Variable Nature of Magnetically-Driven Ultra-Fast Outflows (UFOs) from AGN Accretion Disks
Abstract
With an increasing number of high-S/N spectroscopic observations of nearby radio-quiet Seyfert galaxies in the past decade, it is becoming more obvious that a sub-class of X-ray winds, known as ultra-fast outflows (UFOs) could be more ubiquitous than ever thought while quite variable in nature. It is suggested that the observed UFOs, especially those detected in Fe K band, may exhibit some likely correlations. For example, in the case of a luminous nearby quasar, PDS 456, and a narrow-line Seyfert 1, IRAS 13224-3809, the observed velocity of the Fe K UFOs (i.e. Fe xxv/xxvi) and its equivalent width (EW) are found to be (both) correlated with the ionizing X-ray flux/luminosity across different epochs. Attributed to their extreme brightness particularly in O/UV band most likely powered by accretion process, it is speculated that strong UV radiation field responsible for producing sufficiently high line-opacity due to bound-bound transition may effectively drive the observed UFOs in these systems. However, a certain optimal ionization condition is necessary in such a scenario to acquire high velocity by avoiding overionization, while the observed UFOs are typically characterized by high ionization parameter. In this work, it is shown that the action of global magnetic fields, another equally promising launching mechanism, can naturally explain the observed UFO conditions in PDS456 as a case study. By detailed photoionization calculations with xstar code, the MHD-driven disk-wind model predicts that H/He-like Fe are formed at smaller radius where the velocity is higher with higher X-ray luminosity, while the net ionic columns drop due to its higher ionization parameter thus reducing the EW. We successfully fit the 2013-2014 XMM-Newton/NuSTAR campaign spectrum with this model and also show a simulated XARM/micro-calorimeter spectrum. We further demonstrate that the observed multi-epoch correlations of UFOs are well explained within the framework of MHD-driven disk-winds.The anticipated scientific advances made possible by XARM and Athena in the forthcoming decades will allow us to answer these fundamental questions by exploring AGN winds more throughly.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E1134F