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Published January 1, 2016 | Version v1
Journal article Open

GRS 1739-278 OBSERVED AT VERY LOW LUMINOSITY WITH XMM-NEWTON AND NuSTAR

Creators

  • 1. CALTECH, Cahill Ctr Astron & Astrophys, Pasadena, CA 91125 USA
  • 2. Univ Calif Berkeley, Space Sci Lab, Berkeley, CA 94720 USA
  • 3. Dr Karl Remeis Sternwarte & ECAP, Sternwartstr 7, D-96049 Bamberg, Germany
  • 4. Univ Michigan, Dept Astron, Ann Arbor, MI 48109 USA
  • 5. Inst Astron, Madingley Rd, Cambridge CB3 0HA, England
  • 6. Harvard Smithsonian Ctr Astrophys, Cambridge, MA 02138 USA
  • 7. Univ Paris Diderot, CEA DSM IRFU SAp, Lab AIM, CEA IRFU CNRS INSU, F-91191 Gif Sur Yvette, France
  • 8. Univ Iowa, Dept Phys & Astron, Iowa City, IA 52242 USA
  • 9. Sabanci Univ, Fac Engn & Nat Sci, TR-34956 Istanbul, Turkey
  • 10. Curtin Univ, Int Ctr Radio Astron Res, GPO Box U1987, Perth, WA 6845, Australia
  • 11. CALTECH, Jet Prop Lab, Pasadena, CA 91109 USA
  • 12. ESAC, E-28692 Madrid, Spain

Description

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 similar to 0.02% of the Eddington luminosity (for a distance of 8.5 kpc and a mass of 10 M-circle dot). 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 Gamma = 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.

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