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The Ultraviolet Calibration of the Hubble Space Telescope. IV. Absolute IUE Fluxes of Hubble Space Telescope Standard Stars
Bohlin, Ralph C.; Gry, Cecile; Holm, Albert V. +1 more
UV fluxes are presented for 37 stars selected as calibration standards for the Hubble Space Telescope (HST). In order to provide data of maximum precision, new absolute calibrations have been derived for the IUE SWP and LWR cameras for point sources in the large apertures, while the sensitivities of the trail and small aperture observing modes rel…
Optically Thin Thermal Emission as the Origin of the Big Bump in the Spectra of Active Galactic Nuclei
Peterson, B. M.; Korista, K. T.; Ferland, G. J.
The nature of the "big bump," the excess of ultraviolet emission seen in many active nuclei, is a clue to the source of the energy generation in the central regions of quasars. We report observations of the Seyfert galaxy Mrk 590, which show dramatic changes in both the continuum and hydrogen lines, thus offering a clue to the origin of the "big b…
The X-ray emission of 3C 273.
Koyama, K.; Nandra, K.; Tanaka, Y. +12 more
The X-ray emission of 3C273 in the 0.2-35 keV band has been studied, with EXOSAT and Ginga, over the period 1983 December to 1985 December. The overall flux is variable on a time-scale of weeks by a factor of 2. The spectral index in the 2-10 keV band (α ~0.5) is significantly different from the 'canonical' AGN index of α = 0.7. There are small bu…
Planetary nebulae with a high degree of ionization : NGC 2242 and NGC 4361.
Pena, M.; Torres-Peimbert, S.; Peimbert, M.
New observations in the 3400 - 7500 A range are presented for NGC 2242 and NGC 4361, and new IUE observations are presented for NGC 2242. The physical conditions and chemical composition for these nebulae are derived. The ionization structure of both PN are very similar, the PN are very optically thin to Lyman continuum photons, and more than 90 p…
Ultraviolet and Optical Variations in Active Galactic Nuclei: The Seyfert 1 Galaxy NGC 5548
Talavera, A.; Netzer, H.; Wamsteker, W. +8 more
We describe the results of a project to study the combined ultraviolet- optical spectrum of the Seyfert I galaxy NGC 5548 over the period 1978- 1986. Of the many spectrophotometric observations made with a typical resolution of 5-8 A, 11 epochs were simultaneous and nine cover the range from 1150 to 7000 A. The combined IUE and ground-based observ…
A model for the planetary nebula NGC 7027.
Middlemass, D.
Extensive photoionization modeling of NGC 7027 is presented using the most recent observations available. The effects of the differential extinction caused by the local dust cloud are considered and corrections are made to large-aperture line fluxes. Abundances are derived from a large number of collisionally excited and recombination emission lin…
The Spectrum of NGC 7027
Feibelman, W. A.; Aller, L. H.; Keyes, Charles D.
IUE and echelle spectrograph observations of the planetary nebular NGC 7027 are analyzed to obtain nebular plasma diagnostics and chemical abundances. It is found that most of the nebular plasma has a density of about 60,000/cu cm and a temperature of about 14,000 K. The results suggest that the stellar radiation field is greatly enhanced shortwar…
Hot sublumous stars at high galactic latitudes. II. Physical parameters and distances of 37 sdB stars.
de Boer, K. S.; Heber, U.; Moehler, S.
Spectra and photometry of 37 subdwarf B stars mainly selected from Green et al. (1986), are analyzed to derive effective temperatures, surface gravities and helium abundances. Effective temperatures lie between 23,500 K and 32,000 K with values for log g between 4.5 and 5.6. The helium abundances vary between 0.03 and about 6 percent (number fract…
X-Ray Eclipse Mapping of AR Lacertae
Parmar, A. N.; White, N. E.; Culhane, J. L. +2 more
Exosat data of the binary cycle of the eclipsing RS CVn system AR Lac are studied. The data reveal that at energies of less than 1 ke V a factor of 2 intensity modulation is detected and at energies greater than 1 ke V no orbital modulation is observed. The chi-squared and maximum entropy methods are used to model low-energy modulation. The relati…
Simultaneous multifrequency observations of the Seyfert 1 galaxy NGC 4051 : constant optical-infrared emission observed during large-amplitude X-ray variability.
Done, C.; Fabian, A. C.; Wamsteker, W. +5 more
We report simultaneous infrared, optical, ultraviolet and X-ray observations of the low-luminosity Seyfert 1 galaxy, NGC 4051. A new method of reduction was developed to correct the optical flux for atmospheric variations due to seeing and extinction. The X-ray flux varied by factors of up to 2 on time-scales of tens of minutes whilst the optical …