Can accreting isolated neutron stars be detected?

We perform population synthesis modeling of isolated neutron stars in the Milky Way over its lifetime. Compared with previous studies, we use more detailed models of the interstellar medium and the magneto-rotational evolution of neutron stars. We demonstrate that presently, the spin-down rate at the propeller stage is the main uncertain factor that influences the number of accreting isolated neutron stars. If the propeller stage duration allows neutron stars to begin accreting matter from the interstellar medium and if the efficiency of accretion is high, then the number of accreting isolated neutron stars in eROSITA data can reach ~a few thousand. Still, uncertainties in spin-down at the propeller stage and in the accretion process can drastically decrease this number. We suggest that future observations of neutron stars in wide low-mass binaries recently discovered by Gaia can clarify these issues.

💡 Research Summary

The paper “Can accreting isolated neutron stars be detected?” presents a sophisticated population synthesis study aimed at estimating the observability of isolated neutron stars (INS) within the Milky Way. The primary objective of this research is to evaluate whether current and upcoming X-ray surveys, specifically eROSITA, can identify a significant population of neutron stars that are accreting matter from the interstellar medium (ISM).

To achieve this, the authors employed an advanced population synthesis modeling technique, covering the entire evolutionary history of the Milky Way. A significant advancement over previous studies lies in the integration of more nuanced models regarding the interstellar medium (ISM) and the magneto-rotational evolution of neutron stars. By incorporating detailed ISM density distributions and a more precise treatment of how a neutron star’s magnetic field and rotation period evolve over time, the study provides a more physically grounded prediction of the INS population.

The core of the scientific investigation centers on the “propeller stage” of neutron star evolution. During this phase, the neutron star’s rotation is sufficiently rapid that its magnetic field acts as a centrifugal barrier, preventing the surrounding ISM from accreting onto the stellar surface. The researchers identify the spin-down rate during this propeller stage as the most critical source of uncertainty in their model. The duration of this stage dictates when a neutron star can transition into the accretion stage. If the spin-down process is efficient enough to shorten the propeller phase, and if the subsequent accretion efficiency from the ISM is high, the study predicts that eROSITA could potentially detect several thousand accreting INS.

However, the study also highlights a significant degree of scientific uncertainty. The authors emphasize that the predicted number of detectable neutron stars is highly sensitive to the physics of the spin-down rate and the efficiency of the accretion process itself. If the propeller stage persists longer than expected or if the accretion efficiency is lower than the upper-bound estimates, the number of detectable sources could drop drastically, potentially making them nearly impossible to distinguish from background noise.

In conclusion, the paper provides a roadmap for future observational verification. The authors suggest that studying wide low-mass binaries, which have been recently identified by the Gaia mission, could serve as a vital laboratory. Observations of these binaries can provide the necessary empirical constraints on the spin-down and accretion physics, thereby reducing the uncertainties in the population synthesis models and clarifying the true census of isolated neutron stars in our galaxy.