Active galactic nuclei (AGN) emit radiation via accretion across the entire energy spectrum. While the standard disk and corona model can somewhat describe this emission, it fails to predict specific features such as the soft X-ray excess, the short-term optical/UV variability, and the observed UV/X-ray correlation in AGN. In this context, the fraction of AGN emission in different bands (i.e., bolometric corrections) can be useful to better understand the accretion physics of AGN. Past studies have shown that the X-ray bolometric corrections are strongly dependent on the physical properties of AGN, such as their luminosities and Eddington ratios. However, since these two parameters depend on each other, it has been unclear which is the main driver of the X-ray bolometric corrections. We present here results from a large study of hard-X-ray-selected (14-195 keV) nearby (z < 0.1) AGN. Based on our systematic analysis of the simultaneous optical-to-X-ray spectral energy distributions of 236 unobscured AGN, we found that the primary parameter controlling the X-ray bolometric corrections is the Eddington ratio. Our results show that, while the X-ray bolometric correction increases with the bolometric luminosity for sources with intermediate Eddington ratios (0.01-1), this dependence vanishes for sources with lower Eddington ratios (<0.01). This could be used as evidence for a change in the accretion physics of AGN at low Eddington ratios.

The origin of compact millimeter (mm) continuum emission from radio-quiet active galactic nuclei (RQAGNs) remains not fully understood. Changing-state AGNs (CSAGNs) exhibit rapid and strong variability, which allows us to investigate the origin of the mm emission. We present here the results of a first study of the mm continuum variability of a CSAGN using archival ALMA band 6 (∼230 GHz) observations of NGC 1566 obtained from 2014 to 2023. We find a positive correlation between the mm and X-ray flux with an intrinsic scatter of 0.05 dex (1σ), suggesting a common origin. The mm spectral index (αmm) is found to be in the range of 0.13±0.38 to - 0.26±0.53, consistent with a compact optically thick synchrotron source. No significant correlation was found between the αmm and the mm flux. The mm/X-ray ratio also shows no clear link to the Eddington ratio but is higher in the low-accretion state. We discuss several scenarios about the origin of the mm emission in NGC 1566. We find that synchrotron emission in the magnetized X-ray corona appears to be the most probable origin of the mm emission, confirming that mm emission can be used as a tracer of AGN activity in RQAGNs.

We present the results of the broadband timing and spectral analysis of the poorly understood SMC pulsar RX J0032.9-7348 (=SXP 7.02) using NuSTAR and NICER observations during its X-ray brightening in 2024. Our timing analysis revealed a pulsation period of approximately 7.02 s in the X-ray light curve. The pulse profile obtained in the broad energy range is double-peaked and asymmetric in nature and shows moderate variation with the energy. An absorbed power-law model describes the 0.5-8 keV NICER spectra well. The 3-50 keV NuSTAR spectrum is best described with an absorbed power-law modified with a high-energy cutoff model. We find no evidence of iron or cyclotron line features in the energy spectrum. During our observation period, the 0.5-50 keV luminosity varies in the range of erg s. We also discuss the dependence of spectral parameters on the rotational phase of the pulsar through phase-resolved spectroscopy.

We present an extensive temporal and spectral study of the Seyfert 1 AGN Mrk 50 using 15 yr (2007-2022) of multiwavelength observations from XMM-Newton, Swift, and NuSTAR for the first time. From the timing analysis, we found that the source exhibited variability of ∼20% during the 2007 observation, which reduced to below 10% in the subsequent observations and became nonvariable in the observations from 2010 onward. From the spectral study, we found that the spectra are nearly featureless. Nondetection of absorption in the low-energy domain during the 15 yr of observation infers the absence of obscuration around the central engine, rendering the nucleus a “bare” type. A prominent soft X-ray excess below 2 keV was detected in the source spectrum during the observations between 2007 and 2010, which vanished during the later observations. To describe the nature of the soft excess, we use two physical models, such as warm Comptonization and blurred reflection from the ionized accretion disk. Both of the physical models explain the nature and origin of the soft excess in this source. Our analysis found that Mrk 50 accretes at a sub-Eddington accretion rate (λEdd = 0.13-0.02) during all of the observations used in this work.

Context. Broad emission lines in the UV/optical spectra of changing-look active galactic nuclei (CLAGNs) appear and disappear on timescales of months to decades. Aims. We investigate how changing-look (CL) transitions depend on several active galactic nucleus (AGN) parameters, such as the accretion rate, obscuration properties, and black hole mass. Methods. We studied a sample of 20 nearby optically identified CLAGNs from the BAT AGN Spectroscopic Survey (BASS) using quasi-simultaneous optical and X-ray observations taken in the last ∼40 years. Results. We find that for all CLAGNs, the transition is accompanied by a change in the Eddington ratio. The CL transitions are not associated with changes in the obscuration properties of the AGNs. CLAGNs are found to have a median Eddington ratio lower than that of the AGNs in the BASS sample in which CL transitions were not detected. The median transition Eddington ratio (the Eddington ratio at which an AGN changes its state) is found to be ∼0.01 for type 1 ↔ 1.8, 1.9, and 2 transitions, which is consistent with the hard ↔ soft state transition in black hole X-ray binaries. Most CL events are constrained to have occurred within 3-4 years, which is considerably shorter than the expected viscous timescale in AGN accretion disks. Conclusions. The transitions of the optical CLAGNs studied here are likely associated with state changes in the accretion flow, possibly driven by disk instability.

