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

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Abstract

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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Acknowledgements

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.

Corresponding author

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). https://doi.org/10.1038/nphys2640

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