Ultrafast Outflow in Tidal Disruption Event ASASSN-14li

TL;DR

This study reports the detection of a high-velocity ionized outflow in the TDE ASASSN-14li, observed through X-ray absorption features that diminish over time, revealing insights into the outflow's evolution as accretion rates change.

Contribution

It provides the first evidence of a relativistic outflow in a TDE through X-ray spectroscopy and models its evolution over time.

Findings

Detection of a broad, blueshifted OVIII absorption feature at early times.

Outflow velocity estimated at 0.2c from photoionization modeling.

Absence of the outflow feature at late times as accretion decreases.

Abstract

At only 90 Mpc, ASASSN-14li is one of the nearest tidal disruption event (TDE) ever discovered, and because of this, it has been observed by several observatories at many wavelengths. In this paper, we present new results on archival XMM-Newton observations, three of which were taken at early times (within 40 days of the discovery), and three of which were taken at late times, about one year after the peak. We find that, at early times, in addition to the ~1e5 K blackbody component that dominates the X-ray band, there is evidence for a broad, P Cygni-like absorption feature at around 0.7 keV in all XMM-Newton instruments (CCD detectors and grating spectrometers), and that this feature disappears (or at least diminishes) in the late-time observations. We perform photoionization modelling with XSTAR and interpret this absorption feature as blueshifted OVIII, from an ionized outflow with a velocity of 0.2c. As the TDE transitions from high to low accretion rate, the outflow turns off, thus explaining the absence of the absorption feature during the late-time observations.