A fraction of active galactic nuclei (AGN) have double-peaked Hα, Hβ, and Mg II broad lines attributed to emission from rotating gas in the accretion disk. Using optical spectroscopy of a flux-limited sample of AGN selected via ultrahard X-rays from the BAT AGN Spectroscopic Survey (BASS), we systematically identify 71 double-peaked emitters (DPEs) among 343 broad-line AGN with redshifts 0.004 < z < 0.297 and X-ray luminosities of 40 < log L 2 − 10 KeV (erg s−1) < 45.7, and provide their best-fit accretion disk parameters. We find that ∼21% of X-ray-selected broad-line AGN are DPEs, consistent with rates previously reported for z < 0.2 broad-line AGN selected for strong optical variability in the Zwicky Transient Facility. 11 of 71 DPEs (15%) exhibited a single-peaked Gaussian component to the broad line profile in addition to the double-peaked disk profile. In this sample, DPEs have intrinsically higher masses by ∼0.4 dex and lower Eddington ratios by ∼0.3 dex than other broad-line AGN, and have a preference for elliptical host galaxies, higher X-ray luminosities, and higher [O i] λ6302 to narrow Hα flux ratios than other broad-line AGN. We find that DPEs are not segregated from non-DPE broad-line AGN in the Lbol versus MBH relation or their X-ray to radio luminosity ratios, and do not show a preference for intermediate Seyfert types over Seyfert 1s. We do not find differences in a wide range of multiwavelength properties when comparing DPEs to non-DPE broad-line AGN, including optical and mid-IR variability levels, Wide-field Infrared Survey Explorer colors, αox, the column density of neutral obscuring material NH, and the rate of changing-look events. We discuss the two populations in the context of multicomponent disk-wind models of the AGN broad-line region and consider how unrecognized contributions of disk emission to the broad lines may introduce biases in “virial” supermassive black hole mass estimates, with consequences for the inferred MBH-M* relation.

We present a comprehensive analysis of the timing and spectral properties of NGC 7314, a Seyfert 1.9 galaxy, using X-ray observations from XMM-Newton, NuSTAR, and RXTE/proportional counter array (PCA). The timing analysis reveals significant variability across different energy bands, with fractional variability values consistent with previous studies. The highly variable soft photons and comparatively less variable high-energy photons imply different origins of these two types. The soft energy photons come from a hot corona near the center, while the high-energy photons are produced by inverse Compton scattering of these primary X-ray photons in a hot plasma away from the central region. The spectral analysis employs various models to characterize the emission components. The results indicate the presence of a soft energy bump, Fe Kα line emission, and a prominent reflection component. The long-term RXTE/PCA data analysis reveals temporal variations in the photon index (Γ) and power-law flux, suggesting evolving emission properties over time. The signature of both broad and narrow Fe Kα emission line features suggested the broad, variable one coming from the accretion disk (∼10−5 pc), while the nonevolving narrow line cannot be well constrained. The absorption feature could originate in a highly ionized region, possibly closer to the broad-line region. The evolution of the inner accretion properties indicates that NGC 7314 could be a potential changing-state active galactic nucleus.

Soft X-ray transients (SXTs) are a subclass of the low-mass X-ray binaries that occasionally show a sudden rise in their soft X-ray luminosity; otherwise, they remain in an extremely faint state. We investigate the accretion properties of the SXT XTE J1856+053 during its 2023 outburst obtained by NICER and NuSTAR data in July. We present detailed results on the timing and spectral analysis of the X-ray emission during the outburst. The power spectral density shows no quasi-periodic oscillation features. The source’s spectrum on July 19 can be well fitted with a multicolor blackbody component, a power-law component, and a reflection component with a broadened iron emission line. NICER spectra can be well fitted by considering a combination of a blackbody and a power law. The source exhibits a transition within just 5 days from a soft state to an intermediate state during the outburst decline phase. The inner accretion disk has a low inclination angle (∼18°). The spectral analysis also suggests a high-spin (a > 0.9) black hole as the central accreting object.

A B S T R A C T We present a long-term X-ray study of a nearby Active Galactic Nucleus Mrk 6, utilizing observations from XMM–Newton, Suzaku, Swift, and NuSTAR observatories, spanning 22 years from 2001 to 2022. From timing analysis, we estimated variance, normalized variance, and fractional rms amplitude in different energy bands.The temporal study shows fractional rms amplitude (Fvar) below 10 per cent for the shorter time-scale (∼60 ks) and above 20 per cent for the longer time-scale (∼weeks). A complex correlation is observed between the soft (0.5–3.0 keV) and hard (3.0–10.0 keV) X-ray bands of different epochs of observations. This result prompts a detailed investigation through spectral analysis, employing various phenomenological and physical models on the X-ray spectra. Our analysis reveals a heterogeneous structure of the obscuring material surrounding Mrk 6. A partially ionized absorber exhibits a rapid change in location and extends up to the narrow-line regions or torus. In contrast, another component, located far from the central engine, remained relatively stable. During the observation period, the source luminosity in the 3.0–10.0 keV range varies between (3–15) × 1042 erg s−1

We studied the polarization properties of Cygnus X-1 in both hard and soft spectral states with imaging X-ray polarimetry explorer (IXPE) observations. The polarization degree is, and in the hard and soft states, respectively. The polarization angle is observed along the jet axis and remains the same in both states. Energy-dependent analysis revealed an increasing polarization with the energy. The observed polarization rules out the lamp-post corona and prefers a conical corona if the coronal geometry remains the same in the hard and soft states. The disc is not detected in the hard state but is found in the soft state in the IXPE spectra. The spectral analysis revealed that the disc emission contributes of the total emission in the soft state. The scattering of the seed photons inside the corona is likely to reduce the polarization. This could explain the different polarization in different spectral states. Additionally, if the disc polarization is perpendicular to that of the corona, it could also reduce the polarization in the soft state.

We present the results obtained from the spectral studies of black hole X-ray binary GX 339−4 using AstroSat observations during its 2021 outburst. AstroSat observed the source in the intermediate state for ∼600 ks. The combined spectra of SXT and LAXPC in the 0.7−25 keV energy range are studied with phenomenological and physical models. The spectral study reveals a receding disc and a contracting corona during the observation period. The outflow rate is found to be increased though the accretion rates did not vary during the observation period. The X-ray flux decreases as the disc recedes and the spectrum becomes hard. At the same time, the Comptonized flux decreases with increasing fraction of thermal emission. This could be plausible that episodic jet ejection modified the corona and reduced Comptonized flux. An iron emission line at 6.4 keV is observed in the spectra of all the orbits of observation. We find that the equivalent width of the iron emission line correlates with the photon index, indicating a decrease in the reflection strength as the spectrum becomes hard. We observe that the disc flux does not follow FDBB − T4 relation.

