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

TL;DR

This paper reports the discovery of a 4.5-second quasiperiodic spectral modulation in GRB 230307A, providing evidence for a free precessing post-merger magnetar as the burst's central engine.

Contribution

It presents the first detection of a low-frequency quasiperiodic modulation in a GRB, linking it to free precession of a post-merger magnetar and estimating its internal magnetic field.

Findings

Detected a 4.5-s quasiperiodic modulation across multiple gamma-ray instruments.

Interpreted the modulation as free precession, implying a stellar ellipticity of ε ≥ 2.4×10⁻⁴.

Estimated internal magnetic field strength B_t ≥ 1.6×10¹⁶ G and dipole field B_p ≈ 5.6×10¹⁵ G.

Abstract

Millisecond magnetars, rapidly rotating neutron stars with ultra-strong magnetic fields, have long been proposed as central engines of gamma-ray bursts (GRBs). For GRBs produced by neutron star 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 post-merger 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 $\epsilon \gtrsim 2.4 \times 10^{-4}$, corresponding to an internal magnetic field strength of $B_t \gtrsim 1.6 \times 10^{16}$ G, alongside a dipole field of $B_p \approx 5.6 \times 10^{15}$ G inferred from the early X-ray emission. These results suggest that such systems may provide potential sources of post-merger gravitational waves (GWs), motivating targeted searches following GRB triggers.