By the way: what is all that about?
The Science Analysis System (SAS) is a collection of
tasks, scripts and libraries, specifically designed to reduce and
analyze data collected by the
XMM-Newton observatory.
Why do I need it?
XMM-Newton data are available in two formats:
- Observation Data Files (ODF), i.e.
reformatted telemetry in
FITS format. They contain un-calibrated quantities on a chip-by-chip
or science window basis for the X-ray cameras and Optical Monitor,
respectively
-
Pipeline Processing System (PPS)
products, a collection of validated,
top-level scientific products including event and source lists,
multiwavelength images and cross-correlation products, generated
at the Survey Science
Center (SSC).
Even if one starts the analysis of an XMM-Newton datasets with
the PPS products, the
SAS is necessary to extract standard (spectra, light curves) and/or
customized science products. Moreover, SAS allows the users
to reproduce the reduction pipelines run to get the PPS
products (or, at least, a substantial part of them)
from the ODFs files. This step is advisable, whenever
substantial changes in the software and/or instrument calibrations
occurred from the time when the ODF were processed by the SSC.
... so all my FTOOLS/LHEASOFT or SPEX knowledge is not useful?
Don't jump too early to this pessimistic conclusion!
Whenever relevant,
XMM-Newton data files are FITS (or compressed FITS). When appropriate,
data files produced by the SAS tasks (e.g.: images,
spectra, time series) have been designed to be OGIP-compliant.
They provide therefore full compatibility with
the most commonly used analysis packages,
such as: FTOOLS (FITS file manipulation),
XANADU (timing and spectral analysis), SPEX (spectral analysis)
SAOIMAGE, SAOTNG, DS9 (image display and analysis).
However, you do not need to know the FTOOLS/LHEASOFT
package to work with XMM-Newton data.
SAS includes a powerful and extensive suite of FITS file
manipulation packages, based on the
Data Access Layer library.
On the other hand, SAS does not include tools for spectral,
timing or image analysis (although being able to generate all files
- spectra, light curve, response matrices, exposure maps - which
are required for the scientific analysis).
"Bird's eye" view of the SAS capabilities
In extreme summary, you absolutely need the SAS if you want to:
- reduce XMM-Newton data. A set of "reduction metatasks" is
available in the SAS to transform the ODF into different levels of
scientific products, directly usable for science analysis:
- EPIC: epproc
and emproc
produce calibrated and concatenated (i.e.: 1 single file
including events from all the detector chips) event lists for the EPIC
cameras. Similarly, emchain
and epchain
mirror the PPS SSC pipeline (a comparison between the chain
and the proc tasks is discussed here).
- RGS: the SAS reduction pipeline, rgsproc
generates, alongside with calibrated and concatenated event lists,
background-subtracted spectra and response matrices corresponding to a
given sky coordinate or source in an EPIC source list
- OM: the SAS reduction pipelines omichain
and omfchain
(applicable to Image and Fast Mode windows, respectively) generate
flat-fielded, and
geometrically distortion corrected sky images, source lists and
time series light curves
- create, maintain or update your set of XMM-Newton
calibration data, or build a Calibration Index
File (CIF), via the SAS task cifbuild
- create scientific products according to your
own taste and (data screening) criteria. The scientific product extractor in the SAS realm is evselect,
which its GUI interface xmmselect. evselect
is able to extract spectra, images and pseudo-images, light curves,
histograms and filtered event lists. Data screening criteria can be
applied "on-the-fly", using the powerful capabilities of the selectlib
library
- generate the instrument spectral transfer functions ("response matrices" in
the X-ray astronomy jargon) to perform quantitative spectral analysis
of X-ray cameras data, via the arfgen
plus rmfgen,
and rgsrmfgen tasks
for EPIC and RGS, respectively
- generate EPIC exposure maps, through the eexpmap
task
- estimate the amount of pile-up affecting your favorite source in
an EPIC field-of-view, through the epatplot
task
SAS is very useful for you if you want to:
- browse the content of your ODF, through the GUI-based odfbrowser
- manipulate XMM-Newton data files, through the capabilities
offered by the Data
Access Layer library
- visualize the XMM-Newton calibration files, through the GUI-driven calview
- generate Good Time Interval (GTI) files, applicable to later data
screening for the optimization of scientific product extraction, based
on housekeeping (hkgtigen)
or scientific (tabgtigen)
light curves
- merge
EPIC event lists
- perform source detection on EPIC fields, via the source detection
task edetect_chain
- mosaic images from different exposures and/or instruments,
through the emosaic
tasks
- perform barycentric corrections (earthbarycen)
You do not need SAS if you want to:
- perform timing or spectral science analysis of calibrated
scientific products
Again, please refer to the on-line
documentation for a full view of the whole SAS capabilities. The
SAS GUI interface (launched by typing sas on the command
line after a successful installation) includes the complete list of
all the available SAS tasks, with a short description of their
functionality.
A schematic view of the data analysis process for XMM-Newton data, and
of the role the SAS plays in it is given here.
You are probably now curious enough, and want to start playing with
the SAS. The SAS
start-up may be interesting for you. Other recipes to
accomplish specific data reduction tasks with the SAS are available in the
threads page.
Documentation
If you have installed the config-doc tar archive (cf. the SAS installation page), you have a
copy of the documentation on your disk. You can type the command
sashelp to reach the documentation entry page. Note if you
already have a running Netscape session, the disk where the SAS is
installed must be visible from the machine where Netscape was
started.
You may work with any other html browser like for example "mozilla"
by defining the SAS_BROWSER environment variable as pointing
to the exact place where this binary is placed.
Should sashelp not work, or shouldn't you have Netscape
installed, start any browser on $SAS_DIR/README.html.
For each SAS task both a PostScript and an HTML version of the
documentation are available. For those interested, the ChangeLogs of
each SAS package, with a low level description of all the changes to
the code (since the project's inception, roughly October 1997), are
also available.
Release Note
A release note describes the
difference between the current and the former version.
Status
The status of the data reduction and analysis
capabilities of the SAS is summarized in the
SAS Science Validation reports.
Known problems with solutions are listed
in the SAS watchout page
Pages maintained by SAS librarian.
Any question about SAS should be addressed to the
XMM-Newton help desk.
Updated on:
November 18, 2006