PhD Dissertation Defense: Özge Keskin,
pectrotemporal Studies of Magnetar Bursts
and Their Origin Through Crustal Yielding
Özge Keskin
Physics, PhD Dissertation, 2025
Thesis Jury
Prof. Ersin Göğüş (Thesis Advisor),
Prof. Emrah Kalemci, Prof. Ünal Ertan, Prof. Tolga Güver, Prof. Kazım Yavuz Ekşi
Date & Time: 23rd June, 2025 – 3 PM
Place: https://sabanciuniv.
Keywords : High Energy Astrophysics, Neutron Stars, Magnetars, X-ray Bursts
Abstract
One of the most distinctive features of magnetars — highly magnetized neutron stars — is their recurrent emission of brief but highly luminous bursts in hard X-rays/soft gamma-rays. Once an active episode begins, a few to thousands of such bursts may occur over timescales from days to months. The temporal clustering of these events suggests an underlying mechanism triggering multiple bursts in rapid succession, which is likely essential to understanding the processes driving magnetar activity. In this thesis, we investigate the “triggering” mechanism of short magnetar bursts by modeling repetitive burst behavior through crustal interactions and employing a cellular automaton model for the magnetar crust. Our simulations, based on physically motivated criteria, successfully reproduce burst clustering. Additionally, the durations and energetics of active episodes in our simulations agree well with observational data. We discuss the potential physical mechanisms underlying burst clusters observed in numerous magnetars, as well as the reactivations of an individual magnetar.
We also investigate how the “triggered” system generates radiation (bursts) and how this radiation evolves within the magnetosphere via time-resolved spectral analysis of 51 bright bursts from the magnetar SGR J1935+2154. Unlike conventional studies in the literature, we follow a two-step approach to probe true spectral evolution. For each burst, we first extract spectral information from overlapping time segments, fit them with three continuum models, and employ a machine-learning-based clustering algorithm to identify time segments that provide the largest spectral variations during each burst. We then extract spectra from those non-overlapping (clustered) time segments and fit them again with the three models: the exponential cutoff power-law model, the sum of two blackbody functions, and the model considering the emission of a modified black body undergoing resonant cyclotron scattering, which is applied systematically at this scale for the first time. Our novel technique allowed us to establish the genuine spectral evolution of magnetar bursts. We discuss the implications of our results and compare their collective behavior with the average burst properties of other magnetars.div>