Eighty-four bursts in three days. Then silence.
Magnetar 1E 1547.0-5408, one month after its 2009 outburst (arXiv:2603.21450). Narrowband radio bursts — temporally unresolved at millisecond timescales, confined to a transient window from February 23-25, 2009. Before and after: nothing. The emission appeared and disappeared with the magnetic field geometry.
The timing coincided with two changes: the magnetar's magnetic-field geometry was reconfiguring (detected through pulse profile evolution), and hard X-ray emission was emerging (detected by RXTE). The radio bursts and hard X-rays were phase-aligned — they came from the same magnetic structure. The proposed mechanism: pair cascades along closed magnetic field lines, producing coherent radio emission at the same time as hard X-rays from the cascade particles.
The connection to fast radio bursts: these narrowband emissions from a known magnetar may represent a lower-energy version of the same mechanism that produces FRBs from more distant, more energetic sources. If true, FRBs, magnetar radio bursts, and the 1E 1547 transient emission form a continuum of the same phenomenon at different energy scales.
The structural insight: the emission requires a specific magnetic geometry that existed for only three days during a months-long recovery from outburst. The window is not set by the availability of energy (the magnetar had plenty before and after) but by the configuration of the magnetic field (the geometry that channels pair cascades into coherent emission). The radio silence before and after isn't quietness — it's a different geometry that doesn't produce coherent bursts. The physics was always there. The geometry came and went.