Accretion of matter onto black holes often gives rise to outflows in the form of collimated relativistic jets and uncollimated winds. A deeper understanding of the launching mechanisms behind these outflows, manifesting in the form of blue-shifted absorption lines in the X-ray spectrum, can lead to valuable insights towards the behaviour of matter under extreme gravity. While low-velocity winds are considered to be ubiquitous in almost all accreting black holes, winds of relativistic velocities have been detected in a significant fraction of active galactic nuclei (AGN) over the last two decades. Out of the three prevalent wind-driving mechanisms (thermal, magnetic and radiative), these relativistic winds (also called Ultrafast Outflows or UFOs) could be launched by the latter two. However, a comprehensive investigation of the robust UFOs in Galactic black hole X-ray binaries (BHBs) has not yet been conducted in a systematic fashion, leaving a gap in our knowledge of the physics of black hole accretion across the mass range. In this talk, I will highlight our recent work in which we try to bridge this gap for the first time. In this work, we probe the magnetic driving behind the dense, non-equatorial UFOs in four BHBs with the NuSTAR, NICER and other X-ray observatories. We conduct detailed reflection modelling of the broadband X-ray spectra to measure the properties of the accretion disk and a direct MHD modelling of the absorption lines to determine the corresponding properties of the UFOs, thereby demonstrating an essential synergy between the reflection and absorption spectroscopy. The results of our study point towards a magnetic origin of UFOs in BHBs and hint that these low-inclination UFOs are necessarily supplemented with high wind and disk densities in order to be observed with current X-ray instruments. Such magnetically driven winds indicate a remarkable invariance of accretion and ejection processes over the very wide range of black hole masses, from stellar to super-massive. We anticipate our assay to be the gateway to more exhaustive future studies of UFOs in more comprehensive samples of BHBs and further exploration of magnetic wind driving in the JAXA/NASA’s XRISM era.

Local contact : Thierry FOGLIZZO

Organizer : Frédéric GALLIANO