Browsing by Author "Kouveliotou, Chryssa"
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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, NeilWe 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.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 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 SocietyIn 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.Item Pulse Peak Migration during the Outburst Decay of the Magnetar SGR 1830-0645: Crustal Motion and Magnetospheric Untwisting(IOP Publishing, 2022) Younes, George; Lander, Samuel K.; Baring, Matthew G.; Enoto, Teruaki; Kouveliotou, Chryssa; Wadiasingh, Zorawar; Ho, Wynn C. G.; Harding, Alice K.; Arzoumanian, Zaven; Gendreau, Keith; Güver, Tolga; Hu, Chin-Ping; Malacaria, Christian; Ray, Paul S.; Strohmayer, Tod E.Magnetars, isolated neutron stars with magnetic-field strengths typically ≳1014 G, exhibit distinctive months-long outburst epochs during which strong evolution of soft X-ray pulse profiles, along with nonthermal magnetospheric emission components, is often observed. Using near-daily NICER observations of the magnetar SGR 1830-0645 during the first 37 days of a recent outburst decay, a pulse peak migration in phase is clearly observed, transforming the pulse shape from an initially triple-peaked to a single-peaked profile. Such peak merging has not been seen before for a magnetar. Our high-resolution phase-resolved spectroscopic analysis reveals no significant evolution of temperature despite the complex initial pulse shape, yet the inferred surface hot spots shrink during peak migration and outburst decay. We suggest two possible origins for this evolution. For internal heating of the surface, tectonic motion of the crust may be its underlying cause. The inferred speed of this crustal motion is ≲100 m day−1, constraining the density of the driving region to ρ ∼ 1010 g cm−3, at a depth of ∼200 m. Alternatively, the hot spots could be heated by particle bombardment from a twisted magnetosphere possessing flux tubes or ropes, somewhat resembling solar coronal loops, that untwist and dissipate on the 30–40 day timescale. The peak migration may then be due to a combination of field-line footpoint motion (necessarily driven by crustal motion) and evolving surface radiation beaming. This novel data set paints a vivid picture of the dynamics associated with magnetar outbursts, yet it also highlights the need for a more generic theoretical picture where magnetosphere and crust are considered in tandem.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.Item The 2022 High-energy Outburst and Radio Disappearing Act of the Magnetar 1E 1547.0–5408(IOP Publishing, 2023) Lower, Marcus E.; Younes, George; Scholz, Paul; Camilo, Fernando; Dunn, Liam; Johnston, Simon; Enoto, Teruaki; Sarkissian, John M.; Reynolds, John E.; Palmer, David M.; Arzoumanian, Zaven; Baring, Matthew G.; Gendreau, Keith; Göğüş, Ersin; Guillot, Sebastien; Horst, Alexander J. van der; Hu, Chin-Ping; Kouveliotou, Chryssa; Lin, Lin; Malacaria, Christian; Stewart, Rachael; Wadiasingh, ZorawarWe report the radio and high-energy properties of a new outburst from the radio-loud magnetar 1E 1547.0−5408. Following the detection of a short burst from the source with Swift-BAT on 2022 April 7, observations by NICER detected an increased flux peaking at (6.0 ± 0.4) × 10−11 erg s−1 cm−2 in the soft X-ray band, falling to a baseline level of 1.7 × 10−11 erg s−1 cm−2 over a 17 day period. Joint spectroscopic measurements by NICER and NuSTAR indicated no change in the hard nonthermal tail despite the prominent increase in soft X-rays. Observations at radio wavelengths with Murriyang, the 64 m Parkes radio telescope, revealed that the persistent radio emission from the magnetar disappeared at least 22 days prior to the initial Swift-BAT detection and was redetected two weeks later. Such behavior is unprecedented in a radio-loud magnetar, and may point to an unnoticed slow rise in the high-energy activity prior to the detected short bursts. Finally, our combined radio and X-ray timing revealed the outburst coincided with a spin-up glitch, where the spin frequency and spin-down rate increased by 0.2 ± 0.1 μHz and (−2.4 ± 0.1) × 10−12 s−2, respectively. A linear increase in the spin-down rate of (−2.0 ± 0.1) × 10−19 s−3 was also observed over 147 days of postoutburst timing. Our results suggest that the outburst may have been associated with a reconfiguration of the quasi-polar field lines, likely signaling a changing twist, accompanied by spatially broader heating of the surface and a brief quenching of the radio signal, yet without any measurable impact on the hard X-ray properties.Item The Sleeping Monster: NuSTAR Observations of SGR 1806–20, 11 Years After the Giant Flare(IOP Publishing, 2017) Younes, George; Baring, Matthew G.; Kouveliotou, Chryssa; Harding, Alice; Donovan, Sophia; Göğüş, Ersin; Kaspi, Victoria; Granot, JonathanWe report the analysis of five Nuclear Spectroscopic Telescope Array (NuSTAR) observations of SGR 1806−20 spread over a year from 2015 April to 2016 April, more than 11 years following its giant flare (GF) of 2004. The source spin frequency during the NuSTAR observations follows a linear trend with a frequency derivative $\dot{\nu }=(-1.25\pm 0.03)\times {10}^{-12}$ Hz s−1, implying a surface dipole equatorial magnetic field $B\approx 7.7\times {10}^{14}$ G. Thus, SGR 1806−20 has finally returned to its historical minimum torque level measured between 1993 and 1998. The source showed strong timing noise for at least 12 years starting in 2000, with $\dot{\nu }$ increasing one order of magnitude between 2005 and 2011, following its 2004 major bursting episode and GF. SGR 1806−20 has not shown strong transient activity since 2009, and we do not find short bursts in the NuSTAR data. The pulse profile is complex with a pulsed fraction of $\sim 8 \% $ with no indication of energy dependence. The NuSTAR spectra are well fit with an absorbed blackbody, ${kT}=0.62\pm 0.06\,\mathrm{keV}$, plus a power law, ${\rm{\Gamma }}=1.33\pm 0.03$. We find no evidence for variability among the five observations, indicating that SGR 1806−20 has reached a persistent and potentially its quiescent X-ray flux level after its 2004 major bursting episode. Extrapolating the NuSTAR model to lower energies, we find that the 0.5–10 keV flux decay follows an exponential form with a characteristic timescale $\tau =543\pm 75$ days. Interestingly, the NuSTAR flux in this energy range is a factor of ~2 weaker than the long-term average measured between 1993 and 2003, a behavior also exhibited in SGR 1900+14. We discuss our findings in the context of the magnetar model.Item X-Ray and Radio Observations of the Magnetar SGR J1935+2154 during Its 2014, 2015, and 2016 Outbursts(The American Astronomical Society, 2017) Younes, George; Kouveliotou, Chryssa; Jaodand, Amruta; Baring, Matthew G.; van der Horst, Alexander J.; Harding, Alice K.; Hessels, Jason W.T.; Gehrels, Neil; Gill, Ramandeep; Huppenkothen, Daniela; Granot, Jonathan; Göğüş Ersin; Lin, LinWe analyzed broadband X-ray and radio data of the magnetar SGR J1935+2154 taken in the aftermath of its 2014, 2015, and 2016 outbursts. The source soft X-ray spectrum <10 keV is well described with a blackbody+power-law (BB+PL) or 2BB model during all three outbursts. Nuclear Spectroscopic Telescope Array observations revealed a hard X-ray tail, with a PL photon index Γ = 0.9, extending up to 50 keV, with flux comparable to the one detected <10 keV. Imaging analysis of Chandra data did not reveal small-scale extended emission around the source. Following the outbursts, the total 0.5–10 keV flux from SGR J1935+2154 increased in concordance to its bursting activity, with the flux at activation onset increasing by a factor of ~7 following its strongest 2016 June outburst. A Swift/X-Ray Telescope observation taken 1.5 days prior to the onset of this outburst showed a flux level consistent with quiescence. We show that the flux increase is due to the PL or hot BB component, which increased by a factor of 25 compared to quiescence, while the cold BB component kT = 0.47 keV remained more or less constant. The 2014 and 2015 outbursts decayed quasi-exponentially with timescales of ~40 days, while the stronger 2016 May and June outbursts showed a quick short-term decay with timescales of about four days. Our Arecibo radio observations set the deepest limits on the radio emission from a magnetar, with a maximum flux density limit of 14 μJy for the 4.6 GHz observations and 7 μJy for the 1.4 GHz observations. We discuss these results in the framework of the current magnetar theoretical models.Item X-Ray Burst and Persistent Emission Properties of the Magnetar SGR 1830-0645 in Outburst(IOP Publishing, 2022) Younes, George; Hu, Chin-Ping; Bansal, Karishma; Ray, Paul S.; Pearlman, Aaron B.; Kirsten, Franz; Wadiasingh, Zorawar; Göğüş, Ersin; Baring, Matthew G.; Enoto, Teruaki; Arzoumanian, Zaven; Gendreau, Keith C.; Kouveliotou, Chryssa; Güver, Tolga; Harding, Alice K.; Majid, Walid A.; Blumer, Harsha; Hessels, Jason W.T.; Gawroński, Marcin P.; Bezrukovs, Vladislavs; Orbidans, ArtursWe report on NICER X-ray monitoring of the magnetar SGR 1830−0645 covering 223 days following its 2020 October outburst, as well as Chandra and radio observations. We present the most accurate spin ephemerides of the source so far: ν = 0.096008680(2) Hz, Hz s−1, and significant second and third frequency derivative terms indicative of nonnegligible timing noise. The phase-averaged 0.8–7 keV spectrum is well fit with a double-blackbody (BB) model throughout the campaign. The BB temperatures remain constant at 0.46 and 1.2 keV. The areas and flux of each component decreased by a factor of 6, initially through a steep decay trend lasting about 46 days, followed by a shallow long-term one. The pulse shape in the same energy range is initially complex, exhibiting three distinct peaks, yet with clear continuous evolution throughout the outburst toward a simpler, single-pulse shape. The rms pulsed fraction is high and increases from about 40% to 50%. We find no dependence of pulse shape or fraction on energy. These results suggest that multiple hot spots, possibly possessing temperature gradients, emerged at outburst onset and shrank as the outburst decayed. We detect 84 faint bursts with NICER, having a strong preference for occurring close to the surface emission pulse maximum—the first time this phenomenon is detected in such a large burst sample. This likely implies a very low altitude for the burst emission region and a triggering mechanism connected to the surface active zone. Finally, our radio observations at several epochs and multiple frequencies reveal no evidence of pulsed or burst-like radio emission.