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Astronomical objects, such as the Sun, accreting matter around black holes, neutron stars, white dwarfs, etc. are known to emit X-rays and gamma-rays. Some of these sources, such as highly magnetized neutron stars, show periodic pulsations. Even accreting matter around black holes and neutron stars shows timing properties including quasi-periodic oscillations. In this paper, we use the data of low-cost, lightweight, high-altitude (∼42 km) small, stratospheric balloon-borne missions of the Indian Centre for Space Physics. We measure the intensity of the secondary cosmic rays, the radiation fluxes, and the spectra of persistent X-ray sources or transient events such as solar flares and gamma-ray bursts, apart from routine weather parameters. In this study, we present the source detection method without an onboard pointing system and the temporal and spectral behaviour of the quiet Sun. We also analyse the data containing Crab in the field of view to find the pulsation of the Crab pulsar. During the analysis of the source detection and spectrum of the solar X-rays, we calculate the detector's background radiation (mainly secondary cosmic rays) using physical assumptions and also take care of the atmospheric absorption effects. Finally, we show the source detection processes for the strong sources such as the Sun, Crab, and Cyg X-1, obtaining the quiet Sun's spectrum. Among the interesting timing properties, we present the result for the Crab pulsar and find the well-known ∼33 Hz pulsations whenever our instrument pointed to the Crab. [ABSTRACT FROM AUTHOR]

We present the long-term X-ray spectral and temporal analysis of a 'bare-type AGN' Ark 120. We consider the observations from XMM–Newton, Suzaku, Swift , and NuSTAR from 2003 to 2018. The spectral properties of this source are studied using various phenomenological and physical models present in the literature. We report (a) the variations of several physical parameters, such as the temperature and the optical depth of the electron cloud, the size of the Compton cloud, and the accretion properties for the last 15 yr. The spectral variations are explained from the changes in the accretion dynamics; (b) the X-ray time delay between 0.2–2 and 3–10 keV light-curves exhibited zero-delay in 2003, positive delay of 4.71 ± 2.1 ks in 2013, and negative delay of 2.90 ± 1.26 ks in 2014. The delays are explained considering Comptonization, reflection, and light-crossing time; (c) the long-term intrinsic luminosities, obtained using nthcomp , of the soft-excess and the primary continuum show a correlation with a Pearson correlation coefficient of 0.90. This indicates that the soft-excess and the primary continuum originate from the same physical process. From a physical model fitting, we infer that the soft excess for Ark 120 could be due to a small number of scatterings in the Compton cloud. Using Monte Carlo simulations, we show that indeed the spectra corresponding to fewer scatterings could provide a steeper soft-excess power law in the 0.2–3 keV range. Simulated luminosities are found to be in agreement with the observed values. [ABSTRACT FROM AUTHOR]

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⊙. [ABSTRACT FROM AUTHOR]

Using soft (1.5–3 keV) and hard (3–12 keV) photon counts of All Sky Monitor (ASM) in Rossi X-ray Timing Explorer (RXTE) satellite, we have proposed recently that there is a significant time lag between the infall time-scales of two components in the Two-Component Advective Flow paradigm, where a standard slow moving Keplerian disc is surrounded by a fast moving halo. The time lag is clearly due to the difference in viscosity in the flow components and the size of the Keplerian disc may be considered to be proportional to this arrival time lag. In this paper, using RXTE /ASM (1.5–12 keV) data, we examine eight successive outbursts of the low-mass X-ray binary H 1743-322 since 2003 from a new perspective. The day-to-day temporal evolution of a dynamic photon index, Θ, as well as its cross-correlation with the soft and hard energy fluxes show that the aforesaid time lag was the longest during the brightest outburst of 2003 – thereby indicating its largest Keplerian disc. The disc size diminished thereafter during subsequent weaker outbursts. Moreover, Θ decides spectral transitions of any outburst. We show from the behaviour of Θ alone that the outburst of October 2008 was anomalous while the outburst of 2003 was twin (anomalous + normal). In fact, each normal outburst was either preceded or followed by an otherwise premature outburst showing different degrees of anomaly. This makes H 1743-322 an enigmatic source and a subject of further study. [ABSTRACT FROM AUTHOR]

We analyse several outbursts of a few transient sources using Proportional Counter Array data (2.5–25 keV) as well as All Sky Monitor data (1.5–12 keV) of Rossi X-ray Timing Explorer satellite. We find a time delay between the arrival times of the Keplerian disc component and the halo of the Two-Component Advective Flow (TCAF) when the spectral data are fitted with TCAF solution. We compare this time delay from the spectral fits using the TCAF solution of the transient low-mass X-ray binaries (LMXBs) e.g. MAXI J1836−194, GX 339−4, and H 1743−322 during outbursts with that of the high-mass X-ray binary (HMXB) Cyg X-1 during its flare s. We find that several days of time delays are observed in LMXBs while for Cyg X-1 the delay is negligible. We interpret the long delay to be due to the viscous time-scale of a large Keplerian component to reach the inner region as compared to nearly free-fall time taken by the low angular momentum halo component. The delay is of the order of a week for the LMXBs where the feeding is primarily through the Roche lobe. However, it is negligible in a wind-fed system like Cyg X-1 since a very small Keplerian disc is created here by slowly redistributing the low angular momentum of the wind and sporadic soft or intermediate spectral states are observed. [ABSTRACT FROM AUTHOR]

