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XMM-Newton Science Analysis System Page: 1
gtialign
January 10, 2003
Abstract
Align asynchronous GTI les to frame readout boundaries
1 Instruments/Modes
Instrument Mode
EPIC All
RGS All
OM Fast
Note that the primary use of this task is foreseen to be with the EPIC imaging mode and with the RGS.
It will not be used by default for processing EPIC timing mode data, and its applicability to the OM fast
mode is TBD.
2 Use
pipeline processing yes
interactive analysis yes
3 Description
The purpose of this task is to take GTI les, and align them with the frame readout times of a given
instrument, so that they can be used to properly lter events and calculate exposure times for that
instrument. The output GTI's are also useful for timing analysis.
The problem is illustrated in the gure below.
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_________________| |_____________________
time -->
The top line represents the frame readout time boundaries for a given instrument, i.e. each vertical line
represents the end of one frame readout cycle and the beginning of another. The bottom line represents
a GTI, as produced for example from housekeeping data. Up corresponds to a good time interval. Note
that the frame integration time interval is not constant.
Looking at frame integration period 2, we see that the GTI is valid only for part of this interval. However,
all events from this interval will be labeled with the same time. Thus all events from interval 2 should
be excluded. Similarly, all events from interval 5 should be excluded as well. Thus the GTI should be
adjusted as follows:
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| 1 | 2 | 3 | 4 | 5 | 6 |
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____________________
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________________________| |_____________________________
time -->
This is what is meant by GTI alignment.
The gtialign task takes as input the GTI le to be aligned, and the event list to which the GTI's will
eventually be applied. There should be exposure extension(s) in the event list, which will contain the
frame readout times for the events in this le. This data will be used to adjust the GTI start and stop
times as described above. The dierent CCD's in the instruments may have dierent frame integration
start and stop times, so that the GTI le will have to be aligned separately for each CCD. Note that the
frame integration time for a particular CCD can also vary within a given exposure.
Depending on the convention used, the times associated with individual events can be set to the beginning,
end or middle of the frame readout intervals (or, in principle, any value in between). This is also true of
the exposure extension, which stores the frame readout times as an absolute time and an interval.
In the case of the event times, this task adjusts the GTI's such that any event in a frame with a good
GTI will be accepted, no matter where in the frame readout interval this time is set.
However, for the exposure information, it is necessary to know what convention is being used for storing
frame timing information. This will be determined by referring to the OGIP keyword TIMEPIXR in the
header of the exposure extension. A value of 0 corresponds to time stamps referring to the beginning of
the time interval, 1:0 to the end of the time interval, etc. If the keyword is not present, the default value
is 0:5, i.e. the time stamp refers to the middle of the time interval.
The output of this task will be a FITSle, with a GTI extension for each CCD in the event list. Each
extension will have the name STDGTIn, where n is the CCD number for that extension. Where deemed
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useful, the extension will also contain CCD specic keywords such as CCDID, CCDNODE, and FRMTIME. The
primary header of this le will contain all the standard XMM keywords, as copied from the original GTI
le.
All GTI les will be OGIP compliant.
4 Parameters
This section documents the parameters recognized by this task (if any).
Parameter Mand Type Default Constraints
gtitable yes string none must be valid table
specication.
Specication of GTI extension to be aligned. This specication should follow the Dal convention of
set:table.
eventset yes lename none must be valid le
name.
File name of event list containing exposure extension(s).
outset yes string none must be valid le name
File name of output le containing the aligned GTI extensions.
5 Errors
This section documents warnings and errors generated by this task (if any). Note that warnings and
errors can also be generated in the SAS infrastructure libraries, in which case they would not be docu-
mented here. Refer to the index of all errors and warnings available in the HTML version of the SAS
documentation.
OutsideFrames (warning)
Frame times do not span all GTI's.
corrective action: GTI's outside of frame times are not modied.
6 Input Files
1. OGIP compliant GTI le, such as those produced by hkselect.
2. FITSevent list le, containing exposure extension(s), such as that produced by evlistcomb.
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7 Output Files
1. FITSle containing OGIP compliant GTI extensions, one for each CCD in the input event
list.
8 Algorithm
subroutine gtialign
open gti_in
open events_in
open gti_out
read gti_starts
read gti_stops
foreach ccd
create gti_out extension
read frame_times
foreach frame_time
frame_start = get_start(frame_time, time_del, time_offset)
foreach gti_start
if( gti_start < frame_start )
gti_new_start = frame_start
break gti_start loop
endif
next
frame_end = get_end(frame_time, time_del, time_offset)
foreach gti_stop
if( gti_stop < frame_end )
gti_new_stop = get_end(frame_time-1, time_del, time_offset)
break gti_stop loop
endif
next
next
write gti_out extension, gti_new_start, gti_new_stop
next
close files
end subroutine gtialign
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9 Comments
None.
10 Future developments
This task could be enhanced with options such that it will align GTI les using exposure information
which does not necessarily conform to the conventions described in this document.
References
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