We present a spectral analysis of the Crab Nebula obtained with the Chandra X-ray observatory. The X-ray spectrum is characterized by a power law whose index varies across the nebula. The variation can be discussed in terms of the particle injection from the pulsar in two different directions: the equatorial plane containing the torus and the symmetry axis along the jet. In the equatorial plane, spectra within the torus are the hardest, with a photon index % 1:9, and are almost independent of the surface brightness. At the periphery of the torus, the spectrum gradually softens in the outer, lower surface brightness regions, up to % 3:0. This indicates that synchrotron losses become significant to X-ray-emitting particles at the outer boundary of the torus. We discuss the nature of the torus, incorporating information from observations at other wavelengths. Spectral variations are also seen within the southern jet. The core of the jet is the hardest, with % 2:0, and the outer sheath surrounding the core becomes softer with up to 2.5 at the outermost part. Based on the similarity between the spectra of the jet core and the torus, we suggest that the electron spectra of the particles injected from the pulsar are also similar in these two different directions. The brightness ratio between the near and far sides of the torus can be explained by Doppler boosting and relativistic aberration; however, the observed ratio cannot be derived from the standard weakly magnetized pulsar wind model. We also found a site where an optical filament comprising supernova ejecta is absorbing the soft X-ray emission (<2 keV).

keV) spectra of the persistent X-ray emission from 9 magnetars were obtained with Suzaku, including 3 objects in apparent outburst. The soft X-ray component was detected from all of them, with a typical blackbody temperature of kT ∼ 0.5 keV, while the hard-tail component, dominating above ∼10 keV, was detected at ∼1 mCrab intensity from 7 of them. Therefore, the spectrum composed of a soft emission and a hard-tail component may be considered to be a common property of magnetars, both in their active and quiescent states. Wide-band spectral analyses revealed that the hard-tail component has a 1-60 keV flux, F h , comparable to or even higher than that carried by the 1-60 keV soft component, F s . The hardness ratio of these objects, defined as ξ ≡ F h /F s , was found to be tightly anti-correlated with their characteristic age τ c as ξ = (3.3 ± 0.3) × (τ c /1 kyr) −0.67±0.04 with a correlation coefficient of −0.989, over the range from ξ ∼ 10 to ξ ∼ 0.1. Magnetars in outburst states were found to lie on the same correlation as relatively quiescent ones. This hardness ratio is also positively correlated with their surface magnetic fields with a correlation coefficient of 0.873. In addition, the hard-tail component becomes harder towards sources with older characteristic ages, with the photon index changing from ∼1.7 to ∼0.4.

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.

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