We report a detailed spectral study of the Swift J1357.2–0933 low-mass X-ray binary during its 2017 outburst using Swift and NuSTAR observations. We fit the data with a two-component advective flow (TCAF) model and a power-law model. We observe that the source is in a hard state during the outburst, where the size of the Compton cloud changes significantly with disc accretion rate. The typical disc accretion rate for this source is |${\sim } 1.5\hbox{--}2.0\,{{\ \rm per\ cent}}$| of the Eddington accretion rate |$(\skew2\dot{M}_\mathrm{Edd})$|⁠. The model-fitted intermediate shock compression ratio gives an indication of the presence of a jet, which is reported in the literature in different energy bands. We also split the NuSTAR data into three equal segments and fit them with the model. We check spectral stability using a colour–colour diagram and an accretion rate ratio (ARR) versus intensity diagram using different segments of the light curve, but do not find any significant variation in the hardness ratio or in the accretion rate ratio. To estimate the mass of the candidate, we use an important characteristic of TCAF that the the model normalization always remains a constant. We found that the mass comes out to be in the range of 4.0–6.8 M⊙. From the model-fitted results, we study the disc geometry and different physical parameters of the flow in each observation. The count rate of the source appears to decay on a time-scale of |${\sim } 45\, \mathrm{d}$|⁠. [ABSTRACT FROM AUTHOR]

We study time evolution of sub-Keplerian transonic accretion flows on to black holes using a general relativistic numerical simulation code. We perform simulations around the black holes having non-zero rotation. We first compare one-dimensional simulation results with theoretical results and validate the performance of our code. Next, we present results of axisymmetric, two-dimensional simulation of advective flows. In the literature, there is no solution which describes steady shock solutions in two dimensions. However, our simulations produce these centrifugal force supported steady shock waves even in presence of strong dragging of inertial frames. Since the post-shock region could be hot and upscatter photons through Comptonization, these shock would put imprints on the spectra. Thus, our solutions, which represent truly new results, could be useful to measure spins through radiation spectrum of accreting Kerr black holes. [ABSTRACT FROM AUTHOR]

The evolution of the C-type low-frequency quasi-periodic oscillations (QPOs) and associated time lag in transient black hole sources as a function of time can be explained by variation of the Compton cloud size in a two-component advective flow solution. A similar study of a persistent source, GRS 1915+105, has not been attempted. We fit the evolution of QPOs with propagatory oscillating shock solution for two sets of so-called χ-state observations and find that the shock steadily recedes with almost constant velocity when QPO frequency is decreasing and the spectrum is hardening. The shock moves inwards swith constant velocities $v$ 0 = 473.0 cm s−1 and $v$ 0 = 400.0 cm s−1, respectively, in these two cases, when the QPO frequency is increasing and the spectrum softens. This behaviour is similar to what was observed in XTE J1550−564 during the 1998 outburst. The time lag measured at the QPO frequency varies in a similar way as the size of the Compton cloud. Most interestingly, in both the cases, the lag switches sign (hard lag to soft lag) at a QPO frequency of ∼2.3−2.5 Hz irrespective of the energy of photons. We find, at very low frequencies <1 Hz, the Comptonizing efficiency (CE) increases with QPO frequency and at higher QPO frequencies the trend is opposite. The time lags become mostly positive at all energies when CE is larger than ${\sim } 0.85{{\rm \,per\,cent}}$ for both the sources. [ABSTRACT FROM AUTHOR]

Two-component advective flow (TCAF) successfully explains the spectral and temporal properties of outbursting or persistent sources. Images of static TCAF with Compton cloud or centrifugal pressure supported boundary layer due to gravitational bending of photons have been studied before. In this paper, we study time-dependent images of advective flows around a Schwarzschild black hole, which include cooling effects due to Comptonization of soft photons from Keplerian discs as well as the self-consistently produced jets and outflows. We show the overall image of the disc-jet system after convolving with a typical beamwidth. A long exposure image with time-dependent system need not to show the black hole horizon conspicuously, unless one is looking at a soft state with no jet or the system along the jet axis. Assuming these disc-jet configurations are relevant to radio emitting systems also, our results would be useful to look for event horizons in high accretion rate supermassive black holes in Seyfert galaxies, RL Quasars. [ABSTRACT FROM AUTHOR]

X-ray outbursts of stellar-mass black hole candidates are believed to be due to a sudden rise in viscosity, which transports angular momentum efficiently and increases the accretion rates, causing higher X-ray flux. After the viscosity is reduced, the outburst subsides and the object returns back to the pre-outburst quiescence stage. In the absence of a satisfactory understanding of the physical mechanism leading to such a sharp time dependence of viscous processes, we perform numerical simulations where we include the rise and fall of a viscosity parameter at an outer injection grid, assumed to be located at the accumulation radius where matter from the companion is piled up before being released by enhanced viscosity. We use a power-law radial dependence of the viscosity parameter (α ~ rϵ ), but the exponent (ϵ) is allowed to vary with time to mimic a fast rise and decay of the viscosity parameter. Since X-ray spectra of a black hole candidate can be explained by a Keplerian disc component in the presence of a post-shock region of an advective flow, our goal here is also to understand whether the flow configurations required to explain the spectral states of an outbursting source could be obtained by a time-varying viscosity. We present the results of our simulations to prove that low-angular-momentum (sub-Keplerian) advective flows do form a Keplerian disc in the pre-shock region when the viscosity is enhanced, which disappears on a much longer time-scale after the viscosity is withdrawn. From the variation of the Keplerian disc inside an advective halo, we believe that our result, for the first time, is able to simulate the twocomponent advective flow dynamics during an entire X-ray outburst and explain the observed hysteresis effects in the hardness-intensity diagram. [ABSTRACT FROM AUTHOR]

