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

## 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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## 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.

## Author information

Authors

### Contributions

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.

## Ethics declarations

### Competing interests

The authors declare no competing financial interests.

## Rights and permissions

Reprints and Permissions

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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