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The black hole transient GRS 1716-249 was monitored from the radio to thegamma-ray band during its 2016-2017 outburst. This paper focuses on theSpectral Energy Distribution (SED) obtained in 2017 February-March, when GRS1716-249 was in a bright hard spectral state. The soft gamma-ray data collectedwith the INTEGRAL/SPI telescope show the presence of a spectral component whichis in excess of the thermal Comptonisation emission. This component is usuallyinterpreted as inverse Compton emission from a tiny fraction of non-thermalelectrons in the X-ray corona. We find that hybrid thermal/non-thermalComptonisation models provide a good fit to the X/gamma-ray spectrum of GRS1716-249. The best-fit parameters are typical of the bright hard state spectraobserved in other black hole X-ray binaries. Moreover, the magnetised hybridComptonisation model BELM provides an upper limit on the intensity of thecoronal magnetic field of about 1E+06 G. Alternatively, this soft gamma-rayemission could originate from synchrotron emission in the radio jet. In orderto test this hypothesis, we fit the SED with the irradiated disc plusComptonisation model combined with the jet internal shock emission model ISHEM.We found that a jet with an electron distribution of p~2.1 can reproduce thesoft gamma-ray emission of GRS 1716-249. However, if we introduce the expectedcooling break around 10 keV, the jet model can no longer explain the observedsoft gamma-ray emission, unless the index of the electron energy distributionis significantly harder (p<2).

On the nature of the soft gamma-ray emission in the hard state of the black hole transient GRS 1716-249