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Banibrata Mukhopadhyay

Publications and source records attributed to Banibrata Mukhopadhyay.

At least 73 records · Page 4Linked to original sources

Gravitational geometric phase

We show that spinors propagating in curved gravitational background acquire an interaction with spacetime curvature, which leads to a quantum mechanical geometric effect. This is similar to what happens in the case of magnetic fields, known as Pancharatnam-Berry phase. As the magnetic and gravitational fields have certain similar properties, e.g. both contribute to curvature, this result is not difficult to understand. Interestingly, while spacetime around a rotating black hole offers Aharonov-Bohm and Pancharatnam-Berry both kinds of geometric effect, a static spacetime offers only the latter. In the bath of primordial black holes, such gravity induced effects could easily be measured due to their smaller radius.

gr-qc↗

Relativistic Landau quantization in non-uniform magnetic field and its applications to white dwarfs and quantum information

We investigate the two-dimensional motion of relativistic cold electrons in the presence of `strictly' spatially varying magnetic fields satisfying, however, no magnetic monopole condition. We find that the degeneracy of Landau levels, which arises in the case of the constant magnetic field, lifts out when the field is variable and the energy levels of spin-up and spin-down electrons align in an interesting way depending on the nature of change of field. Also the varying magnetic field splits Landau levels of electrons with zero angular momentum from positive angular momentum, unlike the constant field which only can split the levels between positive and negative angular momenta. Exploring Landau quantization in non-uniform magnetic fields is a unique venture on its own and has interdisciplinary implications in the fields ranging from condensed matter to astrophysics to quantum information. As examples, we show magnetized white dwarfs, with varying magnetic fields, involved simultaneously with Lorentz force and Landau quantization affecting the underlying degenerate electron gas, exhibiting a significant violation of the Chandrasekhar mass-limit; and an increase in quantum speed of electrons in the presence of a spatially growing magnetic field.

quant-ph↗

Resolving dichotomy in compact objects through continuous gravitational waves observation

More than two dozen soft gamma-ray repeaters (SGRs) and anomalous X-ray pulsars (AXPs) have been detected so far. These are isolated compact objects. Many of them are either found to be associated with supernova remnants or their surface magnetic fields are directly measured, confirming that they are neutron stars (NSs). However, it has been argued that some SGRs and AXPs are highly magnetized white dwarfs (WDs). Meanwhile, the existence of super-Chandrasekhar WDs has remained to be a puzzle. However, not even a single such massive WD has been observed directly. Moreover, some WD pulsars are detected in electromagnetic surveys and some of their masses are still not confirmed. Here we calculate the signal-to-noise ratio for all these objects, considering different magnetic field configurations and thereby estimate the required time for their detection by various gravitational wave (GW) detectors. For SGRs and AXPs, we show that, if these are NSs, they can hardly be detected by any of the GW detectors, while if they are WDs, Big Bang Observer (BBO), DECi-hertz Interferometer Gravitational wave Observatory (DECIGO) and Advanced Laser Interferometer Antenna (ALIA) would be able to detect them within a few days to a year of integration, depending on the magnetic field strength and its configuration. Similarly, if a super-Chandrasekhar WD has a dominant toroidal field, we show that even Laser Interferometer Space Antenna (LISA) and TianQin would be able to detect it within one year of integration. We also discuss how GWs can confirm the masses of the WD pulsars.

astro-ph.HE↗

Effects of anisotropy on strongly magnetized neutron and strange quark stars in general relativity