X-ray time lag of black hole candidates contains important information regarding the emission geometry. Recently, study of time lags from observational data revealed very intriguing properties. To investigate the real cause of this lag behavior with energy and spectral states, we study photon paths inside a two-component advective flow (TCAF) which appears to be a satisfactory model to explain the spectral and timing properties. We employ the Monte Carlo simulation technique to carry out the Comptonization process. We use a relativistic thick disk in Schwarzschild geometry as the CENtrifugal pressure supported BOundary Layer (CENBOL) which is the Compton cloud. In TCAF, this is the post-shock region of the advective component. Keplerian disk on the equatorial plane which is truncated at the inner edge i.e. at the outer boundary of the CENBOL, acts as the soft photon source. Ray-tracing code is employed to track the photons to a distantly located observer. We compute the cumulative time taken by a photon during Comptonization, reflection and following the curved geometry on the way to the observer. Time lags between various hard and soft bands have been calculated. We study the variation of time lags with accretion rates, CENBOL size and inclination angle. Time lags for different energy channels are plotted for different inclination angles. The general trend of variation of time lag with QPO frequency and energy as observed in satellite data is reproduced. [ABSTRACT FROM AUTHOR]

We explore spectral properties of a two-component advective flow around a neutron star. We compute the effects of thermal Comptonization of soft photons emitted from a Keplerian disc and the boundary layer of the neutron star by the post-shock region of a sub-Keplerian flow, formed due to the centrifugal barrier. The shock location Xs is also the inner edge of the Keplerian disc. We compute a series of realistic spectra assuming a set of electron temperatures of the post-shock region TCE, the temperature of the Normal BOundary Layer (NBOL) TNS of the neutron star and the shock location Xs. These parameters depend on the disc and halo accretion rates (ṁd and ṁh, respectively) that control the resultant spectra. We find that the spectrum becomes harder when ṁh is increased. The spectrum is controlled strongly by TNS due to its proximity to the Comptonizing cloud since photons emitted from the NBOL cool down the post-shock region very effectively. We also show the evidence of spectral hardening as the inclination angle of the disc is increased. [ABSTRACT FROM AUTHOR]

Entangled magnetic fields entering into an accretion flow would very soon be stretched into a dominant toroidal component due to strong differentially rotating motion inside the accretion disc. This is particularly true forweakly viscous, lowangular momentum transonic or advective discs. We study the trajectories of toroidal flux tubes inside a geometrically thick flow that undergoes a centrifugal force supported shock. We also study effects of these flux tubes on the dynamics of the inflow and the outflow. We use a finite difference method (total variation diminishing) for this purpose and specifically focused on whether these flux tubes significantly affect the properties of the outflows such as its collimation and the rate. It is seen that depending upon the cross-sectional radius of the flux tubes that control the drag force, these field lines may move towards the central object or oscillate vertically before eventually escaping out of the funnel wall (pressure zero surfaces) along the vertical direction. A comparison of results obtained with and without flux tubes show these flux tubes could play a pivotal role in collimation and acceleration of jets and outflows. [ABSTRACT FROM AUTHOR]

We study time evolution of sub-Keplerian transonic accretion flows on to black holes using a general relativistic numerical simulation code. We perform simulations in Schwarzschild space-time. We first compare one-dimensional simulation results with theoretical results and validate the performance of our code. Next, we present results of axisymmetric, twodimensional simulation of advective flows. We find that even in this case, for which no complete theoretical analysis is present in the literature, steady-state shock formation is possible. [ABSTRACT FROM AUTHOR]

Two-component advective flow (TCAF) successfully explains spectral and timing properties of black hole candidates. We study the nature of photon trajectories in the vicinity of a Schwarzschild black hole and incorporate this in predicting images of TCAF with a black hole at the Centre. We also compute the emitted spectra. We employ a Monte Carlo simulation technique to achieve our goal. For accurate prediction of the image and the spectra, null trajectories are generated without constraining the motion to any specific plane. Redshift, bolometric flux and corresponding temperature have been calculated with appropriate rela- tivistic consideration. The CENtrifugal pressure supported BOundary Layer or CENBOL near the inner region of the disc, which acts as the Compton cloud, is appropriately modelled as a thick accretion disc in Schwarzschild geometry for the purpose of imaging and computing spectra. The variations of spectra and image with physical parameters such as the accretion rate (...) and inclination angle are presented. We show that the gravitational bending effects of photons do change the spectral shape to some extent. [ABSTRACT FROM AUTHOR]

