National Aeronautics and Space Administration

The EXOSAT Observatory

Mission Overview

The three instruments on board the EXOSAT observatory, (LE, ME, GSPC) were complementary and designed to give complete coverage over a wide energy band pass of 0.05-50 keV. The figure shows an exploded view of the EXOSAT satellite showing the scientific instruments and the principal spacecraft subsystems. The low energy imaging telescopes (0.05-2.5 keV) gave both medium quality spectral resolution using the gratings, or broad band filter spectroscopy. The field of view was about 2 degrees, with a peak effective area of 10 cm² ( reduced by a factor of ten when the gratings were utilized). The ME detector gave spectra with 20% resolution at 6 keV, with a total effective area of 1600 cm². Each half of the ME detector array could be offset to monitor the particle background. A gas scintillation proportional counter (GSPC) provided a factor of 2 improved spectral resolution in the 2-30 keV band, but with a factor of 10-20 less effective area than the ME. The optical axes of the four telescopes (LE1, LE2, ME and GSPC) were coaligned.

The telemetry rate was 8 kbps. An on board computer (OBC), programmable from the ground, preprocessed and compressed the data. The flexibility given by the OBC and the capability to load on board new OBC programs, enhanced the quality of the science obtained, by writing new data modes to address science discoveries not foreseen before launch. It also proved invaluable to work around problems with the instrumentation and the spacecraft.

The orbit of EXOSAT was quite different from that of any previous X-ray astronomy satellite. The initial apogee was 191,000 km and the perigee 350 km. The science instruments were operated above 50,000 km, outside the earth's radiation belts. This allowed scientific operations for up to 76 hr per 90 hr orbit, without interruption. EXOSAT was visible from the ground station at Villafranca in Spain for almost the entire time that the science instruments were operated and there was no need for any onboard data storage. The attitude was controlled, to within 1 arc second, using one of two star trackers (each attached to a low energy telescope), three gyros and a sun sensor.

The first 8 weeks after launch were dedicated to a performance verification phase. After this the regular guest observer program was undertaken. By the end of the operational phase four announcements of opportunity had been made with 98 %, 94 %, 64 % and 18 % of each program completed. During these 3 years scientists around the world used EXOSAT to study most classes of X-ray sources and made many new discoveries. Perhaps the most notable were the discovery of quasi-periodic oscillations (QPO) from low mass X-ray binaries, soft excesses from AGN, the red and blue shifted iron K line from SS433, many orbital periods from low mass X-ray binaries, and several new transient sources. The public availability of these data contributed to many notable systematic studies of individual classes and the archive continues to be heavily used and is still yielding new results.

The Low Energy Telescopes

The on-axis half energy width of the point spread function is 24 arc seconds, which degrades to 4 arc minutes 1 degree off-axis. Vignetting in the telescopes reduces the off-axis effective area to 45 % of its peak value 1 degree off-axis. In the focal plane of each telescope either a CMA, or a PSD detector was interchanged. Also a transmission grating spectrometer (TGS) could be inserted behind each telescope and the dispersed spectrum imaged by each CMA. The gratings were 500 lines mm^-1 in one telescope (LE2+CMA2 8-400Å) and 1000 line mm^-1 in the other (LE1+CMA1 8-200Å) with a spectral resolution of 2Å and 1Å respectively for energies > 0.25 keV, and 5Å at 304Å in both telescopes.

The CMA detector has no intrinsic energy resolution in the X-ray band, but a number of different filters gave coarse spectral information (analogous to UBV photometry). The choice of filter dictates the energy response with the overall energy window covered by the filter combinations ranging from 0.05 to 2.0 keV. The detection efficiency of the CMA 's decreases of 30% at 0.15 keV to 7 % at 1.5 keV. The combined telescope and CMA on-axis effective area is shown in figure as function energy for each filter. The CMA's are sensitive to ultraviolet photons and this caused contamination by bright O and B stars, pointed or serendipitous, in the field. Filters were used to determine the degree of contamination. The boron filter was free of UV contamination and relatively safe, as was the Al/P, except for very brightest and earliest stars. Also, the sum signal distribution of events in the CMA provided a crude method of differentiating between X-ray and UV sources. The most commonly used filters during the mission were 3000 Lexan , Al/P and boron.

