Can accreting isolated neutron stars be detected?

TL;DR

The paper tackles whether accreting isolated neutron stars in the Milky Way can be detected by current X-ray surveys. It introduces a comprehensive population-synthesis framework that couples NS spin and magnetic-field evolution with a realistic Milky Way and two-phase ISM, and then assesses eROSITA observability by modeling accretion-powered X-ray emission and absorption. The central result is that the duration and efficiency of the propeller regime dominantly control the number of observable accretors, producing a wide range of predictions from zero to several thousand depending on model choices. The work emphasizes the need for empirical constraints on low-rate accretion physics and proposes Gaia-detected wide binaries as a promising route to calibrate these models, thereby improving interpretation of future X-ray surveys.

Abstract

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.

Figures (5)

• Figure 1: Evolutionary stages -- ejector (E), propeller (P), accretor (A), and georotator (G) -- and transition conditions between them. For direct transitions (E-P, P-A, and A-G), the characteristic radius of interaction with external matter (the left side of each equation) must be greater than or equal to the right side. For reverse transitions (P-E, A-P, and G-A), it must be less than the right side. The E-P and P-E transitions require two conditions each. The wavy lines are supposed to illustrate the interaction of the magnetosphere and the external matter.
• Figure 2: Duration of the propeller stage over the characteristic velocity $v$ of an NS in different propeller models. The magnetic field is constant. Every propeller model is shown as two curves with filled color in between. In each model, the lower curve is for the parameters that promote a rapid evolution: the constant field $B=10^{14}$ G, the number density of the ISM $n=10$ cm$^{-3}$. The upper curve is for average parameters: $B=10^{12}$ G, $n=0.1$ cm$^{-3}$. Three grey dotted horizontal lines indicate $1$ Myr, $1$ Gyr and $13.6$ Gyr.
• Figure 3: Corner diagram illustrating the fraction of time an NS with specific initial values of magnetic field, kick velocity, and spin period spends at the accretor stage $\tau_\text{A}$. Both one- and two-dimensional histograms show $\tau_\text{A}$. The dash-dotted line at the panel I is the transition condition between the accretor and georotator stages $R_\text{G} = R_\text{A}$ for the number density $n=0.1\text{~cm}^{-3}$. The dotted line in panel III separates NSs born at the ejector and propeller stages and corresponds to the ejector-propeller transition with $n=0.1\text{~cm}^{-3}$, $v=100$ km s$^{-1}$. Single-phase ISM, propeller model A, and constant field (CF) models are used. The grey diagrams are normalized so that each bin illustrates an average $\tau_\text{A}$ corresponding to the value of the bin.
• Figure 4: Extended corner diagram of the accretion time $\tau_\text{A}$ for the propeller model B, the exponentially decaying magnetic field and the two-phase model of the ISM. The diagram is calculated similarly to Fig. \ref{['fig_triangleA']}. The initial parameters are the kick velocity $v_\text{kick}$, the magnetic field $B_0$, the spin period $P_0$, and the coordinates $R_0$ and $z_0$ in the cylindrical coordinate system with the origin at the Galactic center.
• Figure 5: The number of the accreting neutron stars $N$ producing the count rate CR$_0$ or brighter versus CR$_0$. In each panel, only three out of four propeller models are shown, since models A, B, and C produce enough accreting NSs that might be observable, while model D does not. Propeller models A and B yield similar results, so their curves are nearly identical. Objects potentially visible to the eROSITA telescope are located to the right of the faint vertical line at $10^{-2}$ cts s$^{-1}$. The dashed line on the left in each panel shows the dependence $N(\text{CR}>\text{CR}_0)\propto \text{CR}_0^{-1}$, the one on the right is $\propto \text{CR}_0^{-3/2}$. The colored area shows the range of values within one standard deviation of the mean.