Browsing by Author "Kaneko, Yuki"

Now showing 1 - 5 of 5
  • Thumbnail Image
    Item
    BROADBAND SPECTRAL INVESTIGATIONS OF SGR J1550–5418 BURSTS
    (The American Astronomical Society, 2012) Lin, Lin; Baring, Matthew G.; Granot, Jonathan; Kouveliotou, Chryssa; Kaneko, Yuki; Van Der Horst, Alexander; Gruber, David; Von Kienlin, Andreas; Younes, George; Watts, Anna L.; Gehrels, Neil
    We present the results of our broadband spectral analysis of 42 SGR J1550−5418 bursts simultaneously detected with the Swift/X-ray Telescope (XRT) and the Fermi/Gamma-ray Burst Monitor (GBM), during the 2009 January active episode of the source. The unique spectral and temporal capabilities of the XRT windowed timing mode have allowed us to extend the GBM spectral coverage for these events down to the X-ray domain (0.5–10 keV). Our earlier analysis of the GBM data found that the SGR J1550−5418 burst spectra were described equally well with either a Comptonized model or with two blackbody functions; the two models were statistically indistinguishable. Our new broadband (0.5–200 keV) spectral fits show that, on average, the burst spectra are better described with two blackbody functions than with the Comptonized model. Thus, our joint XRT–GBM analysis clearly shows for the first time that the SGR J1550−5418 burst spectra might naturally be expected to exhibit a more truly thermalized character, such as a two-blackbody or even a multi-blackbody signal. Using the Swift and RXTE timing ephemeris for SGR J1550−5418 we construct the distribution of the XRT burst counts with spin phase and find that it is not correlated with the persistent X-ray emission pulse phase from SGR J1550−5418. These results indicate that the burst emitting sites on the neutron star need not to be co-located with hot spots emitting the bulk of the persistent X-ray emission. Finally, we show that there is a significant pulse phase dependence of the XRT burst counts, likely demonstrating that the surface magnetic field of SGR J1550−5418 is not uniform over the emission zones, since it is anticipated that regions with stronger surface magnetic field could trigger bursts more efficiently.
  • Thumbnail Image
    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, George
    The 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.
  • Thumbnail Image
    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, Chryssa
    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 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.
  • Thumbnail Image
    Item
    DETECTION OF SPECTRAL EVOLUTION IN THE BURSTS EMITTED DURING THE 2008–2009 ACTIVE EPISODE OF SGR J1550−5418
    (2012) Von Kienlin, Andreas; Gruber, David; Kouveliotou, Chryssa; Granot, Jonathan; Baring, Matthew G.; Gogus, Ersin; Huppenkothen, Daniela; Kaneko, Yuki; Lin, Lin; Watts, Anna L.; Bhat, Narayana P.; Guiriec, Sylvain; Van Der Horst, Alexander J.; Bissaldi, Elisabetta; Greiner, Jochen; Meegan, Charles A.; Paciesas, William S.; Preece, Robert D.; Rau, Arne; The American Astronomical Society
    In early 2008 October, the soft gamma repeater SGR J1550−5418 (1E 1547.0−5408, AX J155052−5418, PSR J1550−5418) became active, emitting a series of bursts which triggered the Fermi Gamma-ray Burst Monitor (GBM) after which a second especially intense activity period commenced in 2009 January and a third, less active period was detected in 2009 March–April. Here, we analyze the GBM data for all the bursts from the first and last active episodes. We performed temporal and spectral analysis for all events and found that their temporal characteristics are very similar to the ones of other SGR bursts, as well the ones reported for the bursts of the main episode (average burst durations ∼170 ms). In addition, we used our sample of bursts to quantify the systematic uncertainties of the GBM location algorithm for soft gamma-ray transients to 8◦. Our spectral analysis indicates significant spectral evolution between the first and last set of events. Although the 2008 October events are best fitted with a single blackbody function, for the 2009 bursts an optically thin thermal bremsstrahlung is clearly preferred.We attribute this evolution to changes in the magnetic field topology of the source, possibly due to effects following the very energetic main bursting episode.
  • Thumbnail Image
    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, George
    Magnetars 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.