Light-curve modelling constraints on the obliquities and aspect angles of the youngFermipulsars

Abstract

In more than four years of observation the Large Area Telescope on board the Fermi satellite has identified pulsed γ-ray emission from more than 80 young or middle-aged pulsars, in most cases providing light curves with high statistics. Fitting the observed profiles with geometrical models can provide estimates of the magnetic obliquity α and of the line of sight angle ζ, yielding estimates of the radiation beaming factor and radiated luminosity. Using different γ-ray emission geometries (Polar Cap, Slot Gap, Outer Gap, One Pole Caustic) and core plus cone geometries for the radio emission, we fit γ-ray light curves for 76 young or middle-aged pulsars and we jointly fit their γ-ray plus radio light curves when possible. We find that a joint radio plus γ-ray fit strategy is important to obtain (α, ζ) estimates that can explain simultaneously detectable radio and γ-ray emission: when the radio emission is available, the inclusion of the radio light curve in the fit leads to important changes in the (α, ζ) solutions. The most pronounced changes are observed for Outer Gap and One Pole Caustic models for which the γ-ray only fit leads to underestimated α or ζ when the solution is found to the left or to the right of the main α-ζ plane diagonal respectively. The intermediate-to-high altitude magnetosphere models, Slot Gap, Outer Gap, and One pole Caustic, are favoured in explaining the observations. We find no apparent evolution of α on a time scale of 10 6 years. For all emission geometries our derived γ-ray beaming factors are generally less than one and do not significantly evolve with the spin-down power. A more pronounced beaming factor vs. spin-down power correlation is observed for Slot Gap model and radio-quiet pulsars and for the Outer Gap model and radio-loud pulsars. The beaming factor distributions exhibit a large dispersion that is less pronounced for the Slot Gap case and that decreases from radio-quiet to radio-loud solutions. For all models, the correlation between γ-ray luminosity and spin-down power is consistent with a square root dependence. The γ-ray luminosities obtained by using the beaming factors estimated in the framework of each model do not exceed the spin-down power. This suggests that assuming a beaming factor of one for all objects, as done in other studies, likely overestimates the real values. The data show a relation between the pulsar spectral characteristics and the width of the accelerator gap. The relation obtained in the case of the Slot Gap model is consistent with the theoretical prediction.

Gamma-ray pulsars: A gold mine

“…The flux had slightly, but steadily increased during the three years preceding the jump. We are probably viewing this pulsar at large angle from the magnetic axis, with |αB − ζ obs | of order 60 • -80 • [57]. This mode change resembles γ-ray changes recorded in Vela in 1975 and 1981 [62].…”

“…Model fits to the observed light curves show that the current sample encompasses a large variety of stellar configurations (αB magnetic obliquity and ζ obs viewing angle, see Figure 2), with low |αB − ζ obs | values when we intercept both the radio and γ-ray beams, and large |αB − ζ obs | values when we intercept only the wide γ-ray beam [54,64,57]. A useful anti-correlation has been found between the phase separation between the two γ-ray peaks (∆) and the phase separation between the radio and first γ-ray peak (alias radio lag δ).…”

“…This should reflect in the distribution of light curve profiles recorded by Fermi. Yet, no single geometrical model of the beam origin and shape (see Figure 3) can currently reproduce the whole set of observed light curves [57,19]. Only few trends emerge in the large variety of pulse profiles [49].…”

High-energy pulsar light curves in an offset polar cap B-field geometry

“…We next follow the same approach as a previous study [37] to compare the various optimal solutions of the different models. We determine the difference between the scaled χ 2 of the optimal model, ξ 2 opt , and the other models (ξ 2 ) using…”

“…More recent developments include global magnetospheric models such as the force-free (FF) inside and dissipative outside (FIDO) model [24,25], the wind models of, e.g., [36], and particle-in-cell simulations (PIC; [8,9]). Although much progress has been made using these physical (or emission) models, geometric light curve modeling [16,41,42,22,37] still presents a crucial avenue for probing the pulsar magnetosphere in the context of traditional pulsar models. The most commonly used emission geometries include the two-pole caustic (TPC; the SG model may be its physical representation; [15]) and OG models and may be used to constrain the pulsar geometry (i.e., magnetic inclination angle α and the observer viewing angle ζ with respect to the spin axis Ω Ω Ω), as well as the γ-ray emission region's location and extent.…”