Abstract
The primary uncertainty in understanding the structure of neutron stars is our lack of knowledge of the equation of state of condensed matter above nuclear density (3 x 1014 g cm−3). The most rapidly rotating neutron stars, the millisecond pulsars, may offer a way to probe the equation of state in this region. The periods of the two fastest known pulsars, PSR1937 + 214 and the recently discovered1 PSR1957 + 20, are within 3% of each other, and are shorter than the period of any other known pulsars by about a factor of two. This observation substantially strengthens the argument that the fastest pulsars should rotate at the limiting frequency for stable neutron stars2,3 We show here that if the two fastest pulsars are rotating at or near their limiting frequencies, and if their masses are at least 1.4 M⊙, then the equation of state of neutron star matter must be quite stiff.
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Friedman, J., Imamura, J., Durisen, R. et al. Constraints on the equation of state for neutron stars implied by PSR1957 + 20. Nature 336, 560–562 (1988). https://doi.org/10.1038/336560a0
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DOI: https://doi.org/10.1038/336560a0
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