Relativistic Reflection and Reverberation in GX 339–4 with NICER and NuSTAR
We analyze seven Neutron Star Interior Composition Interior Explorer (NICER) and Nuclear Spectroscopic Telescope Array epochs of the black hole X-ray binary GX 339–4 in the hard state during its two most recent hard-only outbursts in 2017 and 2019. These observations cover the 1–100 keV unabsorbed luminosities between 0.3% and 2.1% of the Eddington limit. With NICER's negligible pileup, high count rate, and unprecedented time resolution, we perform a spectral-timing analysis and spectral modeling using relativistic and distant reflection models. Our spectral fitting shows that as the inner disk radius moves inward, the thermal disk emission increases in flux and temperature, the disk becomes more highly ionized, and the reflection fraction increases. This coincides with the inner disk increasing its radiative efficiency around ~1% Eddington. We see a hint of the hysteresis effect at ~0.3% of Eddington; the inner radius is significantly truncated during the rise (>49R_g ), while only a mild truncation (~5R_g ) is found during the decay. At higher frequencies (2–7 Hz) in the highest-luminosity epoch, a soft lag is present whose energy dependence reveals a thermal reverberation lag with an amplitude similar to previous findings for this source. We also discuss the plausibility of the hysteresis effect and the debate of the disk truncation problem in the hard state.
Additional Information© 2020 The American Astronomical Society. Received 2019 October 2; revised 2020 June 17; accepted 2020 June 19; published 2020 August 11. J.W., E.K., and J.A.G. acknowledges support from NASA grant 80NSSC17K0515 and thank the International Space Science Institute (ISSI) and participants of the ISSI Workshop "Sombreros and Lampposts: The Geometry of Accretion onto Black Holes" for fruitful discussions. J.A.G. is thankful for support from the Alexander von Humboldt Foundation. E.M.C. gratefully acknowledges support from the National Science Foundation, CAREER award No. AST-1351222. R.M.L. acknowledges the support of NASA through Hubble Fellowship Program grant HST-HF2-51440.001.
Published - Wang_2020_ApJ_899_44.pdf
Accepted Version - 1910.01245.pdf