Multi-Wavelength Constraints on the Outflow Properties of the Extremely Bright Millisecond Radio Bursts from the Galactic Magnetar SGR 1935+2154

Abstract: Extremely bright coherent radio bursts with millisecond duration, reminiscent of cosmological fast radio bursts (FRBs), were co-detected with anomalously-hard X-ray bursts from a Galactic magnetar SGR 1935$+$2154. We investigate the possibility that the event was triggered by the magnetic energy injection inside the magnetosphere, thereby producing magnetically-trapped fireball (FB) and relativistic outflows simultaneously. The thermal component of the X-ray burst is consistent with a trapped FB with an average temperature of $\sim200$-$300$ keV and size of $\sim10^5$ cm. Meanwhile, the non-thermal component of the X-ray burst and the coherent radio burst may arise from relativistic outflows. We calculate the dynamical evolution of the outflow, launched with an energy budget of $10^{39}$-$10^{40}$ erg comparable to that for the trapped FB, for different initial baryon load $\eta$ and magnetization $\sigma_0$. If hard X-ray and radio bursts are both produced by the energy dissipation of the outflow, the outflow properties are constrained by combining the conditions for photon escape and the intrinsic timing offset $\lesssim10$ ms among radio and X-ray burst spikes. We show that the hard X-ray burst must be generated at $r_{\rm X}\gtrsim10^{8}$ cm from the magnetar, irrespective of the emission mechanism. Moreover, we find that the outflow quickly accelerates up to a Lorentz factor of $10^2\lesssim\Gamma\lesssim10^3$ by the time it reaches the edge of the magnetosphere and the dissipation occurs at $10^{12}$ cm $\lesssim r_{\rm radio,X}\lesssim10^{14}$ cm. Our results imply either extremely-clean ($\eta\gtrsim10^4$) or highly-magnetized ($\sigma_0\gtrsim10^3$) outflows, which might be consistent with the rarity of the phenomenon.