J0453+1559: A Neutron Star–White Dwarf Binary from a Thermonuclear Electron-capture Supernova?

The compact binary radio pulsar system J0453+1559 consists of a recycled pulsar as primary component of 1.559(5) M⊙ and an unseen companion star of 1.174(4) M⊙. Because of the relatively large orbital eccentricity of e = 0.1125, it was argued that the companion is a neutron star (NS), making it the NS with the lowest accurately determined mass to date. However, a direct observational determination of the nature of the companion is currently not feasible. Moreover, state-of-the-art stellar evolution and supernova modeling are contradictory concerning the possibility of producing such a low-mass NS remnant. Here we challenge the NS interpretation by reasoning that the lower-mass component could instead be a white dwarf born in a thermonuclear electron-capture supernova (tECSN) event, in which oxygen–neon deflagration in the degenerate stellar core of an ultra-stripped progenitor ejects several 0.1 M⊙ of matter and leaves a bound ONeFe white dwarf as the second-formed compact remnant. We determine the ejecta mass and remnant kick needed in this scenario to explain the properties of PSR J0453+1559 by a NS–white dwarf system. More work on tECSNe is needed to assess the viability of this scenario.