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
minutes.
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.
A. Peacock
References
L. Stella et al. 1985. Ap.J., 288, L45.
A. Parmar et al. 1985. Nature, 313,
119.
J. Trumper et al. 1985. Talk presented at the Bamberg Meeting on
CV's
T. Courvoisier etal . 1985. XXI I Eslab Symposium
N. White et al.1984.
Ap.J., 283, L9.
A. Parmar et al1985. IAU 4039.
A. Parmar et al 1985. IAU
4066.
M. Van der Klis et al. 1985. Nature in press.
J. Middledietch et
al. 1985. IAU 4060.
M. Van der Klis et al. 1985. IAU 4068.
G. Hasinger et
al. 1985. IAU 4070.
N. White et al.1985a. MNRAS submitted.
N. White et
al.1985b. Ap.J. in press.
P. Barr et al. 1984. XXII Eslab Symposium.
M.
Watson et al. 1984. XXII Eslab Symposium.
J. Osborne et al. 1984. XXII Eslab
Symposium.
M. Watson et al. 1984. XXII Eslab Symposium.
H. Van der Woerd
et al. 1984. Bologna Symposium.
A. Lawrence et al. 1985. IAU 4054.
P.
Barr et al. 1985 IAU 4044.
A. King et al. 1985. MNRAS, 215, IP.
K. Mason,
1985. Review talk at the XII Eslab Symposium.
[EXOSAT Home]
[About EXOSAT]
[Archive]
[Software]
[Gallery]
[Publications]
Page authors: Lorella Angelini Jesse Allen