arXiv subjects

Sandip K. Chakrabarti

Publications and source records attributed to Sandip K. Chakrabarti.

At least 73 records · Page 4Linked to original sources

arXiv2018

Images and Spectra of Time Dependent Two Component Advective Flow in Presence of Outflows

Two Component Advective Flow (TCAF) successfully explains the spectral and tem- poral properties of outbursting or persistent sources. Images of static TCAF with Compton cloud or CENtrifugal pressure supported Boundary Layer (CENBOL) 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 a Keplerian disks well as the self-consistently produced jets and outflows. We show the overall image of the disk-jet system after convolving with a typical beamwidth. A long exposure image with time dependent system need not show the black hole horizon conspicuously, un- less one is looking at a soft state with no jet or the system along the jet axis. Assuming these disk-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.

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arXiv2018

Modeling D-Region Ionospheric Response of the Great American TSE of August 21, 2017 from VLF signal perturbation

Solar eclipse is an unique opportunity to study the lower ionospheric variabilities under a controlled perturbation when the solar ultraviolet and X-ray are temporally occulted by the lunar disk. Sub-ionospheric Very Low Frequency (VLF) radio signal displays the ionospheric response of solar eclipse by modulating its amplitude and phase. During the Total Solar Eclipse (TSE) on August 21, 2017 in North America, data was recorded by a number of receivers as presented in public archive. Out of these, two receiving stations YADA in McBaine and K5TD in Tulsa could procure a reasonable quality of noise free data where the signal amplitude was clearly modulated due to the eclipse. During the lunar occultation, a C3.0 solar flare occurred and the signal received from Tulsa manifested the effect of sudden ionization due to the flare. The VLF amplitude in Tulsa shows the effect which is generally understood by superimposing effects of both the solar eclipse and flare. However, the signal by YADA did not perturb by the solar flare, as the flaring region was totally behind the lunar disk for the entire period. We numerically reproduced the observed signal amplitude variation at both the receiving locations by using Wait's two component D-region ionospheric model and the well-known Long Wavelength Propagation Capability (LWPC) code. The perturbed electron density for both the cases is computed which matches satisfactorily with the true ionospheric conditions.

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arXiv2018

Deuterated Formaldehyde in the low mass protostar HH212

HH212, a nearby (400 pc) object in Orion, is a Class 0 protostellar system with a Keplerian disk and collimated bipolar SiO jets. Deuterated water, HDO and a deuterated complex molecule, methanol (CH2DOH) have been reported in the source. Here, we report the HDCO (deuterated formaldehyde) line observation from ALMA data to probe the inner region of HH212. We compare HDCO line with other molecular lines to understand the possible chemistry and physics of the source. The distribution of HDCO emission suggests it may be associated with the base of the outflow. The emission also shows a rotation but it is not associated with the Keplerian rotation of disk or the rotating infalling envelope, rather it is associated with the outflow as previously seen in C 34 S. From the possible deuterium fractionation, we speculate that the gas phase formation of deuterated formaldehyde is active in the central hot region of the low-mass protostar system, HH212.

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arXiv2017

Properties of X-ray Flux of Jets During 2005 Outburst of Swift J1753.5-0127 Using TCAF Solution

Galactic black hole candidate Swift~J1753.5-0127 was discovered on 2005 June 30 by the Swift Burst Alert Telescope. We study the accretion flow properties during its very first outburst through careful analysis of the evolution of the spectral and the temporal properties using the two-component advective flow (TCAF) paradigm. RXTE proportional counter array spectra in $2.5-25$ keV are fitted with the current version of the TCAF model fits file to estimate physical flow parameters, such as two component (Keplerian disk and sub-Keplerian halo) accretion rates, properties of the Compton cloud, probable mass of the source, etc. The source is found to be in harder (hard and hard-intermediate) spectral states during the entire phase of the outburst with very significant jet activity. Since in TCAF solution, the model normalization is constant for any particular source, any requirement of significantly different normalization to have a better fit on certain days would point to X-ray contribution from components not taken into account in the current TCAF model fits file. By subtracting the contribution using actual normalization, we derive the contribution of X-rays from the jets and outflows. We study its properties, such as its magnitude and spectra. We find that on some days, up to about 32\% X-ray flux is emitted from the base of the jet itself.

