Browsing by Author "Roberts, Oliver J."
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Item Bursts from High-magnetic-field Pulsars Swift J1818.0-1607 and PSR J1846.4-0258(IOP Publishing, 2022) Uzuner, Mete; Keskin, Özge; Kaneko, Yuki; Göğüş, Ersin; Roberts, Oliver J.; Lin, Lin; Baring, Matthew G.; Güngör, Can; Kouveliotou, Chryssa; Horst, Alexander J. van der; Younes, GeorgeThe detection of magnetar-like bursts from highly magnetic (B > 1013 G) rotation-powered pulsars (RPPs) opened the magnetar population to yet another group of neutron stars. At the same time the question arose as to whether magnetar-like bursts from high-B RPPs have similar characteristics to bursts from known magnetar sources. We present here our analyses of the Fermi Gamma-ray Burst Monitor (GBM) data from two magnetar candidates, Swift J1818.0−1607 (a radio-loud magnetar) and PSR J1846.4−0258. Both sources entered active bursting episodes in 2020 triggering Fermi-GBM in 2020 and in early 2021. We searched for untriggered bursts from both sources and performed temporal and spectral analyses on all events. Here, we present the results of our comprehensive burst search and analyses. We identified 37 and 58 bursts that likely originated from Swift J1818.0−1607 and PSR J1846.4−0258, respectively. We find that the bursts from these sources are shorter on average than typical magnetar bursts. In addition, their spectra are best described with a single blackbody function with kT ∼ 10–11 keV; several relatively bright events, however, show higher energy emission that could be modeled with a cutoff power-law model. We find that the correlation between the blackbody emitting area and the spectral temperature for the burst ensemble of each pulsar deviates from the ideal Stefan–Boltzmann law, as it does for some burst-active magnetars. We interpret this characteristic as being due to the significant radiation anisotropy expected from optically thick plasmas in very strong magnetic fields.Item Concise Spectrotemporal Studies of Magnetar SGR J1935+2154 Bursts(IOP Publishing, 2024) Keskin, Özge; Göğüş, Ersin; Kaneko, Yuki; Demirer, Mustafa; Yamasaki, Shotaro; Baring, Matthew G.; Lin, Lin; Roberts, Oliver J.; Kouveliotou, ChryssaSGR 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 yr since its discovery in 2014, it emitted the first Galactic fast radio burst associated with an X-ray burst in 2020, and it 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 nonoverlapping (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 blackbody 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.Item GRB 180128A: A second magnetar giant flare candidate from the Sculptor Galaxy(EDP Sciences, 2024) Trigg, Aaron C.; Burns, Eric; Roberts, Oliver J.; Negro, Michela; Svinkin, Dmitry S.; Baring, Matthew G.; Wadiasingh, Zorawar; Christensen, Nelson L.; Andreoni, Igor; Briggs, Michael S.; Lalla, Niccolò Di; Frederiks, Dmitry D.; Lipunov, Vladimir M.; Omodei, Nicola; Ridnaia, Anna V.; Veres, Peter; Lysenko, Alexandra L.Magnetars are slowly rotating neutron stars that possess the strongest magnetic fields known in the cosmos (10 14 − 10 15 G). They display a range of transient high-energy electromagnetic activity. The brightest and most energetic of these events are the gamma-ray bursts (GRBs) known as magnetar giant flares (MGFs), with isotropic energies E iso ≈ 10 44 − 10 46 erg. Only seven MGF detections have been made to date: three unambiguous events occurred in our Galaxy and the Magellanic Clouds, and the other four MGF candidates are associated with nearby star-forming galaxies. As all seven identified MGFs are bright at Earth, additional weaker events likely remain unidentified in archival data. We conducted a search of the Fermi Gamma-ray Burst Monitor database for candidate extragalactic MGFs and, when possible, collected localization data from the Interplanetary Network (IPN) satellites. Our search yielded one convincing event, GRB 180128A. IPN localizes this burst within NGC 253, commonly known as the Sculptor Galaxy. The event is the second MGF in modern astronomy to be associated with this galaxy and the first time two bursts have been associated with a single galaxy outside our own. Here we detail the archival search criteria that uncovered this event and its spectral and temporal properties, which are consistent with expectations for a MGF. We also discuss the theoretical implications and finer burst structures resolved from various binning methods. Our analysis provides observational evidence of an eighth identified MGF.Item Quasiperiodic Peak Energy Oscillations in X-Ray Bursts from SGR J1935+2154(IOP Publishing Ltd, 2023) Roberts, Oliver J.; Baring, Matthew G.; Huppenkothen, Daniela; Kouveliotou, Chryssa; Göğüş, Ersin; Kaneko, Yuki; Lin, Lin; Horst, Alexander J. van der; Younes, GeorgeMagnetars are young neutron stars powered by the strongest magnetic fields in the Universe (1013–15 G). Their transient X-ray emission usually manifests as short (a few hundred milliseconds), bright, energetic (∼1040–41 erg) X-ray bursts. Since its discovery in 2014, SGR J1935+2154 has become one of the most prolific magnetars, exhibiting very active bursting episodes and other fascinating events, such as pulse timing antiglitches and fast radio bursts. Here we present evidence for possible 42 Hz (24 ms) quasiperiodic oscillations in the ν F ν spectrum peak energy (E p ) identified in a unique burst detected with the Fermi Gamma-ray Burst Monitor in 2022 January. While quasiperiodic oscillations have been previously reported in the intensity of magnetar burst light curves, quasiperiodic oscillations in E p have not. We also find an additional event from the same outburst that appears to exhibit a similar character in E p , albeit of lower statistical quality. For these two exceptional transients, such E p oscillations can be explained by magnetospheric density and pressure perturbations. For burst-emitting plasma consisting purely of e + e − pairs, these acoustic modes propagate along a highly magnetized flux tube of length up to around L ∼ 130 neutron star radii, with L being lower if ions are present in the emission zone. Detailed time-resolved analyses of other magnetar bursts are encouraged to evaluate the rarity of these events and their underlying mechanisms.