Constraints on the formation of magnetars from associated supernova remnants

Jacco Vink

SRON Netherlands Institute for Space Research


It is now generally accepted that Anomalous X-ray Pulsars (AXPs) and Soft Gamma-ray Repeaters (SGRs) are magnetars, i.e. neutron stars with surface magnetic fields of 1014-1015 G. The origin of this magnetic field is uncertain, but one of the hypotheses is that magnetars are born with an initial spin period close to the break-up limit (< 1 ms), which results in a powerful dynamo action, greatly amplifying the seed magnetic field. A neutron star spinning at such a rate has a rotational energy in excess of 1052 erg, and part of that energy will power the supernova through rapid magnetic breaking. In other words it is expected that if magnetars are born with periods of ~ 1 ms their supernova remnants should be very energetic. However, we have investigated two supernova remnants which contain magnetars, Kes 73 (1E 1841-045) and N49 (SGR 0526-66), and they appear to be the results of explosions with the canonical supernova explosion energy of 1051 erg. Converting this to an initial rotation period suggests that the initial period was longer than ~ 6 ms. This poses problems for the theory. However, there is the possibility that the conversion of rotational energy to magnetic field energy is so efficient that ~ 90% of the rotational energy is used to generate an interior magnetic field of ~ 1017 G, far in excess of the surface magnetic field estimated from the spin-down rate.


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