A NuSTAR View of GRS 1716$-$249 in the Hard and Intermediate States

TL;DR

This study analyzes NuSTAR and Swift observations of GRS 1716-249 during its 2016-2017 outburst, revealing spectral states, broad iron lines, and disc properties through high-density reflection modeling.

Contribution

It provides the first detailed high-density disc reflection analysis of GRS 1716-249, constraining disc density and inner radius during different spectral states.

Findings

Source in hard state shows power-law dominated spectrum.

Broad iron line and Compton hump observed in all states.

Disc density constrained to 10^{19}-10^{20} cm^{-3}.

Abstract

We present a detailed analysis of the spectral properties of the black hole transient GRS 1716-249, based on the archival Swift and NuSTAR observations taken during the outburst of this source in 2016-2017. The first six NuSTAR observations show that the source is in a canonical hard state, where the spectrum is dominated by a power-law continuum. The seventh NuSTAR observation is taken during the intermediate state where both a disc thermal component and a power-law continuum are shown. All of our observations show a broad emission line feature in the iron band and a Compton hump above 10 keV. We model the broad band spectra using a high density disc reflection model, where the soft X-ray emission in the hard state is interpreted as part of the disc reflection component. This model enables us to constrain the disc density parameter of GRS 1716-249 in the range of $10^{19}$-$10^{20}$ cm$^{-3}$. We only obtain an upper limit of the inner disc radius using high density disc reflection spectroscopy and the results indicate either a non-truncated disc or a slightly truncated disc with $R_{\rm in}<20r_{\rm g}$.