Application to young pulsars

From observations, several pulsars may have polar caps with effective temperature though the exact value of the temperatures is rather uncertain (for a review, see e.g., Ögelman 1991). For moderately hot polar caps and typical B fields (), the energy loss rate is not rapid enough to compete with the acceleration, and RICS controls the polar gap only through pair production. This in general reduces the gap length, and hence reduces the acceleration efficiency by at least one order magnitude, imposing a severe constraint on the energetics of the polar gap.

For young pulsars with superstrong magnetic fields , the gap acceleration efficiency can increase provided that the polar cap () is hot () and for electrons (or positrons), the energy loss due to RICS can dominate over the acceleration. One of CGRO pulsars, PSR 1509-58, has magnetic field (the magnetic field at the pole can be twice this value). Since neutron stars are hot at birth (after supernova explosion), their polar caps can be hot as result of self-sustained polar cap heating at very young age (e.g. Luo 1996), e.g within hundred years or so after birth. These high field pulsars, assuming they were born with high magnetic fields, must have undergone a phase when thermal emission had an important effect on pulsar electrodynamics.

We suggest that for high field pulsars there can be four phases during the their lifetime as discussed below. (i) At very young age (within a few years after supernova explosion) when the surface or polar cap temperature is high, energy loss due to RICS is so severe that a pair cascade due to electrons or positrons cannot occur in the region close to the polar cap. In the ion zone (), accelerated ions may carry most of the particle luminosity. For heavy ions, photodisintegration may occur in the thermal photon field from the polar cap. At this stage, since pulsars also have a very large spin-down luminosity, they are a potential source of high energy hadrons and electromagnetic radiation. (ii) As pulsars cool down, the energy loss rate due to RICS becomes less than acceleration rate. However, RICS may still be the dominant mechanism for pair production to constrain the gap length (Luo 1996). The high field pulsar, PSR 1509-58, may currently be at this stage since the polar cap temperature inferred from the observation is about (e.g. Kawai et al. 1991). Its gap length is shorter than that controlled by pair production by either curvature radiation or relativistic ions in the photon field. Thus, the acceleration efficiency is less than in phase (i). (iii) As the surface temperature or polar cap temperature becomes much lower, pair production by RICS is not important. The main mechanism for initiating pair cascades is curvature radiation. As the period increases, the gap length increases and hence the efficiency also increases. However, at this stage, the pulsar spin-down luminosity is lower than at their early young age. (iv) The period reaches the critical period at which pair production due to curvature radiation is no longer operative. Although the gap acceleration efficiency is now high (no pairs to screen the electric field), the luminosity is very low.

In summary, for very young pulsars with superstrong magnetic fields () and hot polar caps (), the severe energy loss due to resonant inverse Compton scattering can prevent electrons or positrons starting a pair cascade in the region close to the polar cap but ions can be accelerated by the full potential drop across the gap (provided that free emission of ions can occur). We then have the ions carrying most of the particle luminosity. Thus, very young pulsars with superstrong B field can be a potential source of high-energy hadrons and gamma-rays. In contrast, for pulsars with moderately hot polar caps (), the effect of resonant inverse Compton scattering is to reduce the gap acceleration efficiency. Pair production by accelerated ions interacting with thermal photons strongly depends on P and T, and in general we have and hence the polar gap length is controlled by RICS.

© Copyright Astronomical Society of Australia 1997