Radio Observations of the Tidal Disruption Event XMMSL1 J0740–85

Abstract

We present radio observations of the tidal disruption event candidate (TDE) XMMSL1 J0740−85 spanning 592 to 875 d post X-ray discovery. We detect radio emission that fades from an initial peak flux density at 1.6 GHz of 1.19±0.06 mJy to 0.65±0.06 mJy suggesting an association with the TDE. This makes XMMSL1 J0740−85 at d=75 Mpc the nearest TDE with detected radio emission to date and only the fifth TDE with radio emission overall. The observed radio luminosity rules out a powerful relativistic jet like that seen in the relativistic TDE Swift J1644+57. Instead we infer from an equipartition analysis that the radio emission most likely arises from a non-relativistic outflow similar to that seen in the nearby TDE ASASSN-14li, with a velocity of about 104 km s−1 and a kinetic energy of about 1048 erg, expanding into a medium with a density of about 102 cm−3. Alternatively, the radio emission could arise from a weak initially-relativistic but decelerated jet with an energy of ≲5×1050 erg, or (for an extreme disruption geometry) from the unbound debris. The radio data for XMMSL1 J0740−85 continues to support our previous suggestion of a bimodal distribution of common non-relativistic isotropic outflows and rare relativistic jets in TDEs (in analogy with the relation between Type Ib/c supernovae and long-duration gamma-ray bursts). The radio data also provide a new measurement of the circumnuclear density on a sub-parsec scale around an extragalactic supermassive black hole.