arXiv:2202.04881v1 [astro-ph.HE] 10 Feb 2022 MNRAS 000, 1–?? (0000) Preprint 11 February 2022 Compiled using MNRAS L A T E X style file v3.0 AstroSat Detection of a Quasi-periodic Oscillation at ∼ 42 Hz in Cygnus X-2 Vanzarmawii Chhangte 1 ⋆ , Jayashree Roy 2 , Ranjeev Misra 2 , Lalthakimi Zadeng 1 1 Mizoram University, Tanhril, Aizawl, Mizoram 796004, India 2 Inter-University Center for Astronomy and Astrophysics, Post Bag 4, Pune, Maharashtra 411007, India 11 February 2022 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 ∼ 42 Hz with broadband 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 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 sug- gests that the dynamic origin of the QPO may be in the disk while the noise continuum may originate from the corona. Key words: accretion, accretion disks – stars: individual(Cygnus X-2) – stars: neutron – X-rays: binaries 1 INTRODUCTION X-ray binaries are a class of binary stars, so-called because they emit X-rays. The binary star comprises of a com- panion star and an accretor. These X-rays are produced by matter falling from the donor star to the accretor. The donors are usually normal stars, and accretors are collapsed stars that are compact, e.g., neutron stars (NS), white dwarf (WD), or black holes (BH). The X-ray bina- ries are classified into two classes according to the masses of the companion star- high-mass and low-mass X-ray binary. The X-ray binary system comprising a NS as a compact object and a low-mass star as its companion is classified as a neutron star low-mass X-ray binary (NS LMXB). These can be sub-divided into Z-type and Atoll-type sources based on X-ray spectral and fast timing behaviour (Hasinger & van der Klis 1989). The Z sources trace out a Z-shape in the X-ray color-color diagram (CCD). The branches of the Z-type sources are horizontal, normal, and flaring, from top to bottom. The Z sources are classified into two groups (Kuulkers et al. 1997; Hasinger & van der Klis 1989), the Cyg-like sources- Cyg X-2, GX 5-1, GX 340+0 where the HB, NB, and FB are seen but with weak flaring and the Sco-like sources- Sco X-1, GX 349+2, GX 17+2 ⋆ E-mail: zarichhangte0739@gmail.com where flaring is strong and frequent but with a short or weak HB. A characteristic feature of these sources is their rapid nearly sinusoidal variation which is revealed by peaks in their power spectra known as Quasi-periodic oscillations (QPO). In the horizontal branch (HB), the frequencies of the QPO vary between ∼ 15Hz to ∼ 55Hz (Alpar & Shaham 1985; Lamb 1989; Ghosh & Lamb 1992), while in the normal branch (NB), the frequencies range between 5-7Hz and in the flaring branch (FB), the frequencies are observed to increase from ∼ 6Hz up to ∼ 20Hz along the branch (Hasinger 1990). The sources also exhibit high frequency kHz QPOs, which sometimes occur in pairs (Wijnands et al. 1998; Kuznetsov 2002). Despite several endeavours to characterise the phenomena, there is at present no consensus on the origin of these different kinds of QPOs. Byram et al. 1966 first discovered Cygnus X-2 (henceforth Cyg X-2) using the sounding rocket experiment. It was first observed by EXOSAT for a continuous duration of 14h starting on July 23rd, 1984, using Gas scintillation proportional counter (Peacock et al. 1981) and one-half of medium energy detectors (Turner et al. 1981). The X-ray binary source- Cyg X-2, is a bright, persistent LMXB. Cyg X-2 is classified as a Z-type source because of its behavior and pattern when studied on an X-ray color-color diagram (CCD) and hardness-intensity diagram (HID) © 0000 The Authors