arXiv subjects

Ozge Keskin

Publications and source records attributed to Ozge Keskin.

5 recordsLinked to original sources

arXiv2026

An 11-Year Catalog of Gamma-Ray Transients: A Comprehensive Search with Fermi Gamma-ray Burst Monitor Data

The Gamma-ray Burst Monitor (GBM) on board Fermi Gamma-ray Space Telescope has produced the largest database of all-sky observations in gamma rays with its continuous data with high time and energy resolutions. These data contain a wealth of unidentified transient events that did not trigger the detectors for various reasons. We conducted extensive searches to identify such untriggered transient events observed with GBM in 11 years (July 2010 - June 2021). In particular, we employed four different search modes with various energy ranges (mainly below 300 keV) and time resolutions (from 8 ms to 2 s), utilizing three statistical methods (signal-to-noise ratio, Poisson, and Bayesian statistics), each with different effectiveness in identifying specific classes of transients. Moreover, we developed algorithms for known-event flagging as well as unknown-event classification for our candidate events found in the searches. In this paper, we present our search methodologies, event flagging and classification algorithms and the resulting comprehensive event catalog. The catalog contains more than a million events in total, including known events such as gamma-ray bursts, soft-gamma repeater bursts, galactic X-ray source activities, terrestrial gamma flashes, and solar flares. For each candidate event, the catalog presents the event time, detection significance, event duration, hardness ratios, known-event flagging results, and classification probabilities. Our short-transient catalog significantly expands the currently-existing list of known events and complements the GBM trigger catalog. The event database with filtering capabilities is also publicly available at https://magnetars.sabanciuniv.edu/gbm, which allows users to retrieve event information based on their input queries along with the event lightcurves.

astro-ph.HE↗

arXiv2025

Correlation of Burst Behaviour with Magnetar Age

We analyze a wide set of historical magnetar burst observations detected with five different instruments, calibrating these to the energy range of Fermi-GBM observations for consistency. We find a striking correlation between a magnetar's characteristic age and both its typical burst energy and its burst activity level. Arguing that this bursting behaviour also correlates with true age, we interpret it as the result of a reducing high-stress volume of the crust in an aging magnetar: previous giant flares cause relaxation of large regions of its crust and inhibit burst clustering, whilst the reducing burst energy reflects the progressively shallower region of the crust where Hall drift can build stresses effectively, as the field decays through the range $\sim 10^{12}-10^{13}\,\mathrm{G}$. Low-energy bursts from very young magnetars may represent failures of weak regions of the crust that have only recently solidified.

astro-ph.HE↗

arXiv2025

Investigating the Recursive Short X-ray Burst Behavior of Magnetars Through Crustal Interactions

Energetic bursts from strongly magnetized neutron stars, known as magnetars, are typically detected in clusters. Once an active episode begins, anywhere from a few to thousands of hard X-ray bursts can occur over durations ranging from days to months. The temporal clustering of these recurrent bursts during an active episode suggests an underlying mechanism that triggers multiple bursts in rapid succession. These burst clusters are likely crucial for understanding the processes driving magnetar activity. In this study, we investigate the repetitive short X-ray burst behavior of magnetars through crustal interactions, employing the cellular automaton model for the magnetar crust proposed by Lander (2023). 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.

astro-ph.HE↗

arXiv2024

Concise Spectrotemporal Studies of Magnetar SGR J1935+2154 Bursts

SGR J1935+2154 has truly been the most prolific magnetar over the last decade: It has been entering into burst active episodes once every 1-2 years since its discovery in 2014, it emitted the first Galactic fast radio burst associated with an X-ray burst in 2020, and has emitted hundreds of energetic short bursts. Here, we present the time-resolved spectral analysis of 51 bright bursts from SGR J1935+2154. Unlike conventional time-resolved X-ray spectroscopic 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 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.

astro-ph.HE↗

arXiv2021

Fermi-GBM Observations of the SGR J1935+2154 Burst Forest

During 2020 April and May, SGR J1935+2154 emitted hundreds of short bursts and became one of the most prolific transient magnetars. At the onset of the active bursting period, a 130 s burst "forest," which included some bursts with peculiar time profiles, were observed with the Fermi/Gamma-ray Burst Monitor (GBM). In this Letter, we present the results of time-resolved spectral analysis of this burst "forest" episode, which occurred on 2020 April 27. We identify thermal spectral components prevalent during the entire 130 s episode; high-energy maxima appear during the photon flux peaks, which are modulated by the spin period of the source. Moreover, the evolution of the $νF_ν$ spectral hardness (represented by $E_{\rm peak}$ or blackbody temperature) within the lightcurve peaks is anti-correlated with the pulse phases extrapolated from the pulsation observed within the persistent soft X-ray emission of the source six hours later. Throughout the episode, the emitting area of the high-energy (hotter) component is 1-2 orders of magnitude smaller than that for the low-energy component. We interpret this with a geometrical viewing angle scenario, inferring that the high-energy component likely originates from a low-altitude hotspot located within closed toroidal magnetic field lines.

astro-ph.HE↗