IF THE recently reported 0.5-ms optical pulsar in the remnant of SN1987A is indeed a rotating neutron star, then its very existence may be used to rule out many previously viable equations of state of nuclear matter (ref. 3). We demonstrated that it may be impossible to spin up an initially non-rotating neutron star to a spin period Prot ≤ 0.5 ms for any currently proposed nuclear equation of state. Here, we reverse the argument: a uniformly rotating neutron star with Prot ≤ 0.5 ms may be unable to spin down to become a slowly rotating neutron star, if any of the proposed equations of state is correct. Assuming that the neutron star in SN1987A is 'typical' and does not collapse to a black hole as it spins down to Prot≫ ms, then its existence may actually invalidate all previously viable nuclear equations of state. We propose an approximate, analytical test that can be used to identify untenable equations of state in the future.
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