MoCA: A Monte Carlo code for Comptonisation in Astrophysics

I. Description of the code and first results

¹ Astronomical Institute of the Czech Academy of Sciences, Boční II 1401/1, 14100 Prague, Czech Republic
e-mail: francesco.tamborra@asu.cas.cz
² Dipartimento di Fisica e Matematica,Università degli Studi Roma Tre, via della Vasca Navale 84, 00146 Roma, Italy

Received: 2 October 2017
Accepted: 6 August 2018

Abstract

We present a new Monte Carlo code for Comptonisation in Astrophysics (MoCA). To our knowledge MoCA is the first code that uses a single photon approach in a full special relativity scenario, and including also Klein–Nishina effects as well as polarisation. In this paper we describe in detail how the code works, and show first results from the case of extended coronae in accreting sources Comptonising the accretion disc thermal emission. We explored both a slab and a spherical geometry, to make comparison with public analytical codes more easy. Our spectra are in good agreement with those from analytical codes for low/moderate optical depths, but differ significantly, as expected, for optical depths larger than a few. Klein–Nishina effects become relevant above 100 keV depending on the optical thickness and thermal energy of the corona. We also calculated the polarisation properties for the two geometries, which show that X-ray polarimetry is a very useful tool to discriminate between them.