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A Magnetar-Asteroid Impact Model for FRB 200428 Associated with an X-ray Burst from SGR 1935+2154

Very recently, an extremely bright fast radio burst (FRB) 200428 with two sub-millisecond pulses was discovered to come from the direction of the Galactic magnetar SGR 1935+2154, and an X-ray burst (XRB) counterpart was detected simultaneously. These observations favor magnetar-based interior-driven models. In this Letter, we propose a different model for FRB 200428 associated with an XRB from SGR 1935+2154, in which a magnetar with high proper velocity encounters an asteroid of mass $\sim10^{20}\,$g. This infalling asteroid in the stellar gravitational field is first possibly disrupted tidally into a great number of fragments at radius $\sim {\rm a\,\,few}$ times $10^{10}\,$cm, and then slowed around the Alfv$\acute{\rm e}$n radius by an ultra-strong magnetic field and in the meantime two major fragments of mass $\sim 10^{17}\,$g that cross magnetic field lines produce two pulses of FRB 200428. The whole asteroid is eventually accreted onto the poles along magnetic field lines, impacting the stellar surface, creating a photon-e$^\pm$ pair fireball trapped initially in the stellar magnetosphere, and further leading to an XRB. We show that this gravitationally-powered model can interpret all of the observed features self-consistently.