The CMA particle background counting rate was typically 8 X 10^-6 count/s/pixels² (a pixel is 4 arcsec) in the central region. The background counting rate depends on the strength of the solar wind. In 90% of the observations the average background counting rate is within a factor of 2 of the quiescent value. The average source detection threshold for a 10⁴ second exposure (a typical minimum observation time) within the central 12 arc minute radius region of the detector, using the 3000 Lexan filter, was 2 X 10^-3 count/s. For observations longer than a few thousand seconds the sensitivity of the CMA is background limited. Source positions can be estimated to 6 arc seconds and 8 arcsec error radius at 67 % and 90% confidence levels, respectively within the central 12 arc min.

There were a number of notable problems with the instrumentation associated with the LE's. Both PSD's failed early on during the performance verification phase. One of the CMA (CMA2) failed on 1983 October 28. The mechanism to insert the grating behind the other telescope (LE1) failed in 1983 September 15, eliminating this spectroscopic capability. The surviving CMA functioned well until the end of the mission.

The Medium Energy Instrument

Each proportional counter had two gas chambers separated by a 1.5 mm beryllium window with argon in the top layer and xenon in the lower. Each proportional counter used a multi-wire design with a deltaE/E of 21(E/6 keV)^-0.5 % FWHM for the argon chambers and 18(E/22 keV)^-0.5 % FWHM for the xenon chambers. The argon and xenon spectra were pulse height analysed into 128 channels each, sensitive to 1-20 keV and 5-50 keV energy range respectively.

The ME background was very stable and dominated by particle events from the solar wind and radioactive lines caused by the decay of residual plutonium in the Beryllium windows and detector bodies. The background count rate per detector were in the argon chamber 2.4, 4.3 and 9.4 count/s in the energy range 1-6, 1-10 and 1-20 keV, respectively, and 40.6 and 59.1 count/s in the xenon chamber in the 10-30 and 10-50 keV energy range, respectively. Occasional background flares occur in both the aligned and offset halves caused by enhancements in the solar wind. A major solar storm typically happened every six months, causing the background to increase by several order of magnitude. The detectors were turned off during these events.

To optimize the background subtraction each half of the ME array detector could be offset from the source, pointing a source-free region of the sky to monitor the particle background. The offset half was alternated every few hours (known as 'array swap'). Since the background obtained from the offset half was slightly different for the half of the detector on source, Difference spectra were created to correct this effect. Background was also obtained using the slew on and/or off the source. This technique was used when the detector halves were coaligned and no array swaps made during the observation.

In 1985 August 20 one of the detectors in half-1 failed. Occasionally because of small detector breakdown and/or reduction in the gain, observations were carried out with one or more detectors off for a few hours.

An important component in the operation of the ME instrument was the usage of the OBC. Depending on the objective of the observation, the OBC programs traded time resolution against spectral information and telemetry load. Two or three ME programs were typically run simultaneously. The table gives an overview of most commonly used ME OBC mode.

Program	Output	Time resolution	No of PHA Channels	Comment
HER4	Spectral Intensity	10, 5, or 2.5s 150, 125 or 31ms	128+128	det, quad, or half summed over Ar+Xe
HER5	Spectral Intensity	1s 0.31s	selectable	only half summed over Ar
PULS	Spectral	fold over period	selectable	for fast pulsars
HTR3	Intensity	4,8 or 16ms	Ar and/or Xe	low OBC usage
HTR4	Intensity	0.25ms	1	selectable channel
HER6	Intensity	typically 4ms	1 or 2	selectable channel
HER7	Intensity	typically 4ms	1, 2 or 4	selectable channel

A typical ME observation was carried out with a primary spectral oriented program and a secondary high time resolution program. The OBC introduced a count rate dependent deadtime due to the limited data sampling.

The Gas Scintillation Proportional Counter

Three different electronic gain modes were used: gain 1 = 2-32 keV, gain 2 = 2-16 keV and gain 0.5 = 2-64 keV. The latter was only used on the very bright source Sco X-1. Variations in the intrinsic detector gain caused by temperature variations were removed using two background line features at 10.54 and 12.70 keV caused by fluorescence in the lead collimator and the radioactive decay of residual plutonium in the beryllium window. In addition a Xenon L feature at 4.78 keV could also be used for bright sources to calibrate the gain.

The particle background rejection used burst length discrimination, which rejects events which have exceptionally long or short duration. The background shape spectrum remains constant during the short timescale variations, but shows small changes on longer timescales. The standard OBC mode used in the GSPC gives 256 channel spectra every 8 seconds. Higher time resolution was used only for bright sources and if the telemetry needs of the other experiments was low. The GSPC worked perfectly for the entire mission.