We study transient Galatic black hole candidate MAXI J1836-194 during its 2011 outburst using RXTE/PCA archival data. 2.5-25 keV spectra are fitted with Two Component Advective Flow (TCAF) model fits file as an additive table local model in XSPEC. From TCAF model spectral fits, physical parameters such as Keplerian disk rate, sub-Keplerian halo rate, shock location and compression ratio are extracted directly for better understanding of accretion processes around the BHC during this outburst. Low frequency quasi-periodic oscillation (QPO) are observed sporadically during the entire epoch of the outburst, with a general trend of increasing frequency during rising and decreasing frequency during declining phases of the outburst, as in other transient BHCs. The nature of the variation of the accretion rate ratio (ratio of halo and disk rates) and QPOs (if observed), allows us to properly classify entire epoch of the outburst into following two spectral state, such as hard (HS), hard-intermediate (HIMS). These states are observed in the sequence of HS (Ris.) → HIMS (Ris.) → HIMS (Dec.) → HS (Dec.). This outburst of MAXI J1836-194 could be termed as 'failed' outburst, since no observation of soft (SS) and soft-intermediate (SIMS) spectral state are found during the entire outburst.

We derive accretion flow properties of the transient black hole candidate (BHC) MAXI J1543-564 using the RXTE data. We use Two-Component Advective Flow (TCAF) solution to fit the data of the very initial rising phase of outburst (from 2011 May 10 to 2011 May 15). 2.5 − 25 keV spectra are fitted using the TCAF solution fits file as a local additive table model in XSPEC. We extract physical flow parameters such as the two component (Keplerian disk and sub-Keplerian halo) accretion rates and size and the property of the Compton cloud (post shock region close to a black hole). Similar to other classical transient BHCs, monotonic evolution of low frequency quasi-periodic oscillations (QPOs) are observed during the rising phase of the outburst, which is fitted with the propagating oscillatory shock (POS) model which describes how the Compton cloud properties change from day to day. From the nature of variations of TCAF model fitted physical flow parameters and QPOs, we only found hard-intermediate and softintermediate spectral states during this phase of the outburst under study. We also calculate frequency of the dominating QPOs from the TCAF model fitted shock parameters, and found that they roughly match with the observed and POS model fitted values. From our spectro-temporal study of the source with TCAF and POS models, the most probable mass of the BHC is found to be 12.6 − 14.0 M ⊙ , or 13 +1.0 −0.4 M ⊙ .

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 25th Sept. 2010. Our recent studies of a few transient BHCs including MAXI J1659-152 using Chakrabarti-Titarchuk two-component advective flow (TCAF) solution as an additive table local model in XSPEC revealed details of accretion flow dynamics around the black hole (BH). 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 TCAF fitted normalization parameter in a narrow range, and ii) studying evolution of the Quasi-Periodic Oscillation frequency (ν QPO ) with time, fitted 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 results of these two methods, we obtain a most probable mass range of the source to be 4.7 − 7.8 M ⊙ or 6 +1.8 −1.3 M ⊙ .

We present detailed timing and spectral studies of the black hole candidate MAXI J0637–430 during its 2019-2020 outburst using observations with the Neutron Star Interior Composition Explorer (NICER) and the Neil Gehrels Swift Observatory. We find that the source evolves through the soft-intermediate, high-soft, hard-intermediate and low-hard states during the outburst. No evidence of quasi-periodic oscillations is found in the power density spectra of the source. Weak variability with fractional rms amplitude $<5{{\ \rm per\ cent}}$ is found in the softer spectral states. In the hard-intermediate and hard states, high variability with the fractional rms amplitude of $>20{{\ \rm per\ cent}}$ is observed. The 0.7 − 10 keV spectra with NICER are studied with a combined disk-blackbody and nthcomp model along with the interstellar absorption. The temperature of the disc is estimated to be 0.6 keV in the rising phase and decreased slowly to 0.1 keV in the declining phase. The disc component was not detectable or absent during the low hard state. From the state-transition luminosity and the inner edge of the accretion flow, we estimate the mass of the black hole to be in the range of 5–12 M⊙, assuming the source distance of d < 10 kpc.

Galactic X-ray binary black hole candidate Swift J1753.5-0127 was discovered on June 30 2005 by Swift/BAT instrument. In this paper, we make detailed analysis of spectral and timing properties of its 2005 outburst using RXTE/PCA archival data. We study evolution of spectral properties of the source from spectral analysis with the additive table model fits file of the Chakrabarti-Titarchuk two-components advective flow (TCAF) solution. From spectral fit, we extract physical flow parameters, such as, Keplerian disk accretion rate, sub-Keplerian halo rate, shock location and shock compression ratio, etc. We also study the evolution of temporal properties, such as observation of low frequency quasi-periodic oscillations (QPOs), variation of X-ray intensity throughout the outburst. From the nature of the variation of QPOs, and accretion rate ratios (ARRs=ratio of halo to disk rates), we classify entire 2005 outburst into two harder (hard-intermediate and hard) spectral states. No signature of softer (soft-intermediate and soft) spectral states are seen. This may be because of significant halo rate throughout the outburst. This behavior is similar to a class of other short orbital period sources, such as, MAXI J1836-194, MAXI J1659-152 and XTE J1118+480. Here, we also estimate probable mass range of the source, to be in between 4.75M ⊙ to 5.90M ⊙ based on our spectral analysis.