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-ray emitters, nine objects showed a hard power-law emission dominating at greater than or similar to 10 keV with the 15-60 keV flux of similar to 1-11 x 10(-11) erg s(-1) cm(-2). The hard X-ray luminosity L-h, relative to that of a soft-thermal surface radiation Ls, tends to become higher toward younger and strongly magnetized objects. Their hardness ratio, updated from a previous study and defined as xi = L-h/L-s, is correlated with the measured spin-down rate (P) over dot as xi = 0.62 x ((P) over dot 10(-11) s s(-1))(0.72), corresponding to positive and negative correlations with the dipole field strength B-d (xi proportional to B-d(1.41)) and the characteristic age tau(c) (xi proportional to tau(-0.68)(c)), 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 similar to 10-40 day plateau phase. Transients show the maximum luminosities of L-s similar to 10(35) erg s(-1), which are comparable to those of persistently bright ones, and fade back to less than or similar to 10(32) erg s(-1). Spectral properties are discussed in the framework of the magnetar hypothesis.