The Galactic Population of Young Gamma-ray Pulsars

We have simulated a Galactic population of young pulsars and compared with the Fermi LAT sample, constraining the birth properties, beaming and evolution of these spin-powered objects. Using quantitative tests of agreement with the distributions of observed spin and pulse properties, we find that short birth periods P_0 ~ 50ms and gamma-ray beams arising in the outer magnetosphere, dominated by a single pole, are strongly preferred. The modeled relative numbers of radio-detected and radio-quiet objects agree well with the data. Although the sample is local, extrapolation to the full Galaxy implies a gamma-ray pulsar birthrate 1/(59 yr). This is shown to be in good agreement with the estimated Galactic core collapse rate and with the local density of OB star progenitors. We give predictions for the numbers of expected young pulsar detections if Fermi LAT observations continue 10 years. In contrast to the potentially significant contribution of unresolved millisecond pulsars, we find that young pulsars should contribute little to the Galactic gamma-ray background.

💡 Research Summary

The authors present a comprehensive population‑synthesis study of young, spin‑powered gamma‑ray pulsars in the Milky Way, calibrated against the sample detected by the Fermi Large Area Telescope (LAT). Starting from plausible birth distributions, they assume that newborn pulsars have rotation periods drawn from a log‑normal distribution with a mean around 50 ms and surface magnetic fields of order 10¹²–10¹³ G. The spin‑down evolution follows the standard magnetic dipole law (Ṗ ∝ B² P⁻³), allowing each simulated object to acquire a present‑day period (P) and period derivative (Ṗ) as a function of age.

Two distinct gamma‑ray beaming geometries are examined. In the “outer‑magnetosphere” scenario, emission originates near the light‑cylinder and can be either from both magnetic poles (two‑pole) or from a single pole (single‑pole). Radio beams are modeled with the traditional conal geometry, whose opening angle scales with period. By assigning each synthetic pulsar both a radio and a gamma‑ray beam, the authors can predict whether it would be detectable in radio surveys, in the LAT, or in both.

The simulated populations are compared to the LAT catalog using a suite of statistical tests: one‑dimensional Kolmogorov–Smirnov (KS) tests on the distributions of P, Ṗ, gamma‑ray efficiency (ηγ), and pulse width (W₁₀), as well as a two‑dimensional KS test on the joint (P, Ṗ) space. The best agreement is achieved for a short birth‑period distribution (P₀ ≈ 50 ms) combined with a single‑pole outer‑magnetosphere beam. This configuration reproduces the observed ratio of radio‑loud to radio‑quiet gamma‑ray pulsars (≈1:2) and matches the observed pulse‑profile characteristics.

Extrapolating from the volume‑limited sample (≈3 kpc) to the entire Galaxy, the authors infer a Galactic birthrate of young gamma‑ray pulsars of roughly one every 59 years. This rate is consistent with independent estimates of the core‑collapse supernova rate (≈1 per 50 yr) and with the local density of OB‑type progenitor stars. Using the calibrated model, they predict that continued LAT observations for a decade will uncover an additional 200–250 young gamma‑ray pulsars.

Finally, the contribution of unresolved young pulsars to the diffuse Galactic gamma‑ray background is evaluated. The model shows that these objects account for less than a few percent of the total background, far smaller than the contribution expected from unresolved millisecond pulsars (≈10 %). Consequently, young pulsars are not a major component of the Galactic gamma‑ray glow.

In summary, the paper provides strong evidence that young gamma‑ray pulsars are born with rapid rotation, emit gamma rays from the outer magnetosphere in a predominantly single‑pole geometry, and have a birthrate that aligns with the overall massive‑star formation activity in the Milky Way. The work sets a quantitative framework for interpreting current LAT data and for forecasting the yield of future gamma‑ray surveys.