Correlation Time Scales of the X-ray Flux in Low Mass X-ray Binary Wu Yu Xiang (Tsinghua University) Yu Wenfei (Shanghai Astronomical Observatory) Li Tipei (Tsinghua University)

Outline Background The Problem Data Analysis Results Conclusion Implications

Background Long-term monitoring of black hole and neutron star soft X-ray transients

Background Yu et al. 2003, ApJ, 589, L33

Yu et al. 2006, accepted by ApJ Background a linear relation between the luminosity of hard-to-soft state transition and luminosity of the HS state peak the accretion flow powering the LH state is related to the accretion flow powering the HS state Aql X-1 Yu & Dolence 2006, submitted to ApJ

Background The time lags between the occurrences of the hard-to-soft state transitions and the corresponding flux peaks of the HS states: Aql X-1 ～ 5 days 4U 1705-44 ～ 30 days GX 339-4 ～ 100 days The flux correlation likely holds even after a source reaches its flux peak in the HS state.

The Problem Test the speculation about the relation between of correlation coefficient and the lag time scale Determine the largest correlation time scale → the time scale may correspond to a certain geometrical size in the accretion system

Data Analysis Dividing outbursts into two types of sample groups small number of samples with known state transition time (Aql X-1,GX 339-4) large number of samples with uncertainties in state transition time (Aql X-1, 4U1705-44) Identifying the state transition time and end time for each outburst Calculating Pearson’s correlation coefficient between fluxes of different time lags for each source Estimating uncertainty with bootstrap method

Aql X-1 GX 339-4 Results for the first sample group

Results for the second sample group Aql X-1 Aql X-1 4U 1705-44

Conclusions The correlation indeed exists on longer time scales The longest correlation time scales are: Aql X-1 ～ 40 days 4U 1705-44 ～ 50 days GX 339-4 ～ above 200 days

Implications for a Two Flow Geometry Provide the additional evidence that the flow powering the LH state is related to the flow powering the HS state at a later time during outburst rises of soft X-ray transients If the correlation time scale corresponds to the viscous time scale in the disk flow, the results suggest that a larger disk range than previously thought is involved in powering the LH state

Implications for a Single Flow Geometry The relation between the accretion flow that powers the LH state and that powers the HS state requires a unified relation between M_dot and radius among the outbursts of an individual source. The luminosity corresponding to the state transition is not fixed, thus spectral states are not solely determined by mass accretion rate.

Suggested Accretion Geometry A very large range of the accretion disk is involved in powering the LH state, probably in the form of an inflow or an outflow originated from the disk.