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

TL;DR

This work challenges the helium-star companion interpretation for J1928+1815, a millisecond pulsar binary with a 3.60 hr orbit and a substantial radio eclipse. It integrates deep near-infrared constraints from EMIR/GTC, a gamma-ray pulsation search with Fermi-LAT, and an ablation-driven haze model to test the companion scenario. The non-detection in the NIR challenging a $1\,M_\odot$ stripped He-star, together with the absence of gamma-ray pulsations, motivates an alternative: a heavyweight black widow with a massive ablated white dwarf whose irradiation-induced haze can obscure the system in radio wavelengths. If correct, this identifies a new BW subtype and implies eclipses may arise for a broader WD mass range than previously thought, underscoring the need for spectroscopic confirmation and high-resolution NIR imaging (e.g., with HST) to cement J1928+1815's nature.

Abstract

We present the results of deep near-infrared imaging of the recently discovered helium star pulsar binary J1928$+$1815 situated in the Galactic plane. Our observations did not achieve significant detections, providing limiting magnitudes of J=23.7 and H=22.2, which are both 2.4\,magnitudes deeper than the expected J and H magnitudes for a modeled stripped helium star with a mass of 1\,$\rm M_{\odot}$ 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$+$1815 with other pulsar binaries, we propose a natural alternative solution: that J1928$+$1815 is a heavyweight black widow system with a massive ablated white dwarf. Due to the pulsar's relatively high spin-down power and short orbital separation, the irradiation heating timescale is uniquely shorter than the cooling timescale for the WD companion. As a result, the WD effectively boils, with its outer layers expanding, overfilling the Roche lobe and producing low-density binary-scale haze opaque in the radio band. If this interpretation is correct, J1928$+$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, an \textit{HST} imaging in the near-infrared band would provide even better constraints.

Figures (2)

• Figure 1: Zoom in the J, left, and H, right, final frames around the position of J1928$+$1815, which is in the center of the blue circle. The annulus in white surrounding the aperture sets the area to measure local background. To illustrate the astrometric accuracy of the analysis, the open and filled red circles and the green point in the bright source on the lower left represent the positions of that object in Gaia, EMIR frame WCS and the centroid measured by SExtractor, respectively.
• Figure 2: The distribution of pulsars in Table 1 on the $\dot{E}-P_{\text{b}}$ plane. The colour map shows the ratio of heating timescale, $T_{\text{haze,form}}$, and the longest possible cooling timescale, $T_{\text{haze,cool,max}}$ (which is largest value for $T_{\text{haze,cool}}$ in Table 2), on a log scale. One may see that J1928$+$1815 is unique in having its pulsar spin-down power and orbital separation prevent it from cooling efficiently. We highlight the region facilitating the process, assuming a system with a $1\,\text{M}_\odot$ WD and a $1.4\,\text{M}_\odot$ NS, and show the boundary of the region with a black dashed line. For comparison, we show the same boundary for the case of $0.6\,\text{M}_\odot$ WD with a grey dashed line. Newly discovered pulsars in the highlighted region are likely to also exhibit similar phenomena.