Banibrata Mukhopadhyay

Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts, United States

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Publications (59)183.4 Total impact

  • Debbijoy Bhattacharya · Parameswaran Sreekumar · Banibrata Mukhopadhyay · Ishan Tomar
    [Show abstract] [Hide abstract] ABSTRACT: Blazars are characterized by large intensity and spectral variations across the electromagnetic spectrum It is believed that jets emerging from them are almost aligned with the line-of-sight. The majority of identified extragalactic sources in γ-ray catalogs of EGRET and Fermi are blazars. Observationally, blazars can be divided into two classes: flat spectrum radio quasars (FSRQs) and BL Lacs. BL Lacs usually exhibit lower γ-ray luminosity and harder power law spectra at γ-ray energies than FSRQs. We attempt to explain the high energy properties of FSRQs and BL Lacs from Fermi γ-ray space telescope observations. It was argued previously that the difference in accretion rates is mainly responsible for the large mismatch in observed luminosity in γ-ray. However, when intrinsic luminosities are derived by correcting for beaming effects, this difference in γ-ray luminosity between the two classes is significantly reduced. In order to explain this difference in intrinsic luminosities, we propose that spin plays an important role in the luminosity distribution dichotomy of BL Lacs and FSRQs. As the outflow power of a blazar increases with increasing spin of a central black hole, we suggest that the spin plays a crucial role in making BL Lac sources low luminous and slow rotators compared to FSRQ sources.
    No preview · Article · Apr 2016 · Research in Astronomy and Astrophysics
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    Upasana Das · Banibrata Mukhopadhyay
    [Show abstract] [Hide abstract] ABSTRACT: Our consistent effort to unravel the mystery of super-Chandrasekhar white dwarfs (WDs), by exploiting the potential of magnetic fields, has brought this topic considerable attention. This is also evident from the recent surge in the corresponding literature. In the present work, by means of full-scale general relativistic magnetohydrodynamic (GRMHD) numerical analysis, we confirm the existence of stable, highly magnetized, significantly super-Chandrasekhar WDs having mass exceeding 3 solar mass. We have explored various possible field configurations, namely, poloidal, toroidal and mixed, by self-consistently incorporating the departure from spherical symmetry induced by a strong magnetic field. Such super-Chandrasekhar WDs can be ideal progenitors of peculiar, over-luminous type Ia supernovae.
    Preview · Article · Feb 2016
  • Upasana Das · Banibrata Mukhopadhyay
    [Show abstract] [Hide abstract] ABSTRACT: We establish the importance of modified Einstein’s gravity (MG) in white dwarfs (WDs) for the first time in the literature. We show that MG leads to significantly sub- and super-Chandrasekhar limiting mass WDs, depending on a single model parameter. However, conventional WDs on approaching Chandrasekhar’s limit are expected to trigger Type Ia supernovae (SNeIa), a key to unravel the evolutionary history of the universe. Nevertheless, observations of several peculiar, under- and over-luminous SNeIa argue for the limiting mass widely different from Chandrasekhar’s limit. Explosions of MG induced sub- and super-Chandrasekhar limiting mass WDs explain under- and over-luminous SNeIa respectively, thus unifying these two apparently disjoint sub-classes. Our discovery questions both the global validity of Einstein’s gravity and the uniqueness of Chandrasekhar’s limit.
    No preview · Article · Nov 2015 · International Journal of Modern Physics D
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    Upasana Das · Banibrata Mukhopadhyay
    [Show abstract] [Hide abstract] ABSTRACT: We establish the importance of modified Einstein's gravity (MG) in white dwarfs (WDs) for the first time in the literature. We show that MG leads to significantly sub- and super-Chandrasekhar limiting mass WDs, depending on a single model parameter. However, conventional WDs on approaching Chandrasekhar's limit are expected to trigger type Ia supernovae (SNeIa), a key to unravel the evolutionary history of the universe. Nevertheless, observations of several peculiar, under- and over-luminous SNeIa argue for the limiting mass widely different from Chandrasekhar's limit. Explosions of MG induced sub- and super-Chandrasekhar limiting mass WDs explain under- and over-luminous SNeIa respectively, thus unifying these two apparently disjoint sub-classes. Our discovery questions both the global validity of Einstein's gravity and the uniqueness of Chandrasekhar's limit.
    Preview · Article · Jun 2015
  • Upasana Das · Banibrata Mukhopadhyay
    No preview · Article · Jan 2015 · Physical Review D
  • Upasana Das · Banibrata Mukhopadhyay
