2017ApJS..231....8E

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^–11ss^–1)^0.72, corresponding to positive and negative correlations with the dipole field strength B_d (ξ∝B_d^1.41) and the characteristic age τ_c (ξ∝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.