We study time evolution of rotating, axisymmetric, two-dimensional inviscid accretion flows around black holes using a grid-based finite difference method. We do not use reflection symmetry on the equatorial plane in order to inspect if the disc along with the centrifugal barrier oscillated vertically. In the inviscid limit, we find that the CENtrifugal pressure supported BOundary Layer (CENBOL) is oscillating vertically, more so, when the specific angular momentum is higher. As a result, the rate of outflow produced from the CENBOL, also oscillates. Indeed, the outflow rates in the upper half and the lower half are found to be anticorrelated. We repeat the exercise for a series of specific angular momentum λ of the flow in order to demonstrate effects of the centrifugal force on this interesting behaviour. We find that, as predicted in theoretical models of discs in vertical equilibrium, the CENBOL is produced only when the centrifugal force is significant and more specifically, when λ > 1.5. Outflow rate itself is found to increase with λ as well and so is the oscillation amplitude. The cause of oscillation appears to be due to the interaction among the back flow from the centrifugal barrier, the outflowing winds and the inflow. For low angular momentum, the back flow as well as the oscillation are missing. To our knowledge, this is the first time that such an oscillating solution is found with a well-tested grid-based finite difference code, and such a solution could be yet another reason of why quasi-periodic oscillations should be observed in black hole candidates that are accreting low angular momentum transonic flows. [ABSTRACT FROM AUTHOR]

Numerical hydrodynamic simulation of inviscid and viscous flows have shown that significant outflows could be produced from the CENtrifugal pressure-supported BOundary Layer or CENBOL of an advective disc. However, this barrier is weakened in presence of viscosity, more so, if there are explicit energy dissipations at the boundary layer itself. We study effects of viscosity and energy dissipation theoretically on the outflow rate and show that, as the viscosity or energy dissipation (or both) rises, the prospect of formation of outflows is greatly reduced, thereby verifying results obtained through observations and numerical simulations. Indeed, we find that in a dissipative viscous flow, shocks in presence of outflows can be produced only if the Shakura-Sunyaev viscosity parameter α is less than 0.2. This is a direct consequence of modification of the Rankine-Hugoniot relation across the shock in a viscous flow, when the energy dissipation and mass-loss in the form of outflows from the post-shock region are included. If we ignore the effects of mass-loss altogether, the standing dissipative shocks in viscous flows may occur only if α < 0.27. These limits are tighter than the absolute limit of α = 0.3 valid for a situation when the shock itself neither dissipates energy nor any outflow is formed. We compute typical viscosity parameters required to understand spectral and temporal properties of several black hole candidates such as GX399-4, MAXI J1659-152 and MAXI J1836-194 and find that required α are indeed well within our prescribed limit. [ABSTRACT FROM AUTHOR]

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 4.7-7.8Mʘ and 5.1-7.4Mʘ, respectively. Combining the results of these two methods, we obtain a most probable mass range of the source of 4.7-7.8Mʘ or 6+1.8-1.3Mʘ. [ABSTRACT FROM AUTHOR]

We carry out Monte Carlo simulation to study deuterium enrichments of interstellar grain mantles under various physical conditions. Based on the physical properties, various types of clouds are considered. We find that in diffuse cloud regions, very strong radiation fields persists and hardly a few layers of surface species are formed. In translucent cloud regions with a moderate radiation field, significant number of layers would be produced and surface coverage is mainly dominated by photo-dissociation products such as, C,CH3, CH2D,OH and OD. In the intermediate dense cloud regions (having number density of total hydrogen nuclei in all forms ~2 × 104 cm-3), water and methanol along with their deuterated derivatives are efficiently formed. For much higher density regions (~106 cm-3), water and methanol productions are suppressed but surface coverages of CO, CO2, O2 and O3 are dramatically increased. We find a very high degree of fractionation of water and methanol. Observational results support a high fractionation of methanol but surprisingly water fractionation is found to be low. This is in contradiction with our model results indicating alternative routes for defractionation of water. Effects of various types of energy barriers are also studied. Moreover, we allow grain mantles to interact with various charged particles (such as H+, Fe+, S+ and C+) to study the stopping power and projected range of these charged particles on various target ices. [ABSTRACT FROM AUTHOR]

It has long been proposed that low-frequency quasi-periodic oscillations (QPOs) in stellar-mass black holes or their equivalents in supermassive black holes are the result of resonances between infall and cooling timescales. We explicitly compute these two timescales in a generic situation to show that resonances are easily achieved. During an outburst of a transient black hole candidate, the accretion rate of the Keplerian disc as well as the geometry of the Comptonizing cloud change very rapidly. During some period, a resonance condition between the cooling timescale (predominantly by Comptonization) and the infall timescale of the Comptonizing cloud is roughly satisfied. This leads to low-frequency quasi-periodic oscillations (LFQPOs) of the Compton cloud and the consequent oscillation of hard X-rays. In this paper, we explicitly follow black hole candidate H1743-322 during its 2010 outburst. We compute the Compton cooling time and infall time over several days and show that QPOs take place when these two roughly agree within ~50 percent, i.e., the resonance condition is generally satisfied. We also confirm that for the sharper LFQPOs (i.e. higher Q-factors) the ratio of the two timescales is very close to 1. [ABSTRACT FROM AUTHOR]