    [Show abstract] [Hide abstract] ABSTRACT: We show that the upper bound for the central magnetic field of a super-Chandrasekhar white dwarf calculated by Nityananda and Konar [Phys. Rev. D 89, 103017 (2014)] and in the concerned comment, by the same authors, against our work [U. Das and B. Mukhopadhyay, Phys. Rev. D 86, 042001 (2012)] is erroneous. This in turn strengthens the argument in favor of the stability of the recently proposed magnetized super-Chandrasekhar white dwarfs. We also point out several other numerical errors in their work. Overall we conclude that the arguments put forth by Nityananda and Konar are misleading.
    No preview · Article · Jan 2015 · Physical Review D
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    Upasana Das · Banibrata Mukhopadhyay
    [Show abstract] [Hide abstract] ABSTRACT: The topic of magnetized super-Chandrasekhar white dwarfs is in the limelight, particularly in the last few years, since our proposal of their existence. By full-scale general relativistic magnetohydrodynamic (GRMHD) numerical analysis, we confirm in this work the existence of stable, highly magnetized, significantly super-Chandrasekhar white dwarfs with mass more than 3 solar mass. While a poloidal field geometry renders the white dwarfs oblate, a toroidal field makes them prolate retaining an overall quasi-spherical shape, as speculated in our earlier work. These white dwarfs are expected to serve as the progenitors of over-luminous type Ia supernovae.
    Preview · Article · Nov 2014 · Journal of Cosmology and Astroparticle Physics
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    Upasana Das · Banibrata Mukhopadhyay
    [Show abstract] [Hide abstract] ABSTRACT: Type Ia supernovae (SNeIa), a key to unravel the evolutionary history of the universe, are believed to be triggered in white dwarfs having mass close to the Chandrasekhar limit. However, observations of several peculiar, under- and over-luminous SNeIa argue for exploding masses widely different from this limit. We show that modification of Einstein's gravity leads to significantly sub- and super-Chandrasekhar limiting masses, determined by a single model parameter. Explosions of these sub- and super-Chandrasekhar limiting mass white dwarfs explain under- and over-luminous SNeIa respectively, thus unifying these two apparently disjoint sub-classes. Our discovery raises two fundamental questions. Is the Chandrasekhar limit unique? Is Einstein's gravity the ultimate theory for understanding astronomical phenomena? Both answers appear to be no!
    Preview · Article · Nov 2014 · Journal of Cosmology and Astroparticle Physics
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    Upasana Das · Banibrata Mukhopadhyay
    [Show abstract] [Hide abstract] ABSTRACT: We show that the upper bound for the central magnetic field of a super-Chandrasekhar white dwarf calculated by Nityananda and Konar [Phys. Rev. D 89, 103017 (2014)] is completely erroneous. This in turn strengthens the argument in favor of the stability of the recently proposed magnetized super-Chandrasekhar white dwarfs. We also point out several other numerical errors in their work. Overall we conclude, based on our calculations, that the arguments put forth by Nityananda and Konar are fallacious and misleading.
    Preview · Article · Jun 2014
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    Upasana Das · Banibrata Mukhopadhyay
    [Show abstract] [Hide abstract] ABSTRACT: We address the issue of stability of recently proposed significantly super-Chandrasekhar white dwarfs. We present stable solutions of magnetostatic equilibrium models for super-Chandrasekhar white dwarfs pertaining to various magnetic field profiles. This has been obtained by self-consistently including the effects of the magnetic pressure gradient and total magnetic density in a general relativistic framework. We estimate that the maximum stable mass of magnetized white dwarfs could be more than 3 solar mass. This is very useful to explain peculiar, overluminous type Ia supernovae which do not conform to the traditional Chandrasekhar mass-limit.
    Preview · Article · Apr 2014 · Journal of Cosmology and Astroparticle Physics
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    Upasana Das · Banibrata Mukhopadhyay
    [Show abstract] [Hide abstract] ABSTRACT: Is the Chandrasekhar mass limit for white dwarfs (WDs) set in stone? Not anymore -- recent observations of over-luminous, peculiar type Ia supernovae can be explained if significantly super-Chandrasekhar WDs exist as their progenitors, thus barring them to be used as cosmic distance indicators. However, there is no estimate of a mass limit for these super-Chandrasekhar WD candidates yet. Can they be arbitrarily large? In fact, the answer is no! We arrive at this revelation by exploiting the flux freezing theorem in observed, accreting, magnetized WDs, which brings in Landau quantization of the underlying electron degenerate gas. This essay presents the calculations which pave the way for the ultimate (significantly super-Chandrasekhar) mass limit of WDs, heralding a paradigm shift 80 years after Chandrasekhar's discovery.