We performed a detailed broad-band spectral and timing analysis of a small flaring event of ∼120 ks in the narrow-line Seyfert 1 galaxy NGC 4051 using simultaneous XMM-Newton and NuSTAR observations. The ∼300 ks long NuSTAR observation and the overlapping XMM-Newton exposure were segregated into pre-flare, flare, and post-flare segments. During the flare, the NuSTAR count rate peaked at 2.5 times the mean count rate before the flare. Using various physical and phenomenological models, we examined the 0.3-50 keV X-ray spectrum, which consists of a primary continuum, reprocessed emission, warm absorber and ultrafast outflows at different time-scales. The mass of the central black hole is estimated to be ≥1.32 × 105 M⊙ from spectral analysis. The absence of correlation between the flux in the 6-7 keV and 10-50 keV bands suggests different origins of the iron emission line and the Compton hump. From the spectral analysis, we found that the reflection fraction drops significantly during the flare, accompanied by an increase in the coronal height above the disc. The spectrum became soft during the flare, supporting the 'softer when brighter' nature of the source. After the alleviation of the flare, the coronal height drops and the corona heats up. This indicates that there could be inflation of the corona during the flare. We found no significant change in the inner accretion disc or the seed photon temperature. These results suggest that the flaring event occurred due to a change in coronal properties rather than any notable change in the accretion disc.

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 the release of the leftover matter from the pile-up radius due to a sudden rise of viscosity causing a rebrightening. We show a possible configuration of the evolution of accretion flow during the outburst. From the spectral analysis with TCAF model, we estimate the probable mass of the black hole to lie in the range 6.31 M⊙ to 13.65 M⊙, and the source distance is estimated to be 1.9-8.3 kpc from transition luminosity considerations.

We report results obtained from the optical and X-ray studies of the Be/X-ray binary 1A 0535+262/HD 245770 during the 2020 October giant X-ray outburst, using the 1.2-m telescope at Mount Abu Infrared observatory and AstroSat, respectively. The peak flux of the outburst was recorded to be ∼11 Crab in the 15–50 keV range, the highest ever observed from the pulsar. We performed optical observations in the 6000–7200 Å band before, during, and after the outburst to investigate the evolution of the circumstellar disc of the Be star between 2020 February and 2022 February. Our optical spectra exhibit prominent emission lines at 6563 Å (H I), 6678 Å (He I), and 7065 Å (He I). We found a significantly variable H α line in the spectra. The single-peaked line profile appeared asymmetric with broad red- and blue-wings in the data before and during the outburst. The post-outburst observations, however, resulted in a double-peaked profile with asymmetry in the blue-wing. Our observations before the outburst confirmed a larger Be disc that decreased in size as the outburst progressed. Furthermore, the observed variabilities in the H α line profile and parameters suggest the presence of a highly misaligned, precessing, and warped Be disc. AstroSat observation of the pulsar detected pulsations at ∼103.55 s in the light curve up to 110 keV. We found strongly energy-dependent pulse profiles with increasing contribution of the pulsing component in hard X-rays. The broad-band spectral fitting in the 0.7–90.0 keV range confirmed the presence of the known cyclotron resonance scattering feature at ∼46.3 keV.

We studied the broad-band X-ray spectra of Swift/Burst Alert Telescope selected low-Accreting active galactic nuclei (AGNs) using the observations from XMM-Newton, Swift, and NuSTAR in the energy range of 0.5-150 keV. Our sample consists of 30 AGNs with Eddington ratio, ?Edd < 10-3. We extracted several coronal parameters from the spectral modelling, such as the photon index, hot electron plasma temperature, cutoff energy, and optical depth. We tested whether there exist any correlation/anticorrelation among different spectral parameters. We observe that the relation of hot electron temperature with the cutoff energy in the low accretion domain is similar to what is observed in the high accretion domain. We did not observe any correlation between the Eddington ratio and the photon index. We studied the compactness-Temperature diagram and found that the cooling process for extremely low-Accreting AGNs is complex. The jet luminosity is calculated from the radio flux, and observed to be related to the bolometric luminosity as, which is consistent with the stand ard radio-X-ray correlation.

Black hole X-ray binaries (BHXRBs) play a crucial role in understanding the accretion of matter onto a black hole. Here, we focus on exploring the transient BHXRB Swift J1728.9-3613 discovered by Swift/BAT and MAXI/GSC during its January 2019 outburst. We present measurements on its accretion properties, long time-scale variability, and spin. To probe these properties, we make use of several NICER observations and an unexplored data set from NuSTAR, as well as long-term light curves from MAXI/GSC. In our timing analysis, we provide estimates of the cross-correlation functions between light curves in various energy bands. In our spectral analysis, we employ numerous phenomenological models to constrain the parameters of the system, including flavours of the relativistic reflection model Relxill to model the Fe Kα line and the >15 keV reflection hump. Our analysis reveals that: (i) Over the course of the outburst, the total energy released was ∼5.2 × 1044 ergs, corresponding to roughly 90 per cent of the mass of Mars being devoured. (ii) We find a continuum lag of 8.4 ± 1.9 d between light curves in the 2-4 and 10-20 keV bands, which could be related to the viscous inflow time-scale of matter in the standard disc. (iii) Spectral analysis reveals a spin parameter of ∼0.6-0.7 with an inclination angle of ∼45°-70° and an accretion rate during the NuSTAR observation of.

We analyze a sample of 21 “bare” Seyfert 1 active galactic nuclei, a subclass of Seyfert 1 galaxies, with intrinsic absorption N H ∼ 1020 cm−2, in the local Universe (z < 0.2) using XMM-Newton and Swift/XRT observations. The luminosities of the primary continuum, the X-ray emission in the 3-10 keV energy range, and the soft excess—the excess emission that appears above the low-energy extrapolation of the power-law fit of 3-10 keV X-ray spectra—are calculated. Our spectral analysis reveals that the long-term intrinsic luminosities of the soft excess and the primary continuum are tightly correlated ( L PC ∝ L SE 1.1 ± 0.04 ) . We also found that the luminosities are correlated for each source. This result suggests that both the primary continuum and soft excess emissions exhibit a dependency on the accretion rate in a similar way.

