AstroSat Detection of a Quasi-periodic Oscillation at ~ 42 Hz in Cygnus X-2

TL;DR

This paper reports the detection of a 42 Hz quasi-periodic oscillation in Cygnus X-2 using AstroSat, revealing energy-dependent timing and spectral properties that suggest different origins for the QPO and broadband noise.

Contribution

First detection of a 42 Hz QPO in Cygnus X-2 with AstroSat, providing detailed energy-dependent timing and spectral analysis to understand their origins.

Findings

QPO at ~42 Hz with energy-dependent fractional rms.

QPO exhibits hard time lags, while broadband noise shows soft lags.

QPO and noise variability dominated by Comptonized component.

Abstract

We report the results of AstroSat observations of Cygnus X-2 during February 2016. The source's power density spectrum generated using LAXPC data revealed the presence of a prominent Quasi-periodic Oscillation (QPO) at $\sim42$ Hz with broadband continuum noise at lower frequencies at $\sim10$ Hz. The large effective area of LAXPC at $\gtrsim$30 keV allowed for an unprecedented study of the energy dependence of the QPO and the broad noise continuum. The fractional r.m.s 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 behavior. The photon spectrum from SXT 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 r.m.s 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 disk while the noise continuum may originate from the corona.