Search for quasi-periodic signals in magnetar giant flares

Bayesian inspection of SGR 1806-20 and SGR 1900+14^★

Max Planck Institute for Astrophysics, Karl-Schwarzschild-Str. 1, 85741 Garching, German
e-mail: dpumpe@mpa-garching.mpg.de

Received: 18 August 2017
Accepted: 17 November 2017

Abstract

Quasi-periodic oscillations (QPOs) discovered in the decaying tails of giant flares of magnetars are believed to be torsional oscillations of neutron stars. These QPOs have a high potential to constrain properties of high-density matter. In search for quasi-periodic signals, we study the light curves of the giant flares of SGR 1806-20 and SGR 1900+14, with a non-parametric Bayesian signal inference method called D³PO. The D³PO algorithm models the raw photon counts as a continuous flux and takes the Poissonian shot noise as well as all instrument effects into account. It reconstructs the logarithmic flux and its power spectrum from the data. Using this fully noise-aware method, we do not confirm previously reported frequency lines at ν ≳ 17 Hz because they fall into the noise-dominated regime. However, we find two new potential candidates for oscillations at 9.2 Hz (SGR 1806-20) and 7.7 Hz (SGR 1900+14). If these are real and the fundamental magneto-elastic oscillations of the magnetars, current theoretical models would favour relatively weak magnetic fields B̅ ~ 6× 10¹³–3 × 10¹⁴ G (SGR 1806-20) and a relatively low shear velocity inside the crust compared to previous findings.