A New Low Magnetic Field Magnetar: The 2011 Outburst of Swift J1822.3-1606

The Astrophysical Journal (Impact Factor: 5.99). 07/2012; 754(1):27. DOI: 10.1088/0004-637X/754/1/27


We report on the long-term X-ray monitoring with Swift, RXTE, Suzaku, Chandra, and XMM-Newton of the outburst of the newly discovered magnetar Swift J1822.3-1606 (SGR 1822-1606), from the first observations soon after the detection of the short X-ray bursts which led to its discovery, through the first stages of its outburst decay (covering the time span from 2011 July until the end of 2012 April). We also report on archival ROSAT observations which detected the source during its likely quiescent state, and on upper limits on Swift J1822.3-1606's radio-pulsed and optical emission during outburst, with the Green Bank Telescope and the Gran Telescopio Canarias, respectively. Our X-ray timing analysis finds the source rotating with a period of P = 8.43772016(2) s and a period derivative \dot{P}=8.3(2)\times 10^{-14} s s-1, which implies an inferred dipolar surface magnetic field of B ~= 2.7 × 10^13 G at the equator. This measurement makes Swift J1822.3-1606 the second lowest magnetic field magnetar (after SGR 0418+5729). Following the flux and spectral evolution from the beginning of the outburst, we find that the flux decreased by about an order of magnitude, with a subtle softening of the spectrum, both typical of the outburst decay of magnetars. By modeling the secular thermal evolution of Swift J1822.3-1606, we find that the observed timing properties of the source, as well as its quiescent X-ray luminosity, can be reproduced if it was born with a poloidal and crustal toroidal fields of Bp ~ 1.5 × 10^14 G and B tor ~ 7 × 10^14 G, respectively, and if its current age is ~550 kyr.

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    • "It is believed that the radiation of a magnetar (AXP or SGR) is powered by the energy stored in the super strong magnetic field with B s > 10 14 G [11]. However, the discoveries of the low magnetic field soft γ-ray repeater [12] [13], the radio emission from magnetar [14] [15] [16] and the magnetar-like outburst of spin-down powered pulsar with high magnetic field [9] challenge the existing theoretical models . The study of the radiation mechanism of the very high magnetic field spin-down powered pulsar, and the connection between the canonical spin-down powered pulsar and magnetar, can help us to understand the physics of the two groups of neutron stars. "
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    ABSTRACT: The outer gap model is used here to explain the spectrum and the energy dependent light curves of the X-ray and soft gamma-ray radiations of the spin-down powered pulsar PSR B1509-58.In the outer gap model, most pairs inside the gap are created around the null charge surface and the gap's electric field separates the two charges to move in opposite directions. Consequently, the region from the null charge surface to the light cylinder is dominated by the outflow of particles and that from the null charge surface to the star is dominated by the inflow of particles. The inflow and outflow of particles move along the magnetic field lines and emit curvature photons, and the incoming curvature photons are converted to pairs by the strong magnetic field of the star. These pairs emit synchrotron photons. We suggest that the X-rays and soft gamma-rays of PSR B1509-58 result from the synchrotron radiation of these pairs, and the viewing angle of PSR B1509-58 only receives the inflow radiation. The magnetic pair creation requires a large pitch angle, which makes the pulse profile of the synchrotron radiation distinct from that of the curvature radiation. We carefully trace the pulse profiles of the synchrotron radiation with different pitch angles. We find that the differences between the light curves of different energy bands are due to the different pitch angles of the secondary pairs, and the second peak appearing at E>10MeV comes from the region near the star, where the stronger magnetic field allows the pair creation to happen with a smaller pitch angle.
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    ABSTRACT: Swift J1822.3-1606 was discovered on 2011 July 14 by the Swift Burst Alert Telescope following the detection of several bursts. The source was found to have a period of 8.4377 s and was identified as a magnetar. Here we present a phase-connected timing analysis and the evolution of the flux and spectral properties using RXTE, Swift, and Chandra observations. We measure a spin frequency of 0.1185154343(8) s$^{-1}$ and a frequency derivative of $-4.3\pm0.3\times10^{-15}$ at MJD 55761.0, in a timing analysis that include significant non-zero second and third frequency derivatives that we attribute to timing noise. This corresponds to an estimated spin-down inferred dipole magnetic field of $B\sim5\times10^{13}$ G, consistent with previous estimates though still possibly affected by unmodelled noise. We find that the post-outburst 1--10 keV flux evolution can be characterized by a double-exponential decay with decay timescales of $15.5\pm0.5$ and $177\pm14$ days. We also fit the light curve with a crustal cooling model which suggests that the cooling results from heat injection into the outer crust. We find that the hardness-flux correlation observed in magnetar outbursts also characterizes the outburst of Swift J1822.3-1606. We compare the properties of Swift J1822.3-1606 with those of other magnetars and their outbursts.
    The Astrophysical Journal 04/2012; 761(1). DOI:10.1088/0004-637X/761/1/66 · 5.99 Impact Factor
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    ABSTRACT: Anomalous X-ray pulsars (AXPs) and soft gamma-ray repeaters (SGRs) are magnetar candidates. During their studies, the magnetic dipole braking mechanism is often assumed. This will result in a high surface dipole field for most AXPs and SGRs. It will also bring several problems challenging the magnetar interpretation. Alternatively, it is possible that AXPs and SGRs are braked down by a particle wind which also originates from magnetic field decay. In the wind braking scenario, magnetars are neutron stars with strong multipole field. A strong dipole field is no longer required. Recent challenging observations of magnetars may be explained naturally in the wind braking scenario: (1) The supernova energetics of those associated with magnetars are of normal value; (2) The non-detection in Fermi observations of magnetars; (3) The problem posed by the low-magnetic field soft gamma-ray repeater; (4) The relation between magnetars and high magnetic field pulsars ; (5) A decreasing period derivative during magnetar outbursts etc. For magnetars with $L_{\rm x}<-\dot{E}_{\rm rot}$, they may still be magnetic dipole braking. This may explain the "fundamental plane" of magnetar radio emissions. A magnetism-powered (instead of rotation-powered) pulsar wind nebula will be one of the consequences of wind braking. For a magnetism-powered pulsar wind nebula, we should see a correlation between the nebula luminosity and the magnetar luminosity. This may be the case of the extended emission around AXP 1E 1547.0-5408. A braking index different from three is also calculated. Future braking index measurement of a magnetar may tell us whether magnetars are wind braking or magnetic dipole braking.
    The Astrophysical Journal 05/2012; 768(2). DOI:10.1088/0004-637X/768/2/144 · 5.99 Impact Factor
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