Is the 6-ms binary pulsar the remnant of a bright galactic X-ray source?

Abstract

It is argued here that the recently discovered 6-ms radio pulsar (ref. 1) with orbital period P_b = 120≠4 days, a sin i = (0.92≠0.08) ×10¹² cm and e ≈ 0) is the spun up remnant of a formerly heavily accreting (M ≈ 10⁻⁸M_⊙ yr⁻¹) X-ray source, belonging to a new type of low-mass X-ray binaries. This new type of X-ray binary consists of a critical lobe filling (lower) branch giant star of ≈ 1M_⊙ that spills mass to its neutron star companion. Systems of this kind were recently introduced² to explain the existence of the very bright (L_x ≈ 10³⁸ erg s⁻¹) X-ray sources located in the bulge of M31 and in the bulge of our Galaxy. Such systems will end their lives as wide binary systems (P_b ≳ 100 days), consisting of a≈0.3M_⊙ helium white dwarf (the remnant core of the giant) in orbit about a rapidly spinning neutron star. For a suitable combination of magnetic field strength and accretion rate onto the neutron star, it will appear as a millisecond radio pulsar once the giant has lost its envelope and the heavy mass transfer ceases abruptly.