Evidence of hard power-law spectral cutoff and disc reflection features from the X-ray transient XTE J1739-285

TL;DR

This study presents the first sensitive hard X-ray spectrum of the neutron star transient XTE J1739-285, revealing a spectral cutoff and disc reflection features, and constrains the inner disc radius and magnetic field strength.

Contribution

It provides the first detection of a spectral cutoff and disc reflection features in XTE J1739-285, with detailed modeling of the accretion disc and magnetic field.

Findings

Detection of a spectral cutoff around 34-40 keV.

Identification of relativistically broadened Fe Kα line and Compton hump.

Constraint on the inner disc radius and magnetic field strength.

Abstract

We report on the nearly simultaneous \nicer{} and \nustar{} observations of the known X-ray transient XTE~J1739-285. These observations provide the first sensitive hard X-ray spectrum of this neutron star X-ray transient. The source was observed on 2020 February 19 in the hard spectral state with a luminosity of $0.007$ of the Eddington limit. The broadband $1-70 \kev{}$ \nicer{} and \nustar{} observation clearly detects a cutoff of the hard spectral component around $34-40 \kev{}$ when the continuum is fitted by a soft thermal component and a hard power-law component. This feature has been detected for the first time in this source. Moreover, the spectrum shows evidence for disc reflection -- a relativistically broadened Fe K$\alpha$ line around $5-8 \kev{}$ and a Compton hump in the $10-20 \kev{}$ energy band. The accretion disc reflection features have not been identified before from this source. Through accretion disc reflection modeling, we constrain the radius of the inner disc to be $R_{in}=3.1_{-0.5}^{+1.8}\;R_{ISCO}$ for the first time. In addition, we find a low inclination, $i\sim 33^{0}$. Assuming the magnetosphere is responsible for such truncation of the inner accretion disc above the stellar surface, we establish an upper limit of $6.2\times 10^8$ G on the magnetic field at the poles.