Spectral and temporal properties of black hole candidates can be explained reasonably well using Chakrabarti–Titarchuk solution of two-component advective flow (TCAF). This model requires two accretion rates, namely the Keplerian disc accretion rate and the halo accretion rate, the latter being composed of a sub-Keplerian, low-angular-momentum flow which may or may not develop a shock. In this solution, the relevant parameter is the relative importance of the halo (which creates the Compton cloud region) rate with respect to the Keplerian disc rate (soft photon source). Though this model has been used earlier to manually fit data of several black hole candidates quite satisfactorily, for the first time, we made it user friendly by implementing it into xspec software of Goddard Space Flight Center (GSFC)/NASA. This enables any user to extract physical parameters of the accretion flows, such as two accretion rates, the shock location, the shock strength, etc., for any black hole candidate. We provide some examples of fitting a few cases using this model. Most importantly, unlike any other model, we show that TCAF is capable of predicting timing properties from the spectral fits, since in TCAF, a shock is responsible for deciding spectral slopes as well as quasi-periodic oscillation frequencies.

The well-known black hole candidate (BHC) H 1743-322 exhibited temporal and spectral variabilities during several outbursts. The variation of the accretion rates and flow geometry that change on a daily basis during each of the outbursts can be very well understood using the recent implementation of the two-component advective flow solution of the viscous transonic flow equations as an additive table model in XSPEC. This has dramatically improved our understanding of accretion flow dynamics. Most interestingly, the solution allows us to treat the mass of the BHC as a free parameter and its mass could be estimated from spectral fits. In this paper, we fitted the data of two successive outbursts of H 1743-322 in 2010 and 2011 and studied the evolution of accretion flow parameters, such as two-component (Keplerian and sub-Keplerian) accretion rates, shock location (i.e., size of the Compton cloud), etc. We assume that the model normalization remains the same across the states in both these outbursts. We used this to estimate the mass of the black hole and found that it comes out in the range of . For the sake of comparison, we also estimated mass using the Photon index versus Quasi Periodic Oscillation frequency correlation method, which turns out to be using GRO J1655-40 as a reference source. Combining these two estimates, the most probable mass of the compact object becomes .

We present spectral and timing properties of the newly discovered X-ray transient source, MAXI J1535-571, which is believed to be a Galactic X-ray binary containing a black hole candidate (BHC) as the primary object. Since its discovery on 2017 September 2, it has been monitored regularly in multi-wavelength bands by several satellites. We use archival data of the Swift (XRT and BAT) and MAXI (GSC) satellite instruments to study accretion flow dynamics of the source during the outburst. During its outburst, the source became very bright in the sky with a maximum observed flux of 5 Crab in the 2–10 keV GSC band. Similar to other transient BHCs, it also shows signatures of low-frequency quasi-periodic oscillations (QPOs) during the outburst. Spectral data of different instruments are fitted with the transonic flow, solution-based two-component advective flow model fits file to find the direct accretion flow parameters. The evolution of spectral states and their transitions are understood from the model-fitted physical flow parameters and nature of QPOs. We also estimate the probable mass of the black hole from our spectral analysis as 7.9–9.9 M _⊙ or 8.9 ± 1.0 M _⊙.

The Galactic transient X-ray binary MAXI J1836-194 was discovered on 2011 August 29. Here we make a detailed study of the spectral and timing properties of its 2011 outburst using archival data from the RXTE Proportional Counter Array instrument. The evolution of accretion flow dynamics of the source during the outburst through spectral analysis with Chakrabarti–Titarchuk’s two-component advective flow (TCAF) solution as a local table model in XSPEC. We also fitted spectra with combined disk blackbody and power-law models and compared it with the TCAF model fitted results. The source is found to be in hard and hard-intermediate spectral states only during the entire phase of this outburst. No soft or soft-intermediate spectral states are observed. This could be due to the fact that this object belongs to a special class of sources (e.g., MAXI J1659-152, Swift J1753.5-0127, etc.) that have very short orbital periods and that the companion is profusely mass-losing or the disk is immersed inside an excretion disk. In these cases, flows in the accretion disk are primarily dominated by low viscous sub-Keplerian flow and the Keplerian rate is not high enough to initiate softer states. Low-frequency quasi-periodic oscillations (QPOs) are observed sporadically although as in normal outbursts of transient black holes, monotonic evolutions of QPO frequency during both rising and declining phases are observed. From the TCAF fits, we find the mass of the black hole in the range of 7.5–11 M _⊙, and from time differences between peaks of the Keplerian and sub-Keplerian accretion rates we obtain a viscous timescale for this particular outburst, ∼10 days.

We study spectral properties of GX 339-4 during its 2010–11 outburst with two component advective flow (TCAF) model after its inclusion in xspec as a table model. We compare results fitted by TCAF model with combined disc blackbody and power-law model. For a spectral fit, we use 2.5–25 keV spectral data of the Proportional Counter Array instrument onboard RXTE satellite. From our fit, accretion flow parameters such as Keplerian (disc) rate, sub-Keplerian (halo) rate, location and strength of shock are extracted. We quantify how the disc and the halo rates vary during the entire outburst. We study how the halo to disc accretion rate ratio (ARR), quasi-periodic oscillations (QPOs), shock locations and its strength vary when the system passes through hard, hard-intermediate, soft-intermediate and soft states. We find pieces of evidence of monotonically increasing and decreasing nature of QPO frequencies depending on the variation of ARR during rising and declining phases. Interestingly, on days of transition from hard state to hard-intermediate spectral state (during the rising phase) or vice-versa (during decline phase), ARR is observed to be locally maximum. Non-constancy of ARR while obtaining reasonable fits points to the presence of two independent components in the flow.

The Galactic transient black hole candidate (BHC) MAXI J1659−152 exhibited temporal and spectral evolution during its very first X-ray outburst (2010) after its discovery on 2010 September 25. Our recent studies of a few transient BHCs, including MAXI J1659−152, using the Chakrabarti–Titarchuk two-component advective flow (TCAF) solution as an additive table local model in the xspec software revealed details of the accretion-flow dynamics around the black holes. The TCAF model-fitted normalization (N) comes out to be almost constant throughout the entire outburst, consisting of several spectral states. We introduce two independent methods to determine the mass (M_BH) of the BHC: namely (i) keeping the TCAF fitted normalization parameter in a narrow range and (ii) studying the evolution of the quasi-periodic oscillation frequency (ν_QPO) with time, fitting with the propagating oscillatory shock (POS) model. The predicted mass ranges of the source with these two methods are 4.7–7.8 M_⊙ and 5.1–7.4 M_⊙, respectively. Combining the results of these two methods, we obtain a most probable mass range of the source of 4.7–7.8 M_⊙ or $$6^{+1.8}_{-1.3}\,\mathrm{M}_{{\odot }}$$.