degree as the molecular-cloud distribution. High angular resolution mapping will be necessary to confirm this hypothesis.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; vol. 212
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  • 10
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    An X-ray survey of BL Lacertae objects (1976)
    In:  Other Sources
    Details
    Publication Date: 2019-06-27
    Description: Results are reported for X-ray observations and nearly simultaneous photographic photometry of four BL Lacertae objects (BL Lac, W Com, ON 325, and AP Lib). The X-ray data were obtained in the band from 2.5 to 7.5 keV using a collimated proportional counter aboard OAO-C. Although none of the sources was detected above background at a statistically significant level, two-sigma upper limits on the X-ray flux incident at earth are given for each object. The photometric data indicate that all four objects were near the faint end of their visibility ranges during the time of the X-ray observations. The X-ray upper limits and B magnitudes are employed to calculate upper limits on the brightness temperatures and angular sizes of the radio synchrotron sources; upper limits on the distances to the objects are obtained from limits on the physical source sizes deduced from the radio variabilities. The strictest distance limit is 240 Mpc for BL Lac. Substantial evidence is noted for variability of ON 325 on a timescale of hours.
    Keywords: ASTROPHYSICS
    Type: Astrophysical Journal; 207; July 15
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