State transitions of GX 339-4 during its outburst rising phase

We investigate systematically four outbursts of black hole system GX 339-4 observed by the Rossi X-ray Timing Explorer (RXTE) in both spectral and timing domains and find that these outbursts have some common properties, although they experience different 'q' tracks in the hardness-intensity diagram (HID). While the spectral indices are around 1.5 in the low/hard state (LHS) and 2.4 in the soft intermediate state (SIMS) and high/soft state (HSS), the spectral parameters of thermal, non-thermal, and reflection components vary significantly in transitions from the LHS to HIMS. Also, the quasi-periodic oscillation (QPO) shows a peculiar behaviour during the state transition between the LHS and HIMS: the RMS drop of the type C fundamental QPO is accompanied by the appearance of the second harmonic. Interestingly, the QPO RMS is found to have a similar linear relationship with the non-thermal fraction of emission in different outbursts. These findings provide more clues to aid our understanding of the outbursts of a black hole X-ray binary system.