arXiv subjects

Samir Mandal

Publications and source records attributed to Samir Mandal.

36 records · Page 2Linked to original sources

arXiv2019

Investigating Particle Acceleration in the Wolf-Rayet Bubble G2.4+1.4

The supersonic winds produced by massive stars carry a large amount of kinetic power. In numerous scenarios such winds have been proven to produce shocks in which relativistic particles are accelerated emitting non-thermal radiation. Here, we report the first detection of non-thermal emission from a single stellar bubble, G2.4+1.4, associated with a WO star. We observed this source with the uGMRT in Band 4 ($550-850$ MHz) and Band 5 ($1050-1450$ MHz). We present intensity and spectral index maps for this source that are consistent with synchrotron emission (average spectral index, $α= -0.83 \pm 0.10$). The fraction of the available kinetic wind power that is converted into cosmic ray acceleration is estimated to be of the order of a few per cent. This finding constitutes an observational breakthrough and gives new insight on the non-thermal physical processes taking place in the environments of isolated massive stars. In particular, our results show that non-runaway isolated massive stars are capable of accelerating relativistic particles and are therefore confirmed as sources of Galactic cosmic rays.

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arXiv2019

AstroSat view of MAXI J1535-571: broadband spectro-temporal features

We present the results of Target of Opportunity (ToO) observations made with AstroSat of the newly discovered black hole binary MAXI J1535-571. We detect prominent C-type Quasi-periodic Oscillations (QPOs) of frequencies varying from 1.85 Hz to 2.88 Hz, along with distinct harmonics in all the AstroSat observations. We note that while the fundamental QPO is seen in the 3 - 50 keV energy band, the harmonic is not significant above ~ 35 keV. The AstroSat observations were made in the hard intermediate state, as seen from state transitions observed by MAXI and Swift. We attempt spectral modelling of the broadband data (0.7-80 keV) provided by AstroSat using phenomenological and physical models. The spectral modelling using nthComp gives a photon index in the range between 2.18-2.37 and electron temperature ranging from 21 to 63 keV. The seed photon temperature is within 0.19 to 0.29 keV. The high flux in 0.3 - 80 keV band corresponds to a luminosity varying from 0.7 to 1.07 L_Edd assuming the source to be at a distance of 8 kpc and hosting a black hole with a mass of 6 M$_{\odot}$. The physical model based on the two-component accretion flow gives disc accretion rates as high as ~ 1 $\dot{m}_{Edd}$ and halo rate ~ 0.2 $\dot{m}_{Edd}$ respectively. The near Eddington accretion rate seems to be the main reason for the unprecedented high flux observed from this source. The two-component spectral fitting of AstroSat data also provides an estimate of a black hole mass between 5.14 to 7.83 M$_{\odot}$.

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arXiv2018

Constraining the mass of the black hole GX 339-4 using spectro-temporal analysis of multiple outbursts

We carried out spectro-temporal analysis of the archived data from multiple outbursts spanning over the last two decades from the black hole X-ray binary GX 339-4. In this paper, the mass of the compact object in the X-ray binary system GX 339-4 is constrained based on three indirect methods. The first method uses broadband spectral modelling with a two component flow structure of the accretion around the black hole. The broadband data are obtained from {\it RXTE (Rossi X-ray Timing Explorer)} in the range 3.0 to 150.0 keV and from {\it Swift} and {\it NuSTAR (Nuclear Spectroscopic Telescope Array)} simultaneously in the range 0.5 to 79.0 keV. In the second method, we model the time evolution of Quasi-periodic Oscillation (QPO) frequencies, considering it to be the result of an oscillating shock that radially propagates towards or away from the compact object. The third method is based on scaling a mass dependent parameter from an empirical model of the photon index ($Γ$) - QPO ($ν$) correlation. We compare the results at 90 percent confidence from the three methods and summarize the mass estimate of the central object to be in the range $8.28 - 11.89~ M_{\odot}$.

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arXiv2018

Observational aspects of Outbursting Black Hole Sources - Evolution of Spectro-Temporal features and X-ray Variability

We report on our attempt to understand the outbursting profile of Galactic Black Hole (GBH) sources, keeping in mind the evolution of temporal and spectral features during the outburst. We present results of evolution of Quasi-periodic Oscillations (QPOs), spectral states and possible connection with Jet ejections during the outburst phase. Further, we attempt to connect the observed X-ray variabilities (i.e., `class' / `structured' variabilities, similar to GRS 1915+105) with spectral states of BH sources. Towards these studies, we consider three Black Hole sources that have undergone single (XTE J1859+226), a few (IGR J17091-3624) and many (GX 339-4) outbursts since the start of RXTE era. Finally, we model the broadband energy spectra (3 - 150 keV) of different spectral states using RXTE and NuSTAR observations. Results are discussed in the context of two component advective flow model, while constraining the mass of the three BH sources.

