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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]

Solar flare intensity is strongly dependent on the phase in the solar cycle, the structure and dynamics of the magnetic field near sunspots, and also on occasional solar coronal mass ejections. In this paper we study some of the solar flares detected by the stratospheric balloon borne experiments of Indian Centre for Space Physics. We also observe a gamma-ray burst which is believed to be originated from sudden energy release in gamma rays. In the hard X-ray region of 10 − 100 keV, we present and analyze data from various classes of solar flares and a gamma-ray burst. Because of natural constraints present in balloon borne experiments we receive data up to about a height of ∼ 42 km. The Earth's residual atmosphere at this height absorbs the lower energy part of the spectrum. Moreover, the background radiation (mainly secondary cosmic rays) introduces noise. We show how we circumvent these limitations and create the accurate light curves and the spectra of a few solar flares and a gamma-ray burst. [ABSTRACT FROM AUTHOR]

Secondary cosmic rays are produced from the interaction of primary cosmic rays, which are high-energy (∼ GeV) particles originating from outer space, primarily, protons (∼ 89%) and alpha particles (∼ 10%) with atoms (mainly nitrogen and hydrogen) present in our Earth's atmosphere. Such rays are dependent on solar activity and geomagnetic latitude through magnetic rigidity and are found to be modulated by these factors. For analysis of secondary cosmic ray data, we use the Indian Centre for Space Physics's data obtained through small stratospheric balloon-borne experiments. In the present paper, we have measured the secondary cosmic ray intensity at low geomagnetic latitude of ∼ 14.50 ∘ N in the low energy range of 25–60 keV from the ground to 30 km altitude for different detectors used in various balloon missions because such missions are almost detecting the X-ray sources above 30 km till 42 km. We also calculated background flux of the detector with the help of some justifiable physical assumptions, secondary cosmic gamma-ray component measurements, and assumed internal background functions. Finally, using our data of 2012–2019, which falls in the 24th solar cycle, we have shown that there is a clear anti-correlation between cosmic ray intensity at Regener–Pfotzer maximum and the solar activity. [ABSTRACT FROM AUTHOR]

We use existing light weight balloon facility of Indian Centre for Space Physics to detect the X-ray radiation from Crab pulsar with a phoswich detector. We present the design considerations and characterization of the detector used for this purpose. We model the background radiation in the detector environment at various altitudes and use this in spectral analysis. The background radiation level and limitations on the detector allowed us to calculate minimum detection limit for extrasolar radiation sources with our set up. [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 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]

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]

We study the effects of space weather on the ionosphere and low Earth orbit (LEO) satellites’ orbital trajectory in equatorial, low- and mid-latitude (EQL, LLT and MLT) regions during (and around) the notable storms of October/November, 2003. We briefly review space weather effects on the thermosphere and ionosphere to demonstrate that such effects are also latitude-dependent and well established. Following the review we simulate the trend in variation of satellite’s orbital radius ( r ), mean height ( h ) and orbit decay rate ( ODR ) during 15 October–14 November 2003 in EQL, LLT and MLT. Nominal atmospheric drag on LEO satellite is usually enhanced by space weather or solar-induced variations in thermospheric temperature and density profile. To separate nominal orbit decay from solar-induced accelerated orbit decay, we compute r , h and ODR in three regimes viz. (i) excluding solar indices (or effect), where r = r 0 , h = h 0 and ODR = ODR 0 (ii) with mean value of solar indices for the interval, where r = r m , h = h m and ODR = ODR m and (iii) with actual daily values of solar indices for the interval ( r , h and ODR ). For a typical LEO satellite at h  = 450 km, we show that the total decay in r during the period is about 4.20 km, 3.90 km and 3.20 km in EQL, LLT and MLT respectively; the respective nominal decay ( r 0 ) is 0.40 km, 0.34 km and 0.22 km, while solar-induced orbital decay ( r m ) is about 3.80 km, 3.55 km and 2.95 km. h also varied in like manner. The respective nominal ODR 0 is about 13.5 m/day, 11.2 m/day and 7.2 m/day, while solar-induced ODR m is about 124.3 m/day, 116.9 m/day and 97.3 m/day. We also show that severe geomagnetic storms can increase ODR by up to 117% (from daily mean value). However, the extent of space weather effects on LEO Satellite’s trajectory significantly depends on the ballistic co-efficient and orbit of the satellite, and phase of solar cycles, intensity and duration of driving (or influencing) solar event. [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]

