arXiv subjects

Dipankar Bhattacharya

Publications and source records attributed to Dipankar Bhattacharya.

At least 73 records · Page 4Linked to original sources

arXiv2019

AstroSat-CZTI detection of variable prompt emission polarization in GRB 171010A

We present spectro-polarimetric analysis of \thisgrb\ using data from \asat, \fermi, and \swift, to provide insights into the physical mechanisms of the prompt radiation and the jet geometry. Prompt emission from \thisgrb\ was very bright (fluence $>10^{-4}$~ergs~cm$^{-2}$) and had a complex structure composed of the superimposition of several pulses. The energy spectra deviate from the typical Band function to show a low energy peak $\sim 15$~keV --- which we interpret as a power-law with two breaks, with a synchrotron origin. Alternately, the prompt spectra can also be interpreted as Comptonized emission, or a blackbody combined with a Band function. Time-resolved analysis confirms the presence of the low energy component, while the peak energy is found to be confined in the range of 100--200~keV. Afterglow emission detected by \fermi-LAT is typical of an external shock model, and we constrain the initial Lorentz factor using the peak time of the emission. \swift-XRT measurements of the afterglow show an indication for a jet break, allowing us to constrain the jet opening angle to $>$ 6$\degr$. Detection of a large number of Compton scattered events by \asat-CZTI provides an opportunity to study hard X-ray polarization of the prompt emission. We find that the burst has high, time-variable polarization, with the emission {\bf have higher polarization} at energies above the peak energy. We discuss all observations in the context of GRB models and polarization arising due to {\bf due to physical or geometric effects:} synchrotron emission from multiple shocks with ordered or random magnetic fields, Poynting flux dominated jet undergoing abrupt magnetic dissipation, sub-photospheric dissipation, a jet consisting of fragmented fireballs, and the Comptonization model.

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arXiv2018

AstroSat and Chandra view of the high soft state of 4U 1630-47 (4U 1630-472): evidence of the disk wind and a rapidly spinning black hole

We present the X-ray spectral and timing analysis of the transient black hole X-ray binary 4U 1630-47, observed with the AstroSat, Chandra and MAXI space missions during its soft X-ray outburst in 2016. The outburst, from the rising phase until the peak, is neither detected in hard X-rays (15-50 keV) by the Swift/BAT nor in radio. Such non-detection along with the source behavior in the hardness-intensity and color-color diagrams obtained using MAXI data confirm that both Chandra and AstroSat observations were performed during the high soft spectral state. The High Energy Grating (HEG) spectrum from the Chandra high-energy transmission grating spectrometer (HETGS) shows two strong, moderately blueshifted absorption lines at 6.705$_{-0.002}^{+0.002}$ keV and 6.974$_{-0.003}^{+0.004}$ keV, which are produced by Fe XXV and Fe XXVI in a low-velocity ionized disk wind. The corresponding outflow velocity is determined to be 366$\pm$56 km/s. Separate spectral fits of Chandra/HEG, AstroSat/SXT+LAXPC and Chandra/HEG + AstroSat/SXT+LAXPC data show that the broadband continuum can be well described with a relativistic disk-blackbody model, with the disk flux fraction of $\sim 0.97$. Based on the best-fit continuum spectral modeling of Chandra, AstroSat and Chandra+AstroSat joint spectra and using the Markov Chain Monte Carlo simulations, we constrain the spectral hardening factor at 1.56$^{+0.14}_{-0.06}$ and the dimensionless black hole spin parameter at 0.92 $\pm$ 0.04 within the 99.7% confidence interval. Our conclusion of a rapidly-spinning black hole in 4U 1630-47 using the continuum spectrum method is in agreement with a previous finding applying the reflection spectral fitting method.

