GRS 1739-278 OBSERVED AT VERY LOW LUMINOSITY WITH XMM-NEWTON AND NuSTAR
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Fürst, F., J. A. Tomsick, K. Yamaoka, T. Dauser, J. M. Miller, M. Clavel, S. Corbel, et al. “GRS 1739-278 Observed at Very Low Luminosity with XMM-Newton and NuSTAR.” The Astrophysical Journal 832, no. 2 (November 2016): 115. https://doi.org/10.3847/0004-637X/832/2/115.
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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 Rᵣₑfₗ = 0.043⁺⁰.⁰³³₋₀.₀₂₃. 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 Rg (5–12 RᵢₛCₒ) at the 90% confidence limit.
These values depend strongly on the assumptions and we discuss possible systematic uncertainties.