Cosmic ray flux in our planetary system is primarily modulated by solar activity. Radiation effects of cosmic rays on the Earth strongly depend on latitude due to the variation of the geomagnetic field strength. To study these effects we carried out a series of measurements of the radiation characteristics in the atmosphere due to cosmic rays from various places (geomagnetic latitude: ∼14.50°N) in West Bengal, India, located near the Tropic of Cancer, for several years (2012–2016) particularly covering the solar maximum in the 24th solar cycle. We present low energy (15–140 keV) secondary radiation measurement results extending from the ground till the near space (∼40 km) using a scintillator detector on board rubber weather balloons. We also concentrate on the cosmic ray intensity at the Regener-Pfotzer maxima and find a strong anti-correlation between this intensity and the solar activity even at low geomagnetic latitudes. [ABSTRACT FROM AUTHOR]

We present effects of the annular solar eclipse of 15th January 2010 on the topside ionosphere using the DEMETER satellite data. Measurements of the electron-ion density and electron temperature by the ISL (Instrument Sonde de Langmuir) and IAP (Instrument Analyseur de Plasma) instruments on board the DEMETER satellite during the eclipse time over the low latitude (±40) Indian ocean area are presented. We found the peak decrease in electron density to be about 35% and the peak decrease in ion density to be about 40% from the reference orbits at the altitude of the satellite (∼660 km). Electron and ion temperatures were found to have decreased by 200–300 K at the same altitude. Instead of simple decrease as in ion density, electron temperature showed a complex wave-like oscillation as the solar eclipse progressed. Electron density decreased to a minimum value before the maximum obscuration and starts to increase before passing through another minimum at the time of maximum obscuration. Both the minima are located at the ±10° geomagnetic latitude. Variations of electron and ion densities were found to follow the average solar illumination experienced by the satellite and its conjugate points at satellite altitude, while the electron temperature showed no such correlation. [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]

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 amplitude and phase of VLF/LF radio signals are sensitive to changes in electrical conductivity of the lower ionosphere which imprints its signature on the Earth–ionosphere waveguide. This characteristic makes it useful in studying sudden ionospheric disturbances, especially those related to prompt X-ray flux output from solar flares and gamma ray bursts (GRBs). However, strong geomagnetic disturbance and storm conditions are known to produce large and global ionospheric disturbances, which can significantly affect VLF radio propagation in the D region of the ionosphere. In this paper, using the data of three propagation paths at mid-latitudes (40–54°), we analyse the trend in variation of aspects of VLF diurnal signal under varying solar and geomagnetic space environmental conditions in order to identify possible geomagnetic footprints on the D region characteristics. We found that the trend of variations generally reflected the prevailing space weather conditions in various time scales. In particular, the ‘dipping’ of mid-day signal amplitude peak (MDP) occurs after significant geomagnetic perturbed or storm conditions in the time scale of 1–2 days. The mean signal amplitude before sunrise (MBSR) and mean signal amplitude after sunset (MASS) also exhibit storm-induced dipping, but they appear to be influenced by event's exact occurrence time and the highly variable conditions of dusk-to-dawn ionosphere. We also observed few cases of the signals rise (e.g., MDP, MBSR or MASS) following a significant geomagnetic event. This effect may be related to storms associated phenomena or effects arising from sources other than solar origin. The magnitude of induced dipping (or rise) significantly depends on the intensity and duration of event(s), as well as the propagation path of the signal. The post-storm day signal (following a main event, with lesser or significantly reduced geomagnetic activity) exhibited a tendency of recovery to pre-storm day level. In the present analysis, we do not see a well-defined trend in the variation of the post-storm sunrise amplitude terminator (SRT) and sunset terminator (SST). The SRT and SST signals show more dipping in GQD-A118 propagation path but generally an increase along DHO-A118 propagation path. Thus the result could be propagation path dependent and detailed modelling is required to understand these phenomena. [ABSTRACT FROM AUTHOR]

