The extremes of accretion: understanding super-Eddington accretion and feedback phenomena in Ultraluminous X-ray Sources

Soutenance de thèse de Andrés Gúrpide

Lieu : Salle de Conférence, IRAP Roche

Résumé de la thèse

The deepest infrared surveys have revealed that supermassive black holes (SMBHs) with masses 10 ⁸⁻⁹ M_☉ were already present when the Universe was only 5% of its current age, suggesting that these black holes (BHs) must have grown extremely fast, through multiple BH mergers or extreme episodes of accretion. After two decades of studies, there is now firm evidence supporting that most of the Ultraluminous X-ray sources (ULXs) typically found in external galaxies are accreting binary systems undergoing sustained super-Eddington accretion. Much like a scaled-down version of a SMBH, the copious amounts of radiation and outflows these mighty sources produce can ionize large nebulae of gas extending hundreds of parsecs in size, offering an observational template to study the feedback and efficiency of the super-Eddington regime.

This thesis presents a multi-wavelength study combining X-rays and optical observations, in an effort to shed light on the engines behind ULXs, the properties of their inflows and outflows as well the feedback signatures in their surroundings. The two main lines of investigation are presented. In the first one, I exploited the extensive archival data offered by the major X-ray telescopes (XMM-Newton, Chandra and NuSTAR) to present a comprehensive study of the long-term X-ray spectral transitions of a broad sample of high-quality data ULXs, in order to test which of the current models to explain their extreme luminosities fit the data best. This work is followed by an in-depth study of two archetypal ULXs, Holmberg II X–1 and NGC 5204 X–1, where I exploited the high-observing cadence of Swift-XRT to reveal that these two sources follow a spectral recurrent evolutionary cycle. The spectral transitions and variability associated with the cycle is discussed in the framework of super-Eddington accretion, by invoking mass-accretion rate changes and varying degree of obscuration by the super-Eddington winds.

The second part of this thesis focuses on exploring the archival data from the Multi-Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope (VLT) to study the feedback properties of super-Eddington winds at larger scales, with focus on characterising the main sources of ionization of the nebular gas through state-of-the art spatially-resolved gas diagnostics and comparisons with emission radiative-shock models. The main result of this work focuses around the discovery of a shock-powered bubble nebula extending hundreds of parsecs around the ULX NGC 1313 X-1. The estimated mechanical power attributed to the outflow is in excess of the radiative power, suggesting that in the super-Eddington regime the mechanical feedback by outflows and jets may be more prevalent than the radiative feedback.

Composition du jury de thèse

• Timothy Paul Roberts, Durham University, Reviewer
• Christian Motch, Observatoire Astronomique de Strasbourg, Reviewer
• Matteo Bachetti, INAF-Osservatorio Astronomico di Cagliari, Examiner
• Victoria Grinberg, European Space Agency, ESTEC, Examiner
• Geneviève Soucail, IRAP, Examiner
• Olivier Godet, IRAP, PhD supervisor
• Jean-François Olive, IRAP, PhD supervisor