When you download quick-look or archived Swift data from the UKSDDC, you have the option to download the SDC processed data, or locally reprocessed data. The difference between these is only in the XRT data.
The SDC-processed data are those provided by the NASA Swift Data Center (SDC), which for uniformity are available from the SDC, ASDC (Italy) and our website. These are not guaranteed to have been processed with the most recent calibration and software. Also, older datasets are not routinely reprocessed, so the software and calibration versions used for the SDC data are not necessarily consistent across the archive. At the current time (2017 November) a ‘grand reprocessing’ of the entire archive using HEASoft v6.17 is underway, but will take years to complete.
The software and CALDB versions used in the
SDC processing are stored as SOFTVER
and CALDBVER
FITS keywords in
the data files; these will also be reported on the data download page, provided the
version is the same for all of the datasets selected. The SDC standard advice is
that users should reprocess the data themselves after downloading, using
xrtpipeline
.
At the UKSSDC, as well as serving the SDC-provided data we reprocess the
XRT data using a more recent version of HEASoft and the CALDB, and allow
you to download this. As part of this we also create exposure maps
(with and without vignetting included) to download as part of the standard products.
The version of HEASoft used for locally reprocessed data is reported on the download page.
Normally the entire archive will be locally reprocessed using the same version of HEASoft,
however when a new version of HEASoft is adopted at the UKSSDC, it takes several months
for the entire archive to be fully reprocessed; during this time the locally-reprocessed
archive will comprise a combination of the old and new software version. You will
be warned of this on the download pages, and the LEICSVER
keyword in the
XRT files indicates the version used.
Normally the locally-reprocessed data will be completely up-to-date, however if for some reason the SDC data are newer than the locally-reprocessed data you will be warned on the download page. For new data, this may indicate a problem or delay in the UKSSDC processing. For older data this typically means that the SDC data have been reprocessed as part of the grand reprocessing. We do not generally reprocess the data locally after such SDC reprocessing because we have already used a newer version of HEASoft than was employed in the SDC reproecssing.
Windowed Timing (WT) mode data have only one-dimensional spatial information,
thus to ensure that the source is placed at the correct RA and Dec, for ease of
data analysis, xrtpipeline
must be given the correct source co-ordinates. The local reprocessing supplies the values in the
RA_OBJ
and DEC_OBJ
FITS keywords. If you are
analysing WT mode data therefore you should verify that these values are
correct for the source you are interested in, and if not, you should
re-run xrtpipeline
yourself, even if you downloaded the
locally-processed data.
Not every update to HEASoft affects the XRT data processing, similarly not all CALDB updates affect all XRT data (for example, new gain calibration tends to affect only recent data). After a new release, UKSSDC scientists will assess whether the new version of the software merits a reprocessing of the archive, and whether this should be partial or complete. There is also a period of testing to verify that the new software interfaces as expected with our local analysis tools. Should we decide to reprocess at least some data, this is done in the following order.
If you have any questions about this, please contact the UKSSDC helpdesk on swifthelp@le.ac.uk.