Magnetar Broadband X-Ray Spectra Correlated with Magnetic Fields: Suzaku Archive of SGRs and AXPs Combined with NuSTAR, Swift, and RXTE

Abstract

The 1-70 keV persistent spectra of 15 magnetars, observed with Suzaku from 2006 to 2013, were studied as a complete sample. Combined with early NuSTAR observations of four hard X-ray emitters, nine objects showed a hard power-law emission dominating at ≳ 10 keV with the 15-60 keV flux of ∼1-11× {10}^-11 erg s^-1 cm^-2. The hard X-ray luminosity {L}_{{h}}, relative to that of a soft-thermal surface radiation {L}_{{s}}, tends to become higher toward younger and strongly magnetized objects. Their hardness ratio, updated from a previous study and defined as ξ ={L}_{{h}}/{L}_{{s}}, is correlated with the measured spin-down rate \dot{P} as ξ =0.62× {(\dot{P}/{10}^-11{{s}}{{{s}}}^-1)}^0.72, corresponding to positive and negative correlations with the dipole field strength {B}_{{d}} (ξ \propto {B}_{{d}}^1.41) and the characteristic age {τ }_{{c}} (ξ \propto {τ }_{{c}}^-0.68), respectively. Among our sample, five transients were observed during X-ray outbursts, and the results are compared with their long-term 1-10 keV flux decays monitored with Swift/XRT and RXTE/PCA. Fading curves of three bright outbursts are approximated by an empirical formula used in the seismology, showing a ∼10-40 day plateau phase. Transients show the maximum luminosities of {L}_{{s}} ∼ 10³⁵ erg s^-1, which are comparable to those of persistently bright ones, and fade back to ≲10³² erg s^-1. Spectral properties are discussed in the framework of the magnetar hypothesis.