Constrainingγ-ray pulsar gap models with a simulated pulsar population

Abstract

With the large sample of young γ-ray pulsars discovered by the Fermi Large Area Telescope (LAT), population synthesis has become a powerful tool for comparing their collective properties with model predictions. We synthesised a pulsar population based on a radio emission model and four γ-ray gap models (Polar Cap, Slot Gap, Outer Gap, and One Pole Caustic). Applying γ-ray and radio visibility criteria, we normalise the simulation to the number of detected radio pulsars by a select group of ten radio surveys. The luminosity and the wide beams from the outer gaps can easily account for the number of Fermi detections in 2 years of observations. The wide slot-gap beam requires an increase by a factor of ∼10 of the predicted luminosity to produce a reasonable number of γ-ray pulsars. Such large increases in the luminosity may be accommodated by implementing offset polar caps. The narrow polar-cap beams contribute at most only a handful of LAT pulsars. Using standard distributions in birth location and pulsar spin-down power (Ė), we skew the initial magnetic field and period distributions in a an attempt to account for the highĖ Fermi pulsars. While we compromise the agreement between simulated and detected distributions of radio pulsars, the simulations fail to reproduce the LAT findings: all models under-predict the number of LAT pulsars with highĖ, and they cannot explain the high probability of detecting both the radio and γ-ray beams at highĖ. The beaming factor remains close to 1.0 over 4 decades inĖ evolution for the slot gap whereas it significantly decreases with increasing age for the outer gaps. The evolution of the enhanced slot-gap luminosity withĖ is compatible with the large dispersion of γ-ray luminosity seen in the LAT data. The stronger evolution predicted for the outer gap, which is linked to the polar cap heating by the return current, is apparently not supported by the LAT data. The LAT sample of γ-ray pulsars therefore provides a fresh perspective on the early evolution of the luminosity and beam width of the γ-ray emission from young pulsars, calling for thin and more luminous gaps.

THE SECOND FERMI LARGE AREA TELESCOPE CATALOG OF GAMMA-RAY PULSARS

“…This is because the radio beam, increasingly aligned to the spin pole, is seldom visible when one views the gamma-ray beam, which is concentrated to the spin equator. Pierbattista et al (2012) also discuss the lack of high-Ė radio-quiet pulsars and the increase in the radio-quiet fraction at low-Ė, plausibly addressed by an evolution toward spin alignment. However, these authors note that such alignment on Myr timescales cannot address the discrepancy seen at high-E because of the young age of those objects.…”

“…This catalog is a milestone in the progress toward the longterm goal of acquiring the most uniform sample of neutron stars possible, so that comparisons with model predictions (e.g., Gonthier et al 2007;Watters & Romani 2011;Pierbattista et al 2012) will allow improved tests of emission models and of their links with their parent population of massive stars or with diffuse Galactic emission. Selection biases can be subtle and the advantage of pulsar searches in the coming years is not so much to increase the absolute numbers, but to be sure to have explored the dark corners of parameter space.…”

Gamma-ray pulsars: A gold mine

“…The slot gap [127,128] is an extension of the polar cap where acceleration occurs in a narrow charge-depleted zone along the last open field lines from 13 the neutron star surface to near the light cylinder. Both of these models can account for the variety and major characteristics of the Fermi pulsar light curves, but do not provide enough power to account for the observed luminosities [37]. Furthermore, neither of these models is consistent with the global magnetosphere current closure.…”

“…[55,56,37]). The expectations in total pulsar counts and in the share between γ-only and γ+radio pulsars agree reasonably well with the data for the outer-magnetospheric Figure 8 but for the FIDO models.…”

The Tale of the Two Tails of the Oldish PSR J2055+2539

“…The discovery of such a large sample of high-energy-emitting pulsars has enabled numerous population studies (see, e.g., Watters & Romani 2011;Pierbattista et al 2012). This has resulted in a better understanding of the physical phenomena behind the multiwavelength emission from pulsars.…”