NOTE: lightcurve has been deprecated as of CIAO 3.0; users are
encouraged to migrate to dmextract ("ahelp dmextract").
Development on this tool has stopped, but any new bugs found
will be reported on the CIAO bugs page.
`lightcurve' will first get the good time intervals from the GTI
(Good Time Interval) extension of the source file. From these it
determines the data start time (start_time) and data stop time
(stop_time) as the beginning of the first good time interval, and the
end of the last good time interval. If the input source file does not
have the GTI extension, start_time will be set to TSTART (listed in
the header of the source file), and stop_time will be set to TSTOP
(also listed in the header of the source file).
After getting the good time intervals from the source file, lightcurve
will determine the bin size according to the "nbins", "binlength",
and "counts_per_bin" user parameters.
There are three binning options in lightcurve. The user can
bin by one
of the following options:
1) total number of bins ("nbins" greater than 0):
binlength = (stop_time - start_time) / (nbins)
2) bin length in seconds ("binlength" greater than 0, and
"nbins" less or equal to 0): nbins = (stop_time -
start_time)/(binlength) + 1
3) minimum number of counts per bin ("counts_per_bin" greater
than 0; and "nbins" less or equal to 0; and "binlength" less
or equal to 0): This option requires the tool to find out how
many events are in the source file, and how many events are
in the background file (if specified). NOTE: If command line
filtering is applied on the input files, this option is not
recommended because it requires going through the whole file
to find out the total number of events and this is time
consuming.
The total number of bins, and bin length can
then be determined from the following equations:
binlength=(counts_per_bin/total_net_counts) * exposure
where total_net_cnts=total_source_cnts - source_area/bkgd_area * bkgfactor* bkgd_source_cnts
(Note that bkgfactor is the background normalization factor
entered by the user, and the default value is 1.)
nbins = (stop_time - start_time) / (bin_length) + 1
`lightcurve' proceeds as follows for each bin: a)Read and sum
the first bin of data from the source file. b)Read and sum
the first bin of data from the background file if there is a
background file. c)Determine the good exposure in the first
bin. This is just the time in the bin that also lies within
the good time intervals. d)Calculate the net counts, net
count error, net count rate, and net count rate error in the
bin. Note: Net count rate = (net counts in the bin)/(good
exposure in the bin). Also Note: There are 2 different
methods used for calculating the uncertainties on counts (or
count rate), namely Poisson statistics and Gaussian
statistics. User can pick either method by setting the
"errtype" user parameter. For Poisson statistics (set
errtype= poisson): error=1+sqrt(counts + 0.75) For Gaussian
statistics (set errtype=gaussian): error=sqrt(counts) e)Write
the results to the output lightcurve file.
Chandra event files contain data blocks which represent the
"good-time intervals" - or GTIs - for an observation, and
each ACIS chip can have a different set of GTIs. For
pipeline-produced data, the GTIs give the time periods when
the mission time line parameters fell within acceptable
ranges (see "ahelp mtl" for more information). Additional
temporal filtering by the user - such as to remove background
flare events - can further restrict these periods; the data
subspace of a file (see "ahelp subspace") lists the GTI
ranges that have been applied to it.
For single-chip data, the event file should be filtered on
the ccd_id (ACIS) column to ensure that the correct GTI is
used. This can be done either before passing the file to
lightcurve or by using the DM virtual-file syntax for the
infile parameter, e.g.
infile="acis_evt2.fits[ccd_id=7]"
to select the data, and GTI, for ACIS-S3. If no filtering on
chip number has been applied, then the GTI for the aim-point
chip will be used.
The situation is much-more complicated for regions that span
chips. In most cases the GTI ranges for the different chips
will not be significantly different, so the use of a single
GTI (lightcurve will use the GTI for the chip at the
aim-point) will not significantly affect the result. If there
are significant differences in the GTI ranges between chips,
then great care must be taken, such as analysing chips
individually. For ACIS data, the ONTIME series of keywords in
the header of the event file list the exposure times of the
different chips.
There have been no changes made in 'lightcurve' for CIAO 3.0.
As of CIAO 3.0, 'lightcurve' will no longer be actively maintained
or enhanced. Users instead are encouraged to create lightcurves
with the new lightcurve functionality of 'dmextract', which more
carefully and accurately applies good time interval (GTI)
information. For users choosing bin option 2 above, i.e., binning
by setting a bin length in seconds, 'dmextract' is the recommended
choice. For users choosing binning method 1 (i.e., binning by
total number of bins), or method 3 (i.e., binning by a minimum number
of counts per bin), 'lightcurve' is still viable, as
'dmextract' does not directly support those options.
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