Abstract

In this thesis, we present an analysis of highly magnetized neutron stars â magnetarsâ , 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 the soft gamma repeater SGR 1806-20 using Rossi X-ray Timing Explorer (RXTE) observations. By searching a large sample of 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 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. These QPOs are detected in archival observations that took place eight years before the giant 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 (1095, 1230, 2785 and 3690 Hz) 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 magnetar crustquake of the neutron star crust. Other models have been proposed to explain the QPO phenomena we observe in magnetars including magnetospheric oscillations, magnetic flux tubes and modes of a passive debris disk. After understanding the real nature of quasi-periodic X-ray oscillations, their observation will be very useful to put stringent constraints on neutron star masses and radii, which define the neutron star equation of state (EOS). We discuss the connection between the QPOs we report and those detected in the giant flares as well as their implications to the neutron star properties.

School

School of Sciences and Engineering

Degree Name

MS in Physics

First Advisor

Ibrahim, Alaa

Document Type

Thesis

Rights

The American University in Cairo grants authors of theses and dissertations a maximum embargo period of two years from the date of submission, upon request. After the embargo elapses, these documents are made available publicly. If you are the author of this thesis or dissertation, and would like to request an exceptional extension of the embargo period, please write to thesisadmin@aucegypt.edu

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