Variability classes in the enigmatic black hole candidate GRS 1915+105 are known to be correlated with the variation of the Comptonizing Efficiency (CE) which is defined to be the ratio between the number of power-law (hard) photons and seed (soft) photons injected into the Compton cloud. Similarities of light curves of several variability classes of GRS 1915+105 and IGR 17091-3624, some of which are already reported in the literature, motivated us to compute CE for IGR 17091-3624 as well. We find that they are similar to what were reported earlier for GRS 1915+105, even though masses of these objects could be different. The reason is that the both the sizes of the sources of the seed photons and of the Comptonizing corona scale in the same way as the mass of the black hole. This indicates that characterization of variability classes based on CE is likely to be black hole mass independent, in general. [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]

The upper atmosphere changes significantly in temperature, density and composition as a result of solar cycle variations, which causes severe storms and flares, and increases in the amount of absorbed solar radiation from solar energetic events. Satellite orbits are consequently affected by this process, especially those in low Earth orbit (LEO). In this paper, we present a model of atmospheric drag effects on the trajectory of two hypothetical LEO satellites of different ballistic coefficients, initially injected at h = 450 km. We investigate long-term trends of atmospheric drag on LEO satellites due to solar forcing induced atmospheric perturbations and heating at different phases of the solar cycle, and during short intervals of strong geomagnetic disturbances or magnetic storms. We show dependence of orbital decay on the severity of both solar cycle and phase and the extent of geomagnetic perturbations. The result of the model compares well with observed decay profile of some existing LEO satellites and provide a justification of the theoretical considerations used here. [ABSTRACT FROM AUTHOR]

Abstract: The Galactic black hole candidate H 1743-322 exhibited two X-ray outbursts in rapid succession: one in August 2010 and the other in April 2011. We analyze archival data of this object from the PCA instrument on board RXTE (2–25keV energy band) to study the evolution of its temporal and spectral characteristics during both the outbursts, and hence to understand the behavioral change of the accretion flow dynamics associated with the evolution of the various X-ray features. We study the evolution of QPO frequencies during the rising and the declining phases of both the outbursts. We successfully fit the variation of QPO frequency using the Propagating Oscillatory Shock (POS) model in each of the outbursts and obtain the accretion flow parameters such as the instantaneous shock locations, the shock velocity and the shock strength. Based on the degree of importance of the thermal (disk black body) and the non-thermal (power-law) components of the spectral fit and properties of the QPO (if present), the entire profiles of the 2010 and 2011 outbursts are subdivided into four different spectral states: hard, hard-intermediate, soft-intermediate and soft. We attempt to explain the nature of the outburst profile (i.e., hardness-intensity diagram) with two different types of mass accretion flow. [Copyright &y& Elsevier]

Abstract: The Galactic microquasar GRS 1915+105 exhibits at least seventeen types of variability classes. Intra and inter class transitions are reported to be observed within seconds to hours. Since the observation was not continuous, these classes appeared to be exhibited in a random order. Our goal is to predict a sequence of these classes. In this paper, we compute the ratio of the photon counts obtained from the power-law component and the blackbody component of each class and call this ratio as the ‘Comptonizing efficiency’ (CE) of that class. We sequence the classes in the ascending order of CE and find that this sequence matches with a few class transitions observed by RXTE satellite and IXAE instruments on board IRS-P3. A change in CE corresponds to a change in the optical depth of the Compton cloud. Our result implies that the optical depth of the Compton cloud gradually rises as the variability class becomes harder. [Copyright &y& Elsevier]

