Millisecond pulsars are born in binary systems where a neutron star is spun up by accreting mass and angular momentum from a companion star. The initial accretion phases and the final products as radio pulsars are well studied, but the intermediate stages, when the system has just finished active mass transfer, remain sparsely observed. This work fits into the broader effort of studying optical companions of millisecond pulsars in globular clusters, where several similar systems have been found, but none have been caught at such an early stage of white dwarf evolution.
The study used deep multi-band, multi-epoch observations from the Hubble Space Telescope in nine filters spanning from the near-ultraviolet to the near-infrared, collected over ten years. The pulsar's position from MeerKAT radio observations was transferred to the epoch of the optical images, accounting for proper motion. Within a radius of 0.1 arcseconds, a single source was found coincident with the pulsar to an accuracy of 0.02 arcseconds. To determine the companion's physical parameters, its position on the color-magnitude diagram was compared with theoretical cooling tracks for helium white dwarfs of various masses using a maximum likelihood method.
The companion COM-NGC362D has a mass of 0.18 ± 0.01 solar masses and an effective temperature of about 13,000 K, corresponding to the proto-white dwarf phase, 180 million years before the onset of cooling. Since the proto-white dwarf phase for such a mass lasts about 0.87 billion years, the detachment from the Roche lobe occurred only about 0.69 billion years ago. This makes it the youngest known white dwarf companion of a millisecond pulsar. Significant color variations with wavelength were detected: the difference between observed and model colors changes monotonically and reaches 0.8 magnitudes when going from blue to red filters. This trend is consistent with self-absorption of light by residual circumstellar material surrounding the companion. A self-absorption model with optical depth τ10 = 0.18 and dust grain size of 0.05 μm fits most of the data points well.
This work directly shows for the first time that residual material can persist around a millisecond pulsar companion for a significant portion of the pre-cooling phase and produce observational signatures not predicted by standard models. This distinguishes the system from previously studied neighbors, where such effects were not seen, and points to a possible systematic bias in classifying young millisecond pulsars with non-degenerate companions. The resemblance to the enigmatic system PSR J1816+4510 and the recently studied companion of pulsar M2C suggests that such transitional phases may not be rare.
This topic could advance by searching for other systems in the transitional phase using deep multi-color surveys of globular clusters and the Galaxy. High-resolution spectroscopic observations would allow measuring the companion's radial velocity curve and constraining the mass ratio of the system, as well as detecting narrow metal lines characteristic of low-pressure material. Further high-time-resolution radio observations could reveal inhomogeneities in the distribution of residual material through changes in dispersion measure.
The results will affect the interpretation of optical and radio observations of millisecond pulsars in binary systems, especially the classification of 'redback' and 'black widow' systems. They are also important for understanding the evolution of low-mass helium white dwarfs and the connection between low-mass X-ray binaries and millisecond pulsars.
Obtain spectra of the companion COM-NGC362D to measure radial velocities and search for narrow emission lines. Conduct monitoring of the radio pulsar NGC362D to look for signs of inhomogeneous absorption or scattering of the signal by the residual material.
This work directly addresses the unresolved problem of the connection between low-mass X-ray binaries and millisecond pulsars: what observational signatures do systems exhibit immediately after mass transfer ends? It also touches on the origin and lifetime of residual disks or shells around white dwarf companions, which is important for understanding the evolution of close binaries in globular clusters.
The orbital period of the NGC362D system is only 0.17 days, or about 4 hours. In that time, the companion completes a full orbit around the neutron star, and the distance between them is smaller than the distance from Earth to the Moon.
In Robert Wilson's novel 'Spin', there are star systems where residual disks of material around compact objects affect observational properties and create false signatures. Here, a similar idea plays out in real astrophysics: residual material around a young white dwarf can mimic the signatures of a non-degenerate companion.