Magnetar fraction in Core-Collapse Supernovae

TL;DR

This work quantitatively constrains how often core-collapse supernovae produce magnetars by combining observational NS samples with comprehensive population synthesis that tracks dynamical, spin-down, and magneto-thermal evolution across all isolated Galactic NS classes. By linking a near-complete census of young NSs and a nearby XDINS/X-ray–bright magnetar population to forward-modelled birth properties (including a bimodal magnetic-field distribution at birth), the authors infer a Galactic CCSN rate near 3 per century and a magnetar birth fraction of about 50% on average. Their results require magnetars to be a common outcome of core collapse and support scenarios in which magnetars act as central engines for a large fraction of related extragalactic transients. The findings hinge on self-consistent treatment of magnetic-field decay, magnetospheric spin-down torques, and observational biases, offering a robust framework for interpreting the magnetar–NS landscape in the Milky Way and other galaxies.

Abstract

Magnetars are extreme neutron stars powered by ultra-strong magnetic fields ($\sim10^{14}$ Gauss) and are compelling engines for some of the most powerful extragalactic transients such as Super Luminous Supernovae, Gamma-Ray Bursts, and Fast Radio Bursts. Yet their formation rate relative to ordinary neutron stars remains uncertain, often precluding direct comparisons with the rates of these extragalactic transients. Furthermore, magnetars have been recently shown to be evolutionarily related to other neutron star classes, complicating the estimate of the exact magnetar fraction within the neutron star population. We study the magnetar birth fraction in core-collapse supernovae using pulsar population synthesis of all isolated neutron star classes in our Galaxy, incorporating self-consistently the Galactic dynamical evolution, spin-down and magneto-thermal evolution. This approach allows us to derive strong constraints from small close-to-complete observational samples. In particular, looking at the age-limited young ($<$2 kyr) neutron star population in the Milky Way we find 24 detected young neutron stars, with only 10 of them (41%) being classical rotational powered pulsars, while the others (59%) are either magnetars or central compact objects, the latter believed to be equally magnetically powered. We further compare the results with the nearby volume-limited class ($<$500 pc) of X-ray Dim Isolated Neutron stars, old nearby magnetars. We conclude that the observed population of isolated neutron stars in the Galaxy can be reproduced only by assuming a core-collapse supernova rate larger than two, and a larger magnetar fraction than previously inferred. By assuming a bimodal initial magnetic field ($B_0$) distribution at birth, we find that the magnetar class peaks between $B_0\sim 1-2.5\times10^{14}$ Gauss and represents on average $\sim50$% of the entire neutron star population.

Figures (9)

• Figure 1: Simulated magnetars (purple contour lines) and XDINSs (orange contour lines) for different mean initial magnetar magnetic field strength at birth compared to the observed magnetars young population from Table \ref{['tab:young_ns']} (purple crosses) and the observed XDINSs (orange crosses). The observed young RPPs from Table \ref{['tab:young_ns']} are indicated with gray stars. The entire observed magnetar and RPP populations are also shown as a reference with purple and gray circles respectively. The top row displays the $P-\dot{P}$ diagrams, while the bottom row shows the spin period versus absorbed X-ray flux ($S_{X,\rm abs}$). The contours indicate the density of detected magnetically powered neutron stars obtained from 100 simulations, filtered according to the criteria described in section "Inferring the magnetar birth fraction from population synthesis" of the Methods.
• Figure 1: Ratio of characteristic age, $\tau_c$, vs. real age, $t$, for four different initial magnetic fields and two spin periods as a function of real age, $t$. The rows correspond to initial spin periods of 0.01 s (top) and 0.1 s (bottom). Colors indicate the initial magnetic field strengths. The left and right panels show cases of a constant and a decaying magnetic field, respectively.
• Figure 2: Pie chart summarizing the inferred magnetar fractions for different CC-SN rates and initial mean magnetar magnetic-field strengths. The chart is divided into three macro-sectors corresponding to a CC-SN rate of 1 (brown), 2 (blue) and 3 (green), respectively. Each sector is then further divided into different magnetic field values representing the mean values of the magnetar initial magnetic field distribution. The parts of the sectors highlighted in white represent the inferred ranges of magnetar fractions at birth that are compatible with observations for each combination of CC-SN rate and initial mean magnetar magnetic-field strength. For CC-SN rate of 1 and 2 there is no combination of magnetic field distributions and magnetar fraction that are compatible with the observed population.
• Figure 2: Neutron star characteristic age, $\tau_c$, versus the age inferred from the associated Astron. Comput.SNR, $\tau_{\mathrm{SNR}}$. The color scale represents the dipolar magnetic field strength $B$ in Gauss. The dashed line indicates where $\tau_{\mathrm{SNR}} = \tau_c$.
• Figure 3: Cumulative age distribution of SNRs with neutron star associations. The green shaded area represents the cumulative distribution of observed Astron. Comput.SNR ages, incorporating the lower and upper bounds derived from observational uncertainties. A green dashed line indicates a constant supernova birthrate of 0.6 events per century. The orange shaded region shows the volume-scaled birthrate estimated from a full evolutionary model of Galactic SNRs. The darker and lighter orange bands correspond to the $1\sigma$ and $3\sigma$ confidence intervals, respectively, while the orange dashed line marks the median of the estimated distribution obtained from 500K simulations.