Is SGR 0418+5729 indeed a waning magnetar?

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Title: Is SGR 0418+5729 indeed a waning magnetar?
Authors: Turolla, Roberto | Zane, Silvia | Pons, José A. | Esposito, Paolo | Rea, Nanda
Research Group/s: Astrofísica Relativista
Center, Department or Service: Universidad de Alicante. Departamento de Física Aplicada
Keywords: Pulsars: individual (SGR 0418+5729) | Stars: magnetic field | Stars: neutron
Knowledge Area: Astronomía y Astrofísica
Issue Date: 4-Oct-2011
Publisher: IOP Publishing
Citation: The Astrophysical Journal. 2011, 740(2): 105. doi:10.1088/0004-637X/740/2/105
Abstract: SGR 0418+5729 is a transient soft gamma-ray repeater which underwent a major outburst in 2009 June, during which the emission of short bursts was observed. Its properties appeared quite typical of other sources of the same class until long-term X-ray monitoring failed to detect any period derivative. The present upper limit on P implies that the surface dipole field is Bp lsim 7.5 × 1012 G, well below those measured in other soft gamma-ray repeaters (SGRs) and in the Anomalous X-ray Pulsars (AXPs), a group of similar sources. Both SGRs and AXPs are currently believed to be powered by ultra-magnetized neutron stars (magnetars, Bp ≈ 1014-1015 G). SGR 0418+5729 hardly seems to fit in such a picture. We show that the magneto-rotational properties of SGR 0418+5729 can be reproduced if this is an aged magnetar, ≈1 Myr old, which experienced substantial field decay. The large initial toroidal component of the internal field required to match the observed properties of SGR 0418+5729 ensures that crustal fractures, and hence bursting activity, can still occur at the present time. The thermal spectrum observed during the outburst decay is compatible with the predictions of a resonant Compton scattering model (as in other SGRs/AXPs) if the field is low and the magnetospheric twist is moderate.
Sponsor: This research was partially funded through grants AAE I/088/06/0 (R.T.), AYA 2010-21097-C03-02, GVPROMETE02009-103 (J.P.) and AYA2009-07391, SGR2009-811, TW2010005 (N.R.). N.R. also acknowledges support from a Ramon y Cajal Fellowship. P.E. acknowledges financial support from the Autonomous Region of Sardinia through a research grant under the program PO Sardegna FSE 2007-2013, L.R. 7/2007 “Promoting scientific research and innovation technology in Sardinia.”
ISSN: 0004-637X (Print) | 1538-4357 (Online)
DOI: 10.1088/0004-637X/740/2/105
Language: eng
Type: info:eu-repo/semantics/article
Rights: © 2011. The American Astronomical Society
Peer Review: si
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Appears in Collections:INV - Astrofísica Relativista - Artículos de Revistas

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