Discovery of a cyclotron absorption line in the transient X-ray pulsar XTE J1829-098

TL;DR

This paper reports the discovery of a cyclotron absorption line at 15 keV in the X-ray pulsar XTE J1829-098, revealing the neutron star's magnetic field and showing phase-dependent spectral variability during an outburst.

Contribution

First detection of a cyclotron line in XTE J1829-098, providing direct measurement of its magnetic field and detailed phase-resolved spectral analysis.

Findings

Cyclotron line at 15 keV detected in the spectrum.

Neutron star magnetic field estimated at 1.7×10^{12} G.

Cyclotron line parameters vary with pulse phase.

Abstract

We report results of a spectral and timing analysis of the X-ray pulsar XTE J1829-098 using data obtained with the NuSTAR observatory during an outburst in August 2018. A strong absorption feature was detected at the energy of $E_{cyc}\simeq 15$ keV in the source spectrum. This feature was interpreted as a cyclotron resonance scattering line corresponding to the magnetic field strength of the neutron star surface $B\simeq1.7\times10^{12}$ G. The pulse phase-resolved spectroscopy shows that the cyclotron line is significantly detected at all phases of the pulse and its energy and other parameters are variable over the pulse period. The timing analysis of the source emission revealed strong pulsations with a period of $P = 7.84480(2)$ s. The pulsed fraction is changed with the energy, including its local increase in the vicinity of the cyclotron line. Using the archival data of the RXTE observatory the presence of the cyclotron line in the spectrum of XTE J1829-098 was independently confirmed.