The black hole X-ray binary GX 339-4 showed an X-ray outburst during 2021. The AstroSat captured this outburst when the source entered into the intermediate flux state while the count rate was declining. The source showed an alternating flux profile in a time-scale of ≤100 ks, where the hard energy band was more variable than the soft band. The energy-dependent timing study showed that the observed quasi-periodic oscillation (QPO) was prominent in the low-energy bands, with its nearly sub-harmonic and harmonic components. These components appear and disappear with time, as observed in the orbit-wise QPO study. The Q-value, fractional rms, and 4.8-5.6 Hz frequency infer the QPOs as type-B and the spectral state as soft intermediate. The rms spectra of all orbits exhibiting QPOs show an increase in amplitude till ∼10 keV, beyond which it starts decreasing. This may indicate that ∼10 keV photons contributed relatively more in QPOs than other energy band photons. The Lorentzian normalization of the type-B QPO in different energy bands is consistent with the 10 keV peak. The energy-dependent time lag is complex and could be associated with the Comptonizing corona or jet. Finally, we discuss possible reasons behind the origin of different timing properties observed.

Black hole X-ray binaries (BHXBs) show rich phenomenology in the spectral and timing properties. We collected the spectral data of 20 BHXBs from the literature across different spectral states. The spectral properties are studied in the forms of the inner disc temperature (Tin), photon index (Γ), hot electron temperature (kTe), X-ray flux (FX), and luminosity (LX). We studied various correlations among different spectral parameters to understand the accretion process on a global scale. In the thermal soft states (TSS), we find most of the sources followed Fdisc ∝ Tin4 relation. A 'V'-shaped correlation is found between Γ and total luminosity (Ltot) in the hard Comptonized state (HCS). The Comptonized luminosity is observed to be correlated with the disc luminosity in the HCS and TSS. No notable correlation is observed in the intermediate state (IMS). The evolution of the inner disc radius (Rin) is unclear in the HCS and IMS. We also discuss how the hot electron temperature changes with other spectral parameters. We observe that the iron line flux correlates with disc and Comptonized fluxes. The strength of the reprocessed emission is found to vary across spectral states.

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 present the results of a broadband (0.5-78 keV) X-ray spectral study of the persistent Galactic black hole X-ray binary GRS 1758-258 observed simultaneously by Swift and NuSTAR. Fitting with an absorbed power-law model revealed a broad Fe line and reflection hump in the spectrum. We used different flavors of the relativistic reflection model for the spectral analysis. All models indicate the spin of the black hole in GRS 1758-258 is >0.92. The source was in the low hard state during the observation, with the hot electron temperature of the corona estimated to be kT e ∼ 140 keV. The black hole is found to be accreting at ∼1.5% of the Eddington limit during the observation, assuming the black hole mass of 10 M ⊙ and distance of 8 kpc.

We present the results obtained from timing and spectral studies of the highly obscured low luminosity active galactic nucleus NGC 4941 using data obtained from the nuclear spectroscopic telescope array and the Neil Gehrels Swift observatories. We find similar variability in 3–10 keV and 10–60 keV energy ranges with fractional rms variability of ∼ 14%. We investigate broad-band spectral properties of the source in 3–150 keV range, using data from NuSTAR and Swift/BAT, with phenomenological slab model and physically motivated mytorus model. From the spectral analysis, we find heavy obscuration with global average column density of the obscured material as 3.09-1.01+1.68×1024cm - 2. Evidence of a strong reflection component is observed in the spectrum. We detect a strong iron line with equivalent width of ∼ 1 keV. From the slab model, we obtain the exponential cutoff energy as 177-16+92 keV. From this, we estimate the Compton cloud properties with the hot electron temperature kTe=59-5+31 keV and the optical depth τ=2.7-1.6+0.2.

We present a detailed study of the highly obscured active galaxy NGC 4507, performed using four Nuclear Spectroscopic Telescope Array (NuSTAR) observations carried out between May and August in 2015 (∼130 ks in total). Using various phenomenological and physically motivated torus models, we explore the properties of the X-ray source and those of the obscuring material. The primary X-ray emission is found to be non-variable, indicating a stable accretion during the period of the observations. We find the equatorial column density of the obscuring materials to be ∼2 × 1024 cm-2 while the line-of-sight column density to be ∼7-8 × 1023 cm-2. The source is found to be deeply buried with the torus covering factor of ∼0.85. We observe variability in the line-of-sight column density on a time-scale of <35 d. The covering factor of the Compton-Thick material is found to be ∼0.35 in agreement with the results of recent X-ray surveys. From the variability of the line-of-sight column density, we estimate that the variable absorbing material is likely located either in the BLR or in the torus.

We present the results obtained from broad-band X-ray timing and spectral analysis of black hole (BH) candidate MAXI J1803-298 using an AstroSat observation on 2021 May 11-12. Four periodic absorption dips with a periodicity of 7.02 ±0.18 h are detected in the light curve. AstroSat observe the source when it was undergoing a transition from hard-intermediate state to soft-intermediate state. Our timing analysis reveals the presence of a sharp Type-C quasi-periodic oscillation (QPO) in the power-density spectra (PDS) with an evolving QPO frequency ranging from 5.31 ±0.02 to 7.61 ±0.09 Hz. We investigate the energy dependence of the QPO and do not find this feature in the PDS abo v e 30 keV. The combined 0.7-80 keV SXT and LAXPC spectra are fitted with a model consisting of thermal multi-colour blackbody emission and Comptonized emission components. We perform time-resolved spectroscopy by extracting spectra during the dip and non-dip phases of the observation. A neutral absorber is detected during the dip and non-dip phases though a signature of an ionized absorber is also present in the dip phases. The spectral and temporal parameters are found to evolve during our observation. We estimate the mass function of the system as f ( M ) = 2.1-7.2 M ⊙and the mass of the BH candidate in the range of M BH ∼3.5-12.5 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 ${lt}5{{ rm per cent}}$ is found in the softer spectral states. In the hard-intermediate and hard states, high variability with the fractional rms amplitude of ${gt}20{{ rm per cent}}$ is observed. The 0.7-10 keV spectra with NICER are studied with a combined disc-black-body 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.

