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A highly resistive layer within the crust of X-ray pulsars limits their spin periods



The lack of isolated X-ray pulsars with spin periods longer than 12 s raises the question of where the population of evolved high-magnetic-field neutron stars has gone. Unlike canonical radiopulsars, X-ray pulsars are not subject to physical limits to the emission mechanism nor observational biases against the detection of sources with longer periods. Here we show that a highly resistive layer in the innermost part of the crust of neutron stars naturally limits the spin period to a maximum value of about 10–20 s. This highly resistive layer is expected if the inner crust is amorphous and heterogeneous in nuclear charge, possibly owing to the existence of a nuclear ‘pasta’ phase. Our findings suggest that the maximum period of isolated X-ray pulsars may be the first observational evidence for an amorphous inner crust, whose properties can be further constrained by future X-ray timing missions combined with more detailed models.

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Figure 1
Figure 2: Impurity parameter Qimp (left) and electrical resistivity (right) as a function of density for the four models with M = 1.4M.
Figure 3
Figure 4: P−Ṗ diagram for magnetars, X-ray isolated neutron stars and rotation-powered pulsars with X-ray emission.


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This work has been supported by the grants AYA 2010-21097-C03-02, AYA2012-39303, SGR2009-811, TW2010005 and iLINK 2011-0303. N.R. is supported by a Ramon y Cajal Research Fellowship and D.V. by the Prometeo/2009/103 grant.

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J.A.P. and D.V. contributed to developing the model, performed the calculations and wrote the manuscript. N.R. contributed to writing the manuscript and selected and checked the observational data.

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Correspondence to José A. Pons.

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The authors declare no competing financial interests.

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Pons, J., Viganò, D. & Rea, N. A highly resistive layer within the crust of X-ray pulsars limits their spin periods. Nature Phys 9, 431–434 (2013).

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