Recent Studies of Strongly Magnetized Neutron Stars

by Kazuo Makishima (RIKEN)

Monday 14 Dec 2015, 13:30 → 14:30 Asia/Tokyo

Main Research Building 433 (RIKEN Wako)

Date: Dec 14 (Mon) Time: 13:30 - Place: Main Research bldg. 433 Speaker: Kazuo Makishima (RIKEN) Title: Recent Studies of Strongly Magnetized Neutron Stars Abstract: Through recent observations in X-ray energies, we have obtained several new insights into strongly magnetized neutron stars, including magnetars in particular. Through a collaboration with MAXI and Suzaku, several new examples of electron cyclotron resonances have been detected from accreting X-ray pulsars. The surface magnetic fields (MFs), thus measured so far from about 20 objects of this class, are clustered in a narrow range of B=(1-7)x10^{12} G, requiring some non-trivial mechanisms of MF sustainment. Magnetars, which are considered to have B=10^{14}-10^{15} G, have been confirmed to emit characteristic two-component spectra, consisting of a soft thermal component and a mysterious hard component. We propose that their hard component results from a cascade of a higher-order QED process called “photon splitting”, wherein an initial gamma-ray photon (most likely the 511 keV one) splits into two lower-energy photons under the presence of the strong MF. The predicted sharp cutoff above 511 keV will be detected by ASTRO-H, to be launched in early 2016 as a successor to Suzaku. We have also detected periodic phase modulation in the rotation of a few magnetars, and interpreted the effect as a manifestation of free precession of a slightly oblate rigid body. The implied deformation, of the order of 10^{-4} in terms of ΔI/I, is possibly caused by ultra-strong toroidal MF of the order of 10^{16} G. Considering all these results, magnetars are indeed likely to be “magnetically-powered” neutron stars. Further investigations into the nature of magnetars provide attractive challenges both theoretically and observationally. See more QHP seminars