The 2015 Outburst of V404 Cygni is an unusual one with several X-ray and radio flares and rapid variation in the spectral and timing properties. The outburst occurred after 26 years of inactivity of the black hole. We study the accretion flow properties of the source during its initial phase of the outburst using Swift/XRT and Swift/BAT data in the energy range of 0.5–150 keV. We have done spectral analysis with the two component advective flow (TCAF) model fits file. Several flow parameters such as two types of accretion rates (Keplerian disk and sub-Keplerian halo), shock parameters (location and compression ratio) are extracted to understand the accretion flow dynamics. We calculated equipartition magnetic field Beq for the outburst and found that the highest Beq ∼ 900 Gauss. Power density spectra (PDS) showed no break, which indicates no or very less contribution of the Keplerian disk component, which is also seen from the result of the spectral analysis. No signature of prominent quasi-periodic oscillations (QPOs) is observed in the PDS. This is due to the non-satisfaction of the condition for the resonance shock oscillation as we observed mismatch between the cooling timescale and infall timescale of the post-shock matter.

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+0.5 −0.7M ⊙, with 90% confidence.

We present the results obtained from detailed timing and spectral studies of a black hole candidate MAXI J1813-095 using Swift, NICER, and NuSTAR observations during its 2018 outburst. The timing behavior of the source is mainly studied by examining NICER light curves in the 0.5-10 keV range. We did not find any signature of quasi-periodic oscillations in the power density spectra of the source. We carry out spectral analysis with a combined disk blackbody & power law model, and physical two-component advective flow (TCAF) model. From the combined disk blackbody & power-law model, we extracted thermal and non-thermal fluxes, photon index and inner disk temperature. We also find evidence for weak reflection in the spectra. We have tested the physical TCAF model on a broadband spectrum from NuSTAR and Swift/XRT. The parameters like mass accretion rates, the size of Compton clouds and the shock strength are extracted. Our result affirms that the source remained in the hard state during the entire outburst which indicates a 'failed' outburst. We estimate the mass of the black hole as 7.4 ± 1.5 M o˙ from the spectral study with the TCAF model. We apply the LAOR model for the Fe Kα line emission. From this, the spin parameter of the black hole is ascertained as a∗ > 0.76$ The inclination angle of the system is estimated to be in the range of 28° - 45° from the reflection model. We find the source distance to be ∼ 6 kpc.

Galactic black hole candidate GX 339-4 underwent several outbursting phases in the past two and a half decades at irregular intervals of 2-3 years. The nature of these outbursts in terms of the duration, number of peaks, maximum peak intensity, and so on varies. We present a possible physical reason behind the variation of the outbursts. From a physical point of view, if the supply of matter from the companion is roughly constant, the total energy released in an outburst is expected to be proportional to the quiescent period prior to the outburst when the matter is accumulated. We use archival data of RXTE/ASM from 1996 January to 2011 June and of MAXI/GSC from 2009 August to 2020 July. Five initial outbursts of GX 339-4 between 1997 and 2011 were observed by ASM and showed a good linear relation between the accumulation period and the amount of energy released in each outburst, but the outbursts after 2013 behaved quite differently. The 2013, 2017-2018, and 2018-2019 outbursts were of short duration and incomplete or "failed"in nature. We suggest that the matter accumulated during the quiescent periods prior to these outbursts was not cleared through accretion due to a lack of viscosity. The leftover matter was cleared in the very next outbursts. Our study thus sheds light on long-term accretion dynamics in outbursting sources.

We performed a detailed spectral and timing analysis of a Seyfert 1 galaxy Mrk 509 using data from the Neil Gehrels Swift observatory that spanned over13 years between 2006 and 2019. To study the variability properties from the optical/UV to X-ray emission, we used a total of 275 pointed observations in this work. The average spectrum over the entire duration exhibits a strong soft X-ray excess above the power law continuum. The soft X-ray excess is well described by two thermal components with temperatures of kTBB1 120 eV and kTBB2 460 eV. The warmthermal component is likely due to the presence of an optically thick and warmComptonizing plasma in the inner accretion disk. The fractional variability amplitude is found to be decreasing with increasing wavelength, i.e., from the soft X-ray to UV/optical emission. However, the hard X-ray (2-8 keV) emission shows very low variability. The strength of the correlation within the UV and the optical bands (0.95-0.99) is found to be stronger than the correlation between the UV/optical and X-ray bands (0.40-0.53). These results clearly suggest that the emitting regions of the X-ray and UV/optical emission are likely distinct or partly interacting. Having removed the slow variations in the light curves, we find that the lag spectrum is well described by the 4/3 rule for the standard Shakura-Sunyaev accretion disk when we omit X-ray lags. All these results suggest that the real disk is complex, and the UV emission is likely reprocessed in the accretion disk to give X-ray and optical emission.

We study the nature of the changing-look active galactic nucleus NGC 1566 during its 2018 June outburst. During the outburst, the X-ray intensity of the source rises up to ∼25-30 times compared to its quiescent state intensity. We perform timing and spectral analysis of the source during pre-outburst, outburst, and post-outburst epochs using semisimultaneous observations with the XMM-Newton, Nuclear Spectroscopic Telescope Array(NuSTAR), and Neil Gehrels Swift Observatories. We calculate variance, normalized variance, and fractional rms amplitude in different energy bands to study the variability. The broad-band 0.5-70 keV spectra are fitted with phenomenological models, as well as physical models. A strong soft X-ray excess is detected in the spectra during the outburst. The soft-excess emission is found to be complex and could originate in the warm Comptonizing region in the inner accretion disc. We find that the increase in the accretion rate is responsible for the sudden rise in luminosity. This is supported by the 'q'-shape of the hardness-intensity diagram that is generally found in outbursting black hole X-ray binaries. From our analysis, we find that NGC 1566 most likely harbours a low-spinning black hole with the spin parameter a∗ ∼0.2. We also discuss a scenario where the central core of NGC 1566 could be a merging supermassive black hole.

