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Analyzing short-term X-ray variability of Cygnus X-1 with Linear State Space Models
Pottschmidt, Katja; Wilms, Joern; Koenig, Michael +1 more
Cyg X-1 exhibits irregular X-ray variability on all measured timescales. The usually applied shot noise models describe the typical short-term behavior of this source by superposition of randomly occuring shots with a distribution of shot durations. We have reanalyzed EXOSAT ME observations of Cyg X-1\ using the more general Linear State Space Mod…
Recurrent Solar Modulation of the Galactic Cosmic Rays and the Anomalous Nuclear Component in Three Dimensions of the Heliosphere
Simpson, J. A.
Ulysses Mission investigations, extending from pole-to-pole of the Sun and inner heliosphere in the period 1993-1996, have led to discoveries that will change dramatically models to account for the physical phenomena underlying the 26-day modulation of galactic cosmic rays and anomalous nuclear components and their propagation modes. These new fin…
Numerical simulation of observations with GOLF on board SOHO
Roca Cortes, T.; Garcia, R. A.; Regulo, C.
The main objective of the GOLF Experiment (Global Oscillations at Low Frequencies) on-board the SOHO (Solar and Heliospheric Observatory) space mission is the quantitative knowledge of the internal structure of the Sun by measuring the spectrum of its global oscillations in a wide frequency range (30 nHz to 6 mHz). There is special interest in det…
A comparison of the fast timing behaviour of 4U 1705-44 to that of 4U 1608-52 and Cygnus X-1
van der Klis, M.; Berger, M.
We studied the fast timing behaviour of the atoll source \leavevmode4U 1705-44 using the entire \mboxEXOSAT dataset, four observations covering a total of 230 000 seconds of 1-20 keV spectral and timing data. In one of the observations, \leavevmode4U 1705-44 was in a low intensity ``island'' state and had an unusually hard spectrum. The fast timin…
Solar EUV and UV Emission Line Observations Above a Polar Coronal Hole
Wilhelm, K.; Bodmer, R.
The roll manoeuvre of SOHO on September 3, 1997 provided the opportunity to study the northern coronal hole with SUMER slits in east-west orientation, so that polar plumes and inter-plume lanes could be observed simultaneously. A preliminary analysis of the observations shows that lines emitted by ions with the lowest formation temperatures (with …
The Structure of Cometary Knots in the Helix Nebula
O'Dell, C. R.; Burkert, A.
The Hα surface brightness distribution of cometary knots in the Helix Nebula is investigated. The brightness profiles of their photoionized surfaces (cusps) drop exponentially with increasing distance r from the center of the knots: Σ(r) ~ exp (-r/h). An r-3 profile, which would be expected for a freely expanding ionization region, in g…
The acoustic cut-off frequency of roAp stars
Provost, J.; Morel, P.; Weiss, W. W. +2 more
Some of the rapidly oscillating (roAp) stars, have frequencies which are larger than the acoustic cut-off frequency determined from published stellar models which usually assume a grey atmosphere. As the cut-off frequency depends on the T(tau ) relation, we have computed models and adiabatic frequencies for pulsating Ap stars with more realistic a…
Unexpected stellar velocity distribution in the warped Galactic disk
Lattanzi, M. G.; Drimmel, R.; Smart, R. L. +1 more
It is now over 40 years since radio observations of neutral hydrogen revealed the gaseous disk of our Galaxy to be warped. Subsequently, the warp has been detected in the distribution of Galactic dust, molecular clouds, and luminous stars,. Roughly half of all spiral galaxies have similarly warped disks, which suggests that warps are a common and …
A Color-Magnitude Diagram for a Globular Cluster in the Giant Elliptical Galaxy NGC 5128
Harris, William E.; Harris, Gretchen L. H.; Poole, G. B.
The Hubble Space Telescope has been used to obtain WFPC2 V, I photometry for a large sample of stars in the outer halo of the giant elliptical NGC 5128 (d ~= 4 Mpc). The target region is located at a projected distance 21 kpc from the center of NGC 5128. The globular cluster N5128-C44, at the center of the Planetary Camera field, is well enough re…
A double-frequency dwarf nova oscillation in OY CAR
Horne, Keith; Marsh, T. R.
We have detected coherent oscillations (`dwarf nova oscillations') in Hubble Space Telescope spectra of the dwarf nova OY Car. The oscillations were seen towards the end of a superoutburst of OY Car. The oscillations are extraordinary compared with the many other examples in the literature for two reasons. First, their amplitude is large, with a p…