A black hole accretion is necessarily transonic. In presence of sufficiently high viscosity and cooling effects, a low angular momentum transonic flow can become a standard Keplerian disc except close to the hole where it must pass through the inner sonic point. However, if the viscosity is not high everywhere and cooling is not efficient everywhere, the flow cannot completely become a Keplerian disc. In this paper, we show results of rigorous numerical simulations of a transonic flow having vertically varying viscosity parameter (being highest on the equatorial plane) and optical depth dependent cooling processes to showthat the flowindeed segregates into two distinct components as it approaches a black hole. The component on the equatorial plane has properties of a standard Keplerian disc, though the flow is not truncated at the innermost stable circular orbit. This component extends till the horizon as a sub- Keplerian flow. This standard disc is found to be surrounded by a hot, low angular momentum component forming a centrifugal barrier dominated oscillating shock wave, consistent with the Chakrabarti-Titarchuk two-component advective flow configuration. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We study self-consistently the hydrodynamic and spectral properties of a general class of steady-state accretion discs where we couple both the hydrodynamics and the radiative transfer. We consider a two-component accretion flow in which the Keplerian disc is immersed inside an accreting low angular momentum flow (halo) around a black hole. The injected soft photons from the Keplerian disc are reprocessed by the electrons in the halo. We study the transonic properties of such a Comptonized flow. We use the Rankine–Hugoniot relation to obtain the shock locations in the disc and compute the radiated spectrum from this shocked disc. We identify the boundary of the parameter space spanned by the specific energy and angular momentum which allows the formation of the standing shocks. We show how the boundary changes in the presence of Compton cooling. Due to the radiative loss, some energy is removed from the accreting matter and the shock moves towards the black hole to maintain the pressure balance condition. We solve the two-temperature equations with Coulomb energy exchange between the protons and the electrons, and the radiative processes such as the bremsstrahlung and thermal Comptonization. We study the variation of the hydrodynamical and spectral properties as a function of the accretion rates of the Keplerian and sub-Keplerian components. Ours is the most accurate transonic solution of an inviscid flow around a black hole to date. [ABSTRACT FROM AUTHOR]

We carry out a series of numerical simulations of viscous accretion flows having a reasonable spatial distribution of the viscosity parameter. We add the power-law cooling throughout the flow. We show that, in agreement with the theoretical solutions of viscous transonic flows, matter having the viscosity parameter above a critical value becomes a Keplerian disc while matter having lesser viscosity remains a low angular momentum, sub-Keplerian flow. The latter component produces centrifugal pressure supported shock waves. Thus, for instance, a flow having sufficiently high viscosity on the equatorial plane and low viscosity above and below would produce a two-component advective flow where a Keplerian disc is surrounded by a rapidly infalling sub-Keplerian halo. We find that the post-shock region of the relatively cooler Keplerian disc is evaporated and the overall configuration is quite stable. This agrees with the theoretical model with two components, which attempt to explain the spectral and timing properties of black hole candidates. [ABSTRACT FROM AUTHOR]

ABSTRACT The spectral properties of black hole candidates and outflow rates depend crucially on the models of accretion flow, and thus they are interconnected. In a model of transonic flow, the centrifugal barrier forms a shock wave in the accretion disc at a few tens of Schwarzschild radii. The post-shock region (i.e. the region between the shock and the innermost critical point) can act as a Compton cloud as well a reservoir of outgoing jets/outflows. In order to compute the parameter space in which the outflow can form, we use a suitable Mach number relation that can be satisfied at the shock derived in our earlier work in the presence of dissipation and outflows. Assuming three models of accretion flow (i.e. models of vertical equilibrium, conical wedge shape and constant height), we examine the parameter space of the specific energy and the specific angular momentum for dissipative shock with mass loss. In the first two models, we find that the parameter space is reduced as the cooling rate is increased. However, in the case of the constant height model, the parameter space initially increases with the cooling process, but starts to decrease with a further increase in cooling. One common property in all three accretion flow models is that, above a critical amount of cooling, the parameter space disappears completely. This indicates that the spectrally soft states might not have a significant amount of outflows from the accretion disc. [ABSTRACT FROM AUTHOR]

ABSTRACT We study the time evolution of a rotating, axisymmetric, viscous accretion flow around black holes using a grid-based finite difference method. We use the Shakura-Sunyaev viscosity prescription. However, we compare with the results obtained when all the three independent components of the viscous stress are kept. We show that the centrifugal pressure supported shocks became weaker with the inclusion of viscosity. The shock is formed farther out when the viscosity is increased. When the viscosity is above a critical value, the shock disappears altogether and the flow becomes subsonic and Keplerian everywhere except in a region close to the horizon, where it remains supersonic. We also find that as the viscosity is increased, the amount of outflowing matter in the wind is decreased to less than a percentage of the inflow matter. Since the post-shock region could act as a reservoir of hot electrons or the so-called 'Compton cloud', the size of which changes with viscosity, the spectral properties are expected to depend on viscosity strongly: the harder states are dominated by low angular momentum and the low-viscosity flow with significant outflows while the softer states are dominated by the high-viscosity Keplerian flow having very few outflows. [ABSTRACT FROM AUTHOR]

ABSTRACT We studied the chemical evolution of interstellar grain mantle by varying the physical parameters of the interstellar medium (ISM). To mimic the actual interstellar condition, gas-grain interactions via accretion from the gas phase and desorption (thermal evaporation and photoevaporation) from the grain surface were considered. We found that the chemical composition of the interstellar grain mantle is highly dependent on the physical parameters associated with molecular cloud. Interstellar photons have been found to play an important role in the growth and structure of the interstellar grain mantle. We considered the effects of interstellar photons (photodissociation and photoevaporation) in our simulation under various interstellar conditions. We noticed that the effects of interstellar photons dominate around the region of lower visual extinction. These photons contribute significantly to the formation of the grain mantle. The energy of the incoming photon is attenuated by the absorption and scattering by the interstellar dust. The topmost layers are assumed to be affected mainly by the incoming radiation. We have studied the effects of photodissociation by varying the number of layers which could be affected by it. Model calculations were carried out for the static (extinction parameter is changing with the density of the cloud) as well as the time-dependent case (i.e. both extinction parameter and number density of the cloud are changing with time) and the results are discussed in detail. Different routes to the formation of water molecules have been studied and it has been noticed that production of water molecules via O3 and H2O2 contributes significantly around the dense region. At the end, various observational evidences for the condensed phase species are summarized with their physical conditions and are compared with our simulation results. [ABSTRACT FROM AUTHOR]