Galactic short orbital period black hole candidate (BHC) XTE J1752-223 was discovered on 2009 October 21 by the Rossi X-ray Timing Explorer (RXTE). We study the spectral properties of this outburst using transonic flow solution based two component advective flow (TCAF) model. TCAF model fitted spectrum gives an estimation of the physical flow parameters, such as the Keplerian disc rate, sub-Keplerian halo rate, properties of the so-called Compton cloud, other than the mass of the source and normalization (N). N is a standardized ratio of emitted to observed photon flux in TCAF that does not include X-ray emission from jets. In the presence of jets, this ratio changes and this deviation is used to obtain the estimation of X-ray contribution from the jets. Nature of the jet is found to be compact during low luminous hard state and discrete or blobby during high luminous intermediate states. We find a correlation between the radio (5.5 GHz) and X-ray (2.5-25 keV) fluxes from different components. The radio (FR) and jet X-ray (Fouf) fluxes are found to be correlated within the acceptable range of the standard correlation (0.6 to 0.7). A similar correlation indices were reported by our group for three other short orbital period transient BHCs (Swift J1753.5-0127, MAXI J1836-194, and XTE J1118+480).

Galactic transient black hole candidate (BHC) MAXI J1535-571 was discovered on 2017 September 02 simultaneously by MAXI/GSC and Swift/BAT instruments. It has also been observed by India’s first multi-wavelength astronomy-mission satellite AstroSat, during the rising phase of its 2017-18 outburst. We make both the spectral and the temporal analysis of the source during 2017 September 12–17 using data of AstroSat’s Large Area X-ray Proportional Counter (LAXPC) in the energy range of 3–40 keV to infer the accretion flow properties of the source. Spectral analysis is done with the physical two-component advective flow (TCAF) solution-based fits file. From the nature of the variation of the TCAF model fitted physical flow parameters, we conclude and confirm that the source was in the intermediate spectral state during our analysis period. We observe sharp type-C quasi-periodic oscillations (QPOs) in the frequency range of ∼ 1.75 – 2.81 Hz. For a better understanding of the nature and evolution of these type-C QPOs, a dynamic study of the power density spectra is done. We also investigate the origin of these QPOs from the shock oscillation model. We find that non-satisfaction of Rankine-Hugoniot conditions for non-dissipative shocks and not their resonance oscillations is the cause of the observed type-C QPOs.

We present the results obtained from timing and spectral studies of Be/X-ray binary pulsar EXO 2030+375 using observations with the Large Area Xenon Proportional Counters and Soft X-ray Telescope of AstroSat, at various phases of its Type-I outbursts in 2016, 2018, and 2020. The pulsar was faint during these observations as compared to earlier observations with other observatories. At the lowest luminosity of 2.5× 1035 erg s- 1 in 0.5–30 keV energy range, ≈ 41.3 s pulsations were clearly detected in the X-ray light curves. This finding establishes the first firm detection of pulsations in EXO 2030+375 at an extremely low mass accretion rate to date. The shape of the pulse profiles is complex due to the presence of several narrow dips. Though pulsations were detected up to ∼ 80 keV when the source was brighter, pulsations were limited up to ∼ 25 keV during the third AstroSat observation at lowest source luminosity. A search for quasi-periodic oscillations in 2 × 10 - 4 Hz to 10 Hz yielded a negative result. Spectral analysis of the AstroSat data showed that the spectrum of the pulsar was steep with a power-law index of ∼ 2. The values of photon-indices at observed low luminosities follow the known pattern in sub-critical regime of the pulsar.

We present the results obtained from analysis of two AstroSat observations of the high mass X-ray binary pulsar OAO 1657-415. The observations covered 0.681–0.818 and 0.808–0.968 phases of the ∼ 10.4 day orbital period of the system, in March and July 2019, respectively. Despite being outside the eclipsing regime, the power density spectrum from the first observation lacks any signature of pulsation or quasi-periodic oscillations. However, during July observation, X-ray pulsations at a period of 37.0375 s were clearly detected in the light curves. The pulse profiles from the second observation consist of a broad single peak with a dip-like structure in the middle across the observed energy range. We explored evolution of the pulse profile in narrow time and energy segments. We detected pulsations in the light curves obtained from 0.808–0.92 orbital phase range, which is absent in the remaining part of the observation. The spectrum of OAO 1657-415 can be described by an absorbed power-law model along with an iron fluorescent emission line and a blackbody component for out-of-eclipse phase of the observation. Our findings are discussed in the frame of stellar wind accretion and accretion wake at late orbital phases of the binary.

We present the results obtained from detailed spectral and timing studies of extragalactic black hole X-ray binaries LMC X-1 and LMC X-3, using simultaneous observations with Nuclear Spectroscopic Telescope Array (NuSTAR) and Neil Gehrels Swift observatories. The combined spectra in the 0.5-30 keV energy range, obtained between 2014 and 2019, are investigated for both sources. We do not find any noticeable variability in 0.5-30 keV light curves, with 0.1-10 Hz fractional rms estimated to be <2 per cent. No evidence of quasi-periodic oscillations is found in the power density spectra. The sources are found to be in the high soft state during the observations with disc temperature Tin ∼1 keV, photon index, Γ > 2.5 and thermal emission fraction, fdisc > 80 per cent. An Fe Kα emission line is detected in the spectra of LMC X-1, though no such feature is observed in the spectra of LMC X-3. From the spectral modelling, the spins of the black holes in LMC X-1 and LMC X-3 are estimated to be in the range of 0.92-0.95 and 0.19-0.29, respectively. The accretion efficiency is found to be, η ∼0.13 and η ∼0.04 for LMC X-1 and LMC X-3, respectively.

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 4.50–5.93 M⊙ 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.

We present the results obtained from a detailed X-ray timing and spectral analysis of the Seyfert 2 galaxy NGC 6300 by using observations from the Suzaku observatory, theChandra X-ray Observatory and the Nuclear Spectroscopic Telescope Array(NuSTAR) mission between 2007 and 2016. We calculate the variance and the rms fractional variability of the source in different energy bands and we find variabilities in various energy bands. Spectral properties of the source are studied by using various phenomenological and physical models. The properties of the Compton clouds, reflection, Fe Kα line emission and soft X-ray excess are studied in detail. Several physical parameters of the source are extracted and investigated to establish the presence/absence of any correlation between them. We also investigate the nature of the circumnuclear 'torus' and we find that the torus is not uniform, but clumpy. The observed changes in the line-of-sight column density can be explained in terms of transiting clouds. The iron line-emitting region is found to be different in the different epochs of observations. We also observe that the torus and the nucleus independently evolve over the years.

