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The Galactic X-ray transient EXO 1846-031 was first discovered during an outburst in 1985 by the EXOSAT mission. The source remained in a quiescent state for nearly 34 years after the first outburst. The source started its second outburst on 23 July 2019. We studied the accretion flow properties using the Two Component Advective Flow (TCAF) paradigm of this 2019 outburst. During the outburst, the source went through all the four spectral states, though, due to data constraints, it was not possible to define the date of the state transitions during the declining intermediate states. During this outburst, the black hole candidate (BHC) exhibited significant jet activity. In the TCAF solution, the model normalization is expected to remain constant for a given source. Therefore, any need for a significantly different normalization to achieve a better spectral fit suggests the presence of additional X-ray contributions from components not accounted for in the current TCAF model fit’s file. By comparing with the expected normalization, we estimate the X-ray contribution originating from jets and outflows. We further analyze the origin of the jet. Our analysis shows that, on some days, up to ∼92% of the total X-ray flux originates from the base of the jet itself.

The newly discovered galactic black hole candidate (BHC) MAXI J1348-630 showed two major outbursts in 2019, just after its discovery. Here, we provide a detailed spectral and temporal analysis of the less-studied second outburst using archive data from multiple satellites, namely Swift, MAXI, NICER, NuSTAR and AstroSat. The outburst continued for around two and a half months. Unlike the first outburst from this source, this second outburst was a ‘failed’ one. The source did not transition to soft or intermediate spectral states. During the entire outburst, the source was in the hard state with high dominance of non-thermal photons. The presence of strong shocks are inferred from spectral fitting using a TCAF model. In NuSTAR spectra, weak reflection is observed from spectral fitting. Low-frequency quasi-periodic oscillations are also detected in AstroSat data.

We study the properties of the faint X-ray activity of Galactic transient black hole candidate XTE J1908+094 during its 2019 outburst. Here, we report the results of detailed spectral and temporal analysis during this outburst using observations from Nuclear Spectroscopic Telescope Array (NuSTAR). We have not observed any quasi-periodic-oscillations (QPOs) in the power density spectrum (PDS). The spectral study suggests that the source remained in the softer (more precisely, in the soft–intermediate) spectral state during this short period of X-ray activity. We notice a faint but broad Fe Kα emission line at around 6.5 keV. We also estimate the probable mass of the black hole to be 6.5−0.7+0.5M⊙, with 90% confidence.

In 2016–17, the Galactic transient black hole candidate GRS 1716-249 exhibited an outburst event after a long quiescence period of almost 23 years. The source remained in the outbursting phase for almost 9 months. We study the spectral and temporal properties of the source during this outburst using archival data from four astronomy satellites, namely MAXI, Swift, NuSTAR and AstroSat. Initial spectral analysis is done using combined disk black body and power-law models. For a better understanding of the accretion flow properties, we studied spectra with the physical two component advective flow (TCAF) model. Accretion flow parameters are extracted directly from the spectral fits with the TCAF model. Low frequency quasi periodic oscillations are also observed in the Swift/XRT and AstroSat/LAXPC data. From the spectral fit, we also estimate the probable mass of GRS 1716-249 to be in the range of 4.50–5.93M⊙ or 5.01−0.51+0.92M⊙. Refitting of all spectra is done by freezing the mass at its average value. An insignificant deviation of the TCAF model parameters is observed. From the nature of the variation of the newly fitted spectral and temporal properties, we find that the source stays in only the harder (hard and hard-intermediate) states during the outburst. It does not make a transition to the softer states which makes it a ‘failed’ outburst.

The recently discovered bright transient black hole candidate Swift J1727.8-1613 is studied in a broad energy range (\\(0.5-79\\) keV) using combined NICER and NuSTAR data on 29 August 2023. A promonient type-C Quasi-Periodic Oscillation (QPO) at \\(0.89 \\pm 0.01\\) Hz with its harmonic was observed in NICER data of \\(0.5-10\\) keV. Interestingly, the harmonic becomes weaker in the lower energy bands (\\(0.5-1\\) & \\(1-3\\) keV). We also report the first detection of a soft time-lag of \\(0.014 \\pm 0.001\\) s at the QPO frequency between harder (\\(3-10\\) kev) and softer (\\(0.5-3\\) keV) band photons observed with the NICER/XTI instrument. This indicates that the inclination of the accretion disk in the binary system might be high. From the detailed spectral analysis with the relxill reflection model, we found the disk inclination angle of source to be \\(\\sim 85^\\circ\\). We discuss how the accretion flow configuration inferred from spectral analysis can help us to understand the origin of QPOs and soft lag in this source.

