Search Publications
The IUE Survey for Damped Lyman- alpha and Lyman-Limit Absorption Systems: Evolution of the Gaseous Content of the Universe
Turnshek, David A.; Lanzetta, Kenneth M.; Wolfe, Arthur M.
We present a spectroscopic survey for moderate-redshift absorption features from damped Lyα and Lyman- limit absorption systems. The survey is based on a set of optimally extracted and co-added International Ultraviolet Explorer spectra of QSOs, BL Lac objects, and Seyfert galaxies. Results of this survey are combined with those of previous survey…
Optical and Ultraviolet Analyses of ZZ Ceti Stars and Study of the Atmospheric Convective Efficiency in DA White Dwarfs
Fontaine, G.; Bergeron, P.; Wesemael, F. +3 more
New high signal-to-noise optical spectrophotometry is presented for 22 ZZ Ceti stars. The atmospheric parameters (Teff log g) and the mass are derived for each object using new model atmospheres and synthetic spectra calculated within the mixing-length theory, as well as recent published mass-radius relationships. Various parameterizati…
Ultraviolet Radiation from Evolved Stellar Populations. II. The Ultraviolet Upturn Phenomenon in Elliptical Galaxies
O'Connell, Robert W.; Dorman, Ben; Rood, Robert T.
We discuss the far-ultraviolet upturn phenomenon (UVX) observed in elliptical galaxies and spiral galaxy bulges. Our premise is the the UV radiation from these systems emanates primarily from extreme horizontal branch (EHB) stars and their progeny. Such objects have zero-age horizontal branch envelope masses M_env_^0^<~0.05 M_sun_. Local exampl…
Multiwavelength Observations of Markarian 421 During a TeV/X-Ray Flare
Kafatos, M.; Takahashi, T.; Kondo, Y. +38 more
A TeV flare from the BL Lac object Mrk 421 was detected in May of 1994 by the Whipple Observatory air Cherenkov experiment during which the flux above 250 GeV increased by nearly an order of magnitude over a 2-day period. Contemporaneous observations by ASCA showed the X-ray flux to be in a very high state. We present these results, combined with …
Testing Unified X-Ray/Ultraviolet Absorber Models with NGC 5548
Elvis, Martin; Mathur, Smita; Wilkes, Belinda
The bright Seyfert galaxy NGC 5548 shows absorption features in its X-ray and UV spectra. The large amount of optical/UV and X-ray monitoring data available for this object makes it an ideal candidate to test our model of a common X-ray/UV absorber. We show that the X-ray/UV absorption is caused by a highly ionized (2.2 < U < 2.8), high colu…
The Pulsation Index, Effective Temperature, and Thickness of the Hydrogen Layer in the Pulsating DA White Dwarf G117-B15A
Koester, D.; Percival, J. W.; van Citters, G. W. +6 more
We have measured the amplitude of the 215 s pulsation of the pulsating DA white dwarf, or ZZ Ceti star, G117-B15A in six passbands with effective wavelengths from 1570 to 6730 A. We find that the index of the pulsation is l = 1 with a high degree of confidence, the first unambiguous determination of l for a pulsation of a ZZ Ceti star. We also fin…
The Nature of the Ultraviolet Continuum in Type 2 Seyfert Galaxies
Leitherer, C.; Calzetti, D.; Meurer, G. +6 more
Type 2 Seyfert nuclei are well known to contain a "featureless continuum" which makes a significant contribution in the optical and ultraviolet. However, the nature of this featureless continuum is not clear. Recent optical spectropolarimetry shows that only a minor part of the optical featureless continuum can be light from a hidden Seyfert 1 nuc…
The Hubble Space Telescope Quasar Absorption Line Key Project. V. Redshift Evolution of Lyman Limit Absorption in the Spectra of a Large Sample of Quasars
Turnshek, David A.; Schneider, Donald P.; Steidel, Charles C. +10 more
Using a sample of 119 QSOs, containing objects we have selected having previously available high quality ground-based and IUE spectral observations, together with Hubble Space Telescope (HST) observations of 26 QSOs from Bahcall et al. (1993, 1995) and Impey et al. (1995) and new optical observations of 41 objects by Steidel & Sargent (1995), …
The IUE MEGA Campaign: Wind Variability and Rotation in Early-Type Stars
Koenigsberger, G.; Howarth, I. D.; Owocki, S. P. +30 more
Wind variability in OB stars may be ubiquitous, and a connection between projected stellar rotation velocity and wind activity is well established. However, the origin of this connection is unknown. To probe the nature of the rotation connection, several of the attendees at the workshop on Instability and Variability of Hot-Star Winds drafted an I…
Multiwavelength Monitoring of the BL Lacertae Object PKS 2155-304. IV. Multiwavelength Analysis
Maraschi, L.; Tagliaferri, G.; Pian, E. +47 more
Simultaneous X-ray, ultraviolet, optical, infrared, and radio monitoring data were used to test and constrain models of continuum emission from the BL Lacertae object PKS 2155-304. Intensively sampled ultraviolet and soft X-ray light curves showed a clear temporal correlation, with the X-rays leading the ultraviolet by 2-3 hr. This lag was found t…