We study spectral and temporal properties of Galactic short orbital period transient black hole XTE J1118+480 during its 2005 outburst using archival data of RXTE PCA and HEXTE instruments in the combined energy range of 3-100 keV. Spectral analysis with the physical two component advective flow (TCAF) model allows us to understand the accretion flow properties of the source. We found that this outburst of XTE J1118+480 is an unconventional outburst as the source was only in the hard state (HS). Our spectral analysis suggests that during the entire outburst, the source was highly dominated by the low angular momentum sub-Keplerian halo rate. Since the source was active in radio throughout the outburst, we make an effort to estimate X-ray contribution of jets to total observed X-ray emissions from the spectral analysis with the TCAF model. The total X-ray intensity shows a similar nature of evolution as that of radio and jet X-ray fluxes. This allowed us to define this 'outburst' also as a jet dominated 'outburst'. Total X-ray flux is also found to subside when jet activity disappears. Our detailed spectral analysis also indicated that although the source was only in the HS during the outburst, in the late declining phase the spectrum became slightly softer due to the slow rise in the Keplerian disk rate.

Galactic black hole (BH) candidate MAXI J1348-630 was recently discovered by MAXI and Swift/BAT satellites during its first outburst in 2019 January, which continued for ∼4 months. We study the spectral and timing properties of the source in detail. The combined 1-150 keV Swift/XRT, Swift/BAT, and MAXI/GSC spectra are investigated with the two-component advective flow (TCAF) solution. Physical flow parameters of TCAF, such as the Keplerian disk accretion rate, the sub-Keplerian halo accretion rate, the shock location, and the shock compression ratio, are estimated from our spectral fits. Based on the variation of flux in soft and hard X-ray ranges, the hardness ratio, TCAF model fitted accretion rates, and the accretion rate ratio, we show how the source evolved through four spectral states, viz., hard, hard-intermediate, soft-intermediate, and soft, in rising and declining states. Low-frequency quasi-periodic oscillations are observed in two observations during the rising phase of the outburst. From the spectral analysis, we estimate the mass of the BH to be 9.1-1.2+1.6 M o˙. We also find that the viscous timescale in this outburst is ∼3.5 days. The distance of the source is also estimated as 5-10 kpc from state transition luminosity.

Spectral and timing properties of the stellar-mass black hole candidate XTE J1752-223 during its 2009-10 outburst are studied using RXTE PCA data in the 2.5-25 keV energy range. Low frequency quasi-periodic oscillations are seen during outburst. The spectral analysis is done using two types of models: one is the combined disc blackbody plus power-law model and the other is Transonic flow solution based Two Component Advective Flow (TCAF) model. Light-curve profiles and evolution of hardness ratios are studied using MAXI GSC and Swift BAT data. Based on the evolution of the temporal and the spectral properties, we find that the object evolved through the following spectral states: hard, hard-intermediate, and soft-intermediate/soft. From the TCAF model fitted spectral analysis, we also estimate the probable mass of the black hole in the range of 8.1-11.9 M, and more precisely, the mass appears to be 10 ± 1.9 M.

Galactic transient black hole candidate (BHC) MAXI J1836-194 was discovered on 2011 Aug 30, by MAXI/GSC and Swift/BAT. The source activity during this outburst continued for ∼ 3 months before entering into the quiescent state. It again became active in March 2012 and continued for another ∼ 2 months. In this paper, 3-25 keV RXTE/PCA spectra from the 2011 outburst and 0.5-10.0 keV Swift/XRT data during its 2012 outburst are analyzed with the two-component advective flow (TCAF) model based fits files in XSPEC. We calculate the X-ray contributions coming from jets/outflow using a newly developed method based on the deviation of the TCAF model normalization. We also study the correlation between observed radio and estimated jet X-ray fluxes. The correlation indices (b) are found to be 1.79 and 0.61, when the 7.45GHz Very Large Array (VLA) radio flux is correlated with the total X-ray and jet X-ray fluxes in 3-25 keV range respectively. It has been found that the jet contributes in X-rays up to a maximum of 86% during its 2011 outburst. This makes the BHC MAXI J1836-194 strongly jet dominated during the initial rising phase.

The galactic black hole candidate (BHC) XTE J1118+480 during its 2000 outburst has been studied in a broad energy range using the archival data of PCA and HEXTE payloads of Rossi X-ray Timing Explorer. Detailed spectral and temporal properties of the source are studied. Low and very low frequency quasi-periodic oscillations (QPOs), with a general trend of increasing frequency are observed during the outburst. Spectral analysis is done using the combined data of the PCA and HEXTE instruments with two types of models: the well-known phenomenological power-law model and the current version of the fits file of two-component advective flow (TCAF) solution as an additive table model in XSPEC. During the entire period of the outburst, a non-thermal power-law component and the TCAF model fitted to the sub-Keplerian halo rate were found to be highly dominant. We suggest that this so-called outburst is due to enhanced jet activity. Indeed, the ‘outburst’ subsides when this activity disappears. We estimated the X-ray fluxes coming from the base of the jet and found that the radio flux is correlated with this X-ray flux. Though the object was in the hard state in the entire episode, the spectrum becomes slightly softer with the rise in the Keplerian disk rate in the late declining phase. We also estimated the probable mass of the source from our spectral analysis with the TCAF solution. Our estimated mass of XTE J1118+480 is 6.99−0.74+0.50M⊙ i.e., in the range of 6.25–7.49M⊙.

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 o or 8.9 ± 1.0 M o.

It is well established that the accretion disk and jet are coupled. Jet and accretion disk arise self-consistently from any transonic flow solution and clearly from its generalized version, the two-component advective flow (TCAF) solution. Observationally, strong signatures of disk-jet connections are seen for many black hole binaries. The nature and evolution of jets depend on accretion flows. Correlation between X-ray and radio fluxes is also seen, which indicates a coupling between accretion disk and jet. In this review, we discuss developments of theoretical models to describe the origin of the jets or outflows and the nature of the disk-jet coupling on the basis of the transonic flows around black holes. Then we discuss the observational evidences of the disk-jet connections.