We carry out a time-dependent numerical simulation where both the hydrodynamics and the radiative transfer are coupled together. We consider a two-component accretion flow in which the Keplerian disc is immersed inside an accreting low angular momentum flow (halo) around a black hole. The injected soft photons from the Keplerian disc are reprocessed by the electrons in the halo. We show that in presence of an axisymmetric soft-photon source the spherically symmetric Bondi flow loses its symmetry and becomes axisymmetric. The low angular momentum flow was observed to slow down close to the axis and formed a centrifugal barrier which added new features into the spectrum. Using the Monte Carlo method, we generated the radiated spectra as functions of the accretion rates. We find that the transitions from a hard state to a soft state is determined by the mass accretion rates of the disc and the halo. We separate out the signature of the bulk motion Comptonization and discuss its significance. We study how the net spectrum is contributed by photons suffering different number of scatterings and spending different amounts of time inside the Compton cloud. We study the directional dependence of the emitted spectrum as well. [ABSTRACT FROM AUTHOR]

We find a self-consistent solution for the outflow rate from an accretion disc around a black hole. The centrifugal pressure dominated shock in a transonic accretion flow can act as a Compton cloud by emitting radiation in the form of hard X-rays. It is also the base of an outflow where considerable matter is ejected. We modify the Rankine-Hugoniot relationship in the accretion flow when the post-shock region suffers energy as well mass-loss. After connecting the post-shock solution in the disc with the sonic surface properties of the outflow, we obtain the ratio of the outflow rate and inflow rate analytically. Our conclusions are (i) the outflow rate is at the most a few per cent of the inflow rate, (ii) the outflow is absent when the shock is relatively weak (more precisely, the compression ratio is less than about 2) and (iii) the outflow rate decreases with the increase of the energy loss at the post-shock region. Thus spectrally soft states will have lesser outflows. [ABSTRACT FROM AUTHOR]

The evolution of grain mantles in various interstellar environments is studied. We concentrate mainly on water, methanol and carbon dioxide, which constitute nearly 90 per cent of the grain mantle. We investigate how the production rates of these molecules depend on the relative gas-phase abundances of oxygen and carbon monoxide and constrain the relevant parameter space that reproduces these molecules close to the observed abundances. Allowing the accretion of only H, O and CO on the grains and using the Monte Carlo method, we follow the chemical processes for a few million years. We allow the formation of multilayers on the grains and incorporate the freeze-out effects of accreting O and CO. We find that the formation of these molecules depends on the initial conditions as well as on the average cloud density. Specifically, when the number density of accreting O is less than three times that of CO, methanol is always overproduced. Using the available reaction pathways it appears to be difficult to match the exact observed abundances of all three molecules simultaneously. Only in a narrow region of parameter space are all three molecules produced within the observed limits. Furthermore, we found that the incorporation of the freeze-outs of O and CO leads to an almost steady state on the grain surface. The mantle thickness grows anywhere between 60 and 500 layers in a period of two million years. In addition, we consider a case in which the gas number density changes with time owing to the gradual collapse of the molecular cloud and present the evolution of the composition of different species as a function of the radius of the collapsing cloud. [ABSTRACT FROM AUTHOR]

We study the spectral properties of the accretion disc in the Galactic black hole candidate XTE J 564 during the 1998–1999 outburst when the source exhibited double-peaked eruptions. This outburst lasted for 250 d and the 2.5–25.0 keV spectral state varied smoothly from one to another several times. We show that the spectral features of the 1998–1999 outburst could be clearly understood by a two-component (Keplerian and sub-Keplerian) advective flow (TCAF). We concentrate on the spectral data from a Proportional Counter Array instrument on the RXTE satellite for the black hole XTE J 564 and fit them quite satisfactorily using TCAF model. From the spectral fit we calculate the disc parameters, such as the Keplerian rate, the sub-Keplerian rate (halo rate), the shock location and the inner edge of the Keplerian disc. This observation points to the presence of two independent components in the accretion flow and that the accretion rate at all radii need not be constant in an evolving disc. [ABSTRACT FROM AUTHOR]

We study the accretion processes on a black hole by a numerical simulation. We use a grid-based finite difference code for this purpose. We scan the parameter space spanned by the specific energy and the angular momentum and compare the time-dependent solutions with those obtained from theoretical considerations. We found several important results. (a) The time-dependent flow behaves close to a constant height model flow in the pre-shock region and a flow with vertical equilibrium in the post-shock region. (c) The infall time-scale in the post-shock region is several times higher than the free-fall time-scale. (b) There are two discontinuities in the flow, one being just outside of the inner sonic point. Turbulence plays a major role in determining the locations of these discontinuities. (d) The two discontinuities oscillate with two different frequencies and behave as a coupled harmonic oscillator. A Fourier analysis of the variation of the outer shock location indicates higher power at the lower frequency and lower power at the higher frequency. The opposite is true when the analysis of the inner shock is made. These behaviours will have implications in the spectral and timing properties of black hole candidates. [ABSTRACT FROM AUTHOR]

