Evidence For Quasi-Periodic Oscillations In The Recurrent Bursts From SGR 1806-20

TL;DR

This study provides evidence for quasi-periodic oscillations in recurrent bursts from SGR 1806-20, with frequencies close to those observed in a giant flare, suggesting a possible physical connection.

Contribution

First detection of QPOs in recurrent SGR bursts at frequencies similar to giant flare QPOs, expanding understanding of neutron star oscillations.

Findings

QPO at 648 Hz with 5.17σ confidence

Additional QPOs at 84 and 103 Hz with over 4σ confidence

Candidate QPOs at 1096, 1230, 2785, and 3690 Hz with 4.14-4.46σ confidence

Abstract

We present evidence for Quasi Periodic Oscillations (QPOs) in the recurrent outburst activity from SGR 1806-20 using Rossi X-ray Timing Explorer (RXTE) observations during November 1996. Searching for QPOs in a sample of 30 bursts at similar frequencies to those previously reported in the December 27, 2004 giant flare, we find evidence for a QPO in a burst at 648 Hz at 5.17{\sigma} confidence level, lying within 3.75% from the 625 Hz QPO discovered in the giant flare. Two additional features are also detected around 84 and 103 Hz in two other bursts at 4.2{\sigma} and 4.8{\sigma} confidence level, respectively, which lie within 8.85% and 11.83% respectively from the QPO at 92.5 Hz also detected in the giant flare. Accounting for the number of bursts analyzed the confidence levels for the 84, 103 and 648 Hz becomes 3{\sigma}, 3.6{\sigma} and 3.4{\sigma} respectively. Extending our search to other frequency ranges, we find candidates at 1096, 1230, 2785 and 3690 Hz in 3 different bursts with confidence levels lying between 4.14{\sigma}-4.46{\sigma}, which is reduced to 2.3{\sigma}-3{\sigma} after accounting for a certain confirmation bias in each case. 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. We examine the candidate QPOs in relation with those found in the giant flare and discuss their possible physical origin.