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arXiv2017

Properties of Two-Temperature Dissipative Accretion Flow Around Black Holes

We study the properties of two-temperature accretion flow around a non-rotating black hole in presence of various dissipative processes where pseudo-Newtonian potential is adopted to mimic the effect of general relativity. The flow encounters energy loss by means of radiative processes acted on the electrons and at the same time, flow heats up as a consequence of viscous heating effective on ions. We assumed that the flow is exposed with the stochastic magnetic fields which leads to Synchrotron emission of electrons and these emissions are further strengthen by Compton scattering. We obtain the two-temperature global accretion solutions in terms of dissipation parameters, namely, viscosity ($α$) and accretion rate (${\dot m}$), and find for the first time in the literature that such solutions may contain standing shock waves. Solutions of this kind are multi-transonic in nature as they simultaneously pass through both inner critical point ($x_{\rm in}$) and outer critical point ($x_{\rm out}$) before crossing the black hole horizon. We calculate the properties of shock induced global accretion solutions in terms of the flow parameters. We further show that two-temperature shocked accretion flow is not a discrete solution, instead such solution exists for wide range of flow parameters. We identify the effective domain of the parameter space for standing shock and observe that parameter space shrinks as the dissipation is increased. Since the post-shock region is hotter due to the effect of shock compression, it naturally emits hard X-rays and therefore, the two-temperature shocked accretion solution has the potential to explain the spectral properties of the black hole sources.

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arXiv2017

Properties of magnetically supported dissipative accretion flow around black holes with cooling effects

We investigate the global structure of the advection dominated accretion flow around a Schwarzschild black hole where the accretion disc is threaded by toroidal magnetic fields. We consider synchrotron radiative process as an effective cooling mechanism active in the flow. With this, we obtain the global transonic accretion solutions by exploring the variety of boundary conditions and dissipation parameters, namely accretion rate (${\dot m}$) and viscosity ($α_B$). The fact that depending on the initial parameters, steady state accretion flows can possess centrifugally supported shock waves. These global shock solutions exist even when the level of dissipation is relatively high. We study the properties of shock waves and observe that the dynamics of the post-shock corona (hereafter, PSC) is regulated by the flow parameters. Interestingly, we find that shock solution disappears completely when the dissipation parameters exceed their critical values. We calculate the critical values of viscosity parameter ($α^{\rm cri}_B$) adopting the canonical values of adiabatic indices as $γ=4/3$ (ultra-relativistic) and $1.5$ (semi-non-relativistic) and find that in the gas pressure dominated domain, $α^{\rm cri}_B \sim 0.4$ for $γ=4/3$ and $α^{\rm cri}_B \sim 0.27$ for $γ=1.5$, respectively. We further show that global shock solutions are relatively more luminous compared to the shock free solutions. Also, we have calculated the synchrotron spectra for shocked solutions. When the shock is considered to be dissipative in nature, it would have an important implication as the available energy at PSC can be utilized to power the outflowing matter escaped from PSC. Towards this, we calculate the maximum shock luminosity and discuss the observational implication of our present formalism.

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arXiv2015

Radiatively driven relativistic jets with variable adiabatic index equation of state

We investigate a relativistic fluid jet driven by radiation from a shocked accretion disc around a non-rotating black hole approximated by Paczyński-Wiita potential. The sub-Keplerian and Keplerian accretion rates control the shock location and therefore, the radiation field around the accretion disc. We compute the radiative moments with full special relativistic transformation. The effect of a fraction of radiation absorbed by the black hole has been approximated, over and above the special relativistic transformations. We show that the radiative moments around a super massive black hole are different compared to that around a stellar mass black hole. We show that the terminal speed of jets increases with the mass accretion rates,synchrotron emission of the accretion disc and reduction of proton fraction of the flow composition. To obtain relativistic terminal velocities of jets, both thermal and radiative driving are important. We show for very high accretion rates and pair dominated flow, jets around super massive black holes are truly ultra-relativistic, while for jets around stellar mass black holes, terminal Lorentz factor of about $10$ is achievable.

