SIROCCO: A Publicly Available Monte Carlo Ionization and Radiative Transfer Code for Astrophysical Outflows

• Main Authors: Matthews, James H, Long, Knox S, Knigge, Christian, Sim, Stuart A, Parkinson, Edward J, Higginbottom, Nick, Mangham, Samuel W, Scepi, Nicolas, Wallis, Austen, Hewitt, Henrietta A, Mosallanezhad, Amin
• Format: Journal Article
• Language: English
• Published: 25-10-2024
• Subjects: Physics - Astrophysics of Galaxies; Physics - High Energy Astrophysical Phenomena; Physics - Instrumentation and Methods for Astrophysics; Physics - Solar and Stellar Astrophysics
• DOI: 10.48550/arxiv.2410.19908

Outflows are critical components of many astrophysical systems, including
accreting compact binaries and active galactic nuclei (AGN). These outflows can
significantly affect a system's evolution and alter its observational
appearance by reprocessing the radiation produced by the central engine.
Sirocco (Simulating Ionization and Radiation in Outflows Created by Compact
Objects - or "the code formerly known as Python") is a Sobolev-based Monte
Carlo ionization and radiative transfer code. It is designed to simulate the
spectra produced by any system with an azimuthally-symmetric outflow, from
spherical stellar winds to rotating, biconical accretion disc winds. Wind
models can either be parametrized or imported, e.g. from hydrodynamical
simulations. The radiation sources include an optically thick accretion disc
and various central sources with flexible spectra and geometries. The code
tracks the "photon packets" produced by the sources in any given simulation as
they traverse and interact with the wind. The code assumes radiative
near-equilibrium, so the thermal and ionization state can be determined
iteratively from these interactions. Once the physical properties in the wind
have converged, Sirocco can be used to generate synthetic spectra at a series
of observer sightlines. Here, we describe the physical assumptions, operation,
performance and limitations of the code. We validate it against tardis, cmfgen
and cloudy, finding good agreement, and present illustrative synthetic spectra
from disc winds in cataclysmic variables, tidal disruption events, AGN and
X-ray binaries. Sirocco is publicly available on GitHub, alongside its
associated data, documentation and sample input files covering a wide range of
astrophysical applications.