An ISIS Tutorial
As part of the Lecture on X-ray Astronomy data analysis classes are offered. Since these data analysis classes might also be useful to other people, instead of giving them out in the lectures, we are posting them on the net with the hope that these will also be useful to others.
For the moment, these pages are maintained by Joern Wilms, Ingo Kreykenbohm, Felicia Krauss, and Maria Obst. Other users at Remeis should feel free to change typos and/or clarify claims in the text, but please do not add exercises and things like that, unless you want to annoy the students in the lecture :-).
The structure of the tutorial is the following:
- Introduction (this page)
- Installing Isis
- A Walk Through Isis:
- Loading your data
- A simple model fit
- Further Data Fitting:
- Data archives
- Multiple Data sets
- More complicated data modeling
- Calculation of error bars
- Advanced X-ray Spectral Analysis, I:
- Data from multiple instruments
- Advanced error calculations
- Advanced X-ray Spectral Analysis, II:
- Multi-wavelength fitting
- Advanced error calculations
- Defining your own models
- Advanced plotting
- Time Series Analysis
- Image Analysis
- Statistics - Chi-squared, Poisson, and all that
- Programming in S-Lang
Because of the complicated data model in X-ray astronomy, data analysis is generally done with a X-ray data analysis package, which allows astronomers to concentrate on solving the astrophysics instead of having to worry about the details of the data analysis. Currently three data analysis packages are available:
- XSPEC: The leader of the pack. Xspec has been around for more than 30 years and is available as part of Heasoft, X-ray astronomy's standard data extraction system (i.e., if you want to reduce data from a high-energy astrophysics satellite, you will need heasoft). XSPEC is very good for quick and dirty data analysis and with some external scripting can also be used to attack quite advanced data analysis issues. In many areas, however, one starts to feel the age of this software, which has been around since the 1980s. This might change with new developments that combine XSPEC with python, though. The big advantage of XSPEC is that it is very easy to learn, the big disadvantage is that after learning the initial steps with XSPEC it is very difficult to attack more complicated problems without outside help.
- ISIS: Developed initially by MIT for the analysis of data from high resolution gratings spectrometers, ISIS is a full fledged data analysis system that also contains high-level scripting capabilities. Its learning curve is steeper than that of XSPEC, however, after some training many of the things that would be very difficult to achieve with XSPEC are very easy to achieve with ISIS.
- Sherpa: An analysis package developed at CfA. I have not used it.
The reason why we are concentrating on isis here as a first X-ray data analysis system is that the isis data model is somewhat more transparent for first-time users. In addition, since a full programming language is integrated into isis, in addition to X-ray spectral fitting also other aspects of X-ray data analysis such as time series analysis are part of the system. This means that all relevant aspects of X-ray data analysis are covered by one data analysis package. Note that all concepts addressed in this tutorial are easily transferred into any of the other data analysis systems. We encourage all astronomers to familiarize themselves with more than one system - choose the best tool for the job instead of limiting yourself by thinking that one tool is the best for all jobs!
This tutorial can only give you an overview of isis and slang that should be sufficient for you to get you started with your initial data analysis. We strongly recommend that you also familiarize you with the following documents if you want to use the full power of isis:
- Mike Nowak at MIT has written a good introduction to isis which is complementary to the information given here. This introduction assumes some familiarity with X-ray data analysis using XSPEC, but it should in the most parts also be understandable without that.
- Also from Mike Nowak are the tutorials he has given at the multi-wavelength astronomy summer schools of ITN 215212 "Black Hole Universe":
- The practical information associated with these lectures:
- The full isis manual. Not for the faint-hearted, but required reading if you want to get the most out of isis.
- For making full use of all capabilities of isis, knowledge of the slang scripting language is required.
Helpful FAQs for X-ray spectral analysis including a discussion on statistics:
This tutorial builds heavily on these materials, which are gracefully acknowledged.