    Preview · Article · May 2013 · International Journal of Modern Physics D
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    Upasana Das · Banibrata Mukhopadhyay
    [Show abstract] [Hide abstract] ABSTRACT: We clarify important physics issues related to the recently established new mass limit for magnetized white dwarfs which is significantly super-Chandrasekhar. The issues include, justification of high magnetic field and the corresponding formation of stable white dwarfs, contribution of the magnetic field to the total density and pressure, flux freezing, variation of magnetic field and related currents therein. We also attempt to address the observational connection of such highly magnetized white dwarfs.
    Preview · Article · Apr 2013 · Modern Physics Letters A
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    Upasana Das · Banibrata Mukhopadhyay · A. R. Rao
    [Show abstract] [Hide abstract] ABSTRACT: Several recently discovered peculiar Type Ia supernovae seem to demand an altogether new formation theory that might help explain the puzzling dissimilarities between them and the standard Type Ia supernovae. The most striking aspect of the observational analysis is the necessity of invoking super-Chandrasekhar white dwarfs having masses ~2.1-2.8 M ☉, M ☉ being the mass of Sun, as their most probable progenitors. Strongly magnetized white dwarfs having super-Chandrasekhar masses have already been established as potential candidates for the progenitors of peculiar Type Ia supernovae. Owing to the Landau quantization of the underlying electron degenerate gas, theoretical results yielded the observationally inferred mass range. Here, we sketch a possible evolutionary scenario by which super-Chandrasekhar white dwarfs could be formed by accretion on to a commonly observed magnetized white dwarf, invoking the phenomenon of flux freezing. This opens multiple possible evolution scenarios ending in supernova explosions of super-Chandrasekhar white dwarfs having masses within the range stated above. We point out that our proposal has observational support, such as the recent discovery of a large number of magnetized white dwarfs by the Sloan Digital Sky Survey.
    Preview · Article · Mar 2013 · The Astrophysical Journal Letters
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    Dataset: NUPHA19373
    Monika Sinha · Banibrata Mukhopadhyay · Armen Sedrakian
    Full-text · Dataset · Mar 2013
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    Upasana Das · Banibrata Mukhopadhyay
    [Show abstract] [Hide abstract] ABSTRACT: We consider a relativistic, degenerate, electron gas under the influence of a strong magnetic field, which describes magnetized white dwarfs. Landau quantization changes the density of states available to the electrons, thus modifying the underlying equation of state. We obtain the mass-radius relations for such white dwarfs and show that it is possible to have magnetized white dwarfs with a mass significantly greater than the Chandrasekhar limit in the range 2.3 - 2.6 M_sun. Recent observations of peculiar type Ia supernovae - SN 2006gz, SN 2007if, SN 2009dc, SN 2003fg - seem to suggest super-Chandrasekhar-mass white dwarfs with masses up to 2.4 - 2.8 M_sun, as their most likely progenitors and interestingly our results lie within the observational limits.
    Preview · Article · Feb 2013
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    Banibrata Mukhopadhyay · Monika Sinha
    [Show abstract] [Hide abstract] ABSTRACT: The magnetars are believed to be highly magnetized neutron stars having surface magnetic field 10^{14} - 10^{15} G. It is believed that at the center, the magnetic field may be higher than that at the surface. We study the effect of the magnetic field on the neutron star matter. We model the nuclear matter with the relativistic mean field approach considering the possibility of appearance of hyperons at higher density. We find that the effect of magnetic field on the matter of neutron stars and hence on the mass-radius relation is important, when the central magnetic field is atleast of the order of 10^{17} G. Very importantly, the effect of strong magnetic field reveals anisotropy to the system. Moreover, if the central field approaches 10^{19} G, then the matter becomes unstable which limits the maximum magnetic field at the center of magnetars.
    Preview · Article · Feb 2013
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    Upasana Das · Banibrata Mukhopadhyay
