Swift J1753.5-0127: understanding the accretion geometry through frequency-resolved spectroscopy

Abstract

The Black Hole Binary source Swift J175.5-0127 remained in outburst for $\sim$ 12 years from May 2005 to April 2017. For most part of the outburst, the source remained in the Low Hard State (LHS) displaying transitions to softer states only towards the end of the outburst for short periods of time. Quasi periodic Oscillations (QPOs) were observed in the Power Density Spectrum (PDS) only during the decay. A soft thermal component was required to model the spectrum in LHS, which does not conform to the generally accepted disc truncation theory. In this work, we attempt to obtain a clearer picture of the accretion disc geometry by studying the QPO variability using frequency resolved spectroscopy (FRS). We obtain the QPO rms spectrum of the source during the bright-hard state and model it with physical components. We find that the QPO rms spectrum can be described only by a Comptonisation component with no contribution from the thermal disc. This indicates that the variability observed in the PDS originates in the Comptonisation component and the evolution of the QPOs is likely to be a result of localization of the variabilities to different radii of the hot inner flow rather than disc truncation. The minimal variation in disc parameters also points to the existence of a stable disc throughout the outburst.