Letter to the Editor

Circular polarization of gravitational waves from magnetorotational supernovae

¹ Incubator of Scientific Excellence – Centre for Simulations of Superdense Fluids, University of Wrocław, 50-204 Wrocław, Poland
² Institute of Theoretical Physics, Wrocław University of Science and Technology, W. Wyspiaṅskiego 27, 50-370 Wrocław, Poland
³ National Astronomical Observatory of Japan (NAOJ), 2-21-1, Osawa, Mitaka, Tokyo 181-8588, Japan
⁴ Department of Applied Physics, Fukuoka University, 8-19-1, Nanakuma, Jonan, Fukuoka 814-0180, Japan
⁵ Max-Planck-Institut für Gravitationsphysik, Am Mühlenberg 1, D-14476 Potsdam-Golm, Germany
⁶ Research Center for Computational Physics and Data Processing, Institute of Physics, Silesian University in Opava, Bezručovo nám. 13, CZ-746-01 Opava, Czech Republic

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 20 March 2026
Accepted: 11 June 2026

Abstract

Context. Gravitational waves (GWs) provide a unique probe of the explosion mechanism of massive stars and the evolution of nascent proto-neutron stars (PNSs). Magnetorotational explosions are one of the promising noncanonical core-collapse supernova scenarios, and they might be linked to magnetar formation and energetic supernova explosions. However, the GW signatures of such events currently remain incompletely understood.

Aims. We investigate the origin and nature of GW polarization arising from a magnetorotational core-collapse model and examine its potential detectability by current GW observatories.

Methods. We performed a three-dimensional simulation of general-relativistic magnetohydrodynamics of a rapidly rotating, strongly magnetized 20 M_⊙ progenitor, including multi-energy neutrino transport. The GW signals were extracted using the standard quadrupole formalism, and their polarization states were analyzed with Stokes parameters.

Results. Strong circular polarization emerges along the rotation axis during the early post-bounce phase (≲230 ms after core bounce). The characteristic GW spectrum peaks at ∼90 Hz, consistent with the emission at twice the local angular velocity (∼45 Hz) around the PNS surface at cylindrical radii of ∼50 km. These features are attributed to the low-T/|W| instabilities and nonaxisymmetric motions near the PNS and not to the magnetohydrodynamic jets themselves. The polarization signals lie within the sensitivity bands of current detectors such as Advanced LIGO, Advanced Virgo, and KAGRA.

Conclusions. Our study demonstrates that models in which magnetorotationally driven jets are launched can produce circularly polarized GW signals originating from the inner PNS region. This provides an observational signature that complements previous findings from nonmagnetized rotating models. Thus, our novel findings establish that the GW polarization is a promising diagnostic of noncanonical core-collapse supernovae. Future third-generation detectors will be crucial to fully exploit this potential.