Search Publications
Non--LTE Model Atmosphere Analysis of Nova Cygni 1992
Sonneborn, G.; Hauschildt, P. H.; Shore, S. N. +3 more
We use spherically symmetric non-local thermodynamic equilibrium (non-LTE), line-blanketed, expanding model atmospheres to analyze the International Ultraviolet Explorer (IUE) and optical spectra of Nova Cygni 1992 during the early phases of its outburst. We find that the first IUE spectrum obtained just after discovery on 1992 February 20, is bes…
Multi-wavelength monitoring of the BL Lacertae Object PKS 2155-304 II. The ROSAT Observations
Maraschi, L.; Treves, A.; Brinkmann, W. +7 more
Extensive ROSAT PSPC observations of the BL Lac object PKS 2155-304 carried out between November 12 and November 15, 1991 revealed the source to be in a bright state with a soft X-ray flux of F_x_(0.1-2.4keV)=~7x10^-10^erg/cm^2^/s. Rapid variations in the X-ray flux of up to 30% within a day were apparent but at the sampling rate of ~96 min all of…
Hubble Space Telescope Images of the Central Star-Forming Region in NGC 1140
O'Connell, Robert W.; Gallagher, John S., III; Hunter, Deidre A.
We present broadband images taken with the Hubble Space Telescope's Planetary Camera of the central supergiant H II region in the amorphous galaxy NGC 1140. These images allow observations to a resolution of about 13 pc at the galaxy, and they reveal that its central 1/2 kpc contains 6- 7 blue, luminous, compact super star clusters, many of which …
Monitoring Mars with the Hubble Space Telescope: 1990-1991 Observations
Clancy, R. Todd; James, Philip B.; Lee, Steven W. +5 more
Images of Mars obtained using the Planetary Camera on the Hubble Space Telescope during 1990 and 1991 are described and analyzed. Multispectral images of Mars record the martian season between Ls = 348° and 60° corresponding to late winter and spring in the northern hemisphere. The wavelengths of these observations varied between 230 nm…
The Coronal Electron Density Distribution Determined from Dual-Frequency Ranging Measurements during the 1991 Solar Conjunction of the ULYSSES Spacecraft
Bird, M. K.; Asmar, S. W.; Volland, H. +3 more
Dual-frequency ranging and Doppler measurements were conducted in support of the Ulysses Solar Corona Experiment (SCE) at and around the spacecraft's first solar conjunction in 1991 August. The differential group delay time between range codes on the two downlink carrier signals at the wavelengths 13.1 and 3.6 cm, a direct measure of the total ele…
The Discovery of 8.7 Second Pulsations from the Ultrasoft X-Ray Source 4U 0142+61
Mereghetti, S.; Stella, L.; Israel, G. L.
We discovered a periodicity at about 8.7s from the X--ray sources 4U0142+61, previously considered a possible black hole candidate on the basis of its ultrasoft spectrum. The pulsations are visible only in the 1--4 keV energy range, during an observation obtained with the EXOSAT satellite in August 1984. A search for delays in the pulse arrival ti…
A preliminary analysis of the planetary nebula central star V477 Lyrae.
Pollacco, D. L.; Bell, S. A.
We present a preliminary analysis of V477 Lyr, the eclipsing central star of Abell 46. Medium-resolution spectroscopy has been obtained, providing radial-velocity measurements for both components for the first time. New V-band charge coupled devices (CCD) photometry has also been obtained and the first full light-curve analysis is presented. The s…
Density Contrast Shell Models for the Planetary Nebula IC 2165
Hyung, S.
Since planetary nebulae are spatially extended sources, measurements of individual line fluxes may pertain to different areas of their images. Photoelectric or infrared observations of the strongest lines may embrace the entire image; International Ultraviolet Explorer (IUE) observations involve an elliptical aperture, 10 sec x 23 sec; while a spe…
Evidence for a quasar in the radio galaxy Cygnus A from observation of broad-line emission
Antonucci, Robert; Hurt, Todd; Kinney, Anne
ACTIVE galaxies are thought to be powered by the accretion of surrounding dust and gas onto central black holes. The unified model1-3 of active galaxies predicts that narrow-line radio galaxies (NLRGs), which are very luminous at radio wavelengths and have narrow optical emission lines, would appear as quasars—which have broad emission …
Hubble Space Telescope Goddard High-Resolution Spectrograph Observation of U Geminorum during Quiescence: Evidence for a Slowly Rotating White Dwarf
Szkody, Paula; Sion, Edward M.; Long, Knox S. +1 more
We have obtained a pair of consecutive far-ultraviolet Goddard High-Resolution Spectrograph (GHRS) exposures of the Si IV region of the dwarf nova U Geminorum in early quiescence, 8 days after its return to optical quiescence when the underlying white dwarf dominates the ultraviolet light of the system. Our GHRS observation revealed a fully resolv…