arXiv subjects

Sandip K. Chakrabarti

Publications and source records attributed to Sandip K. Chakrabarti.

At least 163 records · Page 9Linked to original sources

arXiv2007

Properties of accretion shock waves in viscous flows with cooling effects

We study the properties of the shock waves for a viscous accretion flow having low angular momentum in presence of synchrotron cooling. We present all possible accretion solutions in terms of flow parameters. We identify the region of the parameter space for steady and oscillating shocks and show the effect of various energy dissipation processes on it. We discuss the role of the shock waves while explaining the observations from black hole candidates.

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arXiv2005

Propagating Oscillatory Shock Model for QPOS in GRO J1655-40 During the March 2005 Outburst

GRO 1655-40, a well known black hole candidate, showed renewed X-ray activity in March 2005 after being dormant for almost eight years. It showed very prominent quasi-periodic oscillations. We analysed the data of two observations in this {\it Rapid Communication}, one taken on March 2nd, 2005 and the other taken on the March 11th, 2005. On March 2nd, 2005 the shock was weak and the QPO was seen in roughly all energies. On March 11th, 2005 the power density spectra showed that quasi-periodic oscillations (QPOs) were exhibited in harder X-rays. On the first day, the QPO was seen at 0.13Hz and on the second day, the QPO was seen at $\sim 6.5$Hz with a spectral break at $\sim 0.1$Hz. We analysed the QPOs for the period 25th Feb. 2005 to 12th of March, 2005 and showed that the frequency of QPO increased monotonically from 0.088Hz to 15.01Hz. This agrees well if the oscillating shock is assumed to propagate with a constant velocity. On several days we also noticed the presence of very high frequency QPOs and for the first time we detected QPOs in the 600-700Hz range, the highest frequency range so far reported for any black hole candidate.

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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

VLF observation during Leonid Meteor Shower-2002 from Kolkata

Using a Gyrator-II Loop antenna tuned at 19.0Khz, we monitored the meteor shower during 17-24th November, 2002. We observe the primary peak at 3h58m (UT) on the 19th of November, 2002. We distinctly observed several `beadlike' and `exponentially dropping' signals. The `beadlike' signals were more in abundance on the 18th of November, 2002, one day prior to the actual encounter.

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arXiv2005

Monitoring of Sudden Ionospheric Disturbances (SID) from Kolkata (INDIA)

We report our first results of monitoring sudden ionospheric disturbances (SID). We present data taken continuously for two weeks during 20th Sept. 2002 and 4th Oct. 2002. We compare the effects of solar flares of the VLF signal with those obtained by GOES Satellite of NASA monitoring in X-rays and found excellent agreement.

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arXiv2005

A GHz Flare in a Quiescent Black Hole and A Determination of the Mass Accretion Rate

If the total energy of a radio flare is known, one can estimate the mass accretion rate of the disk by assuming equipartition of magnetic energy and the gravitational potential energy of accreting matter. We present here an example of how such an estimate could be done. Our recent radio observation using the Giant Meter Radio Telescope (GMRT) of the galactic black hole transient A0620-00 at 1.280 GHz revealed a micro-flare of a few milli-Jansky. Assuming a black hole mass of $10 M_\odot$ for the compact object, we find the accretion rate to be at the most ${\dot M} = (8.5 \pm 1.4) \times 10^{-11} (\frac{x}{3})^{5/2} M_\odot$ yr$^{-1}$, where, $x$ is the distance from the hole in units of Schwarzschild radius. This is consistent with the earlier estimate of the accretion rate based on optical and X-ray observations. We claim that this procedure is general enough to be used for any black hole candidate.

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arXiv2005

Results of Recent Multi-wavelength Campaign of SS433

We conducted multi wavelength campaign on SS433 in Sept. 2002 using X-ray, B, Infra-Red and radio telescopes. We observed variabilities on a time-scale of few minutes in all the wavelengths. We interpret them to be due to bullet-like features from the accretion disk. We also present X-ray properties as obtained by RXTE.

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arXiv2005

Class Transitions in Black Holes

A black hole spectrum is known to change from the hard state to the soft state when the energy spectral index $α$ ($F_E \propto E^{-α}$) in, say, 2-20 keV range changes from $α\sim 0.5$ to $\sim 1.5$. However, this `classical' definition which characterizes black holes like Cyg X-1, becomes less useful for many objects such as GRS 1915+105 in which the spectral slope is seen to vary from one to the other in a matter of seconds and depending on whether or not winds form, the spectral slope also changes. The light curves and the colour-colour diagrams may look completely different on different days depending on the frequency and mode of switching from one spectral state to the other. Though RXTE observations have yielded wealth of information on such `variability classes' in GRS 1915+105, very rarely one has been able to observe how the object goes from one class to the other. In the present review, we discuss possible origins of the class transition and present several examples of such transitions. In this context, we use mostly the results of the Indian X-ray Astronomy Experiment (IXAE) which observed GRS 1915+105 more regularly.

