Radiation Spectra and Polarization in Magnetar Bursts

We present Monte Carlo simulations of radiative transfer in magnetaratmospheres. We include the effects of vacuum polarization, electron and protonscattering, and free-free absorption. Simulations are performed for theatmosphere model with the magnetic field perpendicular and also tilted withrespect to the neutron star surface, and we show that the average spectrum doesnot strongly depend on the orientation of the magnetic field. We investigatethe region of the parameter space where the vacuum absorption-like featureappears in the spectrum and we analyze the shape of the proton cyclotron line.Our results indicate that the existence of the vacuum polarization featureshould be a general attribute of soft gamma-ray repeaters burst spectra,provided that the energy release takes place at the sufficiently dense region,and the atmosphere scaleheight is large enough. We discuss the existence ofsuch a feature in recent observational data on these sources.Comment: submitted to Ap