The correlation timescale of the X-ray flux during the outbursts of soft X-ray transients

TL;DR

This study analyzes the correlation timescale of X-ray flux during outbursts in soft X-ray transients, revealing that accretion rate correlations persist over weeks to months, influenced by accretion disk dynamics.

Contribution

It demonstrates that the correlation between transition flux and subsequent flux persists over long timescales, linking it to viscous timescales in accretion disks around compact objects.

Findings

Correlation persists up to 250 days after transition in GX 339-4.

Correlation persists up to 40-50 days in Aql X-1 and 4U 1705-44.

Viscous timescales in accretion disks explain the long-term correlation.

Abstract

Recent studies of black hole and neutron star low mass X-ray binaries (LMXBs) show a positive correlation between the X-ray flux at which the low/hard(LH)-to-high/soft(HS) state transition occurs and the peak flux of the following HS state. By analyzing the data from the All Sky Monitor (ASM) onboard the Rossi X-ray Timing Explorer (RXTE), we show that the HS state flux after the source reaches its HS flux peak still correlates with the transition flux during soft X-ray transient (SXT) outbursts. By studying large outbursts or flares of GX 339-4, Aql X-1 and 4U 1705-44, we have found that the correlation holds up to 250, 40, and 50 d after the LH-to-HS state transition, respectively. These time scales correspond to the viscous time scale in a standard accretion disk around a stellar mass black hole or a neutron star at a radius of ~104-5 Rg, indicating that the mass accretion rates in the accretion flow either correlate over a large range of radii at a given time or correlate over a long period of time at a given radius. If the accretion geometry is a two-flow geometry composed of a sub-Keplerian inflow or outflow and a disk flow in the LH state, the disk flow with a radius up to ~105 Rg would have contributed to the nearly instantaneous non-thermal radiation directly or indirectly, and therefore affects the time when the state transition occurs.