A one-day neutron-star orbit may link two kinds of binary systems

Timing and radio-polarization observations suggest the system could be a transition between a redback-like pair and a detached white-dwarf binary.

Editorial illustration — not from the study.

The neutron star, PSR J2101−4802, spins once every 9.48 milliseconds and was discovered with the Giant Meterwave Radio Telescope. Timing observations over 3.7 years place it in an approximately one-day orbit with a likely helium-white-dwarf companion whose median estimated mass is about 0.15 times the Sun’s mass, assuming a 60-degree orbital inclination.

The system’s orbital period is changing unusually quickly, at about 10−11 seconds per second. The researchers also used radio-polarization observations taken across the orbit to study the neutron star’s emission geometry and found signatures of magnetized material linked to the orbital phase. Together, these properties suggest that the system may connect redback-like spider systems with detached millisecond-pulsar–helium-white-dwarf binaries.

What the pulsar reveals

Phase-connected timing identified PSR J2101−4802 as a millisecond pulsar in an approximately one-day binary orbit. The pulsar’s spin period is 9.48 milliseconds, and its likely helium-white-dwarf companion has a median estimated mass of about 0.15 solar masses when a 60-degree inclination is assumed.

The timing data show an unusually large orbital-period derivative of about 10−11 seconds per second. The researchers report that this change cannot be explained by the Shklovskii effect, which arises from the system’s motion across the sky, by Galactic acceleration, or by general-relativistic orbital damping.

Wideband, full-polarization radio observations traced changes in the linear and circular polarization across the orbit. A rotating-vector model fit to the polarization-angle swing provided constraints on the system’s magnetic and viewing geometry. The orbital-phase-linked polarization behavior also showed signatures of magnetized material between the stars.

Why this system matters

The system combines a roughly one-day orbit, a low-mass likely helium-white-dwarf companion, modest spin-down power, a large unexplained orbital-period change and signs of magnetized material within the binary. This combination suggests that PSR J2101−4802 may be a transitional system linking redback-like spider binaries and detached binaries containing a millisecond pulsar and a helium white dwarf.

If that interpretation is correct, the system could help researchers study how these two classes of binary neutron-star systems are related. The observations identify a possible link, rather than establishing the system’s evolutionary history.

Evidence and caveats

The study is based on phase-connected pulsar timing spanning 3.7 years, together with orbital phase-resolved, wideband full-Stokes radio-polarization observations. The companion is described as likely a helium white dwarf, and its mass estimate depends on assuming a 60-degree inclination. The proposed transitional interpretation is inferred from the system’s combined timing, orbital and polarization properties; the abstract does not establish a specific cause for the unusually large orbital-period derivative or prove an evolutionary sequence.

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