NEWBORN PULSARS WITH A HIDDEN MAGNETIC FIELD

Image: Crab nebula as seen by Chandra. Credit: NASA/CXC/SAO/F. Seward et al.

In the center of several supernova remnants there are pulsars with significantly lower values of the dipolar magnetic field than the average radio-pulsar population (10^{12}G). A possible explanation requires the slow rotation of the proto-neutron star at birth, which is unable to amplify its magnetic field to typical pulsar levels.

However, recent studies have shown that, even in the absence of rapid rotation, magnetic fields in pulsars can be amplified by other mechanisms such as convection and the standing accretion shock instability.
An alternative possibility, the hidden magnetic field scenario, considers the accretion of the fallback of the supernova debris onto the neutron star as responsible for the submergence (or screening) of the field and its apparently low value. A high accretion rate can compress the magnetic field of the NS which can eventually be buried into the neutron star crust. As a result, the value of the external magnetic field would be significantly lower than the internal 'hidden' magnetic field.

Credit: Torres-Forné et al. 2016

Once the accretion process stops, the magnetic field might eventually reemerge.
The main conclusion of a recent paper (Torres-Forné et al. 2016) is that typical magnetic fields of a few times 10^{12}G can be buried by accreting only 0.001-0.01 solar masses, a relatively modest amount of mass. The field would only reemerge after a few thousand years.
On the contrary, magnetar-like field strengths are much harder to screen and the required accreted mass is very large, in some cases so large that the neutron star would collapse to a black hole. The anomalously weak magnetic fields should be common in very young neutron stars.

Read more>>
http://arxiv.org/pdf/1511.03823v2.pdf
http://mnras.oxfordjournals.org/content/456/4/3813.abstract

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