We investigate the properties of anisotropic, spherically symmetric compact stars, especially neutron stars and strange quark stars, made of strongly magnetized matter. The neutron stars are described by SLy equation of state, the strange quark stars by an equation of state based on the MIT Bag model. The stellar models are based on an a priori assumed density dependence of the magnetic field and thus anisotropy. Our study shows that not only the presence of a strong magnetic field and anisotropy, but also the orientation of the magnetic field itself, have an important influence on the physical properties of stars. Two possible magnetic field orientations are considered, a radial orientation, where the local magnetic fields point in the radial direction, and a transverse orientation, where the local magnetic fields are perpendicular to the radial direction. Interestingly, we find that for a transverse orientation of the magnetic field, the stars become more massive with increasing anisotropy and magnetic field strength and increase in size, since the repulsive, effective anisotropic force increases in this case. In the case of a radially orientated magnetic field, however, the masses and radii of the stars decrease with increasing magnetic field strength, because of the decreasing effective anisotropic force. Importantly, we also show that in order to achieve hydrostatic equilibrium configurations of magnetized matter, it is essential to account for both the local anisotropy effects as well as the anisotropy effects caused by a strong magnetic field. Otherwise, hydrostatic equilibrium is not achieved for magnetized stellar models.

astro-ph.HE↗

Super-Chandrasekhar limiting mass white dwarfs as emergent phenomena of noncommutative squashed fuzzy spheres

The indirect evidence for at least a dozen massive white dwarfs violating the Chandrasekhar mass-limit is considered to be one of the wonderful discoveries in astronomy for more than a decade. Researchers have already proposed a diverse amount of models to explain this astounding phenomenon. However, each of these models always carries some drawbacks. On the other hand, noncommutative geometry is one of the best replicas of quantum gravity, which is yet to be proved from observations. Madore introduced the idea of a fuzzy sphere to describe a formalism of noncommutative geometry. This article shows that the idea of a squashed fuzzy sphere can self-consistently explain the super-Chandrasekhar limiting mass white dwarfs. We further show that the length-scale beyond which the noncommutativity is prominent is an emergent phenomenon, and there is no prerequisite for an ad-hoc length-scale.

gr-qc↗

Forced linear shear flows with rotation: rotating Couette-Poiseuille flow, its stability and astrophysical implications

We explore the effect of forcing on the linear shear flow or plane Couette flow, which is also the background flow in the very small region of the Keplerian accretion disk. We show that depending on the strength of forcing and boundary conditions suitable for the systems under consideration, the background plane shear flow and, hence, accretion disk velocity profile modifies to parabolic flow, which is plane Poiseuille flow or Couette-Poiseuille flow, depending on the frame of reference. In the presence of rotation, plane Poiseuille flow becomes unstable at a smaller Reynolds number under pure vertical as well as threedimensional perturbations. Hence, while rotation stabilizes plane Couette flow, the same destabilizes plane Poiseuille flow faster and forced local accretion disk. Depending on the various factors, when local linear shear flow becomes Poiseuille flow in the shearing box due to the presence of extra force, the flow becomes unstable even for the Keplerian rotation and hence turbulence will pop in there. This helps in resolving a long standing problem of sub-critical transition to turbulence in hydrodynamic accretion disks and laboratory plane Couette flow.

physics.flu-dyn↗

Geometric phase for Dirac Hamiltonian under gravitational fields in the non-relativistic regime

We show the appearance of geometric phase in a Dirac particle traversing in non-relativistic limit in a time-independent gravitational field. This turns out to be similar to the one originally described as a geometric phase in magnetic fields. We explore the geometric phase in the Kerr and Schwarzschild geometries, which have significant astrophysical implications. Nevertheless, the work can be extended to any spacetime background including that of time-dependent. In the Kerr background, i.e. around a rotating black hole, geometric phase reveals both the Aharonov-Bohm effect and Pancharatnam-Berry phase. However, in a Schwarzschild geometry, i.e. around a nonrotating black hole, only the latter emerges. We expect that our assertions can be validated in both the strong gravity scenarios, like the spacetime around black holes, and weak gravity environment around Earth.

gr-qc↗

Thermal irradiation induced wind outflow in a geometrically thin accretion disk: A hydrodynamic study