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arXiv2017

Possible range of viscosity parameter to trigger black hole candidates to exhibit different states of outbursts

In a two component advective flow around a compact object, a high viscosity Keplerian disk is flanked by a low angular momentum and low viscosity flow which forms a centrifugal pressure supported shock wave close to the black hole. The post-shock region which behaves as a Compton cloud becomes progressively smaller during the outburst as the spectra change from the hard state to the soft state in order to satisfy Rankine-Hugoniot relation in presence of cooling. The resonance oscillation of the shock wave which causes low frequency quasi-periodic oscillations (QPOs) also allows us to obtain the shock location from each observed QPO frequency. Applying the theory of transonic flow with Compton cooling and viscosity, we obtain the viscosity parameter $α_{SK}$ required for the shock to form at those places in the low-Keplerian component. When we compare the evolution of $α_{SK}$ for each outburst, we arrive at a major conclusion: In each source, the advective flow component typically requires exactly a similar value of $α_{SK}$ when transiting from one spectral state to another (e.g., from hard state to soft state through intermediate states and the other way around in the declining phase). Most importantly, these $α_{SK}$ values in the low-angular momentum advective component are fully self-consistent in the sense that they remain below critical value $α_{cr}$ required to form a Keplerian disk. For a further consistency check, we compute the $α_K$ of the Keplerian component, and find that in each of the objects, $α_{SK}$ $<$ $α_{cr}$ $<$ $α_K$.

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arXiv2017

General Relativistic Numerical Simulation of sub-Keplerian Transonic Accretion Flows onto Black Holes: Schwarzschild Spacetime

We study time evolution of sub-Keplerian transonic accretion flows onto black holes using a general relativistic numerical simulation code. We perform simulations in Schwarzschild spacetime. 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. We find that even in this case, for which no complete theoretical analysis is present in the literature, steady state shock formation is possible.

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arXiv2017

Temporal Evolution of Photon Energy emitted from Two Component Advective Flows: Origin of Time Lag

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.

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arXiv2017

Transient nature of J195754+353513

We have searched for transient and/or variable radio sources in the field of Galactic micro-quasar Cygnus X-1 in 1.4 GHz (L band) using data from the Karl G. Jansky Very Large Array (JVLA). We used twenty years of data between 1983 and 2003. We found a source J195754+353513 showing transient behavior. The source was also mentioned earlier in NVSS and WENSS catalog but its transient nature was not reported earlier. The source is located 23.8 arcminutes far from Cygnus X-1. It is detected many times during the span of our study and it varied between less than 0.3 mJy to 201 mJy. J195754+353513 also showed high intra-day variability. In one occasion, the source rose from ~20 mJy to ~180 mJy 700 second. For limited number of cases circularly polarized emission could be detected from the source with V/I vary between 0.15 to 0.25. 2MASS J19575420+3535152 may be the near-infrared counterpart of the source. We compared the properties of the source with other Galactic transient sources having similar properties. The nature of the source is still unknown. We speculate on its possible nature.

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arXiv2017

Dynamics of Magnetic Flux Tubes in an Advective Flow around a Black Hole

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 for weakly viscous, low angular momentum transonic or advective discs. We study the trajectories of toroidal flux tubes inside a geometrically thick flow which 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 focussed 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 which 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 pivotal role in collimation and acceleration of jets and outflows.

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arXiv2017

Study of high energy phenomena from near space using low-cost meteorological balloons

Indian Centre for Space Physics has taken a novel strategy to study low energy cosmic rays and astrophysical X-ray sources which involve very light weight payloads up to about five kilograms on board a single or multiple balloons which are usually used for meteorological purposes. The mission duration could be anywhere from 3-12 hours. Our strategy provides extreme flexibility in mission preparation and its operation using a very economical budget. There are several limitations but our innovative approach has been able to extract significant amount of scientific data out of these missions. So far, over one hundred missions have been completed by us to near space and a wealth of data has been collected. The payloads are recovered and are used again. Scientific data is stored on board computer and the atmospheric data or payload location is sent to ground in real time. Since each mission is different, we present here the general strategy for a typical payload and provide some results we obtained in some of these missions.

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arXiv2017

Measurement of secondary cosmic ray intensity at Regener-Pfotzer height using low-cost weather balloons and its correlation with solar activity

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 deg 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.

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arXiv2017

Latest trends in the study of accretion and outflows around compact objects

Study of astrophysics of black holes and neutron stars has taken a new turn in the present decade with the realization that sub-Keplerian flows and the associated centrifugal barrier near the horizon or the surface of a neutron star play a major role in deciding the nature of the emitted spectra and the formation of outflows from the accreting matter. This region may remain steady or oscillate depending on the accretion rate, specific angular momentum and specific energy of the flow. Intricacies of oscillation may depend on the degree of feedback the inflow receives from the outflow. This region may emit hard or soft X-rays depending on relative numbers of hot elections and soft photons intercepted by this region. We discuss how these properties come about and how they explain the observational results of black hole candidates.