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arXiv2018

Violation of synchrotron line of death by the highly polarized $GRB~160802A$

$GRB~160802A$ is one of the brightest gamma-ray bursts (GRBs) observed with $Fermi$ Gamma-ray Burst Monitor (GBM) in the energy range of $10~-~1000$ keV, while at the same time it is surprisingly faint at energies $\gtrsim2$ MeV. An observation with $AstroSat$/CZT Imager (CZTI) also provides the polarisation which helps in constraining different prompt emission models using the novel joint spectra-polarimetric data. We analyze the $Fermi$/GBM data, and find two main bursting episodes that are clearly separated in time, one particularly faint in higher energies and having certain differences in their spectra. The spectrum in general shows a hard-to-soft evolution in both the episodes. Only the later part of the first episode shows intensity tracking behaviour corresponding to multiple pulses. The photon index of the spectrum is hard, and in over 90 per cent cases, cross even the slow cooling limit ($α=-2/3$) of an optically thin synchrotron shock model (SSM). Though such hard values are generally associated with a sub-dominant thermal emission, such a component is not statistically required in our analysis. In addition, the measured polarisation in 100--300\,keV is too high, $π=85\pm29\%$, to be accommodated in such a scenario. Jitter radiation, which allows a much harder index up to $α=+0.5$, in principle can produce high polarisation but only beyond the spectral peak, which in our case lies close to $200~-~300$ keV during the time when most of the polarisation signal is obtained. The spectro-polarimetric data seems to be consistent with a subphotospheric dissipation process occurring within a narrow jet with a sharp drop in emissivity beyond the jet edge, and viewed along its boundary.

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arXiv2018

Timing Offset Calibration of CZTI instrument aboard ASTROSAT

The radio as well as the high energy emission mechanism in pulsars is yet not understood properly. A multi-wavelength study is likely to help in better understanding of such processes. The first Indian space-based observatory, ASTROSAT, has five instruments aboard, which cover the electromagnetic spectrum from infra-red (1300 $Å$) to hard X-ray (380 KeV). Cadmium Zinc Telluride Imager (CZTI), one of the five instruments is a hard X-ray telescope functional over an energy range of 20-380 KeV. We aim to estimate the timing offset introduced in the data acquisition pipeline of the instrument, which will help in time alignment of high energy time series with those from two other ground-based observatories, viz. the Giant Meterwave Radio Telescope (GMRT) and the Ooty Radio Telescope (ORT). PSR B0531+21 is a well-studied pulsar with nearly aligned radio and hard X-ray pulse profiles. We use simultaneous observations of this pulsar with the ASTROSAT, the ORT and the GMRT. The pulsar was especially observed using the ORT with almost daily cadence to obtain good timing solutions. We also supplement the ORT data with archival FERMI data for estimation of timing noise. The timing offset of ASTROSAT instruments was estimated from fits to arrival time data at the ASTROSAT and the radio observatories. We estimate the offset between the GMRT and the ASTROSAT-CZTI to be -4716 $\pm$ 50 $μs$. The corresponding offset with the ORT was -29639 $\pm$ 50 $μs$. The offsets between the GMRT and Fermi-LAT -5368 $\pm$ 56 $μs$. (Abridged)

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arXiv2018

Magnetar Signature - The U Curve

This work looks at some definitive signatures of magnetars, in particular of period closures accompanied by a decline of X-ray radiation in two models. We review some of the previous works which are based on the well known dynamo model in which the star is born with a period of a few milliseconds at high temperatures. In such a convection regime the dynamo mechanism can amplify the the magnetic fields to the magnetar value. This is in contrast to a screened core model which posits that a high density phase transition occurs in the inner core of magnetars that dynamically aligns all the neutron magnetic moments producing a large magnetic field in the core. The accompanying change of flux gives rise to shielding or screening currents in the surrounding high conductivity plasma that do not permit the field to exit to the surface. Ambipolar diffusion then transports the field to the crust dissipating energy in neutrinos and X-rays. The up-welling field cleaves the crust resulting in flares and X-ray radiation from ohmic dissipation in the crust till the screening currents are spent and the surface polar field attains its final value. In the dynamo model the polar magnetic field decreases with time whereas in our screened model it increases to its final value. One consequence of this is that in the latter model, as a function of time and period, the ratio of the dipole radiation loss, $\dot E$ to the X-ray luminosity, $ L_X$, is a 'U' curve, indicating that it is the exponential decline in $L_X$, that brings closure to the periods that are observed for magnetars.

