Possible detection of HFQPOs associated with 'unknown' variability class of GRS 1915+105

TL;DR

The paper analyzes two AstroSat observations of GRS 1915+105 in 2017 to identify a new 'unknown' variability class ($\tau$) during a $\rho \rightarrow \kappa$ transition, and to search for high-frequency QPOs. Using broad-band timing and spectral modeling, the study finds possible HFQPOs near $\sim 71$ Hz and a harmonic near $\sim 152$ Hz, with energy dependence indicating origin in the Comptonizing corona. Spectral fits with a cool, optically thick corona ($kT_{\rm e} \sim 1.7$ keV, $\tau \sim 14$) and a significant bolometric luminosity ($L_{\rm bol} \sim 0.42\,L_{\rm Edd}$) support a coronal origin for HFQPOs in a sub-Eddington transitional state. The results imply a link between corona dynamics and the newly identified variability class, offering new insights into the accretion physics around black holes and the production of HFQPOs in GRS 1915+105.

Abstract

We present a comprehensive spectro-temporal analysis of GRS $1915+105$ observed with AstroSat during June, $2017$. A detailed study of the temporal properties reveals the appearance of an `unknown' variability class ($τ$) during $ρ\rightarrow κ$ class transition of the source. This new `unknown' class ($τ$) is characterized by the irregular repetition of low count `dips' along with the adjacent `flare' like features in between two successive steady count rate durations, resulting in uniform `$C$' shaped distribution in the color-color diagram. A detailed comparative study of the variability properties between the $τ$ class and other known variability classes of GRS $1915+105$ indicates it as a distinct variability class of the source. Further, we find evidence of the presence of possible HFQPO features at $\sim 71$ Hz with quality factor $\sim 13$, rms amplitude $\sim 4.69\%$, and significance $3σ$, respectively. In addition, a harmonic-like feature at $\sim 152$ Hz is also seen with quality factor $\sim 21$, rms amplitude $\sim 5.75\%$ and significance $\sim 4.7σ$. The energy-dependent power spectral study reveals that the fundamental HFQPO and its harmonic are present in $3-15$ keV and $3-6$ keV energy ranges, respectively. Moreover, the wide-band ($0.7-50$ keV) spectral modelling comprising of thermal Comptonization component indicates the presence of a cool ($kT_{\rm e}\sim 1.7$ keV) and optically thick (optical depth $\sim 14$) Comptonizing `corona', which seems to be responsible in regulating the HFQPO features in GRS $1915$+$105$. Finally, we find the bolometric luminosity ($L_{\rm bol}$) to be about $42\% L_{\rm Edd}$ within $1-100$ keV, indicating the sub-Eddington accretion regime of the source.

Figures (9)

• Figure 1: (a)MAXI/GSC daily light curve in the energy band of $2-20$ keV in flux units of counts $\rm cm^{-2}$$\rm s^{-1}$. (b) The variation of the hardness ratio defined as the ratio of $6-20$ keV to $2-6$ keV count rates with the time of observation. In panel (b), the insets show the variation of the hardness ratio with the corresponding count rates, indicated by distinct colors (orange and blue). $p$ denotes the Pearson correlation coefficient between the count rate and hardness ratio distributions. Different colored vertical dashed lines represent the Epochs of observations with AstroSat and NuSTAR, respectively. See the text for details.
• Figure 2: Background subtracted and dead-time corrected $1$ s binned light curves ( LAXPC in $3-60$ keV) of GRS 1915$+$105 observed with AstroSat during Epoch AS$1$ (a) and AS$2$ (c), respectively. The SXT light curves of AS$1$ and AS$2$, simultaneous with LAXPC is shown with gray color. The CCD of the same observations are shown at the top right insets, respectively. The variation of the corresponding hardness ratios are shown in panel (b) and (d). The comparison of CCDs in different variability classes under consideration is shown in panel (e). See text for details.
• Figure 3: Light curves from the AS1 and AS2 observations in the $3-6$ keV, $6-15$ keV, and $15-60$ keV energy ranges are shown in the top, middle, and bottom panels, respectively. These segments correspond to the $0.6-1.5$ ks interval of the full light curves shown in Fig. \ref{['fig:lcurvCCD']}, highlighting the structured variability. See text for details.
• Figure 4: RXTE/PCA light curves of $1$ s time bin of the source GRS $1915+105$ in $3-60$ keV energy range corresponding to $\chi_{3}$, $\kappa$ and $\mu$ variability classes. The CCDs of the respective classes are presented in the insets of each panel. See text for details.
• Figure 5: Panel (a): Power density spectra of Epoch AS$1$ and AS$2$ in the broad-band frequency range ($0.01-500$ Hz). Each PDS is obtained in $3-60$ keV energy band using LAXPC10 and LAXPC20 combined observation. Zoomed view of the detected HFQPO and/or harmonic features are shown in the inset. For clarity purpose, PDS of Epoch AS$1$ is re-scaled by multiplying factor $5$. Panel (b): The distribution of $\Delta \chi_{\rm sim}^2$ with the number of occurrences (N), obtained from $1000$ simulated power spectra. The horizontal dash lines denote the $\Delta \chi_{\rm obs}^2$ values obtained using observational date for AS$1$ (blue) and AS$2$ (red), respectively. See text for details.