Testing the Young Neutron Star Scenario with Persistent Radio Emission Associated with FRB 121102

Recently a repeating fast radio burst (FRB) 121102 has been confirmed to be an extragalactic event and a persistent radio counterpart has been identified. While other possibilities are not ruled out, the emission properties are broadly consistent with Murase et al. (2016) that theoretically proposed quasi-steady radio emission as a counterpart of both FRBs and pulsar-driven supernovae. Here we constrain the model parameters of such a young neutron star scenario for FRB 121102. If the associated supernova has a conventional ejecta mass of M-ej greater than or similar to a few M circle dot, a neutron star with an age of t(age) similar to 10-100 yrs, an initial spin period of P-i less than or similar to a few ms, and a dipole magnetic field of B-dip less than or similar to a few x 10(13) G can be compatible with the observations. However, in this case, the magnetically-powered scenario may be favored as an FRB energy source because of the efficiency problem in the rotation-powered scenario. On the other hand, if the associated supernova is an ultra-stripped one or the neutron star is born by the accretion-induced collapse with M-ej similar to 0.1 M circle dot, a younger neutron star with t(age) similar to 1-10 yrs can be the persistent radio source and might produce FRBs with the spin-down power. These possibilities can be distinguished by the decline rate of the quasi-steady radio counterpart.