EXOSAT: TWO YEARS OF ACHIEVEMENT
With the second anniversary of the EXOSAT launch it was felt timely to review the current mission status and mention some of the scientific highlights from the wealth of data obtained over the last two years. Since launch, about 2000 observations have been performed by the Observatory covering the complete range of astrophysical objects. These observations' have come from the AD 1, 2 and 3 observing programmes. The final AO-4 program which will be selected in February 1986 will run to the end of the mission.
The AO-1 program, while providing some good scientific results, suffered from its early selection 1 year prior to launch and did not in general utilise the actual strength of EXOSAT. The program was based on pre-launch sensitivities and a payload complement which differed somewhat from that finally commissioned. Given the limitations of this AO-1 program, the Observatory has produced some excellent science. The single most important mission strength is the long uninterrupted look afforded by the deep orbit, which coupled to the real time control and data processing capability at the Observatory Centre has led to the maximisation of the scientific results from observations made with a well-balanced payload. This is particularly true in the study of classical X-ray binary sources.
One of the first examples of the scientific return that could be achieved with this orbit and near real time data processing was the observation of the transient source V0332+53 by Stella et al. (1984). After notification of a transient, the source was precisely located by the Observatory and an optical counterpart identified. Near real time data analysis revealed rapid Cyg X-1 like variability and stable 4.37s pulsations. Doppler variations of these pulsations indicated an eccentric 34.3 day binary orbit with X-ray outbursts occurring at periastron passage. This predicted outburst period was confirmed when EXOSAT later reobserved the source. Another example was the discovery by Parmar et al. (1984) of an anomalous extended low state from Her X-1 in the summer of 1983. Results by Trumper et-al. (1985) from further Her X-1 observations when it had returned to its normal 35 day behavior suggest the 35 day cycle is not caused by the precession of an acrretion disk, but rather by the precession of the neutron star itself. A long uninterrupted observation of Her X-1, through one complete orbital cycle of 2 days is unprecedented in the history of X-ray astronomy and has paved the way for many more long exposures on galactic binary X- ray sources. Indeed, it is a sobering thought that, post EXOSAT, future missions currently planned will not have this long look capability.In the field of low mass X-ray binaries EXOSAT has had a major impact. These systems, with binary orbits of typically a few hours, are difficult to observe with low earth orbiting satellites which suffer data losses due to earth occultations, Atlantic anomalies etc. A continuous exposure of 9-12 hours is quite typical for EXOSAT. This, coupled with the high sensitivity of the ME detector array, has led to the determination of many binary periods eg. 2SI254-68 (Courvoisier et al. 1984), 4U1755-33 (White et al. 1984).
The ME has also been very successful in discovering many new transient and bursting X-ray sources such as EXO 0748-676 (Parmar et al. 1985), EXO 2030+37 (Parmar et al. 1985).
The study of accretion-powered binary systems has taken a major step forward with the discovery by Van der KIis et al. 1985 of quasi-periodic oscillations (QPO) in the galactic bulge source GX5-1. This discovery has been followed by similar results from Sco X-1 (Middleditch et al. 1985, Van der Klis et al. 1985) and Cygnus X-2 (Hasinger et al. 1985) and we may finally be getting to grips with the nature of these sources. The detection of QPO in Sco X-1 is particularly satisfying since observations of this source essentially founded the subject of Cosmic X-ray astronomy some 20 years ago. A systematic study of the well known bulge sources in the galactic centre region will be carried out to search for similar behaviour. New OBC modes have been written and the observatory mission planning tuned to take maximum advantage of the forthcoming observing window.
The GSPC developed by the European Space Agency and flown successfully on the EXOSAT and TENMA satellites has complemented the sensitive ME array. In the study of bright galactic sources, the ME provides data on the temporal characteristics whilst detailed broad band spectra have been obtained from the GSPC. Some notable results include the discovery of broadened iron emission lines from low mass X-ray binaries by White et al., (1985a))Sco X-1 (White et al. 1985b) and the black hole candidate Cyg X-1 (Barr et al. 1985).
Another area of science reaping benefits from the EXOSAT mission is that of the
study of cataclysmic variables, in particular those containing magnetic white
dwarfs. Here the LE and in some cases that ME instruments provide the bulk of the
results. A systematic study of the light curves of the AM Her binaries (again
made possible for the first time by the uninterrupted coverage) revealed
repeatable features from source to source that has led to new ideas regarding the
geometry of the accretion flow (King et al. 1985, Mason 1984). The discovery of
350 second pulsations from the old nova GK Per by Watson et al. 1984 and the
detection of a 12.4 minute period in the intermediate polar V1223 Sgr by Osborne
et al. 1985, also illustrate the sensitivity to periods of the order of tens of
This type of data in conjunction with spectral information and often simultaneous
coverage at optical and UV wavelengths will provide information on the physics of
the accreting material onto the white dwarf.
It is in this area of science also that the flexibility of the mission has been so well demonstrated, responding with notable success to optical outburst alerts from such organizations as the AAVSO. Some good examples are the observations of SS Cygni in outburst and quiescence by Watson et al. (1985) and VW Hydri during super outburst by Van der Woerd et al. (1984). The latter observation revealed a 14 second coherent period probably associated with the rotation period of the white dwarf.
The other area of science in which considerable effort and observing time has been invested is that of active galactic nuclei. These studies have involved both the long and short term variability of AGN's with particular emphasis on the exposures being performed over as wide a range of the electromagnetic spectrum as possible. In particular UV quasi-simultaneous coverage with IUE has become common place. Only a few examples of short term variability have to date been observed. This is a particularly difficult area of science to address but a notable success is the observation of quasi periodic X-ray variations on timescales of 1 hour from NGC 4051 by the Leicester group. Another important result was the discovery by Barr et al. (1985) that the flux from the nearby emission line galaxy M81 had increased by a factor of 5 compared to measurements made 5 years previously and that during the EXOSAT exposure the flux varied by up to 50% on a timescale of less than an hour. Certain improvements in observing strategy may help to improve the chance of detecting short term variability in these types of objects in future exposures. As EXOSAT moves into its third year the results from the extensive long term monitoring of these AGN's will start to appear in the literature.
At the two year point in the mission, the EXOSAT Observatory is providing high quality scientific results with increasing regularity., These observational results and the associated theoretical interpretation will keep the scientific community engaged for many years to come. Certainly future missions will have to build on these original EXOSAT results. The experience. gained by the Agency in building, flying and operating the EXOSAT Observatory will be utilized in its next major project in High Energy Astrophysics - the high throughput X-ray spectroscopy mission XMM - a cornerstone of the Agency's scientific programme.
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A. Parmar et al1985. IAU 4039.
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G. Hasinger et al. 1985. IAU 4070.
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