Observations of the X-ray pulsar LMC X-4 with NuSTAR: limit on the magnetic field and tomography of the system in the fluorescent iron line

TL;DR

This study uses NuSTAR data to analyze the X-ray pulsar LMC X-4, constraining its magnetic field and mapping the system through spectral and timing analysis, including pulse phase-resolved spectroscopy and iron line delay.

Contribution

First detailed pulse phase-resolved spectral analysis of LMC X-4 with NuSTAR, providing new constraints on the neutron star's magnetic field and system geometry.

Findings

No cyclotron absorption line detected, constraining magnetic field to <3×10^11 G or >6.5×10^12 G.

Detected a delay between emission maxima and iron line equivalent width, indicating photon travel time in the system.

Spectral modeling with Comptonization best fits the observed spectra.

Abstract

We present results of the spectral and timing analysis of the X-ray pulsar LMC X-4 with the NuSTAR observatory in the broad energy range 3-79 keV. Along with the detailed analysis of the averaged source spectrum, the high-precision pulse phased-resolved spectra were obtained for the first time. It has been shown that the comptonization model gives the best approximation of the obtained spectra. The evolution of its parameters was traced depending on the pulse phase as well. The search for the possible cyclotron absorption line was performed for all energy spectra in the 5-55 keV energy range. The obtained upper limit for the depth of the cyclotron absorption line $\tau\simeq0.15$ ($3\sigma$) indicates no cyclotron absorption line in this energy range, which provides an estimate of the magnitude of the magnetic field on the surface of the neutron star: $B < 3 \times 10^{11}$ G or $B > 6.5 \times 10^{12}$ G. The latter one is agree with the estimate of the magnetic field obtained from the analysis of the power spectrum of the pulsar: $B \sim 3 \times 10^{13}$ G. Based on results of the pulse phase-resolved spectroscopy we revealed a delay between maxima of the source emission and the equivalent width of the fluorescent iron line. This delay can be apparently associated with the travel time of photons between the emitting regions in the vicinity of the neutron star and the relatively cold regions where this emission is reflected (presumably, at the inflowing stream or at the place of an interaction of the stream and the outer edge of the accretion disk).