The X-ray pulsar XTE J1858+034 observed with NuSTAR and Fermi/GBM: spectral and timing characterization plus a cyclotron line

TL;DR

This study presents a detailed spectral and timing analysis of the X-ray pulsar XTE J1858+034 during its 2019 outburst, identifying a cyclotron line and estimating the neutron star's magnetic field, orbital period, and distance.

Contribution

The paper reports the first detection of a cyclotron line at 48 keV in XTE J1858+034, providing insights into its magnetic field and accretion properties, along with orbital and distance estimates.

Findings

Candidate cyclotron line at 48 keV indicating a magnetic field of 5.4×10^12 G.

Orbital period estimated at approximately 81 days.

Distance inferred to be 10.9±1.0 kpc.

Abstract

Accreting X-ray pulsars (XRPs) undergo luminous X-ray outbursts during which the spectral and timing behavior of the neutron star can be studied in detail. We analyze a $NuSTAR$ observation of the XRP XTE J1858+034 during its outburst in 2019. The spectrum is fit with a phenomenological, a semi-empirical and a physical spectral model. A candidate cyclotron line is found at $48\,$keV, implying a magnetic field of $5.4\times10^{\rm 12}\,$G at the site of emission. This is also supported by the physical best-fit model. We propose an orbital period of about $81$ days based on the visual inspection of the X-ray outbursts recurrence time. Based on $Fermi$ Gamma-ray Burst Monitor data, the standard disk accretion-torque theory allowed us to infer a distance of $10.9\pm1.0\,$kpc. Pulse profiles are single-peaked and show a pulsed fraction that is strongly energy-dependent at least up to $40$ keV.