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

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.

9 pages, 8 figures, accepted for publication in MNRAS journal

References in corpus (8)

• Thermonuclear (type-I) X-ray bursts observed by the Rossi X-ray Timing Explorer
• An Empirical Background Model for the NICER X-ray Timing Instrument
• Broad relativistic iron emission line observed in SAX J1808.4-3658
• Evidence for 1122 Hz X-Ray Burst Oscillations from the Neutron-Star X-Ray Transient XTE J1739-285
• A Hard Look at the Neutron Stars and Accretion Disks in 4U 1636-53, GX 17+2, and 4U 1705-44 with $\text{NuSTAR}$
• NuSTAR Observations of the Accreting Atolls GX 3+1, 4U 1702-429, 4U 0614+091, and 4U 1746-371
• Study of the reflection spectrum of the LMXB 4U 1702-429
• The X-ray bursts of XTE J1739-285: a NICER sample

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• X-ray spectro-polarimetric characterization of GX 340+0 in the horizontal branch: a highly inclined source?