X-ray and gamma-ray timing of GRB 180720B, GRB 181222B, GRB 211211A and GRB 220910A observed with Fermi and ASIM

TL;DR

This study analyzes gamma-ray and X-ray timing data from four bright GRBs observed with Fermi and ASIM, revealing low-frequency QPOs linked to neutron star spins and binary mergers, including the first detection of such QPOs in magnetars.

Contribution

It reports the first detection of low-frequency QPOs in magnetars and links observed QPOs to neutron star spins and binary merger dynamics in GRBs.

Findings

Detection of low-frequency QPOs (2.5-3.5 Hz) in GRBs, possibly related to neutron star spins.

Identification of a strong 22 Hz QPO in GRB 181222B and its harmonics.

First observation of low-frequency QPOs in magnetars.

Abstract

We present a timing study of the gamma and X-ray observations and analysis of a sample of bright gamma-ray bursts (GRBs; i.e. GRB 180720B, GRB 181222B, GRB 211211A and GRB 220910A), including the very bright and long GRB 211211A (a.k.a. kilonova candidate). They have been detected and observed by the Atmosphere-Space Interactions Monitor (ASIM) installed on the International Space Station (ISS) and the Gamma-ray Burst Monitor (GBM) on-board the Fermi mission. The early (T-T0=s) and high-energy (0.3-20 MeV) ASIM High Energy Detector (HED) and (150 keV-30 MeV) Fermi (BGO) light curves show well-defined peaks with a low quasi-periodic oscillation (QPO) frequency between 2.5-3.5 Hz that could be identified with the spin of the neutron star in the binary mergers (coinciding with the orbital frequency of the binary merger) originating these GRBs. These QPOs consist on the first detection of low-frequency QPOs (<10 Hz) detected in magnetars so far. We also detect a strong QPO at 21.8-22 Hz in GRB 181222B together with its (less significant) harmonics. The low-frequency QPO would correspond to the signal of the orbiting neutron star (NS) previous to the final coalescence giving rise to the gravitational-wave (GW) signal.