Free–free absorption in hot relativistic flows: application to fast radio bursts

ABSTRACT Magnetic flares create hot relativistic shocks outside the light cylinder radius of a magnetized star. Radio emission produced in such shock or at smaller than undergoes free–free absorption while passing through shocked medium. In this work, we demonstrate that can lead to negative drift frequency-time spectra. Whether it is related downward pattern observed fast radio bursts (FRBs) unclear. However, if FRB down-drifting due mechanism then will be pronounced those have isotropic kinetic energies $ \gtrsim 10^{44}$ erg. model, for an internal with Lorentz factor ∼100, normalized rate $|{\rm DR_{\rm obs}}|/\nu _{\rm mean}$ ∼10−2 per ms, where νmean central frequency pulses. The corresponding shell is, therefore, range 1010 cm and 1011 cm. This implies that, outflow consisting hydrogen ion, upper limit on mass few × 10−10 M⊙, which considerably low compared ejected from SGR 1806-20 during 2004 outburst.