Is SGR 0418+5729 Indeed a Waning Magnetar?
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 \dot{P} implies that the surface dipole field is Bp <~ 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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 2011
- DOI:
- 10.1088/0004-637X/740/2/105
- arXiv:
- arXiv:1107.5488
- Bibcode:
- 2011ApJ...740..105T
- Keywords:
-
- pulsars: individual: SGR 0418+5729;
- stars: magnetic field;
- stars: neutron;
- Astrophysics - High Energy Astrophysical Phenomena
- E-Print:
- 8 pages, 4 figures, accepted for publication in ApJ