The 6 yr Radio Light Curve of the Tidal Disruption Event AT2019azh

Abstract Analysis of radio emission from tidal disruption events (TDEs) allows for detailed constraints on the properties of ejected outflows and the host environment surrounding the black hole. However, the late-time radio behaviour of TDEs is not well studied due to a lack of observations. In this work we present long-term radio monitoring observations of the TDE AT2019azh spanning 1167–2159 days postdisruption. We fit a physically motivated synchrotron model to the radio spectra at each epoch and model the decay of the light curve under the assumption that the outflow transitions into the nonrelativistic Sedov–Taylor regime at late times. Through this modelling we obtain strong constraints on the density profile of the circumnuclear medium, finding an unusually flat density profile proportional to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi>r</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>0.2</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.15</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.11</mml:mn> </mml:mrow> </mml:msubsup> </mml:mrow> </mml:msup> </mml:math> . Overall we find that unlike some TDEs, AT2019azh does not show late time rebrightening up to 6 yr postdisruption. The Sedov–Taylor light-curve decay model provides a good fit to the data, motivating the assumption that the outflow was produced by a single ejection of material close to the time of stellar disruption. From forward modelling the evolution of the radio light-curve decay we predict that the radio afterglow from AT2019azh should be detectable for decades at its current rate of evolution.