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arXiv2017

Mass-radius relation of strongly magnetized white dwarfs

We study the strongly magnetized white dwarf configurations in a self-consistent manner as a progenitor of the over-luminous type-Ia supernovae. We compute static equilibria of white dwarf stars containing a strong magnetic field and present the modification of white dwarf mass-radius relation caused by the magnetic field. From a static equilibrium study, we find that a maximum white dwarf mass of about 1.9 M$_\odot$ may be supported if the interior poloidal field is as strong as approximately $10^{10}$ T. On the other hand if the field is purely toroidal the maximum mass can be more than 5 M$_\odot$. All these modifications are mainly from the presence of Lorenz force. The effects of i) modification of equation of state due to Landau quantization, ii) electrostatic interaction due to ions, iii) general relativistic calculation on the stellar structure and, iv) field geometry are also considered. These strongly magnetised configurations are sensitive to magnetic instabilities where the perturbations grow at the corresponding Alfven time scales.

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arXiv2017

A perturbation study of axisymmetric strongly magnetic degenerate stars : the case of super-Chandrasekhar white dwarfs

In the presence of a strong magnetic field a stellar equilibrium configuration, aided by the Lorentz force, can support a larger mass than a non-magnetic one. This has been considered a possible explanation of the super-Chandrasekhar mass white dwarfs giving rise to over-luminous Type-Ia supernovae. We present here linear and non-linear perturbation studies of such strongly magetised configurations and show that axisymmetric configurations with poloidal or toroidal fields are unstable. The numerical evolution of the perturbations shows instability after about an Alfvén crossing time. This time scale is very short for the magnetically supported super-Chandrasekhar mass white dwarfs. Uniform rotation about the symmetry axis can reduce the growth rate but can not stabilize the super-massive configurations. It is concluded that long-lived super-Chandrasekhar mass white dwarfs supported by magnetic field are unlikely to occur in Nature.

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arXiv2017

Quasi-periodic oscillations from post-shock accretion column of polars

A set of strongly magnetized accreting white dwarfs (polars) shows quasi-periodic oscillations (QPOs) with frequency about a Hz in their optical luminosity. These Hz-frequency QPOs are thought to be generated by intensity variations of the emitted radiation originating at the post-shock accretion column. Thermal instability in the post-shock region, triggered by efficient cooling process at the base, is believed to be the primary reason behind the temporal variability. Here we study the structure and the dynamical properties of the post-shock accretion column including the effects of bremsstrahlung and cyclotron radiation. We find that the presence of significant cyclotron emission in optical band reduces the overall variability of the post-shock region. In the case of a larger post-shock region above the stellar surface, the effects of stratification due to stellar gravity becomes important. An accretion column, influenced by the strong gravity, has a smaller variability as the strength of the thermal instability at the base of the column is reduced. On the other hand, the cool, dense plasma, accumulated just above the stellar surface, may enhance the post-shock variability due to the propagation of magnetic perturbations. These characteristics of the post-shock region are consistent with the observed properties of V834~Cen and in general with Cataclysmic Variable sources that exhibit QPO frequency of about a Hz.

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arXiv2017

Observing Compact Stars with AstroSat

This article presents a brief description of India's AstroSat mission which is a powerful space based observatory for compact star research. An account is given of observational constraints and spectral and timing capabilities as realised post-launch. Some preliminary results of observations of the Crab pulsar and an X-ray binary system GX~301-2 are presented to illustrate some of the capabilities of the mission.

