Temporal and spectral X-ray properties of magnetar SGR 1900+14 derived from observations with NuSTAR and XMM-Newton

Abstract

X-ray observations play a crucial role in understanding the emission mechanism and relevant physical phenomena of magnetars. We report on X-ray observations made in 2016 of a young magnetar, SGR 1900+14, which is famous for a giant flare in 1998 August. Simultaneous observations were conducted with XMM-Newton and NuSTAR on 2016 October 20 with 23 and 123 ks exposures, respectively. The NuSTAR hard X-ray coverage enabled us to detect the source up to 70 keV. The 1-10 keV and 15-60 keV fluxes were 3.11(3)× 10^{-12} {erg s^{-1} cm^{-2}} and 6.8(3)× 10^{-12} {erg s^{-1} cm^{-2}}, respectively. The 1-70 keV spectra were fitted well by a blackbody plus power-law model with a surface temperature of kT=0.52(2) keV, a photon index of the hard power-law of Γ = 1.21(6), and a column density of N_{ H}=1.96(11)× 10^{22} cm^{-2}. Compared with previous observations with Suzaku in 2006 and 2009, the 1-10 keV flux showed a decrease by 25%-40%, while the spectral shape did not show any significant change with differences of kT and N_H being within 10% of each other. Through timing analysis, we found that the rotation period of SGR 1900+14 on 2016 October 20 was 5.22669(3) s. The long-term evolution of the rotation period shows a monotonic decrease in the spin-down rate \dot{P} lasting for more than 15 yr. We also found characteristic behavior of the hard-tail power-law component of SGR 1900+14. The energy-dependent pulse profiles vary in morphology with a boundary of 10 keV. The phase-resolved spectra show the differences between photon indices (Γ = 1.02-1.44) as a function of the pulse phase. Furthermore, the photon index is positively correlated with the X-ray flux of the hard power-law component, which could not be resolved by the previous hard X-ray observations.