1. 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)

2. Outline Background The Problem Data Analysis Results Conclusion
Implications

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

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

5. 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

6. 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 ～ 30 days
GX ～ 100 days
The flux correlation likely holds even after a source reaches its flux peak in the HS state.

7. 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

8. 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, 4U )
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

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

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

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

12. 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

13. 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.

14. 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.