On the origin of the radio emission of Sw 1644+57

Abstract

We apply relativistic equipartition synchrotron arguments to the radio data of the tidal disruption event candidate Sw 1644+57. We find that, regardless of the details of the equipartition scenario considered, the energy required to produce the observed radio (i.e., energy in magnetic field and radio emitting electrons) must increase by a factor of ~20 during the first 200 days. It then saturates. This energy increase cannot be alleviated by a varying geometry of the system. The radio data can be explained by: (i) An afterglow like emission of the X-ray emitting narrow relativistic jet. The additional energy can arise here from a slower moving material ejected in the first few days that gradually catches up with the slowing down blast wave (Berger et al. 2012). However, this requires at least ~4x10^{53} erg in the slower moving outflow. This is much more than the energy of the fast moving outflow that produced the early X-rays and it severely constrains the overall energy budget. (ii) Alternatively, the radio may arise from a mildly relativistic quasi-spherical outflow. Here, the energy for the radio emission increases with time to at least ~10^{51} erg after 200 days. This scenario requires, however, a second X-ray emitting collimated relativistic component. Given these results, it is worthwhile to consider models in which the energy of the magnetic field and/or of the radio emitting electrons increases with time without a continuous energy supply to the blast wave. This can happen, for example, if the energy is injected initially mostly in one form (Poynting flux or baryonic) and it is gradually converted to the other form, leading to a deviation from equipartition. Another intriguing possibility is that a gradually decreasing Inverse Compton cooling modifies the synchrotron emission and leads to an increase of the available energy in the radio emitting electrons (Kumar et al. 2013).