We investigate the dynamical structure of advective accretion flow around stationary as well as rotating black holes. For a suitable choice of input parameters, such as accretion rate and angular momentum (λ), a global accretion solution may include a shock wave. The post-shock flow is located at a few tens times the Schwarzchild radius and is generally very hot and dense. This successfully mimics the so-called Compton cloud, which is believed to be responsible for emitting hard radiation. Owing to the radiative loss, significant energy is removed from the accreting matter and the shock moves forward towards the black hole in order to maintain the pressure balance across it. We identify the effective area of parameter space that allows accretion flows to have some energy dissipation at the shock . As the dissipation is increased, the parameter space is reduced and finally disappears when the dissipation reaches a critical value. The dissipation has a profound effect on the dynamics of post-shock flow. By moving forward, an unstable shock, the oscillation of which causes quasi-periodic oscillations (QPOs) in the emitted radiation, will produce oscillations of high frequency. Such an evolution of QPOs has been observed in several black hole candidates during their outbursts. [ABSTRACT FROM AUTHOR]

Low and intermediate frequency quasi-periodic oscillations (QPOs) are thought to be due to oscillations of Comptonizing regions or hot regions embedded in Keplerian discs. Observational evidence of evolutions of QPOs would therefore be very important as they throw lights on the dynamics of the hotter region. Our aim is to find systems in which there is a well-defined correlation among the frequencies of the QPOs over a range of time so as to understand the physical picture. In this paper, we concentrate on the archival data of XTE J1550−564 obtained during 1998 outburst, and study the systematic drifts during the rising phase from the 1998 September 7 to the 1998 September 19, when the QPO frequency increased monotonically from 81 mHz to 13.1 Hz. Immediately after that, QPO frequency started to decrease and on the 1998 September 26, the QPO frequency became 2.62 Hz. After that, its value remained almost constant. This frequency drift can be modelled satisfactorily with a propagatory oscillating shock solution where the post-shock region behaves as the Comptonized region. Comparing with the nature of a more recent 2005 outburst of another black hole candidate GRO 1655−40, where QPOs disappeared at the end of the rising phase, we conjecture that this so-called ‘outburst’ may not be a full-fledged outburst. [ABSTRACT FROM AUTHOR]

We study the dynamical structure of a cooling dominated rotating accretion flow around a spinning black hole. We show that non-linear phenomena such as shock waves can be studied in terms of only three flow parameters, namely the specific energy , the specific angular momentum (λ) and the accretion rate of the flow. We present all possible accretion solutions. We find that a significant region of the parameter space in the plane allows global accretion shock solutions. The effective area of the parameter space for which the Rankine–Hugoniot shocks are possible is maximum when the flow is dissipation-free. It decreases with the increase of cooling effects and finally disappears when the cooling is high enough. We show that shock forms further away when the black hole is rotating compared to the solution around a Schwarzschild black hole with identical flow parameters at a large distance. However, in a normalized sense, the flow parameters for which the shocks form around the rotating black holes are produced shocks closer to the black hole. The location of the shock is also dictated by the cooling efficiency in that higher the accretion rate , the closer is the shock location. We believe that some of the high-frequency quasi-periodic oscillations may be due to the flows with higher accretion rate around the rotating black holes. [ABSTRACT FROM AUTHOR]

Gravitational wave signal characteristics from a binary black hole system in which the companion moves through the accretion disc of the primary are studied. We chose the primary to be a super-massive Kerr black hole and the companion to be a massive black hole to clearly demonstrate the effects. We show that the drag exerted on the companion by the disc is sufficient to reduce the coalescence time of the binary. The drag is primarily due to the fact that the accretion disc on a black hole deviates from a Keplerian disc and becomes sub-Keplerian due to inner boundary condition on the black hole horizon. We consider two types of accretion rates on to the companion. The companion is deeply immersed inside the disc and it can accrete at the Bondi rate which depends on the instantaneous density of the disc. However, an accretion disc can also form around the smaller black hole and it can accrete at its Eddington rate. Thus, this case is also studied and the results are compared. We find that the effect of the disc will be significant in reducing the coalescence time and one needs to incorporate this while interpreting gravitational wave signals emitted from such a binary system. [ABSTRACT FROM AUTHOR]

We solve the magnetohydrodynamic (MHD) equations governing axisymmetric flows around compact objects and found all possible classes of solutions for non-relativistic adiabatic accretion flows. We divide the parameter space in terms of these classes. We study the possibility of the formation of the MHD shock waves and show how the strength of the shocks depends on the flow parameters. We also show regions of the parameter space where the shock conditions are not satisfied and therefore the shocks may oscillate. These solutions are astrophysically interesting as they could give rise to quasi-periodic oscillations seen in hard X-rays. [ABSTRACT FROM AUTHOR]

Standing, propagating or oscillating shock waves are common in accretion and winds around compact objects. We study the topology of all possible solutions using the pseudo-Kerr geometry. We present the parameter space spanned by the specific energy and angular momentum and compare it with that obtained from the full general relativity to show that the potential can work satisfactorily in fluid dynamics also, provided the polytropic index is suitably modified. We then divide the parameter space depending on the nature of the solution topology. We specifically study the nature of the standing Rankine–Hugoniot shocks. We also show that as the Kerr parameter is increased, the shock location generally moves closer to the black hole. In future, these solutions can be used as guidelines to test numerical simulations around compact objects. [ABSTRACT FROM AUTHOR]

