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

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 $\tau \simeq 0.15$ ($3\sigma$) 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).

16 pages, 8 figures, 2 Tables. Accepted for publication in Astronomy Letters

References in corpus (2)

• The ephemeris, orbital decay, and masses of 10 eclipsing HMXBs
• Near-periodical spin period evolution in the binary system LMC X-4

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• Broadband mHz QPOs and spectral study of LMC X$-$4 with AstroSat
• Near-periodical spin period evolution in the binary system LMC X-4
• Common patterns in pulse profiles of High Mass X-ray Binaries
• Pulsating iron spectral features in the emission of X-ray pulsar V0332+53
• Disk precession to explain the super-orbital modulation of LMC X-4: results from the Swift monitoring campaign
• Study of the X-ray Pulsar XTE J1946+274 with NuSTAR
• Monte-Carlo simulations on possible collimation effects of outflows to fan-beamed emission of ultraluminous accreting X-ray pulsars