As for images, light curves can be extracted for the whole field, or filtered through a region descriptor and /or additional housekeeping filters and phalcut and phahcut specifications. The time bin that will be used is "binsize", the current default can be seen in the "show status" summary and it can be reset using e.g. "set binsize 1000" for 1000 second bins. You can specify a minimum limit for partial bins, as a fraction of "binsize". For example
xselect> extract curve exposure=0.2
will include partial bins of 200 seconds exposure or greater, while binsize is set at 1000 seconds.
xselect> set device /xw (/xterm will also work)
xselect> plot curve
will plot the temporary light curve file. The x-axis is seconds since the beginning of the observation and the start time in spacecraft clock seconds is written at the top of the plot.
You will be left with the ``PLT'' prompt. At this point you can use standard PLT commands, ( i.e. those used in the QDP plotting software- which some users may be familiar with). Useful commands include
PLT> r x 1.234e5 2.468e5
PLT> r y -0.1 5.
which reset the x and y axes to zoom into some interesting part of the lightcurve.
In some cases the user will wish to define a region of interest and extract the events from that region without binning the data. The extracted and saved mini-events file has the highest time resolution possible for those data, and can be read by XRONOS. This is the preferred method when doing complicated timing analysis since it avoids the danger of introducing spurious binning effects.
To define time intervals for inclusion/exclusion having already extracted the light curve
xselect> filter time cursor
plots the light curve in cursor time selection mode; the user will see
Type: - click to include a time region c - to cancel cursor selection e - makes the selection excluded l - puts cursor on the left r - puts cursor on the right x - to exit cursor selection p - to return to PLT prompt - click outside the plot to cancel a selection Enter QUIT at PLT prompt to continue
and after exiting from PLT a file of the time selections will be already added to the filters (type 'show status' to see that this is so).
To enter times by hand use 'filter time' MJD,SCC,UT which allow you to enter timing filters by HAND in MJD, Spacecraft Clock time, or UT. To use times from a (previously saved) file 'filter time' FILE. The associated 'save' commands are now grouped into 'save time cursor', and 'save time hand'. The associated clear commands are grouped under 'clear time'. Once you have saved (say) cursor times to a file, you can use those time selections again in the current or in future sessions by the command 'filter time file' thus
xselect> filter time file gti1.xsl
will apply the times saved in the file gti1.xsl, while
xselect> clear time file gti1.xsl
will clear that filter.
You can check your work by creating a lightcurve using just the selected time intervals :
xselect> extract curve
xselect> plot curve
For detailed timing analysis you will wish to apply a barycentric correction to events of interest.
The correction programs work directly on FITS files and are called are
(Barycenter Correction Table: corrects the
times in the orbit table at the barycenter) and
(Apply Baycentric Correction: applies the correction to
the arrival time of the photons in the events list at the barycenter).
Invoking a script,
rosbary, the two tasks can be run in sequence.
The most likely application is that users will extract a mini-events file for the photons of interest and then correct this before entering XRONOS.
This task computes the barycentric correction on the times contained in the
orbit file. The times in the orbit file are sampled every 60 seconds.
This task takes as input:
This program makes use of the JPL DE200 ephemeris given as a FITS binary table (Standish,M.,1982 Astr. Ap.,114,297).
This task corrects the arrival times of the photons, in the EVENT extension
and the times in the GTI extension, for the SSC-UTC relation and uses the
correction table created from
bct to correct the arrival time
at the barycenter. The Barycentric correction is computed by a linear
interpolation between the corrected orbit times (output from
If a photon falls outside of the orbit interval a warning is issued
together with the photon time.
This task takes as input:
bct. It consists of 4 columns, containing the integer and the fractional part of the uncorrected and corrected times, written as JD.
The correction of the times to UTC is computed with the following formula :
where the coefficients are stored together with the time interval validity in the ssc_to_utc2.fits file (available from the ROSAT caldb area) and spacecraft clock (SCC) photon arrival times. Each row in the ssc_to_utc2.fits file contains the JDref, SSCo and Aj values together with the start and end time for which the listed value are applicable. The start and end time for the interval are not corrected to UTC.
bct needs to be run before
This script runs
abc in sequence
The input paramters are:
orbit filename (FITS ext SPORB or EPHEM) (input file)
output correction table filename
RA hh mm ss.s or degrees in 2000 equinox
DEC dd mm ss.s or degrees in 2000 equinox
event filename (FITS EXTNAME EVENTS or STDEVT) (input file)
output corrected event filename
will work on the Rev0 US files, the RDF format files
and will handle the German orbit files.
However, users need to convert the German events files to RDF format
before running these tasks.
For detailed timing analysis the time series or events file can be read in
xronos, which is brielfy described in Chapter 6.