Accretion flow dynamics during the evolution of timing and spectral properties of GX 339-4 during its 2010–11 outburst^⋆

¹ Space Astronomy Group, ISRO Satellite Centre, HAL Airport Road, 560017 Bangalore, India
e-mail: anuj@isac.gov.in
² Indian Centre for Space Physics, Chalantika 43, Garia Station Rd., 700084 Kolkata, India
e-mail: dipak@csp.res.in
³ Indian Institute of Space Science and Technology, 695547 Thiruvananthapuram, India
e-mail: samir@iist.ac.in
⁴ S. N. Bose National Centre for Basic Sciences, Salt Lake, 700098 Kolkata, India
e-mail: chakraba@bose.res.in

Received: 7 August 2011
Accepted: 28 March 2012

Abstract

Context. The Galactic transient black hole candidate (BHC) GX 339-4 exhibited several outbursts at regular intervals of about two to three years in the Rossi X-ray Timing Explorer (RXTE) era. After remaining in an almost quiescent state for three long years, it again became X-ray active in January, 2010, continuing to be so over the next  ~14 months.

Aims. We study the timing and spectral properties of the BHC during its recent outburst and understand the behavioral change in the accretion flow dynamics associated with the evolution of the various X-ray features.

Methods. The detailed analysis of the temporal and spectral properties of the source during this outburst are carried out using archival data of the RXTE PCA instrument. We analyze a total of 236 observational intervals consisting of 419 days of data observed by RXTE, from 2010 January 12 to 2011 March 6.

Results. Our study provides a comprehensive understanding of the mass accretion processes and properties of the accretion disk of the BHC. The PCA spectra of 2.5–25 keV are mainly fitted with a combination of two components, namely, a disk black body and a power-law. The entire outburst as observed by RXTE, is divided into four spectral states, namely, hard, hard-intermediate, soft-intermediate, and soft. Quasi-periodic oscillations (QPOs) were found in three out of the four states, namely hard, hard-intermediate, and soft-intermediate states. The QPO frequencies increase monotonically from 0.102 Hz to 5.692 Hz in the rising phase of the outburst, while during the declining phase QPO frequencies decrease monotonically from 6.420 to 1.149 Hz. The evolution pattern, i.e. the hardness-intensity diagram, of the present outburst can be reproduced by two different components of the flow of accreting material.

Conclusions. The recent outburst of GX 339-4 gives us an opportunity to understand the evolution of the two-component accretion rates starting from the onset to the end of the outburst phase. We found that the QPO frequency variation could be explained by the propagating oscillatory shock model (POS) and the hardness versus intensity variation can be reproduced if we assume that higher viscosity causes the conversion of a low angular momentum disk component into a Keplerian component during the outburst phase. The decline phase starts because of the reduction in the viscosity.