Abstract

Ultra-luminous X-ray sources (ULXs) are accreting compact objects in nearby galaxies, which emit X-rays in excess of their spherical Eddington limit. Due their brightness, most of them are believed to be black holes. However, recently a new class of ULXs was discovered, through the detection of coherent pulsations: Ultra-luminous X-ray pulsars (ULPs) - The presence of pulsations unambiguously identifies the compact objects as neutron stars, which are typically less massive than black holes. In ULPs the neutron star accretes matter from a companion star at inferred rates much higher than previously expected. Currently three of these systems are known: M82 X-2, NGC 5907 ULX, and NGC 7793 P13 (with NGC 300 ULX-1 a likely fourth). Their existence challenges our understanding of accretion physics and stellar evolution. Today, no widely accepted theory has emerged to explain all their observational properties. Furthermore, a clear path forward to obtain a full sample of the ULP population is missing. In order to further our understanding of ULPs, we will have a workshop at ESAC from June 6 to June 8, 2018 -- At this workshop, we will bring together experts in observations, accretion models, and stellar evolution for an exchange of knowledge, ideas, and expertise. The goal of the workshop is to collect the observational evidence, make progress on our theoretical understanding, and identify open questions and projects to work on for the coming years in order to obtain a better understanding of the physics and evolution of ULPs. To this end, the format of the workshop will consist of a mix between review talks by international experts, talks and posters by all participants, and ample time for discussion and project work. We will in particular discuss observational opportunities with current (e.g. XMM-Newton, Chandra, NuSTAR and the Neil Gehrels Swift Observatory) and upcoming (e.g. XARM and Athena) X-ray telescopes and multi-wavelength follow up.