Emission models for soft gamma-ray Repeaters/Anomalous X-ray pulsars described as white dwarfs

Abstract

Soft Gamma Repeaters (SGR) and Anomalous X-Ray Pulsars (AXP) are considered by a large fraction of the scientific community as magnetars: neutron stars having a huge magnetic field (10^13 to 10^15 G). However, because of this giant magnetic field, greater than the critical limit imposed by the quantum mechanics, other possibilities are also studied: the white-dwarf model is one of them. In this context, this project, which started in 2014 August, focuses on studying the SGR/AXP as white dwarfs. In the first year of the project, we calculated the apparent magnitude for a white dwarf in different bands. This result was compared with the observed magnitudes or limits for all sources in the McGill catalog. In the second year, we revised the characteristics of the optical and infrared counterparts, specially for the sources 4U 0142+61, 1E 1048.1-5937 and SGR 0501+4516, which are the only sources with confirmed optical counterpart. As this emission is pulsed, we studied the possibility of this pulsation being caused by an accretion region in the white-dwarf surface. We estimated the temperature and the radius of this accretion region for a white dwarf of known mass. The dimension of the emitting region required to explain the pulsed emission is 1 to 5% of the white-dwarf radius, which supports the white-dwarf model. In the present proposal, we are going to extrapolate the model for X-rays and to calculate the optical and x-ray light curves. (AU)