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arXiv2016

Neutron Star Physics in the Square Kilometer Array Era : An Indian Perspective

It is an exceptionally opportune time for Astrophysics when a number of next-generation mega-instruments are poised to observe the universe across the entire electromagnetic spectrum with unprecedented data quality. The Square Kilometre Array (SKA) is undoubtedly one of the major components of this scenario. In particular, the SKA is expected to discover tens of thousands of new neutron stars giving a major fillip to a wide range of scientific investigations. India has a sizeable community of scientists working on different aspects of neutron star physics with immediate access to both the uGMRT (an SKA pathfinder) and the recently launched X-ray observatory Astrosat. The current interests of the community largely centre around studies of - a) the generation of neutron stars and the SNe connection}, b) the neutron star population and evolutionary pathways}, c) the evolution of neutron stars in binaries and the magnetic fields}, d) the neutron star equation of state}, e) the radio pulsar emission mechanism}, and, f) the radio pulsars as probes of gravitational physics}. Most of these studies are the main goals of the SKA first phase, which is likely to be operational in the next four years. This article summarises the science goals of the Indian neutron star community in the SKA era, with significant focus on coordinated efforts among the SKA and other existing/upcoming instruments.

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arXiv2016

Of Mountains and Molehills : Gravitational Waves from Neutron Stars

Surface asymmetries of accreting neutron stars are investigated for their mass quadrupole moment content. Though the amplitude of the gravitational waves from such asymmetries seem to be beyond the limit of detectability of the present generation of detectors, it appears that rapidly rotating neutron stars with strong magnetic fields residing in HMXBs would be worth considering for targeted search for continuous gravitational waves with the next generation of instruments.

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arXiv2016

In-orbit performance of AstroSat CZTI

Cadmium-Zinc-Telluride Imager (CZTI) is one of the five payloads on-board recently launched Indian astronomy satellite AstroSat. CZTI is primarily designed for simultaneous hard X-ray imaging and spectroscopy of celestial X-ray sources. It employs the technique of coded mask imaging for measuring spectra in the energy range of 20 - 150 keV. It was the first scientific payload of AstroSat to be switched on after one week of the launch and was made operational during the subsequent week. Here we present preliminary results from the performance verification phase observations and discuss the in-orbit performance of CZTI.

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arXiv2016

SALT observation of X-ray pulse reprocessing in 4U 1626-67

We investigate optical reprocessing of X-rays in the LMXB pulsar 4U 1626-67 in its current spin-up phase using observations with Southern African Large Telescope (SALT), near-simultaneous observations with Swift-XRT and non-simultaneous RXTE-PCA observations and present the results of timing analysis. Using SALT observations carried out on 5th and 6th March, 2014, we detect some interesting reprocessing signatures. We detect a weak optical Quasi Periodic Oscillation (QPO) in the power density spectrum on March 5th at 48 mHz with a fractional rms of 3.3% in spite of the fact that source shows no corresponding X-ray QPO in the spin-up phase. In the light curve obtained on March 5th, we detect a coherent pulsation at the spin period of ~7.677 s. A previously known, slightly down-shifted side-band is also detected at 129.92 mHz. The frequency spacing between main pulse and this side-band is different from earlier observations, though the statistical significance of the difference is limited. The light curve of 6th March displays short time-scale variability in the form of flares on timescales of a few minutes. Folded pulse profiles resulting from data of this night show an interesting trend of pulse peak drifting. This drift could be due to i) rapid changes in the reprocessing agent, like orbital motion of an accretion disk warp around the neutron star or ii) intrinsic pulse phase changes in X-rays. We also examine some X-ray light curves obtained with RXTE-PCA during 2008-2010 for pulse shape changes in short time scales during X-ray flares.