    [Show abstract] [Hide abstract] ABSTRACT: Type Ia supernovae, sparked off by exploding white dwarfs of mass close to Chandrasekhar limit, play the key role to understand the expansion rate of universe. However, recent observations of several peculiar type Ia supernovae argue for its progenitor mass to be significantly super-Chandrasekhar. We show that strongly magnetized white dwarfs not only can violate the Chandrasekhar mass limit significantly, but exhibit a different mass limit. We establish from foundational level that the generic mass limit of white dwarfs is 2.58 solar mass. This explains the origin of over-luminous peculiar type Ia supernovae. Our finding further argues for a possible second standard candle, which has many far reaching implications, including a possible reconsideration of the expansion history of the universe.
    Preview · Article · Jan 2013 · Physical Review Letters
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    Banibrata Mukhopadhyay · Debbijoy Bhattacharya · P. Sreekumar
    [Show abstract] [Hide abstract] ABSTRACT: Since it was theorized by Kerr in 1963, determining the spin of black holes from observed data was paid very little attention until few years back. The main reasons behind this were the unavailability of adequate data and the lack of appropriate techniques. In this article, we explore determining/predicting the spin of several black holes in X-ray binaries and in the center of galaxies, using X-ray and gamma-ray satellite data. For X-ray binaries, in order to explain observed quasi-periodic oscillations, our model predicts the spin parameter of underlying black holes. On the other hand, the nature of spin parameters of black holes in BL Lacs and Flat Spectrum Radio Quasars is predicted by studying the total luminosities of systems based on Fermi gamma-ray data. All sources considered here exhibit characteristics of spinning black holes, which verifies natural existence of the Kerr metric.
    Full-text · Article · Oct 2012 · International Journal of Modern Physics D
  • Banibrata Mukhopadhyay · Debbijoy Bhattacharya · Sreekumar Parameswaran
    [Show abstract] [Hide abstract] ABSTRACT: Several observational evidences and deeper theoretical insights reveal that accretion and outflow/jet are strongly correlated. We model an advective disk-outflow coupled dynamics, incorporating explicitly the vertical flux. Inter-connecting dynamics of outflow and accretion essentially upholds the conservation laws. We investigate the properties of the disk-outflow surface and its strong dependence on the rotation parameter of the black hole. The energetics of the disk-outflow strongly depend on the mass, accretion rate and spin of the black holes. The model clearly shows that the outflow power extracted from the disk increases strongly with the spin of the black hole, inferring that the power of the observed astrophysical jets has a proportional correspondence with the spin of the central object. Now blazars are characterized by large intensity and spectral variations across the electromagnetic waveband. Observationally, blazars can be divided into two classes: flat spectrum radio quasars (FSRQs) and BL Lacs. BL Lacs usually exhibit lower luminosity and harder power law spectra at {γ}-ray energies than those of FSRQs. We attempt to explain the high energy properties of FSRQs and BL Lacs from Fermi {γ}-ray space telescope observations, based on our model. We propose that spin plays an important role in the luminosity distribution dichotomy of BL Lacs and FSRQs.
    No preview · Article · Jul 2012
  • Monika Sinha · Ritam Mallick · Banibrata Mukhopadhyay
    [Show abstract] [Hide abstract] ABSTRACT: The magnetars are believed to be neutron stars with surface magnetic field 10^{14}-10^{15} G. However, the central magnetic field may be higher than that at the surface. We study the effect of strong magnetic field on neutron star matter. We model the neutron star matter with non-linear Walecka model including hyperons. We find that the effect of magnetic field on the matter is significant when central magnetic field ≥10^{17} G. Moreover, if the central field is of the order of 10^{19} G, then the magnetized matter becomes unstable which limits the maximum central magnetic field of magnetars. We also study the effect of high magnetic field on the phase transition from neutron star to quark star. Magnetic field helps in initiation of the conversion process. The velocity of the conversion front, however, decreases due to the presence of magnetic field.
    No preview · Article · Jul 2012

Publication Stats

630 Citations
183.40 Total Impact Points

Institutions

  • 2004-2007
    • Harvard-Smithsonian Center for Astrophysics
      • Institute for Theory and Computation
      Cambridge, Massachusetts, United States
  • 2002-2003
    • The Inter-University Centre for Astronomy and Astrophysics
      Poona, Mahārāshtra, India
    • Physical Research Laboratory
      Amadavad, Gujarat, India