Search Publications
Ionizing radiation in the variable active galaxy 3C 120.
Gondhalekar, P. M.
The change in the C IV/Lyα line ratio when the continuum intensity changes, in the spectrum of the radio-loud active galaxy 3C 120, has been modelled. This analysis defines (i) the average parameters of the broad- line region in this galaxy and (ii) the ionizing spectrum seen by the clouds in this region. A soft extreme-ultraviolet continuum is re…
Early improvements to the HIPPARCOS Input Catalogue through the accumulation of data from the satellite - Including the NDAC attitude reconstruction description
van Leeuwen, Floor; Lindegren, Lennart; Penston, Margaret J. +2 more
A description of ways in which the Northern Data Analysis Consortium (NDAC) accumulates positional and photometric data as obtained in the early stages of the reduction of data from the Hipparcos instrument is presented. An outline of the closely associated attitude reconstruction process is also given. Positional internal accuracies in arcsec, an…
The Ulysses Mission: In-ecliptic phase
Smith, E. J.; Wenzel, K. -P.
The Ulysses mission is unique in the history of the exploration of our solar system by spacecraft. The path followed by Ulysses will enable us, for the first time, to explore the heliosphere within a few astronomical units of the Sun over the full range of heliographic latitudes. The mission, a collaboration between ESA and NASA, was launched on 6…
Ultraviolet Imaging Telescope Photometry of Massive Stars: The OB Association NGC 206 in M31
O'Connell, Robert W.; Bohlin, Ralph C.; Smith, Andrew M. +6 more
The Ultraviolet Imaging Telescope (UIT) obtained UV images of the giant M31 OB association NGC 206. Magnitudes in bands at 1520 and 2490 A were obtained for 30 massive stars, which demonstrate the effectiveness of UIT for photometry of moderately crowded hot stars to V ~ 21. The UV colors and magnitudes observed for stars in NGC 206 place them in …
X-Ray Observations of the Seyfert 1 Galaxy MCG 2-58-22
Ghosh, K. K.; Soundararajaperumal, S.
Results of the detailed X-ray (0.1-10 keV) spectral analysis of the Seyfert 1 galaxy, MCG 2-58-22, are presented based on the EXOSAT observations on 1984 November 16-26. X-ray spectra of this source were obtained from the EXOSAT archives. Weak, soft flux (0.1-2 keV) variations were found on time scales of 2-3 days, and no variations of the hard fl…
The Cramer-Rao lower bound and stellar photometry with aberrated HST images
Jakobsen, P.; Greenfield, P.; Jedrzejewski, R.
The Cramer-Rao lower bound theorem of statistics is applied to the problem of extracting 2D stellar photometry from aberrated Hubble Space Telescope camera images. The theorem is used to derive a simple and general expression for the best possible photometric precision that can be derived from a given image, regardless of what image processing or …
The UV and IR Jovian aurorae
Prange, Renee
Although the Jovian aurorae have been widely considered as due to the interaction of the Io torus with the magnetosphere, rather little had been learnt until the last few years from the remote UV observations as far as generation mechanisms are concerned, due both to the complexity of the observation analysis and the deficiency of the theoretical …
Cool star flux spectra for population studies in galaxies.
Parthasarathy, M.; Morossi, C.; Malagnini, M. L. +1 more
Five solar-composition cool giant stars have been observed with the IUE low resolution mode. The UV energy distributions were merged with visual spectrophotometry from the literature and compared with predictions from two new grids of model atmospheres. Atmospheric parameters were derived consistent with the literature data by matching observation…
Effective temperature of B-type stars from the SI II lines of the UV multiplet 13.04 at 130.5-130.9 nm.
Castelli, F.; Singh, J.
An analysis was performed for the 129.3-132.0 nm region of ten normal and two helium-weak peculiar B-type stars of spectral types ranging from B2 to B9 and luminosity classes IV-V. Computations of spectra made it possible to determine the Si abundance from the Si III lines and to estimate the effective temperature from the Si III profiles. The rot…
X-ray variability in the Seyfert galaxy Markarian 618.
Singh, K. P.; Rao, A. R.; Vahia, M. N.
We report X-ray observations of the Seyfert type I galaxy Markarian 618, using the low-energy (LE) and the medium-energy (ME) detectors of the EXOSAT observatory. The 0.1-10 keV X-ray spectrum is adequately explained by a simple power law with the photon index {GAMMA} = 1.8^+0.7^_-0.5_( 90 per cent confidence), similar to that observed from variou…