A 4.5 s Quasiperiodic Spectral Oscillation in GRB 230307A: Evidence for Free Precession of a Postmerger Magnetar?

Abstract Millisecond magnetars, rapidly rotating neutron stars (NSs) with ultra-strong magnetic fields, have long been proposed as central engines of gamma-ray bursts (GRBs). For GRBs produced by NS mergers, the survival of a long-lived magnetar remnant remains uncertain, as the merger remnant may rapidly collapse into a black hole. In GRB 230307A, multiwavelength observations together with a previously reported 909 Hz periodic signal consistent with millisecond spin in its prompt emission provide strong evidence that such a postmerger magnetar may power the burst. Here, we report the discovery of a quasiperiodic modulation with a characteristic period of 4.5 s in the spectral evolution of GRB 230307A, detected consistently across multiple gamma-ray instruments. The modulation is manifested as a coherent, energy-dependent variation of the spectral shape, with the strongest signature in the evolution of the peak energy. Within the magnetar-engine framework, such a low-frequency modulation can be interpreted as a manifestation of large-scale periodic variations associated with the central engine. If interpreted in terms of free precession, the observed timescale implies a stellar ellipticity of ϵ ≳ 2.4 × 10 −4 , corresponding to an internal magnetic field strength of B t ≳ 1.6 × 10 16 G, alongside a dipole field of B p ≈ 5.6 × 10 15 G inferred from the early X-ray emission. These results suggest that such systems may provide potential sources of postmerger gravitational waves, motivating targeted searches following GRB triggers.