We investigate the accretion dynamics of the black hole X-ray binary Swift J1727.8-1613 during its \\(2023-2024\\) discovery outburst that lasted for \\(\\sim10\\) months. Insight-HXMT monitored the rising phase of the outburst of Swift J1727.8-1613 roughly continuously from 2023 Aug 25 to 2023 Oct 05. Strong signatures of type-C Quasi-Periodic Oscillations (QPOs) are observed during this phase of the outburst. In our recent paper, nature of the QPOs are studied with the propagating oscillatory shock (POS) model. In this paper, we report on the observation of both positive (or hard) and negative (or soft) time-lags in the \\(4-10\\) keV (LE), \\(10-30\\) keV (ME), and \\(30 -150\\) keV (HE) bands, computed with respect to the \\(2-4\\) keV reference band. We detect a clear transition from hard to soft lags as the outburst evolves. We show the evolution of QPOs and associated time-lags between different X-ray energy bands, correlated with changes in the QPO frequency, spectral state, and the size of the Comptonizing region. Our analysis reveals strong anti-correlations between the time-lags and both QPO frequency and photon index, and a strong positive correlation with the shock location. These evolving lag characteristics and their correlations provide crucial insights into the changing accretion geometry and the interplay of radiative processes, further supporting dynamic models like the POS in explaining the coupled spectro-temporal evolution in black hole X-ray binaries.

The rising phase of the 2023-24 outburst of the recently discovered bright transient black hole candidate Swift J1727.8-1613 was monitored by {\\it Insight}-HXMT. We study the evolution of hard (\\(4\\)-150\\( keV) and soft (\\)2\\(-\\)4\\( keV) band photon count rates, the hardness ratio (HR), QPO frequencies, and spectral features using daily observations from the HXMT/LE, ME, and HE instruments between August 25 and October 5, 2023. The QPO frequency is found to be strongly correlated with the soft-band X-ray count rates, and spectral photon indices. In contrast, a strong anti-correlation is observed between HR and QPO frequency, as well as between HR and photon index. Based on the evolution of the temporal and spectral properties, the rising phase of the outburst is subdivided into six parts. The evolution of the QPOs in parts 1-5 is fitted with the propagating oscillatory shock (POS) solution to understand the nature of the evolution from a physical perspective. An inward-propagating shock with weakening strength (except in part 4) is observed during the period of our study. The probable mass of the source is estimated to be \\)13.5 \\pm 1.9~M_\\odot\\( using the QPO frequency (\\)\\nu\\()-photon index (\\)\\Gamma$) scaling method.

Galactic black hole candidate MAXI J1910-057/Swift J1910.2-0546 was simultaneously discovered by MAXI/GSC and Swift/BAT satellites during its first outburst in 2012. We study the detailed spectral and temporal properties of the source in a broad energy range using archival data from Swift/XRT, MAXI/GSC and Swift/BAT satellites/instruments. Low frequency quasi periodic oscillations are observed during the outburst. The combined 1-50 keV spectra are analyzed using the transonic flow solution based Two Component Advective Flow (TCAF) model. Based on the variations of soft and hard X-ray fluxes, their hardness ratios and the variations of the spectral model fitted parameters, we find that the source has evolved through six spectral states. We interpret this spectral state evolution to be a result of two connected outbursts where the leftover matter from a primary outburst is released from the pile-up radius due to a sudden rise of viscosity causing a reflare/secondary outburst. We show a possible configuration of the evolution of accretion flow during the outburst. From the spectral analysis with TCAF model, we estimate the mass of the black hole to be \\(9.97^{+3.51}_{-3.24}\\) \\(M_\\odot\\) , and the source distance is estimated to be \\(3.4-9.6\\)~kpc from transition luminosity considerations.

The Galactic transient black hole candidate GX 339-4 is a very interesting object to study as it showed both complete and failed types of outbursts. We studied both spectral and temporal properties of the 2017-18 outburst of the source using archival data of NICER and AstroSat instruments. This 2017-18 outburst is found to be failed in nature, as during the entire period of the outburst, the source was only in the hard spectral state. Source spectra were highly dominated with the non-thermal fluxes. When we tried to fit spectra with phenomenological models, most of the spectra were fitted with only the powerlaw model, and only six spectra required disk black body plus powerlaw models. While fitting spectra with the physical two-component advective flow (TCAF) model, we observed that the flow was highly dominated by the sub-Keplerian halo rate. The presence of stronger shock at a larger radius from the black hole was also observed during the rising and declining phases of the outburst. A prominent signature of \\(0.31\\)~Hz QPO is observed with its four harmonics. Mass of the black hole was also estimated from our spectral analysis with the TCAF model as \\(10.76^{+0.77}_{-1.07}~M_\\odot\\).

The Galactic black hole candidate (BHC) 4U~1630-472 has gone through several outbursts (13 to be particular) in the last two and a half decades starting from the RXTE era till date. Like the outbursts of other transient BHCs, the outbursts of this source show variations in duration, peak numbers, highest peak flux, etc. However, unlike any other soft X-ray transients, this source showed outbursts of two types, such as normal and super. The normal outbursts of duration \\(\\sim 100-200\\)~days are observed quasi periodically at an average recurrence/quiescence period of \\(\\sim 500\\)~days. The super outbursts of duration \\(\\sim 1.5-2.5\\)~years contain one or more normal outbursts other than one mega outburst. We make an effort to separate flux contribution of the normal and the mega outbursts from the super outbursts, and tried to understand the nature of evolution of both types (normal and mega) of outbursts, based on the quiescent period prior to the outbursts. Archival data of RXTE/ASM from January 1996 to June 2011, and MAXI/GSC from August 2009 to July 2020 are used for our study. A possible linear relation between the quiescent and outburstsing periods for both types of outbursts are observed. This makes the BHC a special source, and it may contain two companion binaries. Two companions might be responsible for two types of outbursts.