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arXiv2005

Numerical Simulations Reveal the Origin of QPOs in Black Hole Candidates

We present results of various types of numerical simulations of black hole accretion disks and find that those flows which are relatively non-dissipative and which contain accretion shocks are the best candidates so far. The power density spectra (PDS) reveal the aspects which are similar to what are observed in black hole candidates.

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arXiv2005

Spectral Properties of Black Holes in Gamma Rays

Black holes are the most compact objects in the universe. Therefore, matter accreting onto is likely to radiate photons of energy comparable to very high gravitational potential energy. We discuss the nature of the emitted radiation in X-rays and gamma-rays from black hole candidates. We present theoretical solutions which comprise both Keplerian and sub-Keplerian components and suggest that shocks in accretion and outflows

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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

Mass accretion rate of the galactic black hole A0620-00 in its quiescent state

Our recent radio observation using the Giant Meter Radio Telescope (GMRT) of the galactic black hole transient A0620-00 at 1.280GHz revealed a micro-flare of a few milli-Jansky. Assuming an equipartition of magnetic energy and the gravitational potential energy of accreting matter, it is possible to estimate the upper limit of the accretion rate. Assuming a black hole mass of $10M_\odot$ residing at the center, we find this to be at the most ${\dot M} = (8.5 \pm 1.4) \times 10^{-11} M_\odot$ yr$^{-1}$. This is consistent with earlier estimates of accretion rates based on optical and X-ray observations.

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arXiv2005

Class Transitions and Two Component Accretion Flow in GRS 1915+105

The light curve of the galactic micro-quasar GRS 1915+105 changes in at least thirteen different ways which are called classes. We present examples of the transitions from one class to another as observed by the IXAE instrument aboard the Indian Satellite IRS-P3. We find that the transitions are associated with changes in photon counts over a time-scale of only a few hours and they take place through unknown classes. Assuming that the transitions are caused by variation of the accretion rates, this implies that a significant fraction of the matter must be nearly freely falling in order to have such dramatic changes in such a short time.

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arXiv2005

Evidence of Class Transitions in GRS 1915+105 from IXAE Data

GRS 1915+105 shows at least twelve distinct classes of light curves. By analysing the data obtained from Indian X-ray Astronomy Experiment (IXAE) instrument aboard IRS-P3 satellite, we show that in at least two days, transitions between one class to another were observed. In these days the so-called $κ$ class went to $ρ$ class and $χ$ class went to $ρ$ class. In the frequency-time plane such transitions exhibited change in quasi-periodic oscillation (QPO) frequency. We could detect that low-frequency QPOs can occur in anticipation of a class transition several hundred minutes before the actual transition

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arXiv2005

Analytical studies of particle dynamics in bending waves in planetary rings

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 due to radiation drag, collisional effects and Lorentz force due to 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 local vertical direction cannot be ignored and it may be responsible for damping of the bending waves in planetary rings as observed by the Voyager data. We present the wave profile resulting from the dissipation. We estimate that the surface mass density of the C ring to be of the order of $σ\sim 1.2-1.6$gm cm$^{-2}$, and the height $h \sim 2.2-2.4$m. These theoretical results are in agreement with observations.

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arXiv2005

X-ray Observation of SS 433 with RXTE

Apart from regular monitoring by ASM, the compact object SS 433 was observed with RXTE several times last two/three years. We present the first analysis of these observations. We also include the results of the recent exciting TOO campaign made during donour inferior (orbital phase $ϕ=0$) and superior ($ϕ=0.5$) conjunctions which took place on Oct. 2nd, 2003, and on March 13th, 2004 respectively, when the jet itself was directly pointing towards us (i.e., precessional phase $ψ\sim 0$). Generally, we found that two distinct lines fit the spectra taken on all 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 FeXXVI Ly-$α$ transition while the line with lower energy can be identified with a FeXXV (1s2p - 1s$^2$) transition. We observe that the X-ray flux on March 13th, 2004 (when the base of the jet is exposed) is more than twice compared to that on Oct. 2nd, 2003 (when the base is covered by the companion). We find the flux to continue to remain high at least till another orbital period. We believe that this is because SS 433 was undergoing a weak flaring activity during the recent observation.

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arXiv2005

Molecular Hydrogen Formation During Dense Interstellar Cloud Collapse

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 molecules) and the master equation (which tracks the expectation values of molecules). 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=10K and 12K) and two different masses ($1M_\odot$ and $10M_\odot$) of the cloud when olivine grains were used. Since at higher temperatures, the recombination is very small for these grains, we also studied a similar hydrodynamic processes at higher temperatures (T=20K and 25K) when amorphous carbon grains were used. We find that generally, for olivine grains, more than 90% $H$ is converted to $H_2$ within $\sim 10^{5-7}$yr whereas for amorphous grains it takes $\sim 10^{6-7}$yr. $H_2$ formed in this manner can be adequate to produce the observed complex molecules.

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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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