Study of the X-ray Pulsar XTE J1946+274 with NuSTAR

TL;DR

This study analyzes the spectral and timing properties of the X-ray pulsar XTE J1946+274 using NuSTAR and Swift/XRT data, revealing phase-dependent variations in cyclotron and iron line features during a bright outburst.

Contribution

It provides the first detailed phase-resolved analysis of the cyclotron line and iron line variability in XTE J1946+274, linking spectral changes to emission reflection models.

Findings

Cyclotron line energy varies from 34 to 39 keV across the pulse.

Iron line equivalent width changes significantly with pulse phase.

Cyclotron line parameters support a reflection emission model.

Abstract

We present the results of our spectral and timing analysis of the emission from the transient X-ray pulsar XTE J1946+274 based on the simultaneous NuSTAR and Swift/XRT observations in the broad energy range 0.3-79 keV carried out in June 2018 during a bright outburst. Our spectral analysis has confirmed the presence of a cyclotron absorption line at an energy $\sim38$ keV in both averaged and phase-resolved spectra of the source. Phase-resolved spectroscopy has also allowed the variation in spectral parameters with neutron star rotation phase, whose period is $\simeq15.755$ s, to be studied. The energy of the cyclotron line is shown to change significantly (from $\simeq34$ to $\simeq39$ keV) on the scale of a pulse, with the line width and optical depth also exhibiting variability. The observed behavior of the cyclotron line parameters can be interpreted in terms of the model of the reflection of emission from a small accretion column (the source's luminosity at the time of its observations was $\sim 3 \times 10^{37}$ erg s$^{-1}$) off the neutron star surface. The equivalent width of the iron line has been found to also change significantly with pulse phase. The time delay between the pulse and equivalent width profiles can be explained by the reflection of neutron star emission from the outer accretion disk regions.