Top News

New Delhi: Chinese astronomers using the Five hundred metre Aperture Spherical radio Telescope (FAST), the world’s largest filled-aperture radio telescope have discovered a millisecond pulsar with properties that challenge conventional models on how such dense objects form. PSR J0435+3233 has a spin-down rate two orders of magnitude greater than any other millisecond pulsar in the Milky Way. The pulsar also exhibits significant timing noise and an unusual orbital variation rate, suggesting the presence of two other celestial bodies in the system. The pulsar lies above the spin-up line in the period-period-derivative (P-P) diagram, that provides key information about its evolutionary history.

A P-P diagram of known pulsars with the red star marking PSR J0435+3233. (image Credit: Wang Na’s Team).

The Eddington spin-up line marks the equilibrium between outward radiation pressure and inward gravitational force during accretion. It represents the Eddington accretion rate, the maximum stable rate at which a neutron star can sustain accretion. The dense cores of dead stars can siphon of material from a binary companion, increasing in mass and spinning up. The tortured material falling in heats up, with the radiation reducing any more material from falling in. A voraciously feeding neutron reduces its own food supply. This process is called recycling.

How a millisecond pulsar is born

A neutron star accretes material from a binary companion, gains angular momentum, and spins up to millisecond periods. All previously detected binary millisecond pulsars lie below the spin-up line, consistent with models. PSR J0435+3233 however lies above the line, indicating that millisecond pulsar formation does not follow a single accretion-driven channel, but involves multiple physical mechanisms that are not yet fully understood. Typical millisecond pulsars are old, have weak magnetic fields, and a highly stable rotation. These traits make them the most precise natural clocks in the universe. PSR J0435+3233 is young, shows high energy loss, and has a strong surface magnetic field.

While the scientists do not know how PSR J0435+3233 came into existence, they do have some theories. The researchers suggest that super-Eddington accretion onto a strongly magnetised neutron star or accretion-induced collapse of a magnetised white dwarf may explain the formation of this young millisecond pulsar. Here super-Eddington accretion means that the neutron star siphoned off material faster than allowed by the Eddington Limit, with the radiation pushing away any forther infalling material. A paper describing the research has been published in Nature Astronomy.