Evidence of a Massive Stellar Disruption in the X-Ray Spectrum of ASASSN-14li

Abstract

The proximity and duration of the tidal disruption event ASASSN-14li led to the discovery of narrow, blueshifted absorption lines in X-rays and UV. The gas seen in X-ray absorption is consistent with bound material close to the apocenter of elliptical orbital paths, or with a disk wind similar to those seen in Seyfert-1 active galactic nuclei. We present a new analysis of the deepest high-resolution XMM-Newton and Chandra spectra of ASASSN-14li. Driven by the relative strengths of He-like and H-like charge states, the data require [N/C] ≥ 2.4, in qualitative agreement with UV spectral results. Flows of the kind seen in the X-ray spectrum of ASASSN-14li were not clearly predicted in simulations of TDEs; this left open the possibility that the observed absorption might be tied to gas released in prior active galactic nucleus (AGN) activity. However, the abundance pattern revealed in this analysis points to a single star rather than a standard AGN accretion flow comprised of myriad gas contributions. The simplest explanation of the data is likely that a moderately massive star (M ≳ 3 M ⊙) with significant CNO processing was disrupted. An alternative explanation is that a lower mass star was disrupted that had previously been stripped of its envelope. We discuss the strengths and limitations of our analysis and these interpretations.