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.
Highlights
The discovery of quasi-periodic oscillations (QPOs) in the giant flare of the magnetar SGR 1806-20 by Israel et al (2005) may have been the first detection of neutron star oscillations and triggered a wealth of theoretical work explaining the reported frequencies
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
We only found potential periodic signals at 9.2 Hz (SGR1806-20) and 7.7 Hz (SGR1900+14), which are significant in χ0 and in the combination of χ0 and χ1 according to our respective criterium
Summary
The discovery of quasi-periodic oscillations (QPOs) in the giant flare of the magnetar SGR 1806-20 by Israel et al (2005) may have been the first detection of neutron star oscillations and triggered a wealth of theoretical work explaining the reported frequencies. Large amounts of energy are released as an expanding e±-pair plasma, observable as the initial spike of the giant flare. Parts of this plasma are trapped by the ultra-strong magnetic field and form a so-called trapped fireball (Thompson & Duncan 1995), which slowly evaporates on a timescale of up to a few 100 s. Other groups have confirmed this detection, they even found additional oscillation frequencies in different magnetars: 18, 26, 29, 92, 150 , 625, and 1840 Hz in the giant flare of SGR 1806-20, and 28, 53, 84, and 155 Hz in the giant flare of SGR 1900+14 The more frequent but less energetic bursts of several magnetars have been investigated for frequencies, and some candidates were found: 57 Hz in SGR 1806-20
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