Episodic X-Ray Outflows from the Tidal Disruption Event ASASSN-14li

Abstract

ASASSN-14li is a low-redshift (z = 0.0206) tidal disruption event (TDE) that has been studied extensively across the entire electromagnetic spectrum and has provided one of the most sensitive measurements of a TDE to date. Its X-ray spectrum is soft and thermal (kT ∼ 0.05 keV) and shows a residual broad absorption feature between 0.6 and 0.8 keV, which can be associated with a blueshifted O VII line (rest-frame energy 0.57 keV) resulting from an ultrafast outflow at early times (within 40 days of optical discovery). By carefully accounting for photon pileup and using XSTAR photoionization models tailored to the evolving disk continuum properties, we analyze the entire archival X-ray data from XMM-Newton and track the evolution of this absorption feature for ∼4.5 yr post-disruption. Our main finding is that the absorption feature is transient and intermittent. Assuming the same underlying physical model (i.e., outflows) for the recurring absorption feature in ASASSN-14li, the outflow is seen to disappear and reappear multiple times during the first ∼1.5 yr of its evolution. No observable spectral imprint is detected thereafter. While theoretical studies suggest the launch of outflows in the early phases of the outburst during the super-Eddington regime, the outflow's intermittent behavior for multiple years after disruption is unusual. We discuss this peculiar behavior within the context of varying inner-disk truncation, radiation pressure, and magnetically driven outflow scenarios.