Accretion flow behaviour during the evolution of the Quasi Periodic Oscillation Frequency of XTE J1550-564 in 1998 outburst

Low and Intermediate Frequency Quasi-Periodic Oscillations (QPOs) are thought to be due to oscillations of Comptonizing regions or hot regions embedded in Keplerian discs. Observational evidence of evolutions of QPOs would therefore be very important as they throw lights on the dynamics of the hotter region. Our aim is to find systems in which there is a well defined correlation among the frequencies of the Quasi Periodic Oscillations over a range of time so as to understand the physical picture. In this paper, we concentrate on the archival data of XTE J1550-564 obtained during 1998 outburst, and study the systematic drifts during the rising phase from the 7 of September 1998 to the 19 September 1998, when the QPO frequency increased monotonically from 81mHz to 13.1Hz. Immediately after that QPO frequency started to decrease and on the 26 of September 1998, the QPO frequency became 2.62Hz. After that its value remained almost constant. This frequency drift can be modeled satisfactorily with a propagatory oscillating shock solution where the postshock region behaves as the Comptonized region. Comparing with the nature of a more recent 2005 outburst of another black hole candidate GRO 1655-40, where QPOs disappeared at the end of the rising phase, we conjecture that this so-called ’outburst’ may not be a full-fledged outburst.