Paper detail

Delayed X-ray brightening accompanied by variable ionized absorption following a tidal disruption event

Supermassive black holes can experience super-Eddington peak mass fallback rates following the tidal disruption of a star. The theoretical expectation is that part of the infalling material is expelled by means of an accretion disk wind, whose observational signature includes blueshifted absorption lines of highly ionized species in X-ray spectra. To date, however, only one such ultra-fast outflow (UFO) has been reported in the tidal disruption event (TDE) ASASSN-14li. Here we report on the discovery of transient absorption-like signatures in X-ray spectra of the TDE AT2020ksf/Gaia20cjk (at a redshift of $z$=0.092), following an X-ray brightening $\sim 230$ days after UV/optical peak. We find that while no statistically significant absorption features are present initially, they appear on a timescale of several days, and remain detected up to 770 days after peak. Simple thermal continuum models, combined with a power-law or neutral absorber, do not describe these features well. Adding a partial covering, low velocity ionized absorber improves the fit at early times, but fails at late times. A high velocity (v$_w$ $\sim$ 42000 km s$^{-1}$, or -0.15c), ionized absorber (ultra-fast outflow) provides a good fit to all data. The few day timescale of variability is consistent with expectations for a clumpy wind. We discuss several scenarios that could explain the X-ray delay, as well as the potential for larger scale wind feedback. The serendipitous nature of the discovery could suggest a high incidence of UFOs in TDEs, alleviating some of the tension with theoretical expectations.