The return to the hard state of GX 339-4 as seen by Suzaku

¹ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, 38041 Grenoble, France
e-mail: pierre-olivier.petrucci@obs.ujf-grenoble.fr
² Université de Toulouse, UPS-OMP, IRAP, Toulouse, France CNRS, IRAP, 9 Av du Colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
³ Laboratoire AIM (CEA/IRFU – CNRS/INSU – Université Paris Diderot), CEA DSM/IRFU/SAp, 91191 Gif-sur-Yvette, France
⁴ Department of Astrophysics/IMAPP, Radboud University Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
⁵ University of Oxford, Department of Physics, Astrophysics, Denys Wilkinson Building, Keble Road, OX1 3 RH Oxford, UK
⁶ School of Physics and Astronomy, University of Southampton, Highfield, Southampton SO17 1BJ, UK

Received: 12 July 2013
Accepted: 1 February 2014

Abstract

Disk recession is of prime importance to understand the physics of the state transitions in X-ray binaries. The microquasar GX 339-4 was observed by Suzaku five times spaced by a few days during its transition back to the hard state at the end of its 2010−2011 outburst. The 2−10 keV source flux decreases by a factor ~10 between the beginning and the end of the monitoring. Simultaneous radio and optical/infrared (OIR) observations highlighted the re-ignition of the radio emission just before the beginning of the campaign with the maximum radio emission being reached between the first two Suzaku pointings, while the IR peaked a few weeks later. A fluorescent iron line is always significantly detected. Fits with Gaussian or Laor profiles give statistically equivalent results. In the case of a Laor profile, fits of the five data sets simultaneously agree with a disk inclination angle of ~20 degrees. The disk inner radius is < 10−30 R_g in the first two observations but almost unconstrained in the last three due to the lower statistics. A soft X-ray excess is also present in these first two observations. Fits with a multicolored disk component give disk inner radii in qualitative agreement with those obtained with the iron line fits. The use of a physically more realistic model, including a blurred reflection component and a comptonization continuum, give some hints of the increase in the disk inner radius but the significances are always weak (and model dependent), preventing any clear conclusion concerning disk recession during this campaign. Interestingly, the addition of warm absorption significantly improves the fit of OBS1, while it is not needed in the other observations. Given the radio-jet re-ignition that occurs between OBS1 and OBS2, these absorption features may indicate the natural evolution of the accretion outflows transiting from a disk wind, which is a ubiquitous characteristic of soft states, and a jet, which is a signature of hard states. The comparison to a long 2008 Suzaku observation of GX 339-4 in a persistent faint hard state (similar in flux to OBS5), where a narrow iron line clearly indicates a disk recession, is discussed.