The inner region of a transient source is cooled down by the inverse Comptonization of soft photons from the Keplerian disk component. The reduction of pressure forces the outer boundary of the Compton cloud, namely, the centrifugal pressure supported shock to move inward slowly in order to satisfy Rankine-Hugoniot conditions. We consider the transient source H1743-322 to study this movement of the shock. The presence of cooling changes the geometry of the Compton cloud gradually. We also see how the flow parameters of this source change day by day during a complete outburst. As the shock-oscillation could also modulate harder X-rays, we want to resolve the question: are QPOs originated from shock oscillations and can the time variation of the QPO frequency be explained by this slow propagation of the shock? For that we solve the Rankine-Hugoniot conditions and derive the condition of shock formation in presence of Compton cooling. We also compute inward velocity of the shock Compton cloud to be a few meters per second, which agrees well with earlier observational results.

Transient black hole candidate MAXI J1659-152 showed rapid spectral and temporal evolution during its very first outburst. Our understanding about accretion flow dynamics around black hole candidates has improved much more after the inclusion of Chakrabarti-Titarchuk Two Component Advective Flow (TCAF) model as an additive table model in XSPEC. In this paper we make a detail study of temporal and spectral properties of black hole candidate MAXI J1659-152 during its 2010 outburst with TCAF and POS model. From our fit, we extract accretion flow parameters (Keplerian disk and sub-Keplerian halo rates, shock location, shock strength). We find mass of the object to be 4.17-7.74 M.

To understand accretion flow dynamics around black hole candidates (BHCs) one needs to study spectral as well as temporal features in details. After the inclusion of Chakrabarti-Titarchuk two Component Advective Flow (TCAF) model into HEASARC's spectral analysis package XSPEC as a local additive table model, we found that it is quite capable of fitting spectra from different phases of few transient black hole candidates (TBHCs) during their X-ray outbursts. From spectral fits with TCAF model, one can directly extract physical flow parameters, such as two types of accretion (Keplerian disk and sub-Keplerian halo) rates and shock parameters (location and strength of the shock). A much better understanding of spectral and timing properties is achieved by studying evolution of these physical flow parameters during the outbursts of TBHCs. One can also predict frequency of primary dominating QPOs from TCAF fitted shock parameters. Based on a comparison of halo to disk accretion rate ratio (ARR) with quasi-periodic oscillation (QPOs; if present) frequencies, a physical understanding of the classification of the entire outburst phase of the BHCs into different spectral states emerges. We conclude that TCAF in XSPEC, provides us with a better tool to understand accretion flow dynamics during the outbursts of TBHCs.

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.

ASTROSAT is India's first multi-wavelength astronomy satellite, was successfully launched on 2015 September 28 by PSLV in a ∼ 650 km near-equatorial orbit. It can be used to study astronomical objects in a wide range of electromagnetic energy band from UV to hard X-rays. With a very high spectral, timing as well as spacial resolutions from different scientific instruments of the satellite, one can make a detailed spectral and timing study of transient black hole candidates (BHCs) during their outbursts. Recently, we have included Chakrabarti-Titarchuk (1995) Two-Component Advective Flow (TCAF) model in HEASARC's spectral analysis package XSPEC as a local additive table model to fit black hole with the model, and we found that it is quite capable of explaining both the spectral and temporal properties of BHCs very successfully. Multi-wavelength data of ASTROSAT is expected to provide some useful data to understand spectral as well as temporal variability of transient BHCs using TCAF solution. Here, we discuss various aspects of TCAF which can be verified from ASTROSAT data.

Galactic black hole candidate Swift J1753.5-0127 was discovered on 2005 June 30 by the Swift Burst Alert Telescope. We study the accretion flow properties during its very first outburst through careful analysis of the evolution of the spectral and the temporal properties using the two-component advective flow (TCAF) paradigm. RXTE proportional counter array spectra in are fitted with the current version of the TCAF model fits file to estimate physical flow parameters, such as two-component (Keplerian disk and sub-Keplerian halo) accretion rates, properties of the Compton cloud, probable mass of the source, etc. The source is found to be in harder (hard and hard-intermediate) spectral states during the entire phase of the outburst with very significant jet activity. Since in the TCAF solution the model normalization is constant for any particular source, any requirement of significantly different normalization to have a better fit on certain days would point to an X-ray contribution from components not taken into account in the current TCAF model fits file. By subtracting the contribution using actual normalization, we derive the contribution of X-rays from the jets and outflows. We study its properties, such as its magnitude and spectra. We find that on some days, up to about 32% of X-ray flux is emitted from the base of the jet itself.

Galactic X-ray binary black hole candidate Swift J1753.5-0127 was discovered on 2005 June 30 by the Swift/BAT instrument. In this paper, we make a detailed analysis of spectral and timing properties of its 2005 outburst using the archival data of the RXTE/PCA instrument. A simultaneous observation of Swift/XRT with PCA is also used to study the broadband features. Here, we study the evolution of the spectral properties of the source from spectral analysis with an additive table model fits file of the Chakrabarti-Titarchuk two-component advective flow (TCAF) solution. From the spectral fit, we extract physical flow parameters, such as the 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 the observation of low-frequency quasi-periodic oscillations (QPOs), and the 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 the entire 2005 outburst into two harder (hard-intermediate and hard) spectral states. No signatures of softer (soft-intermediate and soft) spectral states are seen. This may be because of a 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. We estimate the probable mass range of the source to be in between based on our spectral analysis.

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 (MBH) 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 (νQPO) 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 6-1.3+1.8 M⊙.

We derive accretion flow properties of the transient black hole candidate (BHC) MAXI J1543-564 using the RXTE data. We use the two-component advective flow (TCAF) solution to fit the data of the initial rising phase of outburst (from 2011 May 10 to 15). The 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) is 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 soft-intermediate spectral states during this phase of the outburst under study. We also calculated the 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 o, or .

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.