Browsing by Author "Arzoumanian, Zaven"

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    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.
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    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, Zorawar
    We 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.
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    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, Arturs
    We 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.