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arXiv2015

Mass-Radius Relation of Strongly Magnetized White Dwarfs: Dependence on Field Geometry, GR effects and Electrostatic Corrections to the EOS

Recent literature has seen an ongoing discussion on the limiting mass of strongly magnetized white dwarfs, since such objects may prove to be a source of over-luminous type-Ia supernovae. In an earlier paper, we have presented the mass-radius relation of white dwarfs with a strong poloidal magnetic field in Newtonian gravity. The inclusion of effects such as general relativistic gravity and many-body corrections to the equation of state can alter the mass-radius relation and the maximum mass. In this work we estimate the extent to which these effects may modify the earlier results. We find that the general relativistic effects tend to reduce the maximum mass by about 2% and many-body corrections by another additional $\sim$2%, for an assumed carbon composition. We also explore field geometries that are purely toroidal or a mixture of poloidal and toroidal and find that the limiting mass of such equilibrium configurations can be substantially higher than in the case of a purely poloidal field.

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arXiv2015

The magnetic field strengths of accreting millisecond pulsars

In this work we have estimated upper and lower limits to the strength of the magnetic dipole moment of all 14 accreting millisecond X-ray pulsars observed with the Rossi X-ray Timing Explorer (RXTE). For each source we searched the archival RXTE data for the highest and lowest flux levels with a significant detection of pulsations. We assume these flux levels to correspond to the closest and farthest location of the inner edge of the accretion disk at which channelled accretion takes place. By estimating the accretion rate from the observed luminosity at these two flux levels, we place upper and lower limits on the magnetic dipole moment of the neutron star, using assumptions from standard magnetospheric accretion theory. Finally, we discuss how our field strength estimates can be further improved as more information on these pulsars is obtained.

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arXiv2015

On the modulation of low frequency Quasi-Periodic Oscillations in black-hole transients

We studied the properties of the low-frequency quasi-periodic oscillations detected in a sample of six black hole candidates (XTE J1550-564, H 1743-322, XTE J1859+226, 4U 1630-47,GX 339-4, XTE J1650-500) observed by the Rossi XTE satellite. We analyzed the relation between the full width half maximum and the frequency of all the narrow peaks detected in power density spectra where a type-C QPO is observed. Our goal was to understand the nature of the modulation of the signal by comparing the properties of different harmonic peaks in the power density spectrum. We find that for the sources in our sample the width of the fundamental and of the first harmonic are compatible with a frequency modulation, while that of the sub-harmonic is independent of frequency, possibly indicating the presence of an additional modulation in amplitude. We compare our results with those obtained earlier from GRS 1915+105 and XTE J1550-564.

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arXiv2014

Mass-radius relation of strongly magnetized white dwarfs: nearly independent of Landau quantization

We compute static equilibria of white dwarf stars containing strong poloidal magnetic field, and present the modification of white dwarf mass-radius relation caused by the magnetic field. We find that a maximum white dwarf mass of about $1.9$~$M\odot$ may be supported if the interior field is as strong as approximately $10^{10}$ T. This mass is over 30 per cent larger than the traditional Chandrasekhar Limit. The equation of state of electron degenerate matter can be strongly modified due to Landau quantization at such high magnetic fields. We find, however, that this does not significantly affect the structure of the white dwarf.

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arXiv2013

Magnetic field structure in accretion columns on HMXB and effects on CRSF

In accreting neutron star binaries, matter is channelled by the magnetic fields from the accretion disc to the poles of neutron stars forming an accretion mound. We model such mounds by numerically solving the Grad-Shafranov equation for axisymmetric static MHD equilibria. From our solutions we infer local distortion of field lines due to the weight of accreted matter. Variation in mass loading at the accretion disc will alter the shape of the accretion mound which will also affect the local field distortion. From simulations of cyclotron resonance scattering features from HMXBs, we conclude that local field distortion will greatly affect the shape and nature of the CRSF. From phase resolved spectral analysis one can infer the local field structure and hence the nature of mass loading of field lines at the accretion disc. We also study the stability of such mounds by performing MHD simulations using the PLUTO MHD code. We find that pressure and gravity driven instabilities depend on the total mass accreted and the nature of mass loading of the field lines.

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