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arXiv2017

Accretion Flow Properties of Swift J1753.5-0127 during its 2005 outburst

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

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arXiv2016

Search for Interstellar monohydric Thiols

It has been pointed out by various astronomers that very interesting relationship exists between interstellar alcohols and the corresponding thiols (sulfur analogue of alcohols) as far as the spectroscopic properties and chemical abundances are concerned. Monohydric alcohols such as methanol and ethanol are widely observed and 1-propanol is recently claimed to have been seen in Orion KL. Among the monohydric thiols, methanethiol (chemical analogue of methanol), has been firmly detected in Orion KL and Sgr B2(N2) and ethanethiol (chemical analogue of ethanol) has been claimed to be observed in Sgr B2(N2) though the confirmation of this detection is yet to come. It is very likely that higher order thiols could be observed in these regions. In this paper, we study the formation of monohydric alcohols and their thiol analogues. Based on our quantum chemical calculation and chemical modeling, we find that `Tg' conformer of 1-propanethiol is a good candidate of astronomical interest. We present various spectroscopically relevant parameters of this molecule to assist its future detection in the Interstellar medium (ISM).

astro-ph.GA↗

arXiv2016

Impacts of photon bending on observational aspects of Two Component Advective Flow

Nature of photon trajectories in a curved spacetime around black holes are studied without constraining their motion to any plane. Impacts of photon bending are separately scrutinized for Keplerian and CENBOL components of Two Component Advective Flow (TCAF) model. Parameters like Red shift, Bolometric Flux, temperature profile and time of arrival of photons are also computed.

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arXiv2016

Images and Spectral Properties of Two Component Advective Flows Around Black Holes: Effects of Photon Bending

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. Red shift, bolometric flux and corresponding temperature have been calculated with appropriate relativistic consideration. The centrifugal barrier dominated boundary layer or CENBOL near the inner region of the disk which acts as the Compton cloud is appropriately modelled as a thick accretion disk 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 ($\dot{m}_d$) and inclination angle are presented. We show that the gravitational bending effects of photons do change the spectral shape to some extent.

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arXiv2016

Estimation of Mass of Compact Object in H 1743-322 from 2010 and 2011 Outbursts using TCAF Solution and Spectral Index - QPO Frequency Correlation

The well known black hole candidate H~1743-322 exhibited temporal and spectral variabilities during several outbursts. Daily variation of the accretion rates and the flow geometry change on a daily basis during each of the outbursts could be understood very well using the recent implementation of two component advective flow (TCAF) solution of the viscous transonic flow equations as an additive table model in XSPEC. This has dramatically improved our understanding about the accretion flow dynamics. Most interestingly, the solution allows to treat mass of the black hole candidate as a free parameter and there mass could be estimated from spectral fits. In this paper, we fit the data of two successive outbursts of H~1743-322 in 2010 and 2011 and studied evolutions of accretion flow parameters, such as, two component (Keplerian and sub-Keplerian) accretion rates, shock location (i.e., size of the Compton cloud), etc. We assume that the model Normalization remains the same accross the states in both these outbursts. We use this to estimate mass of the black hole and found that it comes out in the range of $9.25-12.86 M_\odot$. For the sake of comparison, we also estimated mass using Photon index vs. QPO frequency correlation method which turns out to be $11.65 \pm 0.67 M_\odot$ using GRO J1655-40 as reference source. Combining these two estimates, the most probable mass of the compact object becomes $11.21^{+1.65}_{-1.96} M_\odot$.

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arXiv2016

Temporal variability from the two-component advective flow solution and its observational evidence

In the propagating oscillatory shock model, the oscillation of the post-shock region, i.e., the Compton cloud, causes the observed low-frequency quasi-periodic oscillations (QPOs). The evolution of QPO frequency is explained by the systematic variation of the Compton cloud size, i.e., the steady radial movement of the shock front, which is triggered by the cooling of the post-shock region. Thus, analysis of energy-dependent temporal properties in different variability time scales can diagnose the dynamics and geometry of accretion flows around black holes. We study these properties for the high inclination black hole source XTE J1550-564 during its 1998 outburst and the low-inclination black hole source GX 339-4 during its 2006-07 outburst using RXTE/PCA data, and we find that they can satisfactorily explain the time lags associated with the QPOs from these systems. We find a smooth decrease of the time lag as a function of time in the rising phase of both sources. In the declining phase the time lag increases with time. We find a systematic evolution of QPO frequency and hard lags in these outbursts. In XTE J1550-564, the lag changes from hard to soft (i.e., from a positive to a negative value) at a crossing frequency ($ν_C$) of $\sim 3.4$ Hz. We present possible mechanisms to explain the lag behavior of high and low-inclination sources within the framework of a single two-component advective flow model (TCAF).

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