Many astrophysical sources, e.g., cataclysmic variables, X-ray binaries, active galactic nuclei, exhibit a wind outflow, when they reveal a multicolor blackbody spectrum, hence harboring a geometrically thin Keplerian accretion disk. Unlike an advective disk, in the thin disk, the physical environment, like, emission line, external heating, is expected to play a key role to drive the wind outflow. We show the wind outflow in a thin disk attributing a disk irradiation effect, probably from the inner to outer disks. We solve the set of steady, axisymmetric disk model equations in cylindrical coordinates along the vertical direction for a given launching radius $(r)$ from the midplane, introducing irradiation as a parameter. We obtain an acceleration solution, for a finite irradiation in the presence of a fixed but tiny initial vertical velocity (hence thin disk properties practically do not alter) at the midplane, upto a maximum height ($z^{max}$). We find that wind outflow mainly occurs from the outer region of the disk and its density decreases with increasing launching radius, and for a given launching radius with increasing ejection height. Wind power decreases with increasing ejection height. For $z^{max} < 2r$, wind outflow is ejected tangentially (or parallel to the disk midplane) in all directions with the fluid speed same as the azimuthal speed. This confirms mainly, for low mass X-ray binaries, (a) wind outflow should be preferentially observed in high-inclination sources, (b) the expectation of red and blue shifted absorption lines.

astro-ph.HE↗

Modified Newtonian Gravity: Explaining observations of sub- and super-Chandrasekhar limiting mass white dwarfs

The idea of possible modification to gravity theory, whether it is in the Newtonian or general relativistic premises, is there for quite sometime. Based on it, astrophysical and cosmological problems are targeted to solve. But none of the Newtonian theories of modification has been performed from the first principle. Here, we modify Poisson's equation and propose two possible ways to modify the law gravitation which, however, reduces to Newton's law far away from the center of source. Based on these modified Newton's laws, we attempt to solve problems lying with white dwarfs. There are observational evidences for possible violation of the Chandrasekhar mass-limit significantly: it could be sub- as well as super-Chandrasekhar. We show that modified Newton's law, either by modifying LHS or RHS of Poisson's equation, can explain them.

gr-qc↗

Gravitational wave in f(R) gravity: possible signature of sub- and super-Chandrasekhar limiting mass white dwarfs

After the prediction of many sub- and super-Chandrasekhar (at least a dozen for the latter) limiting mass white dwarfs, hence apparently peculiar class of white dwarfs, from the observations of luminosity of type Ia supernovae, researchers have proposed various models to explain these two classes of white dwarfs separately. We earlier showed that these two peculiar classes of white dwarfs, along with the regular white dwarfs, can be explained by a single form of the f(R) gravity, whose effect is significant only in the high-density regime, and it almost vanishes in the low-density regime. However, since there is no direct detection of such white dwarfs, it is difficult to single out one specific theory from the zoo of modified theories of gravity. We discuss the possibility of direct detection of such white dwarfs in gravitational wave astronomy. It is well-known that in f(R) gravity, more than two polarization modes are present. We estimate the amplitudes of all the relevant modes for the peculiar as well as the regular white dwarfs. We further discuss the possibility of their detections through future-based gravitational wave detectors, such as LISA, ALIA, DECIGO, BBO, or Einstein Telescope, and thereby put constraints or rule out various modified theories of gravity. This exploration links the theory with possible observations through gravitational wave in f(R) gravity.

astro-ph.HE↗

Significantly super-Chandrasekhar mass-limit of white dwarfs in noncommutative geometry

Chandrasekhar made the startling discovery about nine decades back that the mass of compact object white dwarf has a limiting value, once nuclear fusion reactions stop therein. This is the Chandrasekhar mass-limit, which is $\sim1.4M_\odot$ for a non-rotating non-magnetized white dwarf. On approaching this limiting mass, a white dwarf is believed to spark off with an explosion called type Ia supernova, which is considered to be a standard candle. However, observations of several over-luminous, peculiar type Ia supernovae indicate that the Chandrasekhar mass-limit to be significantly larger. By considering noncommutativity among the components of position and momentum variables, hence uncertainty in their measurements, at the quantum scales, we show that the mass of white dwarfs could be significantly super-Chandrasekhar and thereby arrive at a new mass-limit $\sim 2.6M_\odot$, explaining a possible origin of over-luminous peculiar type Ia supernovae. The idea of noncommutativity, apart from the Heisenberg's uncertainty principle, is there for quite sometime, without any observational proof however. Our finding offers a plausible astrophysical evidence of noncommutativity, arguing for a possible second standard candle, which has many far-reaching implications.

