Here is an example of what might be in "file.xco":
For more information on xselect, see the Xselect
To retrieve data for a specific burst, go to the special Swift interface of Browse. (Browse can also be found under the "HEASARC Quick Links" menu on any Swift Web page. Once you're on the Browse page, select the Swift interface.)
Select the GRB that you are interested in. Upon selection, the target id box will be filled with the target-id. Next, click on Start Search. The results page you will see is from Browse, which may be familiar to users of other missions served by the HEASARC.
To actually retrieve the data, click the box on the right for the observation you are interested in, and use one of the data retrieval options listed at the bottom of the page. The results page gives an 11 digit number for each observation, known as its observation id. (Note: most GRBs have many separate observations, and therefore many observation IDs.) For more information on observation IDs and data organization in the Swift Archive, see the Overview of the Swift Data Archive (PDF file, 196 Kb).
If you wish to use FTP instead of Browse to retrieve the data,
you can FTP to legacy.gsfc.nasa.gov as user "anonymous" and cd to
swift/data. From there, follow the directory structure outlined in the Overview
of the Swift
Data Archive guide, mentioned above, to retrieve the file(s) you
are interested in.
It is quite possible for an enable map generated during one observation to be included in the subdirectory of a different observation. The maps are produced on-board as needed, not according to any ground-defined schedule. The ground processing pipeline is meant to find the nearest map in time.
A reprocessing of existing Swift data is planned soon, which
will, among other
things, merge any files into a single one, and rename each one
the observation number. Until then it is safe to take the first file.
This is usually appropriate for GRBs since the GRB occurs at the
beginning of the first observation snapshot.
There is a description of the testing of the XRT absolute timing accuracy in SPIE.
This of course relies on the absolute accuracy of the spacecraft clock. The requirement was that the clock (Mission Elapsed Time plus UTCF) be kept within 200 microseconds of UTC, and it has been comfortably doing so throughout the mission, sometimes much better.
The MOC measures this clock offset routinely, and this data is stored in a calibration file, the clock offset log. This file is part of the Swift caldb and is used for fine the time corrections. See the Swift CALDB for links and documentation. The clock offset log is documented under "Swift Common Calibration Products".
The BAT timetags photons with a relative precision of 100 microseconds.
Once the UTCF correction is applied to BAT data, you should be
0.2 msec. If you want times better than that, use the "swifttime" tool
X-time Web tool (mentioned above), both of which incorporate the clock
XRT data from different data sets can be combined using the following procedure.
Use XSelect to sum the individual event files. Read all the cleaned event files into XSelect and extract the total event file. The "copyall=yes" option must be used when extracting the total event file. For example:
> extract events copyall=yes
Sum the individual exposure maps. The total exposure map can be used for spectral analysis of the total spectrum. You can sum the individual exposure maps using, for example, the command "sum_ima" in the XImage package. (See The XImage User's Guide for details). The following XImage commands may be useful:
XIMAGE>read sw00036093001xpcw2po_ex.img XIMAGE>read sw00036093002xpcw2po_ex.img XIMAGE>sum_ima XIMAGE>save_ima XIMAGE>read sw00036093003xpcw2po_ex.img XIMAGE>sum_ima XIMAGE>save_ima XIMAGE>write_ima/file="sum_01_02_03_ex.img" template=all XIMAGE>exit
At this point one can extract the spectrum (with XSelect) and generate its ARF (with "xrtmkarf" using the option "expofile=sum_01_02_03_ex.img".
Xrtpipeline sometimes returns errors that are similar to the example below.
Running SWIFT XRT pipeline
Task: xrtpipeline Version: 0.12.6 Release Date: 2010-07-01
Retrieving files from Input Target Directory '.'
xrtpipeline_0.12.6: Info: Input parameter 'evtfilesfromarchive' set to 'yes',
xrtpipeline_0.12.6: Info: getting input event files from the Target Directory:
xrtpipeline_0.12.6: Info: '.'
xrtpipeline_0.12.6: Info: Using Telescope Definition File:
xrtpipeline_0.12.6: Info: '/sw/heasoft-6.10.2/caldb/data/swift/xrt/bcf/teldef/swx20050923v001.teldef'
xrtpipeline_0.12.6: Error: CallScQuzcif: Found 4 calibration file(s) instead of 1 file(s)
xrtpipeline_0.12.6: ERROR: cannot retrieve S/C Alignment file name from CALDB
This error (cannot retrieve file from CALDB) usually indicates that there is a problem with the way that CalDB is set up on your system. The first thing to check is if you have the most recent Swift/XRT and Swift/mis (common calibration products) CalDB files The most recent calibration data can be obtained from http://swift.gsfc.nasa.gov/docs/heasarc/caldb/swift/. You must install both the XRT and the common calibration products CalDBs in order to run xrtpipeline. Second, check that your CALDB environment variable is pointing to the correct location.
You can test your local installation of CalDB by temporarily setting your CALDB environment variable to
CALDB = ftp://legacy.gsfc.nasa.gov/caldb/
and rerunning xrtpipeline. You will need an active internet connection to do this. If xrtpipeline works using the online version of CalDB then check your local CalDB installation for errors.