Browsing by Author "Bhattacharya, Dipankar"

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    Compact Objects
    (2013-07-16) Bhattacharya, Dipankar
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    Cyclotron Resonance lines with ASTROSAT
    (2011-01-29) Bhattacharya, Dipankar
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    Evolution of multipolar magnetic field in isolated neutron stars
    (2015-03-13) Mitra, Dipanjan; Konar, Sushan; Bhattacharya, Dipankar
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    Hard Electron Energy Distribution in the Relativistic Shocks of GRB Afterglows
    (2008-04) Resmi, L.; Bhattacharya, Dipankar
    Particle acceleration in relativistic shocks is not a very well understood subject. Owing to that difficulty, radiation spectra from relativistic shocks, such as those in GRB afterglows, have been often modelled by making assumptions about the underlying electron distribution. One such assumption is a relatively soft distribution of the particle energy, which need not be true always, as is obvious from observations of several GRB afterglows. In this paper, we describe modifications to the afterglow standard model to accommodate energy spectra which are ‘hard’. We calculate the overall evolution of the synchrotron and compton flux arising from such a distribution. We also model two afterglows, GRB010222 and GRB020813, under this assumption and estimate the physical parameters.
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    Magnetic field evolution of accreting neutron stars - III
    (2015-03-13) Konar, Sushan; Bhattacharya, Dipankar
    The evolutionary scenario of the neutron star magnetic field is examined assuming a spindown-induced expulsion of magnetic flux originally confined to the core, in which case the expelled flux undergoes ohmic decay. The nature of field evolution, for accreting neutron stars, is investigated incorporating the crustal microphysics and material movement due to accretion. This scenario explains the observed field strengths of neutron stars but only if the crustal lattice contains a large amount of impurity which is in direct contrast to the models that assume an original crustal field.
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    Magnetic field evolution of accreting neutron stars- II
    (2015-03-13) Konar, Sushan; Bhattacharya, Dipankar
    We investigate the evolution of the magnetic field of isolated pulsars and of neutron stars in different kinds of binary systems, assuming the field to be originally confined to the crust. Our results for the field evolution in isolated neutron stars helps us to constrain the physical parameters of the crust. Modelling the full evolution of a neutron star in a binary system through several stages of interaction we compare the resulting final field strength with that observed in neutron stars in various types of binary systems. One of the interesting aspects of our result is a positive correlation between the rate of accretion and the final field strength, for which some observational indication already exists. Our results also match the overall picture of the field evolution in neutron stars derived from observations.
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    Magnetic fields of neutron stars
    (2015-03-01) Konar, Sushan; Bhattacharya, Dipankar
    The evolution of the magnetic field is investigated for isolated as well as binary neutron stars. The overall nature of the field evolution is seen to be similar for an initial crustal field and an expelled flux. The major uncertainties of the present models of field evolution and the directions in which further investigation are required are also discussed in detail.
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    Type IIP Supernova SN 2004et: A Multi-Wavelength Study in X-Ray, Optical and Radio
    (2008-06) Bhattacharya, Dipankar
    Context. We explore the physics behind one of the brightest radio afterglows ever, GRB 030329, at late times when the jet is non- relativistic. Aims. We determine the physical parameters of the blast wave and its surroundings, in particular the index of the electron energy distribution, the energy of the blast wave, and the density (structure) of the circumburst medium. We then compare our results with those from image size measurements. Methods. We observed the GRB030329 radio afterglow with the Westerbork Synthesis Radio Telescope and the Giant Metrewave Radio Telescope at frequencies from 325 MHz to 8.4 GHz, spanning a time range of 268-1128 days after the burst. We modeled all the available radio data and derived the physical parameters. Results. The index of the electron energy distribution is p = 2.1, the circumburst medium is homogeneous, and the transition to the non-relativistic phase happens at tNR ∼ 80 days. The energy of the blast wave and density of the surrounding medium are comparable to previous findings. Conclusions. Our findings indicate that the blast wave is roughly spherical at tNR, and they agree with the implications from the VLBI studies of image size evolution. It is not clear from the presented dataset whether we have seen emission from the counter jet or not. We predict that the Low Frequency Array will be able to observe the afterglow of GRB030329 and many other radio afterglows, constraining the physics of the blast wave during its non-relativistic phase even further.