What can we learn from phase alignment of gamma-ray and radio pulsar light curves?

Abstract

The Fermi Large Area Telescope (LAT) has revolutionized high-energy (HE) astronomy, and is making enormous contributions particularly to γ-ray pulsar science. As a result of the many new pulsar discoveries, the γ-ray pulsar population is now approaching 100. Some very famous millisecond pulsars (MSPs) have also been detected: J1939+2134 (B1937+21), the first MSP ever discovered, as well as J1959+2048 (B1957+20), the first black widow pulsar system. These, along with other MSPs such as PSR J0034−0534 and J2214+3000, are rare among the pulsar population in that they exhibit nearly phasealigned radio and γ-ray light curves (LCs). Traditionally, pulsar LCs have been modelled using standard HE models in conjunction with low-altitude conal beam radio models. However, a different approach is needed to account for phase-aligned LCs. We explored two scenarios: one where both the radio and γ-ray emission originate in the outer magnetosphere, and one where the emission comes from near the polar caps (PCs) on the stellar surface. We find best-fit LCs using a Markov chain Monte Carlo (MCMC) technique for the first class of models. The first scenario seems to be somewhat preferred, as is also hinted at by the radio polarization data. This implies that the phase-aligned LCs are possibly of caustic origin produced in the outer magnetosphere, in contrast to the usual lower-altitude conal beam radio models. Lastly, we constrain the emission altitudes with typical uncertainties of ∼ 10% of the light cylinder radius. The modelled pulsars are members of a third γ-ray MSP subclass, in addition to two others with non-aligned radio and γ-ray LCs.