A Dynamic Photon Index Probes into TCAF

Abstract

Five successive outbursts in each of the two transient low-mass X-ray binaries (LMXBs), viz. GX 339-4 and H 1743-322, during 1997–2010 and 2003–2008 respectively, are analyzed using (1.5–12 keV) data of All Sky Monitor (ASM) in Rossi X-ray Timing Explorer (RXTE) satellite. In LMXBs, a big Keplerian disc having high viscosity and high angular momentum is likely to form via Roche-lobe overflow amid the advective sub-Keplerian halo. However, in high-mass X-ray binaries (HMXBs), almost inviscid and wind-dominated sub-Keplerian flow carrying low angular momentum rushes inwards virtually freely. Two travel timescales in these two components bring about a longer delay of the standard Keplerian disc heading for the black hole in LMXBs than that in HMXBs. Disc and halo components are respectively behind the emission of black-body and power-law photons. A direct cross-correlation between these two fluxes of photon (or energy), in dearth of a linear mutual dependence, may never reveal this arrival time delay; rather these two are interlinked via viscous modulations, which usher the accreting matter to the interior. In order to compute a relative time delay between the two radiation fluxes emitted by the aforesaid two flow components, RXTE/ASM photon counts (GX 339-4) and average energy fluxes (H 1743-322) are cross-correlated with respect to a dynamic photon index (Θ) proposed by Professor Sandip K. Chakrabarti. This viscous time delay dictates the fate of any outburst in all respects, while Θ acts as a dynamical indicator to trace the spectral states throughout the duration of the outburst. It is also found in GX 339-4 that the Keplerian disc varies in size from an outburst to another, whereas the disc size is biggest during the brightest outburst of H 1743-322 in 2003 and diminishes during its subsequent weaker outbursts. Moreover, the time delay and hysteresis property of the disc, as well as the failure of a potential outburst can be evident from the behaviour of Θ alone. All results confirm two dynamical flows in accretion onto a black hole, as proposed in the TCAF paradigm.