Fast Radio Burst Breakouts from Magnetar Burst Fireballs

Abstract

The recent discovery of a Mega-Jansky radio burst simultaneously with short X-ray bursts from the Galactic magnetar (strongly-magnetized neutron star (NS)) SGR 1935+2154 is a smoking gun for the hypothesis that some cosmological fast radio bursts (FRBs) arise from magnetar bursts. We argue that the X-ray bursts with high temperature \(T \sim 80\) keV entail an electron-positron (\(e^{\pm}\)) outflow from a trapped-expanding fireball, polluting the NS magnetosphere before the FRB emission. The \(e^{\pm}\) outflow is opaque to induced Compton scatterings of FRB photons, and is strongly Compton-dragged by the X-ray bursts. Nevertheless the FRB photons can break out the \(e^{\pm}\) outflow with radiation forces if the FRB emission radius is larger than a few tens of NS radii. A FRB is chocked if the FRB is weaker or X-ray bursts are stronger, possibly explaining no FRBs with giant flares and no detectable X-ray bursts with weak FRBs. We also speculate that the \(e^{\pm}\) outflow may be needed to generate FRBs, solving why the FRBs occur only with the high-\(T\) X-ray bursts. The breakout physics is important for constraining the emission mechanism and electromagnetic counterparts to future FRBs.