Constraining the FRB mechanism from scintillation in the host galaxy

Pawan Kumar, Paz Beniamini, Om Gupta, James M. Cordes

Published 2023-07-28Version 1

We propose an observational test to distinguish between the two primary classes of Fast Radio Burst (FRB) radiation models. For an extended FRB source, the strength of spectral amplitude modulation caused by scintillation, which we refer to as the scintillation index, across the measured frequency band is much smaller than the 100% variations expected for a point source. FRB sources easily satisfy the point-source size requirement for scintillation from the Milky Way ISM plasma but may be large enough for certain classes of proposed FRB mechanisms to quench scintillation from plasma in the host galaxy. An FRB source of size smaller than the diffractive scale for scattering in the ISM of the host galaxy, which is about 10$^9$ cm at 1 GHz, would be considered point-like for the purpose of scintillation. The lateral size of the source for a magnetospheric origin of FRBs is expected to be $\lesssim 10^7$ cm, while it is $\gtrsim 10^9$ cm for far-away models, i.e., models in which the emission is generated far from the central engine. Therefore, scintillation in the host galaxy is well-suited for distinguishing between these two types of FRB models. Scatter-broadening of an FRB pulse by plasma in the host galaxy by $\delta t_s$ causes stochastic flux variation with frequency on a scale of $\sim (2\pi\delta t_s)^{-1}$. Determining the scintillation index for an FRB with scatter broadening identified to be from the host galaxy would provide a strong constraint on the FRB radiation mechanism. Since the scintillation bandwidth scales with frequency as $\sim \nu^{4.4}$, it may be observationally easier to determine the flux variation amplitude at a few GHz.