FRB 121102: No supernova-like ejecta or magnetar power, hinting at a binary WD merger

We use measurements of the time-dependent dispersion measure of the repeating FRB 121102, together with earlier radio observations of its associated persistent radio source (PRS), to derive stringent constraints on its underlying ``engine.'' The energy held by the relativistic PRS plasma is >10 49.5 erg, and its age is about60 yr, corresponding to an underlying source power exceeding 10 40 erg s-1. If the underlying source is a neutron star, this implies a rotational rather than a magnetic energy source, consistent with a about10 ms, about10 12.5 G neutron star. Alternatively, accretion may also be a viable energy source. The velocity and the kinetic energy of the cold plasma confining the relativistic PRS plasma are inconsistent with it being typical supernova ejecta (unless a significant fraction of the ejecta mass is carried by high-density clumps of about10-2.5 fractional size)- its expansion speed is limited to a few hundred km/s, which also implies that it was ejected from the source more than about 10 cubed yr preceding the onset of relativistic PRS plasma emission. These constraints may be satisfied by a white dwarf binary merger progenitor system, where a fraction of a solar mass was ejected at a slow speed during and after the merger, and a rapidly rotating neutron star was formed after about10 cubed yr thermal evolution period of the merger remnant.