GRS 1739-278 Observed at Very Low Luminosity with XMM-Newton and NuSTAR
Harrison, F. A.; Stern, D.; Yamaoka, K.; Miller, J. M.; Rahoui, F.; Tomsick, J. A.; Rodriguez, J.; Kaaret, P.; García, J.; Dauser, T.; Wilms, J.; Miller-Jones, J. C. A.; Pottschmidt, K.; Migliari, S.; Walton, D. J.; Fürst, F.; Fabian, A.; Corbel, S.; Kalemci, E.; Stuhlinger, M.; Clavel, M.; Loh, A.
United States, Japan, Germany, France, United Kingdom, Turkey, Spain, Australia
Abstract
We present a detailed spectral analysis of XMM-Newton and NuSTAR observations of the accreting transient black hole GRS 1739-278 during a very faint low hard state at ∼0.02% of the Eddington luminosity (for a distance of 8.5 kpc and a mass of 10 {M}⊙ ). The broadband X-ray spectrum between 0.5 and 60 keV can be well-described by a power-law continuum with an exponential cutoff. The continuum is unusually hard for such a low luminosity, with a photon index of Γ = 1.39 ± 0.04. We find evidence for an additional reflection component from an optically thick accretion disk at the 98% likelihood level. The reflection fraction is low, with {{ R }}{refl}={0.043}-0.023+0.033. In combination with measurements of the spin and inclination parameters made with NuSTAR during a brighter hard state by Miller et al., we seek to constrain the accretion disk geometry. Depending on the assumed emissivity profile of the accretion disk, we find a truncation radius of 15-35 {R}{{g}} (5-12 {R}{ISCO}) at the 90% confidence limit. These values depend strongly on the assumptions and we discuss possible systematic uncertainties.