Broadband X-ray Spectral Investigations of Magnetars, 4U 0142+61, 1E 1841-045, 1E 2259+586, and 1E 1048.1-5937

TL;DR

This study extends a physical magnetar emission model to 100 keV, applying it to four magnetars' broadband spectra, revealing the need for non-thermal processes to explain hard X-ray emission.

Contribution

The paper introduces an extended, physically motivated 3D magnetar emission model, STEMS3D, capable of spectral analysis up to 100 keV, and applies it to multiple magnetars for the first time.

Findings

Hard X-ray emission in 4U 0142+61 was spectrally harder than previously observed.

Surface properties derived from soft X-ray data are consistent with broadband spectral analysis.

Resonant scattering alone cannot explain the hard X-ray emission, indicating additional non-thermal processes.

Abstract

We have generated an extended version of rather simplified but physically oriented three-dimensional magnetar emission model, STEMS3D, to allow spectral investigations up to 100 keV. We have then applied it to the broadband spectral spectra of four magnetars: 4U 0142+61, 1E 1841-045, 1E 2259+586 and 1E 1048.1-5937, using data collected with Swift/XRT or XMM-Newton in soft X-rays, and Nuclear Spectroscopic Telescope Array in the hard X-ray band. We found that the hard X-ray emission of 4U 0142+61 was spectrally hard compared to the earlier detections, indicating that the source was likely in a transition to or from a harder state. We find that the surface properties of the four magnetars are consistent with what we have obtained using only the soft X-ray data with STEMS3D, implying that our physically motivated magnetar emission model is a robust tool. Based on our broadband spectral investigations, we conclude that resonant scattering of the surface photons in the magnetosphere alone cannot account for the hard X-ray emission in magnetars; therefore, an additional non-thermal process, or a population of relativistic electrons is required. We also discuss the implication of the non-detection of persistent hard X-ray emission in 1E 1048.1-5937.