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 X-ray observations of a young magnetar SGR 1900+14 made in 2016, 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 well fitted 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 rateṖ lasting for more than 15 years. We also found a 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.

Extragalactic fast X-ray transient candidates discovered by Chandra (2000-2014)

“…At the distances of our nearby FXRTs, we would presumably only see the most luminous portions of these rare giant bursts (a few seconds at most) and not be sensitive to the fainter bursts or quiescent emission. They should be quite spectrally hard (e.g., SGR 1900+14 has a photon index range Γ≈1.0-2.0; Tamba et al 2019) and given the relation to young, highly-magnetic neutron stars, seen to be originating from young star clusters and Hii regions (Woods & Thompson 2006). In this sense, FXRTs 2, 4, 5, and 6 share some similarities with the SGR/AXP phenomenon.…”

Discovery of 40.5 ks Hard X-ray Pulse-Phase Modulations from SGR 1900+14

“…From 2016 October 20 UT 16:56:08, SGR1900+14 was observed with NuSTAR for an elapsed time of 242 ks (ObsID 30201013002). The archival data were already analyzed by Tamba et al (2019), incorporating XMM-Newton data which were partially simultaneous. We do not utilize the XMM-Newton data set, because it covers only up to 10 keV with a short exposure (23 ks).…”

“…As described in Tamba et al (2019), the NuSTAR data were processed using nupipeline and nuproducts in HEASoft 6.23, and the photon arrival times were corrected for the Earth and spacecraft motions around the Solar system barycenter. The net exposure was 122.6 ks after the standard pipeline processes.…”

“…The net exposure was 122.6 ks after the standard pipeline processes. Since the nearby bright binary GRS 1915+105 caused severe stray light, particularly in FPMB, we utilize only the FPMA data, like in Tamba et al (2019). To suppress the stray light, the on-source FPMA events were extracted from a circular region of R acc = 50 ′′ radius around the image centroid.…”

On the radiation-loud and radiation-quiet anti-glitches in magnetar 1E 2259+586

“…In this way, the star's characteristic age and magnetic field can be estimated. However, the magnetar showed various kinds of timing activities (Younes et al 2015;Camilo et al 2018;Levin et al 2019;Tamba et al 2019;Makishima et al 2019;Archibald et al 2020). Many of these timing event are correlated with radiative changes.…”