Stringent tests of spin-up theories posed by a millisecond pulsar with an extreme spin-down rate

Binary millisecond pulsar systems are superb probes of stellar, binary evolution and the physics of extreme environments. The location of millisecond pulsars relative to the ‘spin-up line’ in the spin period and spin-down rate (P–Ṗ) diagram plays a critical role in understanding population studies of radio millisecond pulsars and accreting neutron stars in X-ray binaries. Here we report the discovery of the pulsar PSR J0435+3233, which exhibits an exceptionally high spin-down rate of 4.8775101(2) × 10−17 s s−1—two orders of magnitude higher than that of other known millisecond pulsars, thus occupying a distinctive position in the P–Ṗ diagram. This discovery implies a formation channel that differs from the classical evolutionary path, challenging the theoretical models of the recycling process and placing stringent constraints on spin-up theories. One plausible scenario is that its progenitor was a neutron star undergoing super-Eddington accretion, with the currently observed pulsar being a remnant of such a system. Another possible mechanism is its formation through the accretion-induced collapse of a magnetized ONeMg white dwarf, although this scenario requires specific theoretical assumptions. Hence, PSR J0435+3233 has an important impact on binary evolution theory. Millisecond pulsar PSR J0435+3233, with a spin-down rate that is 100 times higher than those of other millisecond pulsars, occupies a unique position on the P–Ṗ diagram. This pulsar challenges classical recycling models and constrains spin-up theories. Backer, D. C., Kulkarni, S. R., Heiles, C., Davis, M. M. & Goss, W. M. A millisecond pulsar. Nature 300, 615–618 (1982). Lorimer, D. R. Binary and millisecond pulsars. Living Rev. Relativ. 11, 8 (2008). Ransom, S. M. et al. A millisecond pulsar in a ... [10417 chars]