GRS 1739-278 observed at very low luminosity with XMM-Newton and NuSTAR

F. Fürst (1) , J. A. Tomsick (2) , K. Yamaoka (3) , T. Dauser (4) , J. M. Miller (5) , M. Clavel (2) , Stéphane Corbel (6, 7) , A. C. Fabian (8) , J. García (9) , F. A. Harrison (1) , A. Loh (6) , P. Kaaret (10) , E. Kalemci (11) , S. Migliari (12) , J. C. A. Miller-Jones (13) , K. Pottschmidt (14) , F. Rahoui (15) , J. Rodriguez (6) , D. Stern (16) , M. Stuhlinger (12) , D. J. Walton (1, 16) , J. Wilms (4)

1 Cahill Center for Astronomy and Astrophysics
2 SSL - Space Sciences Laboratory [Berkeley]
3 SolarTerrestrial Environment Laboratory
4 Dr. Karl-Remeis-Sternwarte
5 Department of Astronomy, University of Michigan
6 AIM (UMR7158 / UMR_E_9005 / UM_112) - Astrophysique Interprétation Modélisation
7 USN - Unité Scientifique de la Station de Nançay
8 Institute of Astronomy [Cambridge]
9 CfA - Harvard-Smithsonian Center for Astrophysics
10 Department of Physics and Astronomy, University of Iowa
11 Sabanci University - Faculty of Engineering and Natural Sciences
12 ESAC - European Space Astronomy Centre
13 International Centre for Radio Astronomy Research - Curtin University,
14 GSFC - NASA Goddard Space Flight Center
15 ESO - European Southern Observatory
16 JPL - Jet Propulsion Laboratory

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_sun ). The broad-band X-ray spectrum between 0.5-60 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(+0.033,-0.023). In combination with measurements of the spin and inclination parameters made with NuSTAR during a brighter hard state by Miller and co-workers, 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 R_ISCO ) at the 90% confidence limit. These values depend strongly on the assumptions and we discuss possible systematic uncertainties.

Keywords: accretion disks; stars: black holes; X-rays: binaries; X-rays: individual (GRS 1739-278); accretion

Domains: High Energy Astrophysical Phenomena [astro-ph.HE]

F. Fürst, J. A. Tomsick, K. Yamaoka, T. Dauser, J. M. Miller, et al.. GRS 1739-278 observed at very low luminosity with XMM-Newton and NuSTAR. The Astrophysical Journal, 2016, 832 (2), pp.115. ⟨10.3847/0004-637X/832/2/115⟩. ⟨insu-01432858⟩