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Constraints on the formation of magnetars from associated supernova remnants

Jacco Vink

SRON Netherlands Institute for Space Research

Abstract:

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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