Timing Analysis

Light Curve Extraction

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.

Simple Time filtering

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

Barycentric correction

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 bct (Barycenter Correction Table: corrects the times in the orbit table at the barycenter) and abc (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.

bct

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:

• the orbit file, corresponding to the observation for which the correction is needed. Both format old style corresponding to REV0 and the RDF style file are supported. The orbit data can be found in a file containing the extension SPORB, for US Rev0 this is the file with extension .so, i.e. file number 14); in the RDF case the data are in the _anc.fits file i.e. that containing the extension EPHEM (_anc.fits; in the old style German case the file required is _orbit.tfits.

• the equatorial coordinates of the source given either as hh:mm:ss.s, dd:mm:ss.s or in degrees (epoch 2000.0).

• output filename, this will contain the uncorrected and the corrected orbit time. The output FITS file consists of 4 columns, containing the integer and the fractional part of the uncorrected and corrected times. The times are written as JD. The output file is used by abc.

This program makes use of the JPL DE200 ephemeris given as a FITS binary table (Standish,M.,1982 Astr. Ap.,114,297).

abc

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 bct). If a photon falls outside of the orbit interval a warning is issued together with the photon time. This task takes as input:

• the event list data file (in FITS), corresponding to the observation for which the correction is needed. It is recommended to input the event data file obtained after having applied a region selection, which contains only the photons from the source for which the correction is required. Both format old style produced by REV0 processing and the RDF are supported. Old style German evenets files must be converted to RDF first, using gevt2rdf.

• input correction table: table containing the corrected and uncorrected orbit times (in FITS) created by bct. It consists of 4 columns, containing the integer and the fractional part of the uncorrected and corrected times, written as JD.

• the equatorial coordinates of the source (epoch 2000.0) given either as hh:mm:ss.s, dd:mm:ss.s or in degrees. The coordinates are not used for computation but they are requested in order to test the compatability with the input correction table.

• output file: event output file (in FITS) with the arrival times of the photon corrected at the barycenter.

  The correction of the times to UTC is computed with the following formula :

  
  

  $$UTC = JDref + (Aj\times (SCC-SCCo)^{j-1})/86400.0$$
  
  

  where the $Aj$ 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 abc.

rosbary

This script runs bct and 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

rosbary (and bct and abc) 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.