An Alternative Explanation for the Helium Star Pulsar Binary J1928+1815: The Most Heavyweight Black Widow System to Date

Abstract We present the results of deep near-infrared imaging of the recently discovered helium star pulsar binary J1928+1815 situated in the Galactic plane. Our observations did not achieve significant detections, providing limiting magnitudes of J = 23.7 and H = 22.2, which are both 2.4 mag deeper than the expected J and H magnitudes for a modeled stripped helium star with a mass of 1 M ⊙ after extinction. Although we cannot completely rule out the possibility of more significant extinction and the exact evolutionary status of the supposed helium star is uncertain, by comparing J1928+1815 with other pulsar binaries, we propose a natural alternative solution: that J1928+1815 is a heavyweight black widow system with a massive ablated white dwarf. Due to the pulsar’s relatively high spindown power and short orbital separation, the irradiation heating timescale is uniquely shorter than the cooling timescale for the WD companion. As a result, the WD effectively boils, with its outer layers expanding, overfilling the Roche lobe and producing low-density binary-scale haze opaque in the radio band. If this interpretation is correct, J1928+1815 would represent a new category distinct from canonical lightweight black widow systems. Radio eclipses can occur in pulsar binaries across a wider range of WD companion masses than previously thought. Therefore, they do not serve as a definitive indicator of a helium star without its direct detection. We contend that a spectroscopic identification remains the smoking gun for its existence. Given the crowding in this field, HST imaging in the near-infrared band would provide even better constraints.