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]

We study the spectral properties and accretion flow behavior of an ultraluminous X-ray source M82 X-1 using NuSTAR observations. We use the physical two-component advective flow (TCAF) model to fit the data and to derive the accretion flow properties of the source. From the model fitted parameters, we found that M82 X-1 is harboring an intermediate mass black hole at its centre, where the mass varies from 156. 04 - 15.30 + 13.51 to 380. 96 - 29.76 + 28.38 M ⊙ . The error weighed average mass of the black hole is 273 ± 43 M ⊙ , which accreted in nearly super-Eddington rate. The Compton cloud was compact with a size of ∼ 13 r g and the shock compression ratio had nearly intermediate values except for the epoch four. These indicate a possible significant mass outflow from the inner region of the disk. The quasi periodic oscillation (QPO) frequencies estimated from the model fitted parameters can reproduce the observed QPOs. The robustness of the model parameters is verified by drawing the confidence contours among them. [ABSTRACT FROM AUTHOR]

Abstract: During the total solar eclipse of 2009, a week-long campaign was conducted in the Indian sub-continent to study the low-latitude D-region ionosphere using the very low frequency (VLF) signal from the Indian Navy transmitter (call sign: VTX3) operating at 18.2kHz. It was observed that in several places, the signal amplitude is enhanced while in other places the amplitude is reduced. We simulated the observational results using the well known Long Wavelength Propagation Capability (LWPC) code. As a first order approximation, the ionospheric parameters were assumed to vary according to the degree of solar obscuration on the way to the receivers. This automatically brought in non-uniformity of the ionospheric parameters along the propagation paths. We find that an assumption of 4km increase of lower ionospheric height for places going through totality in the propagation path simulate the observations very well at Kathmandu and Raiganj. We find an increase of the height parameter by km for the VTX-Malda path and km for the VTX-Kolkata path. We also present, as an example, the altitude variation of electron number density throughout the eclipse time at Raiganj. [Copyright &y& Elsevier]

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]

Solutions of black hole accretion flows with axisymmetric shocks are obtained self-consistently when the dissipation at the post-shock flow is taken into account. The Rankine-Hugoniot relationships had to be modified suitably to incorporate the energy loss as well as possible matter loss due to outflows in the post-shock region. The outflow rate from the post-shock region is also computed self-consistently. This was done by considering the quantities in the subsonic post-shock flow as the initial condition for the conical outflow. We have several major results: we find the analytical expression of the ratio of the outflow rate and the inflow rate Rṁ. We find that Rṁ strongly depends on the model assumptions which govern the flow geometry. It appears that, (a) the outflow rate is at most a few percent of the inflow rate, (b) the outflow is absent when the shock is relatively weak, (c) the outflow rate decreases with the increase in the energy loss at the post-shock region. These conclusions are very important as they have direct bearings on the observational effects. Since spectrally soft states are generally believed to be caused by the dominance of the soft photons and almost total loss of thermal energy of the Compton cloud by inverse Comptonization, a spectrally softer state should have less outflows. The opposite is generally true: A spectrally harder state will have a stronger outflow, but the result depends on the compression ratio and the adopted model. The other major result is that the model independence of the transonic properties of the flow does not hold in presence of the loss of the energy (radiation) and mass (outflow). [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]

The Galactic microquasar GRS 1915 + 105 exhibits various types of light curves. There is, however, no understanding of when a certain type of light curve will be exhibited and only in a handful of cases, the transitions from one type to another have actually been observed. We study the detailed spectral properties in these cases to show that different classes have different ratio of the power-law photon and the blackbody photon. Since the power-law photons are from the Compton cloud, and the intensity of the power-law photon component depends on the degree of interception of the soft photons by the Compton cloud, we conclude that not only the accretion rate, but the accretion flow geometry must also change during a class transition. [ABSTRACT FROM AUTHOR]