Simultaneous fits in ISIS on the example of GRO J1008-57

TL;DR

This paper discusses the implementation of simultaneous fitting of multiple datasets in the ISIS tool, demonstrating its application to GRO J1008-57 to improve parameter estimation in X-ray data analysis.

Contribution

It introduces the methodology and implementation of simultaneous fits in ISIS, highlighting advantages over traditional individual dataset fitting.

Findings

Spectral shape depends only on X-ray flux.

Time-independent parameters are determined with high precision.

Simultaneous fitting enhances parameter constraints.

Abstract

Parallel computing and steadily increasing computation speed have led to a new tool for analyzing multiple datasets and datatypes: fitting several datasets simultaneously. With this technique, physically connected parameters of individual data can be treated as a single parameter by implementing this connection into the fit directly. We discuss the terminology, implementation, and possible issues of simultaneous fits based on the X-ray data analysis tool Interactive Spectral Interpretation System (ISIS). While all data modeling tools in X-ray astronomy allow in principle fitting data from multiple data sets individually, the syntax used in these tools is not often well suited for this task. Applying simultaneous fits to the transient X-ray binary GRO J1008-57, we find that the spectral shape is only dependent on X-ray flux. We determine time independent parameters such as, e.g., the folding energy E_fold, with unprecedented precision.