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We report the results of AstroSat observations of GRS 1915+105 obtained using 100 ks guaranteed-time (GT) during the soft state. The Color-Color Diagram (CCD) indicates a variability class of δ with the detection of High Frequency QPO (HFQPO) in the power density spectra (PDS). The HFQPO is seen to vary in the frequency range of 67.96 − 70.62 Hz with percentage rms ∼0.83 − 1.90 % and significance varying from 1.63 − 7.75. The energy dependent power spectra show that the HFQPO features are dominant only in 6 − 25 keV energy band. The broadband energy spectra (0.7 − 50 keV) of SXT (Soft X-ray Telescope) and LAXPC (Large Area X-ray Proportional Counter) modelled with nthComp and powerlaw imply that the source has an extended corona in addition to a compact ‘Comptonizing corona’ that produces high energy emission and exhibits HFQPOs. The broadband spectral modelling indicates that the source spectra are well described by thermal Comptonization with electron temperature (kTe) of 2.07 − 2.43 keV and photon-index (Γnth) between 1.73 − 2.45 with an additional powerlaw component of photon-index (ΓPL) between 2.94 − 3.28. The norm of nthComp component is high (∼8) during the presence of strong HFQPO and low (∼3) during the absence of HFQPO. Further, we model the energy spectra with the kerrbb model to estimate the accretion rate, mass and spin of the source. Our findings indicate that the source accretes at super-Eddington rate of $1.17-1.31~ \dot{M}_{\rm Edd}$. Moreover, we find the mass and spin of the source as 12.44 − 13.09 M⊙ and 0.990 − 0.997 with $90\%$ confidence suggesting that GRS 1915+105 is a maximally rotating stellar mass X-ray binary black hole source.

We study the properties of the dissipative accretion flow around rotating black holes in presence of mass loss. We obtain the complete set of global inflow-outflow solutions in the steady state by solving the underlying conservation equations selfconsistently. We observe that global inflow-outflow solutions are not the isolated solution, instead such solutions are possible for wide range of inflow parameters. Accordingly, we identify the boundary of the parameter space for outflows, spanned by the angular momentum (λ in ) and the energy (E in ) at the inner sonic point (x in ), as function of the dissipation parameters and find that parameter space gradually shrinks with the increase of dissipation rates. Further, we examine the properties of the outflow rate Rṁ (defined as the ratio of outflow to inflow mass flux) and ascertain that dissipative processes play the decisive role in determining the outflow rates. We calculate the limits on the maximum outflow rate (R maẋ m ) in terms of viscosity parameter (α) as well as black hole spin (a k ) and obtain the limiting range as 3% R maẋ m 19%. Moreover, we calculate the viable range of α that admits the coupled inflow-outflow solutions and find that α 0.25 for Rṁ = 0. Finally, we discuss the observational implication of our formalism to infer the spin of the black holes. Towards this, considering the highest observed QPO frequency of black hole source GRO J1655-40 (∼ 450 Hz), we constrain the spin value of the source as a k 0.57.

We examine the dynamical behavior of accretion flow around XTE J1859+226 during the 1999 outburst by analyzing the entire outburst data (∼ 166 days) from RXTE Satellite. Towards this, we study the hysteresis behavior in the hardness intensity diagram (HID) based on the broadband (3−150 keV) spectral modeling, spectral signature of jet ejection and the evolution of Quasiperiodic Oscillation (QPO) frequencies using the twocomponent advective flow model around a black hole. We compute the flow parameters, namely Keplerian accretion rate (ṁ d ), sub-Keplerian accretion rate (ṁ h ), shock location (r s ) and black hole mass (M bh ) from the spectral modeling and study their evolution along the q-diagram. Subsequently, the kinetic jet power is computed as L obs jet ∼ 3 − 6 × 10 37 erg s −1 during one of the observed radio flares which indicates that jet power corresponds to 8 − 16% mass outflow rate from the disc. This estimate of mass outflow rate is in close agreement with the change in total accretion rate (∼ 14%) required for spectral modeling before and during the flare. Finally, we provide a mass estimate of the source XTE J1859+226 based on the spectral modeling that lies in the range of 5.2 − 7.9M ⊙ with 90% confidence.

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In this paper, we employ Machine Learning algorithms on multimission observations for the classification of accretion states of outbursting black hole X-ray binaries for the first time. Archival data from RXTE, Swift, MAXI, and AstroSat observatories are used to generate the hardness intensity diagrams (HIDs) for outbursts of the sources XTE J1859+226 (1999 outburst), GX 339−4 (2002, 2004, 2007, and 2010 outbursts), IGR J17091−3624 (2016 outburst), and MAXI J1535−571 (2017 outburst). Based on variation of X-ray flux, hardness ratios, presence of various types of quasi-periodic oscillations (QPOs), photon indices, and disc temperature, we apply clustering algorithms like K-Means clustering and Hierarchical clustering to classify the accretion states (clusters) of each outburst. As multiple parameters are involved in the classification process, we show that clustering algorithms club together the observations of similar characteristics more efficiently than the ‘standard’ method of classification. We also infer that K-Means clustering provides more reliable results than Hierarchical clustering. We demonstrate the importance of the classification based on machine learning by comparing it with results from ‘standard’ classification.

We present a comprehensive temporal and spectral analysis of the ‘softer’ variability classes (i.e. θ, β, δ, ρ, κ, ω and γ) of the source GRS 1915+105 observed by AstroSat during the 2016−2021 campaign. Wide-band (3−60 keV) timing studies reveal the detection of high-frequency quasi-periodic oscillations (HFQPOs) with frequencies of 68.14−72.32 Hz, significance of 2.75−11σ and rms amplitude of 1.48–2.66 per cent in δ, κ, ω and γ variability classes. Energy-dependent power spectra show that HFQPOs are detected only in the 6−25 keV energy band and rms amplitude is found to increase (1–8 per cent) with energy. The dynamical power spectra of the κ and ω classes demonstrate that HFQPOs seem to be correlated with high count rates. We observe that wide-band (0.7−50 keV) energy spectra can be described by the thermal Comptonization component (nthComp) with a photon index (Γnth) of 1.83−2.89 along with an additional steep (ΓPL ∼ 3) power-law component. The electron temperature (kTe) of 1.82−3.66 keV and optical depth (τ) of 2−14 indicate the presence of a cool and optically thick corona. In addition, nthComp components, 1.97 ≲ Γnth ≲ 2.44 and 1.06 × 10−8 ≲ Fnth (erg cm−2 s−1) ≲ 4.46 × 10−8, are found to dominate in the presence of HFQPOs. Overall, these findings infer that HFQPOs are possibly the result of the modulation of the ‘Comptonizing corona’. Further, we find that the bolometric luminosity (0.3−100 keV) of the source lies within the sub-Eddington (3–34 per cent LEdd) regime. Finally, we discuss and compare the obtained results in the context of existing models on HFQPOs.