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 …
On the Theory of Type I X-Ray Bursts. The Energetics of Bursts and the Nuclear Fuel Reservoir in the Envelope
Lewin, Walter H. G.; Fujimoto, Masayuki Y.; Sztajno, Mirek +1 more
A comparison is made between the observed properties of type I X-ray bursts from 4U/MXB 1636-53 and those of models of thermonuclear flashes on accreting neutron stars. Possible explanations of variations in the burst recurrence properties without an apparent correlation with accretion rate are discussed. The strongest bursts with photospheric exp…
Intensity and Source State Dependence of the Quasi-periodic Oscillations in Scorpius X-1
Stella, L.; Parmar, A. N.; van der Klis, M. +2 more
Quasi-periodic oscillations (QPO) at 6 Hz were discovered from EXOSAT observations of Sco X-1 in its quiescent state by Middleditch and Priedhorsky. The authors report here on the properties of the QPO seen in earlier EXOSAT observations of Sco X-1. During these observations QPO were also detected when Sco X-1 was in an active state, in ≡20 minute…
EXOSAT Observations of Double-peaked Bursts with Radius Expansion from 4U/MXB 1820-30
Haberl, F.; Stella, L.; White, N. E. +2 more
A detailed analysis is presented of seven X-ray bursts from the X-ray source 4U/MXB 1820-30 in the globular cluster NGC 6624. The bursts were double-peaked, and recurred at almost equal time intervals of 3.2 hr. The persistent flux varied by only 10 percent on time scales of 60 minutes. The ratio of the total persistent energy emitted before a bur…
CAL 83: A Puzzling X-Ray Source in the Large Magellanic Cloud
Thompson, I. B.; Crampton, David; Hutchings, J. B. +3 more
Spectroscopic observations of the X-ray point source no. 83 in the Columbia Astrophysics Laboratory Einstein survey of the Large Magellanic Cloud have been accumulated over 4 yr. The optical spectrum shows no stellar absorption features, but only emission lines typical of an accretion disk. Radial velocity measurements reveal a small (K = 30 km/s)…
EXOSAT Observations of 4U 1705-44: Type I Bursts and Persistent Emission
Hasinger, G.; Gottwald, M.; Sztajno, M. +2 more
During four Exosat observations, the bright galactic X-ray source 4U 1705-44 exhibited persistent emission variations from 1.3 to 10.7 x 10 to the -9th ergs/sq cm per sec in the 1-20 keV band. Type I X-ray bursts have been detected from this source whose properties correlate with source intensity. The burst shape changed with increasing intensity …
Correlation between Spectral State and Quasi-periodic Oscillation Parameters in GX 5-1
van Paradijs, J.; van der Klis, M.; Lewin, W. H. G. +3 more
In a series of seven Exosat observations, the bimodal spectral behavior and the quasi-periodic oscillation (QPO)/red noise properties of GX 5-1 show a strict correlation. In one of the two spectral states (characterized by a 'horizontal branch' in the hardness-intensity diagram), strong 20-40 Hz QPO and red noise below about 60 Hz were always pres…
EXOSAT Observations of 4U/MXB 1636-53: On the Relation between the Amount of Accreted Fuel and the Strength of an X-Ray Burst
van Paradijs, J.; van der Klis, M.; Lewin, W. H. G. +4 more
During EXOSAT observations of 4U/MXB 1636-53 in 1985 August, three bursts were observed in 6 hr and 24 bursts in 79 hr of uninterrupted observing. The persistent X-ray flux varied by about a factor of 2.4, the burst intervals by a factor of 24 (from 35 minutes to 14 hr), and the integrated burst fluxes (the burst fluences), and burst peak fluxes b…
Deterministic Chaos in Accreting Systems: Analysis of the X-Ray Variability of Herculis X-1
Voges, W.; Atmanspacher, H.; Scheingraber, H.
The irregular X-ray variability of Her X-1 has been investigated in different phases of the 1.7 day orbital period of the binary system Her X-1/HZ Her. The analyzed data have been sampled by the Exosat satellite during two main-on states of the 35 day cycle of the system. Using new system theoretical methods, the attractors of different processes …
The Low-Luminosity X-Ray Sources in Omega Centauri
Bolte, Michael; Margon, Bruce
The authors report multicolor CCD photometry of the fields of three X-ray sources in ω Centauri; two have very accurate (10arcsec) EXOSAT X-ray positions, in addition to observations from the Einstein Observatory. The crowding in these X-ray fields is moderate and does not preclude accurate photometry of individual objects. Although the data in th…