gr-qc↗

Origin of hydrodynamic instability from noise: from laboratory flow to accretion disk

We attempt to address the old problem of plane shear flows: the origin of turbulence and hence transport of angular momentum in accretion flows as well as laboratory flows, such as plane Couette flow. We undertake the problem by introducing an extra force in Orr-Sommerfeld and Squire equations along with the Coriolis force mimicking the local region of the accretion disk. For plane Couette flow, the Coriolis term drops. Subsequently we solve the equations by WKB approximation method. We investigate the dispersion relation for the Keplerian flow and plane Couette flow for all possible combinations of wave vectors. Due to the very presence of extra force, we show that both the flows are unstable for a certain range of wave vectors. However, the nature of instability between the flows is different. We also study the Argand diagrams of the perturbation eigenmodes. It helps us to compare the different time scales corresponding to the perturbations as well as accretion. We ultimately conclude with this formalism that fluid gets enough time to be unstable and hence plausibly turbulent particularly in the local regime of the Keplerian accretion disks. Repetition of the analysis throughout the disk explains the transport of angular momentum and matter along outward and inward direction respectively.

astro-ph.HE↗

Modified virial theorem for highly magnetized white dwarfs

Generally the virial theorem provides a relation between various components of energy integrated over a system. This helps us to understand the underlying equilibrium. Based on the virial theorem we can estimate, for example, the maximum allowed magnetic field in a star. Recent studies have proposed the existence of highly magnetized white dwarfs, with masses significantly higher than the Chandrasekhar limit. Surface magnetic fields of such white dwarfs could be more than 10^9 G with the central magnitude several orders higher. These white dwarfs could be significantly smaller in size than their ordinary counterparts (with surface fields restricted to about 10^9 G). In this paper we reformulate the virial theorem for non-rotating, highly magnetized white dwarfs (B-WDs) in which, unlike in previous formulations, the contribution of the magnetic pressure to the magnetohydrostatic balance cannot be neglected. Along with the new equation of magnetohydrostatic equilibrium, we approach the problem by invoking magnetic flux conservation and by varying the internal magnetic field with the matter density as a power law. Either of these choices are supported by previous independent work and neither violates any important physics. They are useful while there is no prior knowledge of field profile within a white dwarf. We then compute the modified gravitational, thermal and magnetic energies and examine how the magnetic pressure influences the properties of such white dwarfs. Based on our results we predict important properties of these B-WDs, which turn out to be independent of our chosen field profiles.

astro-ph.SR↗

Gravity-Induced Geometric Phases and Entanglement in Spinors and Neutrinos: Gravitational Zeeman Effect

We show Zeeman-like splitting in the energy of spinors propagating in a background gravitational field, analogous to the spinors in an electromagnetic field, otherwise termed the Gravitational Zeeman Effect. These spinors are also found to acquire a geometric phase, in a similar way as they do in the presence of magnetic fields. However, in a gravitational background, the Aharonov-Bohm type effect, in addition to Berry-like phase, arises. Based on this result, we investigate geometric phases acquired by neutrinos propagating in a strong gravitational field. We also explore entanglement of neutrino states due to gravity, which could induce neutrino-antineutrino oscillation in the first place. We show that entangled states also acquire geometric phases which are determined by the relative strength between gravitational field and neutrino masses.

gr-qc↗

Suppression of luminosity and mass-radius relation of highly magnetized white dwarfs

