Discovery of an outflow from radio observations of the tidal disruption event ASASSN-14li

Abstract

We report the discovery of transient radio emission from the nearby optically-discovered TDE ASASSN-14li (distance of 90 Mpc), making it the first typical TDE detected in the radio, and unambiguously pointing to the formation of a non-relativistic outflow with a kinetic energy of $\approx (4-10)\times10^{47}$ erg, a velocity of $\approx 12,000-36,000$ km s$^{-1}$, and a mass of $\approx 3\times10^{-5}-7\times10^{-4}$ M$_{\odot}$. We show that the outflow was ejected on 2014 August 11--25, in agreement with an independent estimate of the timing of super-Eddington accretion based on the optical, UV, and X-ray observations, and that the ejected mass corresponds to about $1-10\%$ of the mass accreted in the super-Eddington phase. The temporal evolution of the radio emission also uncovers the circumnuclear density profile, $\rho(R)\propto R^{-2.5}$ on a scale of about 0.01 pc, a scale that cannot be probed via direct measurements even in the nearest SMBHs. Our discovery of radio emission from the nearest well-studied TDE to date, with a radio luminosity lower than all previous limits, indicates that non-relativistic outflows are ubiquitous in TDEs, and that future, more sensitive, radio surveys will uncover similar events.