Low frequency QPOs and possible change in the accretion geometry during Aquila X$-$1 outbursts

Wenda Zhang, Wenfei Yu

Published 2014-12-02Version 1

We have studied the evolution of the Low Frequency Quasi-Periodic Oscillations (LFQPOs) during the rise phase of seven outbursts of the neutron star soft X-ray Transient (SXT) Aquila X$-$1 observed with the {\it Rossi X-ray Timing Explorer (RXTE)}. The frequency correlation between the low frequency break and the LFQPO sampled on the time scale of $\sim$2 days is seen. Except for the peculiar 2001 outburst, the frequency of the LFQPOs increased with time before the hard-to-soft state transition up to a maximum $\nu_{max}$ at $\sim$30 Hz, a factor of $\sim$5 higher than those seen in black hole transients such as GX 339$-$4, likely an observational indicator of the mass of the central object. The frequencies increased by around 10 percent per day in the earlier rising phase and accelerated to nearly one hundred percent per day since $\sim$2 days before the hard-to-soft state transition. We examined the dependence of the frequency $\nu_{LF}$ on the source flux $f$ and found in the well-sampled 2009 outburst, during the early stage of the outburst the frequency evolved as $\nu_{LF} \propto f^{1.34\pm0.22}$ while afterwards it evolved as $\nu_{LF}\propto f^{3.58\pm0.34}$. We found that X-ray evaporation can not be the only mechanism that drives the variation of the inner disk radius if either of the twin kHz QPO corresponds to the Keplerian frequency at the truncation radius.