Search Publications
The Discovery of a 685 second Orbital Period from the X-Ray Source 4U 1820-30 in the Globular Cluster NGC 6624
Stella, L.; Priedhorsky, W.; White, N. E.
The discovery of a coherent 685 s periodicity in the X-ray source 4U 1820 -30, which is located in the globular cluster NGC 6624, is reported. The modulation has a peak-to-peak amplitude of up to 3 percent and was independently detected in three observations by the EXOSAT Observatory. This period is unlikely to represent the rotation of a neutron …
Comet P/Halley's nucleus and its activity
Keller, H. U.; Reitsema, H. J.; Delamere, W. A. +2 more
The Halley Multicolour Camera on board ESA's Giotto spacecraft observed the nucleus of comet P/Halley and its environment and returned more than 2000 images. The observations are summarized, their calibration is described, the status of the analysis and the results are discussed. Topographic features on the nuclear surface and areas of activity ar…
To CO and N2 Abundance in Comet p/ Halley
Berthelier, J. J.; Eberhardt, P.; Krankowsky, D. +8 more
A preliminary evaluation of the mass 28 amu/e signal observed in the neutral mode of the Giotto neutral gas mass spectrometer (NMS) is presented. At 1000 km from the nucleus the authors obtain for the CO number density n(CO)/n(H2O) ≤ 0.07. The production rate of CO as parent molecule directly from the nucleus is thus less than 7% of the…
The dust distribution within the inner coma of comet P/Halley 1982i - Encounter by Giotto's impact detectors
Zarnecki, J. C.; Wallis, M. K.; Sekanina, Z. +12 more
Analysis of the data from Giotto's Dust Impact Detection System experiment (DIDSY) is presented. These data represent measurement of the size of dust grains incident on the Giotto dust shield along its trajectory through the coma of comet P/Halley on 1986 March 13/14. First detection occurred at some 287,000 km distance from the nucleus on the inb…
Low-frequency divergent X-ray variability in the Seyfert galaxy NGC4051
Elvis, M.; Watson, M. G.; Lawrence, A. +1 more
The X-ray emission from NGC4051, a nearby low luminosity Type 1 Seyfert galaxy, exhibits variations that are both rapid and apparently quasi-periodic1,2. Many Seyfert galaxies are variable3 but our knowledge of the form of variability has not advanced substantially since early well-resolved observations4 and attemp…
Chromospheric-Coronal Activity at Saturated Levels
Walter, Frederick M.; Vilhu, Osmi
High-resolution IUE observations of Mg II emission lines in W Ursae Majoris were obtained. In addition, estimates of the Mg II emission for other F-type contact binaries were obtained using archival high-resolution k line spectra and some nonstandard methods; the available data were compiled for Mg II, C IV, and X-ray emission in main-sequence F, …
Fractal X-ray time variability and spectral invariance of the Seyfert galaxy NGC5506
McHardy, Ian; Czerny, Bozena
Rapid X-ray variability has long been sought after in active galactic nuclei (AGN) as an indicator of such parameters as the mass-to-energy conversion efficiency and black hole mass. Although some interesting variability information has been gained from early observations (for example see ref. 1 for a review) the faintness of AGN, combined with th…
Ultraviolet observations of cataclysmic variables : the IUE archive.
Verbunt, F.
A comparative study of the ultraviolet properties of cataclysmic variables is made on the basis of the IUE data archive. The reddening to 51 systems is determined from the 2200 Å feature. The spectral flux distribution of cataclysmic variables does not depend on system type, orbital period or (for dwarf novae) average length of interval between ou…
Electron impact ionization in the vicinity of comets
Gombosi, T. I.; Cravens, T. E.; Kozyra, J. U. +2 more
The solar wind interacts very strongly with the extensive cometary coma, and the various interaction processes are initiated by the ionization of cometary neutrals. The main ionization mechanism far outside the cometary bow shock is photoionization by solar extreme ultraviolet radiation. Electron distributions measured in the vicinity of comets Ha…
Highly Ionized Interstellar Gas Located in the Galactic Disk and Halo
Savage, Blair D.; Massa, Derck
High-resolution IUE absorption line spectra have been obtained for 40 distant stars in order to study the distribution of interstellar H I, Si IV, C IV, and N V in the Galactic disk and lower halo. Respective midplane densities of 2 x 10 to the -9th, 7 x 10 to the -9th, and 3 x 10 to the -9th are found for Si IV, C IV, and Ni V. Both column densit…