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arXiv2013

Radiatively and thermally driven self-consistent bipolar outflows from accretion discs around compact objects

We investigate the role of radiative driving of shock ejected bipolar outflows from advective accretion discs in a self consistent manner. Radiations from the inner disc affects the subsonic part of the jet while those from the pre-shock disc affects the supersonic part, and there by constitutes a multi stage acceleration process. We show that the radiation from the inner disc not only accelerate but also increase the mass outflow rate, while the radiation from the pre-shock disc only increases the kinetic energy of the flow. With proper proportions of these two radiations, very high terminal speed is possible. We also estimated the post-shock luminosity from the pre-shock radiations, and showed that with the increase of viscosity parameter the disc becomes more luminous, and the resulting jet simultaneously becomes faster. This mimics the production of steady mildly relativistic but stronger jets as micro-quasars moves from low hard to intermediate hard spectral states.

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arXiv2012

Accretion flow dynamics during the evolution of timing and spectral properties of GX 339-4 during its 2010-11 outburst

The Galactic black-hole candidate GX 339-4 exhibited several outbursts at regular intervals of \sim 2-3 years in the Rossi X-ray Timing Explorer (RXTE) era. After remaining in an almost quiescent state for 3 years, it again became X-ray active in 2010 January, continuing to be so over the next \sim 14 months. We study the timing and spectral properties of the black hole candidate (BHC) during its recent outburst using RXTE PCA data, starting from 2010 January 12 to 2011 March 6. Our study provides a comprehensive understanding of the mass accretion processes and properties of the accretion disk of the black hole candidate. The PCA spectra of 2.5-25 keV are mainly fitted with a combination of two components, namely, a disk black body and a power-law. The entire outburst as observed by RXTE, is divided into 4 spectral states, namely, hard, hard-intermediate, soft-intermediate, and soft. Quasi-periodic oscillations (QPOs) were found in 3 out of the 4 states, namely hard, hard-intermediate, and soft-intermediate. The QPO frequencies increase monotonically from 0.102 Hz to 5.692 Hz in the rising phase of the outburst, while during the declining phase QPO frequencies decrease monotonically from 6.420 to 1.149 Hz. The recent outburst of GX 339-4 gives us an opportunity to understand the evolution of the two-component accretion rates starting from the onset to the end of the outburst phase. We found that the QPO frequency variation could be explained by the propagating oscillatory shock model (POS) and the hardness versus intensity variation can be reproduced if we assume that higher viscosity causes the conversion of a low angular momentum disk component into a Keplerian component during the outburst phase. The decline phase starts because of the reduction in the viscosity.

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arXiv2010

Instruments of RT-2 Experiment onboard CORONAS-PHOTON and their test and evaluation IV: Background Simulations using GEANT-4 Toolkit

Hard X-ray detectors in space are prone to background signals due to the ubiquitous cosmic rays and cosmic diffuse background radiation that continuously bombards the satellites which carry the detectors. In general, the background intensity depends on the space environment as well as the material surrounding the detectors. Understanding the behavior of the background noise in the detector is very important to extract the precise source information from the detector data. In this paper, we carry out Monte Carlo simulations using the GEANT-4 toolkit to estimate the prompt background noise measured with the detectors of the RT-2 Experiment onboard the CORONAS-PHOTON satellite.

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arXiv2010

How unusual is XRF 060218 ?

Light curves are calculated for an off-axis observer due the scattering of primary radiation by extended baryonic material. The unusually long duration and the chromaticity of the light curves above several KeV of XRF 060218 can be explained as a result of the acceleration of the baryonic scattering material by the primary radiation. The observed light curves by our model and detailed fits to the data are presented. The model predicts that $\sim 4\times 10^{48}$ ergs are put into accelerated, mildly relativistic baryons by the radiation pressure at large radii from the central engine. It is suggested that the emission below 3 KeV, which lies {\it below} the Amati relation, is a baryon contaminated fireball.

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arXiv2009

Spectral Properties of M87 Using Two-Component Flow

We fit the observational data for M87 using two-component advective disk model. We show that the flat spectrum from the nucleus of M87 is due to synchrotron radiation produced by non-thermal electrons in the CENBOL. The non-thermal distribution is produced due to acceleration of electrons across the shock in a sub-Keplerian flow.

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arXiv2008

Spectrum of two component flows around a super-massive black hole: an application to M87

We calculate the spectra of two-component accretion flows around black holes of various masses, from quasars to nano-quasars. Specifically, we fit the observational data of M87 very satisfactorily using our model and find that the spectrum may be well fitted by a sub-Keplerian component alone, and there is little need of any Keplerian component. The non-thermal distribution of electrons produced by their acceleration across the standing shock in the sub-Keplerian component is enough to produce the observed flat spectrum through the synchrotron radiation.

