SIMBAD references

We have studied the evolution of low-frequency quasi-periodic oscillations (LFQPOs) during the rising phase of seven outbursts of the neutron star soft X-ray transient Aql X-1 observed with the RXTE. A frequency correlation between the low-frequency break and the LFQPO sampled on a timescale of ∼2 days was observed. Except for the peculiar 2001 outburst, the frequency of LFQPOs increased with time before the hard-to-soft state transition up to a maximum v_max at ∼31 Hz, a factor of ∼5 higher than those seen in BH transients such as GX 339-4, making the maximum quasi-periodic oscillation (QPO) frequency a likely indicator of the mass of the central compact object. The characteristic frequencies increased by around 10% per day in the early rising phase and accelerated to nearly 100% per day since ∼2 days before the hard-to-soft state transition. We examined the dependence of the frequency v_LF on the source flux f and found an anti-correlation between the maximum frequency of the LFQPOs and the corresponding X-ray luminosity of the hard-to-soft transition (or outburst peak luminosity) among the outbursts. We suggest that the X-ray evaporation process cannot be the only mechanism that drives the variation of the inner disk radius if either of the twin kHz QPOs corresponds to the Keplerian frequency at the truncation radius.