Introduction

What is XSTAR?

Xstar is a command-driven computer program for calculating the physical conditions and spectra of ionized and nearly-neutral gases. It has been developed for simulating astrophysical plasmas, and has been used for modeling diverse environments, such as nebulae, stellar winds, accretion disks, and interstellar and intergalactic gases. In addition, it has been benchmarked against laboratory experiments.

Xstar is intended to be flexible, and for this reason it makes simple assumptions about the geometry of the gas, the observer, and any illumination or other soure of energy. Stripped to essentials, its job may be described simply: A spherical gas shell surrounding a central source of ionizing radiation absorbs some of this radiation and reradiates it in other portions of the spectrum; XSTAR computes the effects on the gas of absorbing this energy, and the spectrum of reradiated light. Other sources of heat may be included, such as mechanical compression or expansion, or cosmic ray scattering. The user supplies the shape and strength of the incident continuum, the elemental abundances in the gas, its density or pressure, and its thickness; the code returns the ionization balance and temperature, opacity, and emitted line and continuum fluxes. In addition, Xstar includes Python tools for diagnosing the various atomic rates and internal quantities, running grids of models, and accessing and modifying the raw atomic data.

This document describes how to run xstar and interpret the output. Chapters include: how to obtain and install xstar (chapter 2), a simple introductory tutorial (chapter 3), detailed descriptions of the input parameters and output (chapters 4 and 5), descriptions of the associated tools (chapters 6 thru 8), examples and threads (chapter 10), and descriptions of the internal working and assumptions (chapters 11 thru 13). The new user is encouraged to read chapter 3 and then 4 and 5.

Acknowledgements

We would like to thank many colleagues for their contributions, including Dick McCray, Ian Stevens, Yuan-Kuen Ko, Julian Krolik, Tom Bridgman, James Peachey, Bryan Irby, Bill Pence, Randall Dannen and Abdu Zoghbi. Crucial work on the atomic data was done by Manuel Bautista, Patrick Palmeri, Claudio Mendoza, Javier Garcia, and Mike Witthoeft.

Feedback and Bug Reports

XSTAR is under active development and we appreciate bug reports, requests for additional features and general feedback from the community. Please use the HEASARC Feedback form <https://heasarc.gsfc.nasa.gov/cgi-bin/Feedback> and select “xstar” as mailing list.

Timothy Kallman, & timothy.r.kallman@nasa.gov

Ralf Ballhausen, & ralf.ballhausen@nasa.gov