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arXiv2005

Signature of Accretion Shocks in Emitted Radiation From a Two Temperature Advective Flows Around Black Holes

Centrifugal barrier supported boundary layer (CENBOL) of a black hole affects the spectrum exactly in the same way the boundary layer of a neutron star does. The CENBOL is produced due to standing or oscillating shock waves and these shocks accelerate electrons very efficiently and produce a power-law distribution. The accelerated particles in turn emit synchrotron radiation in presence of the magnetic field. We study the spectral properties of an accretion disk as a function of the shock strength, compression ratio, flow accretion rate and flow geometry. In the absence of a satisfactory description of magnetic fields inside the advective disk, we consider the presence of only stochastic fields and use the ratio of the field energy density to the gravitational energy density to be a parameter. Not surprisingly, stronger fields produce stronger humps due to synchrotron radiation. We not only include `conventional' synchrotron emission and Comptonization due to Maxwell-Bolzmann electrons in the gas, we also compute these effects due to power-law electrons. For strong shocks, a bump is produced at a frequency just above the synchrotron self-absorption frequency at $ν_{bump} \sim ν_{inj} [1+{4/3}\frac{R-1} {R}\frac{1}{x_s^{1/2}}]^{x_s^{1/2}}$, where, $ν_{inj}$ is the frequency of the dominant photons from the pre-shock flow, $R$ is the compression ratio of the shock located at $x_s$. For strong shocks, a bump at a higher frequency appears predominantly due to the power-law electrons formed at the shock front.

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arXiv2005

Identification of Shocks in the Spectra from Black Holes

We study the spectral properties of a low angular momentum flow as a function of the shock strength, compression ratio, accretion rate and flow geometry. In the absence of a satisfactory description of magnetic fields inside the advective disk, we consider the presence of only stochastic fields and use the ratio of the field energy to the gravitational energy density as a parameter. We not only include `conventional' synchrotron emission and Comptonization by Maxwell-Bolzmann electrons in the gas, but we also compute these effects due to power-law electrons. For strong shocks, a bump is produced due to the post-shock flow. A power-law spectral components due to the thermal and non-thermal electrons appear after this bump.

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arXiv2005

SS 433: Results of a Recent Multi-wavelength Campaign

We conducted a multi-wavelength campaign in September-October, 2002, to observe SS 433. We used 45 meter sized 30 dishes of Giant Meter Radio Telescope (GMRT) for radio observation, 1.2 meter Physical Research Laboratory Infra-red telescope at Mt Abu for IR, 1 meter Telescope at the State Observatory, Nainital for Optical photometry, 2.3 meter optical telescope at the Vainu Bappu observatory for spectrum and Rossi X-ray Timing Explorer (RXTE) Target of Opportunity (TOO) observation for X-ray observations. We find sharp variations in intensity in time-scales of a few minutes in X-rays, IR and radio wavelengths. Differential photometry at the IR observation clearly indicated significant intrinsic variations in short time scales of minutes throughout the campaign. Combining results of these wavelengths, we find a signature of delay of about two days between IR and Radio. The X-ray spectrum yielded double Fe line profiles which corresponded to red and blue components of the relativistic jet. We also present the broadband spectrum averaged over the campaign duration.

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arXiv2005

Spectral Properties of a Two Component and Two Temperature Advective Flow

Low angular momentum accretion flows very often have centrifugal pressure supported standing shock waves which can accelerate flow particles. The accelerated particles in turn emit synchrotron radiation in presence of magnetic fields. Efficient cooling of the electrons reduces its temperature in comparison to the protons. In this paper, we assume two temperature flows to explore this property of shocks and present an example of the emitted radiation spectrum.

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arXiv2005

Signatures of Accretion Shocks in Broadband Spectrum of Advective Flows Around Black Holes

We compute the effects of the centrifugal pressure supported shock waves on the emitted spectrum from an accretion disk primarily consisting of low angular momentum matter. Electrons are very efficiently accelerated by the accretion shock and acquire power-law distribution. The accelerated particles in turn emit synchrotron radiation in presence of a stochastic magnetic field in equipartition with the gas. Efficient cooling of the electrons by these soft photons reduces its temperature in comparison to the protons. We explore the nature of the broadband spectra by using Comptonization, bremsstrahlung and synchrotron emission. We then show that there could be two crossing points in a broadband spectrum, one near $\sim 10 keV$ and the other $\sim 300-400$KeV.

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