Long-term radio and X-ray evolution of the tidal disruption event ASASSN-14li

TL;DR

This study presents long-term radio and X-ray observations of the tidal disruption event ASASSN-14li, revealing a plateau in radio emission likely from relic lobes and a decoupled evolution from X-ray emission, providing insights into jet and accretion processes.

Contribution

First detailed long-term radio and X-ray monitoring of ASASSN-14li, showing a stable radio plateau and correlation with X-ray emission, challenging existing models of ejecta evolution.

Findings

Radio emission plateau at 244 μJy after 400 days

Radio and X-ray emissions initially fade together with a correlation of L_R ∝ L_X^1.9

Radio emission likely from relic lobes, not ejecta

Abstract

We report on late time radio and X-ray observations of the tidal disruption event candidate ASASSN-14li, covering the first 1000 days of the decay phase. For the first $\sim200$ days the radio and X-ray emission fade in concert. This phase is better fit by an exponential decay at X-ray wavelengths, while the radio emission is well described by either an exponential or the canonical $t^{-5/3}$ decay assumed for tidal disruption events. The correlation between radio and X-ray emission during this period can be fit as $L_{R}\propto L_{X}^{1.9\pm0.2}$. After 400 days the radio emission at $15.5\,\textrm{GHz}$ has reached a plateau level of $244\pm8\,\mu\textrm{Jy}$ which it maintains for at least the next 600 days, while the X-ray emission continues to fade exponentially. This steady level of radio emission is likely due to relic radio lobes from the weak AGN-like activity implied by historical radio observations. We note that while most existing models are based upon the evolution of ejecta which are decoupled from the central black hole, the radio : X-ray correlation during the declining phase is also consistent with core jet emission coupled to a radiatively efficient accretion flow.