Implications of the lowest frequency detection of the persistent counterpart of FRB121102

Abstract

Context. The repeating FRB121102 is so far the only extragalactic fast radio burst found to be associated with a counterpart, that is to say a steady radio source with a nearly flat spectral energy distribution (SED) in centimeter wavelengths. Aims. Previous observations of the persistent source down to 1.6 GHz have not provided a sign of a spectral turnover. Absorption is expected to eventually cause a turnover at lower frequencies. Better constraints on the physical parameters of the emitting medium can be derived by detecting the self-absorption frequency. Methods. We used the Giant Metre-Wave Radio Telescope during the period of July to December 2017 to observe the source at low radio frequencies down to 400 MHz. Results. The spectral energy distribution of the source remains optically thin even at 400 MHz, with a spectral index of ν−(0.07 ± 0.03), which is similar to what is seen in Galactic plerions. Using a generic synchrotron radiation model, we obtain constraints on properties of the nonthermal plasma and the central engine powering it. Conclusions. We present low frequency detections of the persistent source associated with FRB121102. Its characteristic flat SED extends down to 400 MHz. Similar to models for Galactic plerions, we assume that the energy in the persistent source is carried predominantly by leptons. The emitting plasma has a B < 0.01 G, and its age is >524(B/0.01 G)−3/2 yr. We show that the energetics of the persistent source requires an initial spin period shorter than 36 ms, and the magnetic field of the neutron star must exceed 4.5 × 1012 G. This implies that the persistent source does not necessarily require energetic input from a magnetar.