03.03.2025
Thibault Barnouin
The long awaited come back of UV polarimetry of Active Galactic Nuclei
The last UV detectors capable of measuring the polarized signals from Active Galactic Nuclei (AGN) were the Faint Object Spectrograph (FOS) and Faint Object Camera (FOC) on board the Hubble Space Telescope (HST), decommissioned in 1997 and 2001, respectively, and the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE), which flew on two NASA Space Shuttle missions in December 1990 and March 1995. The FOC’s imaging resolution of 22x22 milliarcseconds squared at wavelengths as short as 115 nm is still unmatched to this day. Used together with polarizing filters, it allowed the study of 25 AGN, providing great insight into the structure of the scattering medium of type 2 AGN and the precise location of their obscured nucleus. The FOS's far- to near-UV high spectral resolution polarimeter — achieved using a prism and rotating waveplate — enabled the study of 50 AGN. During its only two flights, WUPPE obtained spectropolarimetry for only 5 AGN. With the exception of one radio-quiet AGN (NGC 1068), WUPPE's AGN observations had very poor spectral resolution in polarimetry. Although limited in number, these observations provided critical insights into the physics of these highly asymmetrical and spatially unresolved objects. During this presentation, I will go through the AGN samples in the polarimetric archives of HST/FOC, HST/FOS, and WUPPE, highlighting their groundbreaking contributions and the unanswered questions that remain since their decommissioning. Finally, I will discuss how a POLLUX-like instrument — to be proposed by CNES as a fourth instrument for NASA’s Habitable Worlds Observatory — could not only build upon this legacy but also surpass it.