Abstract :
Discovered in 2019, Quasi-Periodic Eruptions (QPEs) from super massive black holes are a new X-ray variability pattern, never seen before, characterized by high-amplitude quasi-periodic X-ray bursts, repeating every few hours, over a stable quiescent level. So far, QPEs have been detected in a handful of active galactic nuclei. The mechanism responsible for the emergence of those is an open question, but Tidal Disruption Events (TDEs) are among the more plausible explanations. In turn, TDEs occur when a supermassive black hole tidally disrupts a star, forming immediately an accretion disc and, sometimes, also a relativistic jet. How this happens also remains a mystery, but observationally TDEs also display flares/bursts along the electromagnetic spectrum. Polarization measurements, although still scarce in the TDE field, have already been proved to discern between theoretical models of accretion disc formation that conventional observations cannot. This potential should also be key to clarify if TDEs are at the origin of QPEs, thus more observational efforts are needed for a better understanding.
In this talk, I will focus on the source where QPEs were discovered: GSN069. It is a high Eddington-ratio Seyfert galaxy that exhibits an unabsorbed highly variable X-ray spectrum typical of a Seyfert 1, but it does not show broad emission lines in its optical spectrum, which leads to a Seyfert 2 classification. It could be then considered as a true Seyfert 2 candidate. Those candidates are thought to accrete at low Eddington rates, unable to sustain the broad line region (BLR). Nevertheless, GSN 069 exceeds the theoretical accretion rate at which a BLR is formed. Apart from probing the disruption mechanism of TDEs, polarimetry is the only technique capable of demonstrating the existence of hidden-BLR. I will report optical imaging polarimetry and spectropolarimetry of GSN069 with FORS2@VLT to explore both scenarios: the true Seyfert 2 candidacy and TDEs as a possible origin of QPEs in GSN 069. This is the first time time that QPEs are observed in polarized light.
