Magnetic photon splitting γ --> γγ, a quantum electrodynamics process that becomes important only in magnetic fields approaching the quantum critical value, Bcr = 4.41 × 1013 G, is investigated as a mechanism for attenuation of γ-rays emitted near the surface of strongly magnetized pulsars. Since splitting has no threshold, it can attenuate photons and degrade their energies below the threshold for one-photon pair production, and in high enough fields it may dominate photon attenuation above pair threshold. We model photon-splitting attenuation and subsequent splitting cascades in γ-ray pulsars, including the dipole field and curved spacetime geometry of the neutron star magnetosphere. We focus specifically on PSR 1509-58, which has the highest surface magnetic field of all the γ-ray pulsars (B0 = 3 × 1013 G). We find that splitting will not be important for most γ-ray pulsars, i.e., those with B0 <~ 0.2Bcr, either in competition with pair production attenuation in pair cascades, or in photon escape cutoffs in the spectrum. Photon splitting will be important for γ-ray pulsars having B0 >~ 0.3Bcr, where the splitting attenuation lengths and escape energies become comparable to or less than those for pair production. We compute Monte Carlo spectral models for PSR 1509-58, assuming that either a full photon-splitting cascade or a combination of splitting and pair production (depending on which splitting modes operate) attenuate a power-law input spectrum. We find that photon splitting, or combined splitting and pair production, can explain the unusually low cutoff energy (between 2 and 30 MeV) of PSR 1509-58, and that the model cascade spectra, which display strong polarization, are consistent with the observed spectral points and upper limits for polar cap emission at a range of magnetic colatitudes up to ~25°.
The Astrophysical Journal
- Pub Date:
- February 1997
- Gamma Rays: Theory;
- Stars: Pulsars: Individual: Alphanumeric: PSR 1509-58;
- Stars: Magnetic Fields;
- 39 pages, 14 embedded figures, AASTEX To appear in ApJ, January 20, 1997