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

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.

💡 Research Summary

The paper investigates the temporal relationship between the X‑ray flux at the low‑hard (LH) to high‑soft (HS) state transition and the subsequent HS flux during outbursts of soft X‑ray transients (SXTs). Earlier work had identified a positive correlation between the transition flux and the peak HS flux, but it remained unclear whether this correlation persisted beyond the immediate post‑transition phase. Using the long‑term monitoring data from the All‑Sky Monitor (ASM) aboard the Rossi X‑ray Timing Explorer (RXTE), the authors selected three well‑studied low‑mass X‑ray binaries (LMXBs) that exhibit clear outbursts: the black‑hole candidate GX 339‑4, and the neutron‑star systems Aql X‑1 and 4U 1705‑44.

For each source the authors measured the 2–12 keV ASM count rate at the moment of the LH→HS transition and then tracked the HS flux at successive time intervals after the transition, up to the point where the HS flux reached its maximum. Pearson correlation coefficients were computed between the transition flux and the HS flux measured at each later epoch. The analysis revealed that the correlation remains statistically significant for a surprisingly long period: up to ~250 days after the transition for GX 339‑4, ~40 days for Aql X‑1, and ~50 days for 4U 1705‑44.

These timescales are comparable to the viscous (diffusion) timescale of a standard thin accretion disc at radii of order 10⁴–10⁵ gravitational radii (R_g). Using the standard expression t_visc ≈ (R³/GM)¹ᐟ² /