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 …
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…
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…
Soft X-ray transients in quiescence : observations of AQL X-1 and CenX-4.
van Paradijs, J.; Verbunt, F.; Arnaud, K. A. +1 more
The authors observed the soft X-ray transients Aql X-1 and Cen X-4 during quiescence with EXOSAT. For Aql X-1 they obtain a 3σ upper limit to the X-ray luminosity Lx (in the energy range between 0.5 and 4.5 keV) of <3.0×1033erg s-1, Cen X-4 was detected at Lx (0.5 - 4.5 keV) ≅ 1.5 to 4.2×1033
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…
X-ray observations of the Ophiuchus, PKS 0745-191 and Cygnus-A clusters of galaxies.
Fabian, A. C.; Mushotzky, R. F.; Nulsen, P. E. J. +4 more
The authors report observations of the X-ray emission from three bright clusters of galaxies at low Galactic latitudes; the Ophiuchus, PKS 0745-191 and Cygnus-A clusters. Temperatures and iron abundances are determined from EXOSAT spectral data. Images of the Ophiuchus cluster show it to have a high central density and a cooling flow. The authors …
Discovery of a strong soft X-ray excess in MKN 335 - evidence for an accretion disc ?
Turner, T. J.; King, A. R.; Pounds, K. A. +2 more
Exosat observations of Mkn 335 reveal a hard power-law spectrum above about 1 keV, typical for Seyfert 1 galaxies, but with unusually strong variability on time-scales of about 1-2 hr. In addition, an intense soft X-ray component is found to dominate the overall spectrum of Mkn 335 below about 0.6 keV. Both soft and hard components increased in st…
EXOSAT observations of X-rays from classical novae during the outburst stage.
Beuermann, K.; Krautter, J.; Oegelman, H.
Soft X-ray light curves of the classical novae Nova Muscae 1983 and Nova Vulpeculae 1984 No. 1 and No. 2 from optical maximum to about 900 days after maximum were sampled with the Exosat satellite, and the implications of the results for models of nova outbursts are considered. The observations seem best explained by the constant bolometric lumino…
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…