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A DESCRIPTION OF THE EXOSAT TIMING ANALYSIS INTERACTIVE SYSTEM


A short description is given of the facilities supported by the EXOSAT Timing Analysis Interactive System and of the improvements which will be implemented in the near future.


1. Rate Buffers and Light Curves

The EXOSAT Timing Analysis Interactive System consists of a set of programs scheduled from a control program (TINTE), which can be executed directly from the File Manager or from the LE, ME or GS interactive system. The Timing Analysis system uses as input files X-ray light curve points stored in the so-called rate buffers by the interactive analysis system of the various instruments. Rate buffers consist of a header containing the basic information concerning the instrument, the PHA channel, the integration time etc., and of a binned light curve whose points are described by: start time of the bin, exposure fraction (to correct for instrumental dead times), count rate and count rate error.


The Timing Analysis Interactive System is designed to operate on one or two rate buffers which partially overlap in time and refer to two different energy bands (or instruments). The latter is to permit the determination of hardness ratio, cross-correlations etc. which can require the use of two (simultaneous) time series.


2. Interaction

The interaction between the system and the user can be handled in 3 different ways which correspond, more or less, to various levels of expertise of the user. In particular:


  1. After issuing a command, all the following parameters are input by replying to the relevant questions.

  2. The command and all the following parameters are input to the system as a single string.

  3. The string(s) as in b. can be written into a macro file and run automatically via a special command.

At the current state of development the following commands are available:

--- Set up commands -------------------

SF set input files
SR set no. of recs
EL set min expos limits
EB set error bar option
BG set manual bkgd subtraction
GA set gap filling options
BU set params for burst search
SI set selection on intensity
SC set params for correlat. anal.
RD reset defaults
PU purge file(s) one by one
PA purge files automatically
MD map good data intervals
HA hardcopy alphanumeric screen
HG hardcopy graphic screen
ED edit a file
RB rebin the data
WI time window(s) handling


IN infos on set up
EX exit

- Execution commands -----------

ST run statistical analysis
VA run detailed RMS variab analysis
HD run hdns ratio vs intensity
PL write data in Pablo-like file
PC prepare correlation input files
PF run phase fitting
LH list rate buffer header
LR list rate buffer points
CC run correlation analysis
FC final correlation analysis
PD plot results or data
FG execute a FMGR command
VE write energy in RMS variab file
PT plot (or fold) rate buffer data
FT FFT power spectrum analysis
F2 FFT with trend rem. & selection
FD folding period search run macro MA
@@,MA run macro MA

??,XX infos on command XX
??,?? general infos

3. Objectives

The purposes of the Timing Analysis Interactive System are as follows:

a. 'Clean' the Data. A series of set-up commands is designed to remove all bad-quality light-curve points from the time series to be analysed. This data, which only marginally affecting the average count rate, can strongly affect the determination of the higher statistical moments. 'Cleaning' is carried out through a set of qualification criteria (such as time windows, intensity windows, exposure fraction windows etc.) which can be enabled and disabled.

b. Remove Long-Term Trends. Trend removal techniques are often needed to exclude the effects of long-term variability in the evaluation of the Correlation Functions and the Power Spectrum (PS). A facility is provided to remove trends by means of a polynomial fit to the data.


c . Bridge Data Gaps. Several timing analysis techniques (e.g. Correlation Functions, PS) require the use of uninterrupted time series. Bad data gaps and/or telemetry dropouts can be filled with simulated data obtained by the use of several different techniques.


d. Test the Hypothesis of Source Constancy. This is done by the use of the 2 and the RMS variability tests and by the determination of the source statistical moments.


e. Search for Flares/Bursts. A search for flares and/or bursts can be performed interactively by specifying the significance level above which an event is revealed. It is also possible to carry out a millisecond burst search by using the local average technique discussed by Rothschild et al. 1974 (Ap.J.Lett., 189, L13) in connection with the very short-term activity of Cyg X-1.


f. Compute Correlation Functions. Partially unbiasd Auto Correlation (ACF) and Time Skewness (TSF) Functions can be calculated for a time series of any length shorter than 5000 bins. When analysing two simultaneous time series from different instruments and/or energy bands, a partially unbiased Cross Correlation Function (CCF) can also be calculated (for details see Stella et al. 1984, Ap.J., 282, 713). The results from different time intervals (5000 bins) can be averaged to produce an average ACF, CCF and TSF. To allow for maximum flexibility the programs which compute the correlation functions do not use Fast Fourier Transform algorithms. Correlation Functions are widely used to characterise the aperiodic source flickering and to investigate the amount of correlation and any possible time lag between the source variations in two different energy hands.


g. Search for Coherent Pulsations. This can be done using 3 different approaches: (1) the Power Spectrum Technique (which uses an FFT algorithm on data intervals long up to 4096 bins), (2) The Folding Analysis Technique (which can use an unlimited number of data bins to search for coherent pulsations over up to 75 trial periods), (3) the Phase Fitting Technique (which is normally used to determine the pulsation period as accurately as possible). The Power Spectrum Technique is rapid and straightforward to search for pulsations over a wide range of frequencies and is the most commonly used technique. The Folding analysis allows a more sensitive search for coherent pulsations when dealing with non-continuous or unevenly spaced data (cf. Leahy et al. 1983, Ap.J., 266, 160), but is highly CPU consuming and is usually applied over a limited number of trial periods.

h. Characterise the RMS Variability as a Function of Energy. This technique is often used to establish whether the source is more variable at high or low energies.

j. Investigate the Hardness Ratio as a Function of Time and/or Source Intensity. This simple investigation of the source spectral changes has proven to be very powerful in establishing the presence of absorption dips, intensity correlated spectral changes etc.

4. Recent Developments

Following the interest in the recent discovery of fast (1-50 Hz) Quasi- Periodic Oscillations (QPO) in the flux of several bright galactic X-ray sources, a dedicated software package has been developed as part of the EXOSAT Timing Interactive System to search for QPO and investigate their properties. A new type of compact rate buffer, the so-called Millisecond-Type Rate Buffers, has been defined and implemented in order to store onto disk a very high number of light curve bins (currently up to 2-4 X 107, depending on disk, space availability). These rate buffers are accessed by a dedicated Power Spectrum Program which uses an FFT algorithm and allows storage of <256 colour-coded power spectra together with the source light curve into an 'PS image'. This type of display has proven to be a very powerful means of investigating QPO. The 'PS images' can be analysed and the power spectra fitted to various models using a dedicated program which allows a fully interactive selection of the power spectra in order to investigate the possible dependence of the QPO parameters on source intensity, activity state etc.


5. Future Developments

A major rewrite of the EXOSAT Timing Analysis Interactive System will be made during 1986 in order to:


  1. make the software more complete and flexible (in particular the limitations on the maximum number of points which can be analysed at once, will be made less stringent).


  2. adapt the package to Vax machines.


  3. make the system more user-friendly, by standardising the interaction with the various programs.


L. Stella

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