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 10 keV with the 15-60 keV flux of ∼1-11 × 10 −11 ergs 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. Updated from the previous study, their hardness ratio, defined as ξ = L h /L s , is correlated with the measured spin-down rateṖ as ξ = 0.62 × (Ṗ /10 −11 s s −1 ) 0.72 , corresponding with positive and negative correlations of the dipole field strength B d (ξ ∝ B d) and the characteristic age τ c (ξ ∝ τ −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 ∼10-40 d plateau phase. Transients show the maximum luminosities of L s ∼1035 erg s −1 , which is comparable to those of the persistently bright ones, and fade back to 10 32 erg s −1 . Spectral properties are discussed in a framework of the magnetar hypothesis.

X-ray Observations of Neutron Stars

“…A prototypical AXP 4U 0142+61 shows the both components [9], and in 2009, we discovered a hard power-law component from a transient source 1E 1547.0−5408 [8]. Based on our Suzaku legacy, we pointed out that the broad-band spectral shape of magnetars changes as the magnetic field decays [6,11]. For example, a wide-band hardness ratio, defined as the 1-60 keV X-ray luminosity L h of the hard X-ray component relative to those of the soft X-ray one L s , shows a positive correlation of the dipole magnetic field B d .…”

“…For example, a wide-band hardness ratio, defined as the 1-60 keV X-ray luminosity L h of the hard X-ray component relative to those of the soft X-ray one L s , shows a positive correlation of the dipole magnetic field B d . Another hint of the photon index of the hard X-ray component was also suggested [11]. These are hints to study the radiation mechanism of magnetars.…”

“…In addition, the observed X-ray luminosity L x = L s + L h of magnetars are clearly shown to exceed the spin-down luminosity derived from the pulsar rotation period. Details are described in [11].…”

“…The X-ray satellite Suzaku was launched in 2005, and we have accumulated magnetar observations of Suzaku in 2006-2013 [11]. Since a hard X-ray radiation was already discovered above 10 keV by long monitoring observations with INTEGRAL [12], we concentrated on wide-band spectral analyses of observed magnetars covering the ∼1-60 keV energy range.…”

Understanding Recent Magnetar Observations from the Magnetospheric Point of View

“…Their spectra also contain a significant fraction of nonthermal emissions [11,12]. Therefore, a physical picture is that: part of the magnetic energy of magnetars is first converted to a system of particles.…”