2021NatAs...5..372R

Fast radio bursts (FRBs) are bright, millisecond-scale radio flashes of unknown physical origin. Young, highly magnetized, isolated neutron stars-magnetars-have been suggested as the most promising candidates for FRB progenitors owing to their energetics and high X-ray flaring activity. Here we report the detection with Konus-Wind of a hard X-ray event of 28 April 2020 temporally coincident with a bright, two-peak radio burst in the direction of Galactic magnetar SGR 1935+2154, with properties remarkably similar to those of FRBs. We show that the two peaks of the double-peaked X-ray burst coincide in time with the radio peaks and infer a common source and the association of these phenomena. An unusual hardness of the X-ray spectrum strongly distinguishes the 28 April event among multiple `ordinary' flares from SGR 1935+2154. A recent non-detection of radio emission from about 100 typical soft bursts from SGR 1935+2154 favours the idea that bright, FRB-like magnetar signals are associated with rare, hard-spectrum X-ray bursts. The implied rate of these hard X-ray bursts (∼0.04 yr^–1 magnetar^–1) appears consistent with the rate estimate of SGR 1935+2154-like radio bursts (0.007-0.04 yr^–1 magnetar^–1).