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We report on the nearly simultaneous \nicer{} and \nustar{} observations ofthe known X-ray transient XTE~J1739-285. These observations provide the firstsensitive hard X-ray spectrum of this neutron star X-ray transient. The sourcewas observed on 2020 February 19 in the hard spectral state with a luminosityof $0.007$ of the Eddington limit. The broadband $1-70 \kev{}$ \nicer{} and\nustar{} observation clearly detects a cutoff of the hard spectral componentaround $34-40 \kev{}$ when the continuum is fitted by a soft thermal componentand a hard power-law component. This feature has been detected for the firsttime in this source. Moreover, the spectrum shows evidence for disc reflection-- a relativistically broadened Fe K$\alpha$ line around $5-8 \kev{}$ and aCompton hump in the $10-20 \kev{}$ energy band. The accretion disc reflectionfeatures have not been identified before from this source. Through accretiondisc 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 alow inclination, $i\sim 33^{0}$. Assuming the magnetosphere is responsible forsuch truncation of the inner accretion disc above the stellar surface, weestablish an upper limit of $6.2\times 10^8$ G on the magnetic field at thepoles.

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