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

摘要: We have studied the evolution of Low Frequency Quasi-Periodic Oscillations (LFQPOs) during rising phase seven outbursts neutron star Soft X-ray Transient (SXT) Aql X$-$1 observed with \textit{Rossi Timing Explorer (RXTE)}. The frequency correlation between low break and LFQPO sampled on time scale $\sim$2 days was seen. Except for peculiar 2001 outburst, LFQPOs increased before hard-to-soft state transition up to a maximum $\nu_{max}$ at $\sim$31 Hz, factor $\sim$5 higher than those seen in black hole transients such as GX 339$-$4, making QPO likely indicator mass central compact object. characteristic frequencies by around ten percent per day early accelerated nearly one hundred since transition. examined dependence $\nu_{LF}$ source flux $f$ found an anti-correlation corresponding luminosity (or outburst peak luminosity) among outbursts. suggest that evaporation process can not be only mechanism drives variation inner disk radius if either twin kHz corresponds Keplerian truncation radius.