Phase-Dependent Spectral Shape Changes in the Ultraluminous X-Ray Pulsar NGC 5907 ULX1

TL;DR

This study analyzes phase-resolved X-ray spectra of the ultraluminous X-ray pulsar NGC 5907 ULX1, revealing phase-dependent spectral changes and suggesting an asymmetric magnetosphere geometry around the neutron star.

Contribution

First broadband spectral analysis of NGC 5907 ULX1 showing phase-dependent spectral shape changes and insights into the magnetosphere's asymmetric structure.

Findings

Disk component is phase-invariant with a low inclination angle.

Power law component varies with phase, indicating asymmetric magnetosphere.

Hotter regions near magnetic poles become visible before pulse peak.

Abstract

Discovery of coherent pulsations from several ultraluminous X-ray pulsars (ULXPs) has provided direct evidence of super-critical accretion flow. However, geometrical structure of such accretion flow onto the central neutron star remains poorly understood. NGC 5907 ULX1 is one of the most luminous ULXPs with the luminosity exceeding $10^{41}~{\rm erg~s^{-1}}$. Here we present a broadband X-ray study of this ULXP using the data from simultaneous observations with XMM-Newton and NuSTAR conducted in July 2014. The phase-resolved spectra are well reproduced by a model consisting of a multicolor disk blackbody emission with a temperature gradient of $p = 0.5~(T \propto r^{-p})$ and a power law with an exponential cutoff. The disk component is phase-invariant, and has an innermost temperature of $\sim 0.3~{\rm keV}$. Its normalization suggests a relatively low inclination angle of the disk, in contrast to the previous claim in other literature. The power law component, attributed to the emission from the accretion flow inside the magnetosphere of the neutron star, indicates phase-dependent spectral shape changes; the spectrum is slightly harder in the pre-peak phase than in the post-peak phase. This implies that the magnetosphere has an asymmetric geometry around the magnetic axis, and that hotter regions close to the magnetic pole become visible before the pulse peak.