AstroSat detection of a quasi-periodic oscillation at 42 Hz in Cygnus X-2

We report the results of AstroSat observations of Cygnus X-2 during 2016 February. The source's power density spectrum generated using Large Area X-ray Proportional Counter (LAXPC) data revealed the presence of a prominent quasi-periodic oscillation (QPO) at ~42 Hz with broad-band continuum noise at lower frequencies at ~10 Hz. The large effective area of LAXPC at ≳30 keV allowed for an unprecedented study of the energy dependence of the QPO and the broad noise continuum. The fractional rms increases with energy, and its shape is similar for both the QPO and the continuum noise, suggesting a common radiative origin. However, while the QPO exhibits hard time-lags, with the high-energy photons lagging the low ones by a few milliseconds, the continuum noise shows the opposite behaviour. The photon spectrum from Soft X-ray Imaging Telescope and LAXPC in 0.7-30 keV band comprises the soft component from a disc and a hard Comptonized component from a hot corona. While the energy dependence of the rms shows that the QPO and the continuum noise variability are dominated by the Comptonized component, the change in sign of the time-lag suggests that the dynamic origin of the QPO may be in the disc while the noise continuum may originate from the corona.