Broadband study with Suzaku of the magnetar class

Abstract

Broadband (0.8-70 keV) spectra of the persistent X-ray emission from nine magnetars were obtained with Suzaku, including three 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 seven 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.Wideband spectral analyses revealed that the hard-tail component has a 1-60 keV flux, Fh, comparable to or even higher than that carried by the 1-60 keV soft component, Fs. The hardness ratio (HR) of these objects, defined as ζ = Fh/Fs, was found to be tightly anti-correlated with their characteristic age τc as ζ = (3.3 ± 0.3) × (τc /1 kyr) -0.67±0.04 with a correlation coefficient of -0.989, over the range from ζ ∼10 to ζ ∼0.1. Magnetars in outburst states were found to lie on the same correlation as relatively quiescent ones. This HR is also positively correlated with their surface magnetic fields with a correlation coefficient of 0.873. In addition, the hard-tail component becomes harder toward sources with older characteristic ages, with the photon index changing from ∼1.7 to ∼0.4. © 2010 The American Astronomical Society. All rights reserved.