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

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-rayemitters, nine objects showed a hard power-law emission dominating at ≳ 10 keV with the 15–60 keV flux of ~1–11 x 10^(-11) ergs^(−1) cm^(−2). The hard X-rayluminosity 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 P as ξ = 0.62 x (P/10^(-11)s s^(-1))^(0.72), corresponding to positive and negative correlations with the dipole field strength B_d (ξ ∝ B^(1.41)_d) 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 ~ 10^(35) ergs^(−1), which are comparable to those of persistently bright ones, and fade back to ≾10^(32) ergs^(−1). Spectral properties are discussed in the framework of the magnetarhypothesis.