Radiative interaction between the relativistic jet and optically thick envelope in tidal disruption events

Abstract

Reverberation observations have uncovered an Fe K{\alpha} fluorescence line in the tidal disruption event (TDE) Swift J1644+57 (Kara et al. 2016). The discovery paper used the lag spectrum to argue that the X-ray continuum source was located very close to the blackhole (~30 gravitational radii) and moved sub-relativistically. We reanalyze the lag spectrum, pointing out that dilution effects cause it to indicate a geometric scale an order of magnitude larger than previously inferred. If the X-ray continuum is produced by a relativistic jet, as suggested by rapid variability, high luminosity and hard spectrum, this larger scale predicts an Fe ionization state consistent with efficient K{\alpha} production. Moreover, the momentum of the jet radiation impinging on the surrounding accretion flow on this larger scale accelerates a layer of gas to speeds ~0.1-0.2c, consistent with the blueshifted line profile. Implications of our results on the global picture of jetted TDEs are discussed. A power-law {\gamma}/X-ray spectrum may be produced by external UV-optical photons being repetitively inverse-Compton scattered by cold electrons in the jet, although our model for the K{\alpha} reverberation does not depend on the jet radiation mechanism (magnetic reconnection in a Poynting jet is still a viable mechanism). The non-relativistic wind driven by jet radiation may explain the late-time radio rebrightening in Swift J1644+57. This energy injection may also cause the thermal UV-optical emission from jetted TDEs to be systematically brighter than in non-jetted ones.