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Recent discoveries made with the EXOSAT medium energy experiment have highlighted the need to improve the on-board high time resolution application software. The Observatory has therefore implemented a number of new on-board computer programs (modes - eg. MHER7, MHTR5, MHTR4) to exploit to the full the capabilities of the system. Naturally, with several ME experiment modes designed for quite specific applications, the selection of a particular combination of modes which would maximise the scientific return from an observation is not always straightforward. To assist the user, a few standard OBC configurations suitable for observing a wide range of classes of X-ray object have therefore been defined and are outlined in this note. Note that the two new OBC modes, MHTR4 and MHTR5, are described on p.34.

1a) Faint ME Source (a)

This is the standard EXOSAT configuration for observations of weak ME sources, if high time resolution is not important. The ME array halves will normally be swapped every 3 hr or so using the standard array swap strategy (eg. HI+, H2-, Hl-, H2+ etc) to give the best estimate of the background counting rate. Unexpected brightening of the source or solar activity should not cause CPU or telemetry overload problems.

OBC Program                  Comments                         % Telementry

  MHER4             det.ID, 256 channel spectra
                    every 10s. Intensity samples                      27
                    (Ar+Xe) every 0.25s.
  MHTR5             4 msec intensity samples summed                   15
                    over all Ar detectors.
  GHBL4             256 channel spectrum every 8s.                     5
  LDIR2             Direct mode                                      ~25

1b) Faint ME Source (b)

This configuration substitutes MHER6 for MHTR5 and provides intensity samples over a selected range of energy channels with a time resolution of 9 msec. If a limited energy range of 2-6 keV is chosen, the background for an 'offset mode' observation in the MHER6 data is about a factor of 10 lower than that in MHTR5. This mode could be used, for example, to search for low level pulsed emission with periods >50 msec from a supernova remnant. The telemetry usage, ~84%, is slightly higher than for Faint Source (a). If higher time resolution is required, it is possible to reduce the telemetry requirements of the LE by using a diamond filter and hence increase the MHER6 resolution to 6 msec. Note that MHER6 has an option to accumulate intensity samples over two selected ranges of energy channels.

A major disadvantage of MHER6 is that CPU overload can occur if the source count rate increases to ~500 cts/sec. This results in a halt of the CPU processor and a potentially dangerous spacecraft operational state since monitoring of 'safety mode' conditions is continuously carried out by the 0BC, notwithstanding the loss of about 20 minutes of data while programs are re-initiated. This mode will therefore not be used if there is the slightest chance of bright flaring or bursting behaviour.

2. Intermediate Source Strength Configuration

This configuration is designed to provide high time. resolution spectra during an X-ray burst. Even during the most intense flares or bursts, the CPU usage will remain comfortably below the level at which the OBC would halt. It is also suited to observations of all sources of medium intensity such as long period pulsars (Vela X-1), dipping sources (X1755-338) etc. The LE is used with a diamond filter in order to reduce the telemetry load. Depending on whether the observer is primarily interested in the continuum or burst properties, the ME configuration can be offset, coaligned or a combination of both. A full LE image can be acquired if MHTR5 data is collected with a time resolution of 4 msec. Note that changing the time resolution of the MHTR5 data would result in a loss of about 20 minutes OBC data (QE rates are still available from the housekeeping telemetry).

OBC Program              Comments                           % Telemetry 

  MHER5       64channel Ar and Xe spectra
              every 1 and 2s respectively and                    31
              intensity samples (Ar only) every
              31 msec per half experiment.
  MHTR5       Ar intensity samples summed over                   30
              all detectors every 2 msec.
  GHBL4       256 channel spectra every 2s                       17
  LDIR2       Diamond filter                                    ~10
                                                              = ~88%

3. High Time Resolution Configuration

This configuration is designed to provide the highest possible time resolution continuous data (0.25 msec) over a selected range of energy channels, together with high time resolution ME spectra and GSPC spectra at the normal time resolution. The ME would normally be coaligned although it is sometimes worth spending 1 hr with each array half offset to obtain good background estimates. This mode should be considered the standing 'QPO hunting' configuration. Although certain circumstances may allow it, the LE is normally not operated because of CPU limitations.

OBC Program             Comments                             % Telemetry
  MHTR4       Intensity samples over the                          58
              whole instrument at 0.25 msec
              for a selected energy band.
  MHER5 64    channel spectra for Ar and                          31
              Xe every 1 and 2s and msec Ar
              intensity samples per half experiment.
  GHBL4       256 channel spectra every 8s.                        5

4. Energy Dependent High Time Resolution Configuration

This configuration is designed to give high time resolution data in four energy bands. It can be used, for example, to determine the energy dependence of any QPO's detected with the High Time Resolution configuration. Normally the ME should be coaligned, although an hour or so in array offset mode (each half) is recommended if spectra are also of prime importance. The maximum incident count rate (source + background) which can be accommodated with both MHER5 and MHER7 active is ~1300 cts/s. 'Since the Xe background is ~600 cts/s, this represents an incident source count rate of ~700 cts/s (or~200 UFU; coaligned). For sources brighter than this, only the Ar data should be routed to the OBC, resulting in a lower deadtime and no Xe data in the HER5 packets. Incident source count rates of up to ~1200 cts/s can be accommodated in this way. If required, MHER7 can be configured to give intensity samples over 2 selectable energy ranges every 2 msec.

Alternatively, MHER2 may be substituted for 14HER5. HHER2 will provide 128 channel Argon and Xenon spectra every Is integrated over the whole instrument. The disadvantage is that if all array is offset to provide background information, MHER2 be stopped and MHER5 started resulting in the loss of ~5 minutes of ME data. Note that MHER2 does not provide intensity samples. MHER2 requires significantly less CPU than MHER5 and can probably be used for all allowable incident count rates.

OBC Program                      Comments                    % Telemetry

MHER7 Intensity samples over 4 selected 60
energy bands every 4 msec.
MHER5 64 channel Ar and Xe spectra every 31
1 and 2s respectively and Ar intensity
samples every 31 msec per half experiment
GHBL4 256 channel spectra every 8s. 5
= 96%
5. Extremely Bright Source Energy Dependent Configuration

This configuration has been designed for observations of extremely bright sources such as Sco X- 1, GX5-1 to provide energy-dependent high time resolution data with a minimum deadtime from 031, sampling losses. To ensure tile safe operation of the detectors, tile ME should be coaligned or slightly offset to provide an incident count rate in each Ar detector of not more than 500 cts/s. Only Ar data should be routed to the OBC to minimise tile deadtime. Xe data is provided by MHTR5. Spectral data is provided by the GSPC only if required MHER7 can be configured to provide intensity samples over 2 selectable energy ranges every 2s.

OBC Program            Comments                       % Telemetry

MHER7 Intensity samples summed over 60
4 selectable Ar energy channel
ranges every 4 msec.
MHTR5 Intensity samples summed over all 30
Xe detectors only every 2 msec.
GHBL4 256 channel spectra every 4s. 9

6. Pulsar Configuration

For sources with known periods <10s it is often advisable to use either MPULS or MPULS2 OBC programs in order to minimise the telemetry load and provide high quality phase resolved spectra. MPULS should be used with sources brighter than ~50 cts/s and MPULS2 with fainter sources. The telemetry usage depends on the pulsar period and the number of times the data is folded over this period prior to telemetry.

Please contact the Observatory Team member planning your observation for more details.

A. Parmar

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