Evidence of oscillating `compact' Comptonized corona in GRS 1915+105: Insights into HFQPOs with AstroSat

TL;DR

This study analyzes AstroSat data of GRS 1915+105, revealing that a compact, oscillating Comptonized corona is responsible for 70 Hz HFQPOs, with spectral and timing properties correlating with QPO presence.

Contribution

First detailed dynamical analysis linking spectral properties of the corona to HFQPOs in GRS 1915+105 using AstroSat data.

Findings

HFQPOs primarily originate from Comptonized photons in the 6-25 keV range.

Detection of HFQPOs correlates with harder spectra and smaller inner disc radii.

The corona size is estimated to be 2.8-16 gravitational radii.

Abstract

We present, for the first time, an in-depth dynamical analysis of the spectro-temporal properties of the soft variability classes ($\delta$, $\kappa$, $\omega$, and $\gamma$) of GRS 1915+105 during the detection of $\sim$70 Hz High-Frequency Quasi-periodic Oscillations (HFQPOs) using AstroSat data. The wide-band spectra ($0.7-50$ keV) are well described by thermal Comptonization along with an extended power-law component. Additionally, power spectra ($0.01-500$ Hz) indicate that Comptonized photons ($6-25$ keV) primarily contribute to the HFQPOs. Our findings reveal that high (low) count rates referred to as `non-dips' (`dips') in the light curves of the variability classes correspond to the detection (non-detection) of HFQPOs. Accumulated `non-dips' (`dips') spectra are modelled separately using thermal Comptonization (\texttt{nthComp}) as well as \texttt{kerrd} which indicates harder spectra and smaller inner disc radius during the detection of HFQPOs. We conduct dynamical analyses (every 32 s) to trace the presence of HFQPOs, and variations in thermal Comptonization parameters ($\Gamma_{\rm nth}$ and ${\rm N}_{\rm nth}$). Moreover, we observe a positive correlation of `non-dips' with QPO strength, ${\rm HR}1$, and ${\rm N}_{\rm nth}$, while $\Gamma_{\rm nth}$ shows an anti-correlation, suggesting that high-energy photons from the Comptonized corona are responsible for the HFQPOs. Furthermore, we estimate the size of the Comptonized corona using \texttt{kerrd} and \texttt{diskpn} to be $\sim 2.8 - 16$ $r_{\rm g}$. Thus, we infer that a `compact' oscillating corona likely modulates the high-energy radiation, exhibiting the $70$ Hz HFQPOs in GRS 1915$+$105.