Effect of the Polarization of Photon Splitting on the Cyclotron Lines in Large-Scale Atmospheres with Super-Strong Magnetic Fields

Abstract

Abstract The cyclotron lines are affected by photon splitting in super-strong magnetized atmospheres ($ \sim 4.4 \times 10^{13} \,\mathrm{G}$) into which a power-law-type flux is injected. We studied the polarization effect of photon splitting on the cyclotron line formation by treating different modes of splitting. We calculated the emergent spectra for two conditions: only one splitting mode and three splitting modes that could operate in a neutron-star atmosphere with a subcritical magnetic field. Photon splitting can also operate over a larger scale than cyclotron resonant scattering in a neutron-star atmosphere, since photon splitting occurs in a super-strong magnetic field even in the absence of a plasma. We therefore considered atmospheres consisting of lower and higher density regions. Cyclotron resonant scattering is dominant in a higher density region, and photon splitting is dominant in a lower density region. In three splitting modes, we found that the cyclotron absorption lines are strongly reprocessed by photon splitting, and thus may not be detected even for a magnetic field of about $ 1.0 \times 10^{13} \,\mathrm{G}$ in the power-law-type spectra. However, we confirmed that the cyclotron absorption lines are not mostly reprocessed by photon splitting in only one splitting mode.