Loving ISIS - Confessions of a Former XSPEC User


Left: An RXTE power spectrum entirely created an analyzed with ISIS. Right: ISIS can also be used to interact with DS9. Data can be passed back and forth between DS9 and ISIS. Here we show an example of Suzaku pileup estimation performed with a simple S-lang script.


With the upgrade of XSPEC to version 12, both ISIS and XSPEC have features such as: support for OGIP Type II files; support of linear combinations of RMFs comprising a response (e.g., for LETG spectra); linking model parameters via polynomial functions (in fact, ISIS model parameters can be arbitrary functions of other parameters - if you want to use a hypergeometric function, download the GSL module and go for it!); extensible fitting methods and fit statistics; easy incorporation of user models, etc.

Except that ISIS doesn't stop there. Thanks to its programmability, it can do a lot more. For example, I use ISIS for timing analysis (obviating any need to use a separate timing package, such as XRONOS, in addition to a spectral fitting package). The power spectrum shown above was performed entirely in ISIS. The data were read, the FFTs were performed and averaged, the Power Spectrum was logarithmically binned over Fourier frequency, the constant + two Lorentzian model was fit, and the results plotted, all in ISIS without ever leaving the program.

With the use of the S-Lang Modules Packages, lots of other cool things can be done. For example, the XPA module allows 2D arrays to be passed back and forth between ISIS and the DS9 imaging package, as also shown above.

The Parallel Virtual Machine (PVM) module allows extremely time consuming calculations (which are becoming more and more common in astrophysics) to be done quickly and efficiently. Here is an example of the PVM module being used to produce a temperature map of an X-ray cluster. The PVM module has been used to do a pixel-by-pixel spectral fit to the Chandra megasecond observation of the Cas A supernova remnant.

Don't have time to learns the ins and outs of setting up a parallelized fit? ISIS now transparently incorporates parallelized fitting and error bar searches on multi-core machines. This functionality has even been tested on a 20-core virtual machine using the Amazon Elastic Compute Cloud, and it works fabulously!

As we've argued elsewhere, the extensibility and flexibility of ISIS allows one to create new and advanced astrophysical analyses that aren't easily achievable with XSPEC.

Next up: Making the Leap

This page was last updated Oct 7, 2013 by Michael Nowak. To comment on it or the material presented here, send email to mnowak@space.mit.edu.
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