Presentation #117.25 in the session Time-Domain Astrophysics.
Tidal disruption events (TDEs) are exotic transients often leading to temporary super-Eddington accretion onto a supermassive black hole. Such accretion mode is naturally expected to result in powerful outflows of ionized matter. However, to date such an outflow has only been confirmed in a single TDE, ASASSN-14li, with a very low velocity of just a few 100 km/s, in disagreement with the violent nature of the TDE phenomenon, where velocities in excess of 10% of the speed of the light are expected. The origin of these low-velocity outflows is currently uncertain – they may originate from shocks during stream-stream collisions, or from the outer accretion disk after the matter has efficiently circularized.
In this talk, I will present the detection of a low-velocity outflow in the second TDE. The high-resolution X-ray spectrum reveals a spectacular array of narrow absorption lines, each blueshifted by a few 100 km/s, which cannot be described by a single photo-ionization phase. For the first time, we thus confirm the multiphase nature of a TDE outflow, with at least two phases and two distinct velocity components. One highly ionized phase is outflowing at 900 km/s, while a lower ionization component is moving at 400 km/s. We perform a time-resolved analysis of the X-ray spectrum and detect that, surprisingly, the colder absorber strongly varies over the course of a single 60 ks observation. I will discuss these findings in the context of TDEs and compare this newly detected outflow with that of ASASSN-14li.