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

Abstract

Studies were made of the 1-70 keV persistent spectra of fifteen magnetars as a complete sample observed with Suzaku from 2006 to 2013. Combined with early NuSTAR observations of four hard X-ray emitters, nine objects showed a hard power-law emission dominating at $\gtrsim$10 keV with the 15--60 keV flux of $\sim$1-$11\times 10^{-11}$ ergs s${}^{-1}$ cm${}^{-2}$. The hard X-ray luminosity $L_{\mathrm{h}}$, relative to that of a soft-thermal surface radiation $L_{\mathrm{s}}$, tends to become higher toward younger and strongly magnetized objects. Updated from the previous study, their hardness ratio, defined as $\xi =L_{\mathrm{h}}/L_{\mathrm{s}}$, is correlated with the measured spin-down rate $\dot{P}$ as $\xi =0.62\times (\dot{P}/10^{-11}\mathrm{s}{\mathrm{s}}^{-1}{)}^{0.72}$, corresponding with positive and negative correlations of the dipole field strength $B_{\mathrm{d}}$ ($\xi \propto B_{\mathrm{d}}^{1.41}$) and the characteristic age ${\tau }_{\mathrm{c}}$ ($\xi \propto {\tau }_{\mathrm{c}}^{-0.68}$), 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 $\sim$10-40 d plateau phase. Transients show the maximum luminosities of $L_{\mathrm{s}}$$\sim$$10^{35}$ erg s${}^{-1}$, which is comparable to those of the persistently bright ones, and fade back to $\lesssim$$10^{32}$ erg s${}^{-1}$. Spectral properties are discussed in a framework of the magnetar hypothesis.

22 pages, 18 figures, 7 tables, accepted for publication in The Astrophysical Journal Supplement

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