New Clues to the Hard X-ray Emission from Magnetars and Prospects for their X-ray Polarimetric Observations

Magnetars provide a laboratory for a series of attractive physics in ultra-high electromagnetic fields, because they are considered to have magnetic fields exceeding the quantum-electro-dynamical critical value of 4e13 G, and thought to emit X-rays by consuming their huge magnetic energies. As revealed with INTEGRAL and Suzaku (Enoto et al. 2010, 2017), the magnetar's emission consists ubiquitously of a Soft X-ray Component (SXC) and a Hard X-ray Component (HXC), which dominate the energies below and above about 10 keV, respectively. The HXC is extremely hard in its spectral slope, and exhibits an interesting scaling dependence on the object's age (Enoto et al. 2010, 2017). Therefore, the HXC is likely to be a result of some exotic non-thermal processes involving the super-critical magnetic fields. Using Suzaku and NuSTAR, we detected slow phase modulations in the HXC pulses of three magnetars; 4U 0142+61, 1E 1547.0-5408, and SGR 1900+14 (Makishima et al. 2014, 2016, 2019). In contrast, their SXC pulses were all free from the modulation. These objects are considered to be axially deformed by their internal magnetic stress, and are undergoing free precession. Then, the HXC pulses will be phase-modulated as its emission pattern is highly anisotropic, whereas the SXC pulses are not, because it is symmetric around the star's symmetry axis. This interpretation implies that the two components are produced by distinct mechanisms, and the HXC photons may be significantly polarized. In addition, the polarization properties could depend on the energy, if the proton cyclotron resonance is in the relevant energy range. Thus, polarimetric observations of magnetars are encouraged. The Imaging X-ray Polarimetry Explorer (IXPE) to be launched in 2021 utilizes, as its key technology to detect photoelectron tracks, Gas Electron Multiplier foils developed in Japan (RIKEN and SciEnergy Co., Ltd; Tamagawa et al. 2009). Although the energy band of IXPE is limited to < 8 keV, the HXC of some magnetars (particularly in high activity) may be accessible with the IXPE.