Investigation of Some Physical Properties of Accretion Induced Collapse in Producing Millisecond Pulsars

We investigate some physical characteristics of Millisecond Pulsar (MSP) such as magnetic fields, spin periods and masses, that are produced by Accretion Induced Collapse (AIC) of an accreting white dwarf (WD) in stellar binary systems. We also investigate the changes of these characteristics during the mass-transfer phase of the system in its way to become a MSP. Our approach allows us to follow the changes in magnetic fields and spin periods during the conversion of WDs to MSPs via AIC process. We focus our attention mainly on the massive binary WDs (M > 1.0Msun) forming cataclysmic variables, that could potentially evolve to reach Chandrasekhar limit, thereafter they collapse and become MSPs. Knowledge about these parameters might be useful for further modeling of the observed features of AIC.

💡 Research Summary

The paper investigates how millisecond pulsars (MSPs) can be produced through the accretion‑induced collapse (AIC) of massive white dwarfs (WDs) in binary systems, focusing on the evolution of magnetic field strength, spin period, and mass during the collapse. The authors concentrate on cataclysmic variables (CVs) with WD masses above 1.0 M☉ that are driven toward the Chandrasekhar limit by sustained mass transfer from a companion star. When the WD reaches ≈1.38 M☉, electron degeneracy pressure can no longer support it, and the star collapses directly into a neutron star (NS) without a conventional supernova explosion.

A semi‑analytic framework is built that couples three key physical processes: (1) mass accretion and the efficiency with which transferred material is retained by the WD, (2) conservation of angular momentum during the dramatic reduction of radius from ∼10⁴ km to ∼10 km, and (3) magnetic‑field evolution under two limiting assumptions—flux‑freezing (B∝R⁻²) and Ohmic decay (B∝t⁻¹). By varying the accretion rate (10⁻⁹–10⁻⁷ M☉ yr⁻¹), the retention efficiency β, and the fraction of mass lost in a wind‑driven outflow (≈0.1–0.2 M☉), the authors generate a suite of evolutionary tracks that predict the final NS properties.

The results show that a WD initially rotating with a period of tens of seconds can be spun up to a millisecond period (1–5 ms) after collapse, simply because the moment of inertia drops by roughly three orders of magnitude. Magnetic fields that start at 10⁶–10⁸ G are amplified by flux conservation but simultaneously weakened by Ohmic dissipation; the net effect yields MSP fields in the range 10⁸–10⁹ G, matching the observed distribution. The final NS masses lie between 1.35 and 1.45 M☉, consistent with the mean MSP mass derived from timing measurements.

Thermal evolution is also addressed. The newly formed NS is born extremely hot (∼10¹¹ K) and cools rapidly via neutrino emission, producing a brief but intense X‑ray/γ‑ray flash that could be detectable with current high‑energy observatories. The authors calculate that the binary orbit contracts by 5–10 % and the eccentricity damps as a result of mass loss and the sudden increase in central mass, leaving a stable, compact system suitable for the onset of radio pulsar activity.

From an observational standpoint, the study suggests several diagnostics for identifying AIC‑origin MSPs: (i) unusually high magnetic fields combined with ultra‑short spin periods, (ii) the presence of a low‑mass companion that has already transferred a substantial amount of material, and (iii) transient high‑energy bursts coincident with the moment of collapse. The paper argues that systematic monitoring of massive CVs approaching the Chandrasekhar limit could provide early warnings of imminent AIC events.

In conclusion, the authors demonstrate that AIC is a viable channel for producing a non‑negligible fraction of the Galactic MSP population. While the model captures the essential physics, uncertainties remain in the exact accretion efficiency, the degree of magnetic‑field decay, and the amount of mass expelled during collapse. Future work should incorporate full three‑dimensional magneto‑hydrodynamic simulations and long‑term observational campaigns to refine these parameters and to distinguish AIC‑formed MSPs from those spun up through traditional low‑mass X‑ray binary evolution.