In this series of papers, we shall present a simplistic approach to the study of particle dynamics, fluid dynamics and numerical simulations of accretion flows and outflows around rotating black holes. We show that with a suitably modified effective potential of the central gravitating rotating object, one can carry out these studies very accurately. In this approach, one need not use the full general relativistic equations to obtain the salient features of the general relativistic flows provided the Kerr parameter remains within . We present the equatorial and the non-equatorial particle trajectories from our potential and compare salient properties in Kerr and in pseudo-Kerr geometries. Our potential naturally produces accurate results for motions around the Schwarzschild geometry when the black hole angular momentum is set to zero. [ABSTRACT FROM AUTHOR]

We conducted a multiwavelength campaign in 2002 September–October, to observe SS 433. We used the Giant Meter Radio Telescope for radio observations, the Physical Research Laboratory Infrared Telescope at Mt Abu for infrared (IR), the ARIES telescope at Nainital for optical photometry, the telescope at the Vainu Bappu observatory for spectral measurements and the Rossi X-ray Timing Explorer for X-ray observations. We find sharp variations in intensity on time-scales of a few minutes in the X-ray, IR and radio wavelengths. Differential photometry in the IR observations clearly indicates significant intrinsic variations on short time-scales of minutes throughout the campaign. Combining the results for these wavelengths, we find a signature of delay of about two days between the IR and radio signals. The X-ray spectrum yielded double Fe line profiles which corresponded to red and blue components of the relativistic jet. We also present the broad-band spectrum averaged over the campaign duration. [ABSTRACT FROM AUTHOR]

We study the evolution of molecular hydrogen on the grain surfaces and in the gas phase using both the rate equation (which tracks the average number of various species) and the master equation (which tracks the expectation values of various species). We show that above a certain critical accretion rate of H on the grains, the results from these two methods become identical. We used this result to follow the collapse of a dense interstellar cloud and studied the formation of molecular hydrogen for two different temperatures (T= 10 and 12 K) and two different masses (1 and 10M⊚) of the cloud when olivine grains were used. Because at higher temperatures, the recombination is very small for these grains, we also studied similar hydrodynamic processes at higher temperatures (T= 20 and 25 K) when amorphous carbon grains were used. We find that generally, for olivine grains, more than 90 per cent H is converted to H2 within ∽ 105-7 whereas for amorphous grains it takes ∽ 106-7 yr . H2 formed in this manner can be adequate to produce the observed complex molecules. [ABSTRACT FROM AUTHOR]

Apart from regular monitoring with the All Sky Monitor, the compact object SS 433 was observed withRXTEseveral times over the last 2 or 3 yr. We present the first analysis of these observations. We also include the results of the recent exciting Target of Opportunity campaign made during donor inferior (orbital phase) and superiorconjunctions which took place on 2003 October 2 and 2004 March 13, respectively, when the jet was directly pointing towards us (i.e. precessional phase). Generally, we found that two distinct lines fit the spectra taken on these days. We present some of the light curves and the X-ray spectra, and show that the Doppler shifts of the emitted lines roughly match those predicted by the kinematic model for the jets. We find that the line with a higher energy can be best identified with a Fe xxviLyman-α transition while the line with lower energy can be identified with a Fe xxv(1s2p–1s2) transition. We observe that the X-ray flux on 2004 March 13 (when the base of the jet was exposed) is more than twice that on 2003 October 2 (when the base is covered by the companion). We find that the flux continues to remain high at least until another orbital period. We believe that this is because SS 433 was undergoing a weak flaring activity during the recent observation. [ABSTRACT FROM AUTHOR]

Particles inside a planetary ring are subject to forcing due to the central planet, moons in inclined orbits, self-gravity of the ring and other forces resulting from radiation drag, collisional effects and the Lorentz force arising from the magnetic field of the planet. We write down the equations of motion of a single particle inside the ring and solve them analytically. We find that the importance of the shear caused by variation of the radial velocity component with the local vertical direction cannot be ignored and it may be responsible for damping of the bending waves in planetary rings as observed by theVoyagerdata. We present the wave profile resulting from the dissipation. We estimate the surface mass density of the C-ring to be of the order of, and the height. These theoretical results are in agreement with observations. [ABSTRACT FROM AUTHOR]

Accretion flows having low angular momentum and low viscosity can have standing shock waves. These shocks arise because of the presence of multiple sonic points in the flow. We study the region of the parameter space in which multiple sonic points occur in viscous flows in the absence of cooling. We also separate the parameter space into regions allowing steady shocks and oscillating shocks. We quantify the nature of two critical viscosities which separate the flow topologies. The post-shock region being hotter, it emits harder X-rays and oscillating shocks cause oscillating X-ray intensities giving rise to quasi-periodic oscillations. We show that with the increase in viscosity parameter, the shock always moves closer to the black hole. This implies an enhancement of the quasi-periodic oscillation frequency as viscosity is increased. [ABSTRACT FROM AUTHOR]

Matter accreting on to black holes has long been known to have standing or oscillating shock waves. The post-shock matter puffs up in the form of a torus, which intercepts soft photons from the outer Keplerian disc and inverse Comptonizes to produce hard photons. The post-shock region also produces jets. We study the interaction of both hard photons and soft photons, with on-axis electron–positron jets. We show that the radiation from post-shock torus accelerates the flow to relativistic velocities, while that from the Keplerian disc has marginal effect. We also show that the velocity at infinity or the terminal velocity ϑ depends on the shock location in the disc. [ABSTRACT FROM AUTHOR]