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Evidence for Quasi-Periodic Oscillations in the Recurrent Emission from Magnetars and their Implications on the Neutron Star Properties and Equation of State

We present an analysis of highly magnetized neutron stars "magnetars", in search for high frequency oscillations in the recurrent emission from the soft gamma repeater SGR 1806-20, and we discuss the physical interpretation of these oscillations and its implications on the neutron star properties and structure. We present evidence for Quasi-Periodic Oscillations (QPOs) in the recurrent outburst activity from SGR 1806-20 using RXTE observations. By searching 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 8.85% and 11.83%, respectively, from the 92 Hz QPO detected in the giant flare. The third QPO lie within 3.75% 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. 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 findings against the available theoretical models and discuss the physical interpretation of these QPOs. The leading interpretation for the origin of magnetar QPOs suggests that these toroidal seismic oscillatory modes are most likely to be excited by a crustquake of the neutron star crust. Other models have been proposed to explain the magnetar QPO phenomena including magnetospheric oscillations, magnetic flux tubes and modes of a passive debris disk. Theoretical modeling and observation of QPOs will be very useful in putting stringent constraints on neutron star equation of state.