Loving ISIS - Confessions of a Former XSPEC User

Example Analysis:

These web pages will outline a detailed example of an ISIS analysis of a set of simultaneous radio and RXTE X-ray observations. The former is stored as an ASCII file giving frequency in Hz and radio flux (with its, presumed gaussian, error) in units of mJy. The latter consists of both PCA and HEXTE data, stored as FITS files, taken from the usual RXTE data extraction procedures. The data can be obtained as a gzipped tar file found here.

A plot of the data is shown below. A detailed description of the observations can be found in Nowak et al. (2005), ApJ, 626, p. 1006. (Every figure and result in that paper was generated with ISIS.)

All results and figures found on these web pages were generated with this analysis script. This script presumes that you have downloaded my .isisrc file. On my 1.5 GHz Apple Power PC G4, the whole script, soup to nuts (four different models, two complete sets of error bars, and a 961 point grid of error contours), runs in about 4 hours. Whereas this might seem like a long time, that's mostly driven by the addition of the radio data points (sitting 8 orders of magnitude away in energy space makes for a slightly statistically odd lever arm; there can be some "fussiness" with the fitting the location of the radio/X-ray spectral break), and the presence of the computationally expensive reflection and diskline models. If one were to restrict this to only X-ray data being fit with broken power laws and gaussians, the whole script would run in minutes. A shortened analysis, which simultaneously fits the radio but only considers broken power law plus gaussian line models, is presented on these pages, and takes only 23 minutes to produce a set of fits, complete error bars, and an error contour grid. I've benchmarked individual steps against XSPEC, and the run times are comparable.

First we discuss five ISIS basics, and then the analysis begins with loading the data.