Spectral variability in Cygnus X-3

TL;DR

This study models the broad-band X-ray spectrum of Cygnus X-3 across all states, revealing it likely hosts a massive black hole rather than a neutron star, based on spectral analysis and evolution.

Contribution

First-time derivation of unabsorbed spectral shapes and luminosities for all spectral states of Cygnus X-3, with interpretation in terms of Comptonization and reflection.

Findings

Unabsorbed spectra and luminosities derived for all states.

Spectral evolution inconsistent with a neutron star accretor.

Evidence supports a ~30 solar mass black hole in Cygnus X-3.

Abstract

We model the broad-band X-ray spectrum of Cyg X-3 in all states displayed by this source as observed by the Rossi X-ray Timing Explorer. From our models, we derive for the first time unabsorbed spectral shapes and luminosities for the full range of spectral states. We interpret the unabsorbed spectra in terms of Comptonization by a hybrid electron distribution and strong Compton reflection. We study the spectral evolution and compare with other black hole as well as neutron star sources. We show that a neutron star accretor is not consistent with the spectral evolution as a function of Ledd and especially not with the transition to a hard state. Our results point to the compact object in Cyg X-3 being a massive, ~30 Msun black hole.