Spectral Constraints for Millisecond Pulsars Due to General Relativistic Frame Dragging |
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Authors: | C Venter O C De Jager |
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Institution: | (1) Unit for Space Physics, North-West University, Potchefstroom Campus, Potchefstroom, South Africa |
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Abstract: | We develop a numerical code for simulating the magnetospheres of millisecond pulsars, which are expected to have unscreened
electric potentials due to the lack of magnetic pair production. We incorporate General Relativistic (GR) expressions for
the electric field and charge density and include curvature radiation (CR) due to primary electrons accelerated above the
stellar surface, whereas inverse Compton scattering (ICS) of thermal X-ray photons by these electrons are neglected as a second-order
effect. We apply the model to PSR J0437-4715, a prime candidate for testing the GR-Electrodynamic theory, and find that the
curvature radiation spectrum cuts off at energies below 15 GeV, which are well below the threshold of the H.E.S.S. telescope,
whereas Classical Electrodynamics predict a much higher cutoff near 100 GeV, which should be visible for H.E.S.S., if standard
assumed Classical Electrodynamics apply. GR theory also predicts a relatively narrow pulse (2φ
L
∼ 0.2 phase width) centered on the magnetic axis, which sets the beaming solid angle to ∼0.5 sr per polar cap (PC) for a magnetic
inclination angle of 35∘ relative to the spin axis, given an observer which sweeps close to the magnetic axis. We also find that EGRET observations
above 100 MeV of this pulsar constrain the polar magnetic field strength to B
pc < 4× 108 G for a pulsar radius of 10 km and moment of inertia of 1045 g cm2. The field strength constraint becomes even tighter for a larger radius and moment of inertia. Furthermore, a reanalysis
of the full EGRET data set of this pulsar, assuming the predicted pulse shape and position, should lead to even tighter constraints
on neutron star and GR parameters, up to the point where the GR-derived potential and polar cap current may be questioned. |
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Keywords: | stars: neutron pulsars: individual (PSR J0437-4715) |
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