Discovery of Quasi-Periodic Oscillations in the Recurrent Burst Emission from SGR 1806-20

TL;DR

This paper reports the detection of quasi-periodic oscillations in recurrent bursts from SGR 1806-20, revealing frequencies close to those in giant flares, which may provide insights into neutron star properties and oscillation origins.

Contribution

It is the first evidence of QPOs in recurrent bursts of SGR 1806-20 at frequencies similar to giant flare QPOs, expanding understanding of neutron star oscillations.

Findings

Detected QPOs at 84, 103, and 648 Hz in recurrent bursts

QPO frequencies are close to those in giant flares

QPOs are statistically significant with confidence ≥ 4.3 σ

Abstract

We present evidence for Quasi-Periodic Oscillations (QPOs) in the recurrent outburst emission from the soft gamma repeater SGR 1806-20 using NASA's Rossi X-ray Timing Explorer (RXTE) observations. By searching a sample of 30 bursts for timing signals at the frequencies of the QPOs discovered in the 2004 December 27 giant flare from the source, we find three QPOs at 84, 103, and 648 Hz in three different bursts. The first two QPOs lie within $\sim$ 1$\: \sigma$ from the 92 Hz QPO detected in the giant flare. The third QPO lie within $\sim$ 9$\: \sigma$ from the 625 Hz QPO also detected in the same flare. The detected QPOs are found in bursts with different durations, morphologies, and brightness, and are vindicated by Monte Carlo simulations, which set a lower limit confidence interval $\geq 4.3 \sigma$. We also find evidence for candidate QPOs at higher frequencies in other bursts with lower statistical significance. The fact that we can find evidence for QPOs in the recurrent bursts at frequencies relatively close to those found in the giant flare is intriguing and can offer insight about the origin of the oscillations. We confront our finding against the available theoretical models and discuss the connection between the QPOs we report and those detected in the giant flares. The implications to the neutron star properties are also discussed.