SGR 0418+5729—How does a Young Neutron Star Spin Down to a 9 s Period with a Dipole Field Less Than 1013 G?

• Published in: Astrophysical journal. Letters 2011-05, Vol.732 (1), p.L4-jQuery1323917767176='48'
• Main Authors: Alpar, M. A, Ertan, Ü, Çalışkan, Ş
• Format: Article
• Language: English
• ISSN: 2041-8205; 2041-8213
• DOI: 10.1088/2041-8205/732/1/L4

The period derivative bound for the soft gamma-ray repeater SGR 0418+5729 establishes the magnetic dipole moment to be distinctly lower than the magnetar range, placing the source beyond the regime of isolated pulsar activity in the diagram and giving a characteristic age >2 X 107 yr, much older than the 105 yr age range of SGRs and anomalous X-ray pulsars. So the spin-down must be produced by a mechanism other than dipole radiation in vacuum. A fallback disk will spin down a neutron star with surface dipole magnetic field in the 1012 G range and initial rotation period P 0 ~ 100 ms to the 9.1 s period of SGR 0418+5729 in a few 104 to ~105 yr. The current upper limit to the period derivative gives a lower limit of ~105 yr to the age that is not sensitive to the neutron star's initial conditions. The total magnetic field on the surface of SGR 0418+5729 could be significantly larger than its 1012 G dipole component.