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. 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 ≳ a few M _⊙, a neutron star with an age of t _age ∼ 10-100 years, an initial spin period of P _I ≲ a few ms, and a dipole magnetic field of B _dip ≲ a few × 10¹³ 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 ∼ 0.1 M _⊙, a younger neutron star with t _age ∼ 1-10 years 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.