We explore the luminosity L of magnetized white dwarfs and its effect on the mass-radius relation. We self-consistently obtain the interface between the electron degenerate gas dominated inner core and the outer ideal gas surface layer or envelope by incorporating both the components of gas throughout the model white dwarf. This is obtained by solving the set of magnetostatic equilibrium, photon diffusion and mass conservation equations in the Newtonian framework, for different sets of luminosity and magnetic field. We appropriately use magnetic opacity, instead of Kramer's opacity, wherever required. We show that the Chandrasekhar-limit is retained, even at high luminosity upto about 10^{-2} solar luminosity but without magnetic field, if the temperature is set constant inside the interface. However there is an increased mass for large-radius white dwarfs, an effect of photon diffusion. Nevertheless, in the presence of strong magnetic fields, with central strength of about 10^{14} G, super-Chandrasekhar white dwarfs, with masses of about 1.9 solar mass, are obtained even when the temperature inside the interface is kept constant. Most interestingly, small-radius magnetic white dwarfs remain super-Chandrasekhar even if their luminosity decreases to as low as about 10^{-20} solar luminosity. However, their large-radius counterparts in the same mass-radius relation merge with Chandrasekhar's result at low L. Hence, we argue for the possibility of highly magnetized, low luminous super-Chandrasekhar mass white dwarfs which, owing to their faintness, can be practically hidden.

astro-ph.SR↗

Hydrodynamical instability with noise in the Keplerian accretion discs: Modified Landau equation

Origin of hydrodynamical instability and turbulence in the Keplerian accretion disc as well as similar laboratory shear flows, e.g. plane Couette flow, is a long standing puzzle. These flows are linearly stable. Here we explore the evolution of perturbation in such flows in the presence of an additional force. Such a force, which is expected to be stochastic in nature hence behaving as noise, could be result of thermal fluctuations (however small be), Brownian ratchet, grain-fluid interactions and feedback from outflows in astrophysical discs etc. We essentially establish the evolution of nonlinear perturbation in the presence of Coriolis and external forces, which is modified Landau equation. We show that even in the linear regime, under suitable forcing and Reynolds number, the otherwise least stable perturbation evolves to a very large saturated amplitude, leading to nonlinearity and plausible turbulence. Hence, forcing essentially leads a linear stable mode to unstable. We further show that nonlinear perturbation diverges at a shorter timescale in the presence of force, leading to a fast transition to turbulence. Interestingly, emergence of nonlinearity depends only on the force but not on the initial amplitude of perturbation, unlike original Landau equation based solution.

astro-ph.HE↗

Timescales for detection of super-Chandrasekhar white dwarfs by gravitational wave astronomy

In about last couple of decades, the inference of the violation of the Chandrasekhar mass-limit of white dwarfs from indirect observation is probably a revolutionary discovery in astronomy. Various researchers have already proposed different theories to explain this interesting phenomenon. However, such massive white dwarfs usually possess very little luminosity, and hence they, so far, cannot be detected directly by any observations. We have already proposed that the continuous gravitational wave may be one of the probes to detect them directly, and in the future, various space-based detectors such as LISA, DECIGO, and BBO, should be able to detect many of those white dwarfs (provided they behave like pulsars). In this paper, we address various timescales related to the emission of gravitational as well as dipole radiations. This exploration sets a timescale for the detectors to observe the massive white dwarfs.

astro-ph.HE↗

Role of magnetically dominated disc-outflow symbiosis on bright hard-state black hole sources: ultra-luminous X-ray sources to quasars

We present optically thin solutions for magnetized, advective disc-outflow symbiosis around black holes (BHs). The main objective is to explain the bright, hard state observations of accreting systems with stellar-mass to supermassive BHs. We include the effects of magnetic fields and radiation counterpart in entropy gradient based on the first law of thermodynamics to represent energy advection. The cooling process includes bremsstrahlung, synchrotron radiation, and inverse Comptonization process. One of our main ventures is to explain some long-standing issues of ultra-luminous X-ray sources (ULXs). The existing physical scenarios to explain their unusual high luminosity are either the existence of the missing class of intermediate-mass BH (IMBH), or super-Eddington accretion around a stellar-mass BH. However, most ULXs with steep power-law spectrum can be well explained through super-Eddington accretion, while the existence of IMBH is indeed disputed extensively. Nevertheless, the interpretation of ULXs with a hard power-law dominated state remains mysterious. Here we show that our magnetically dominated disc-outflow symbiosis around rapidly spinning stellar-mass BHs can achieve such large luminosity even for sub-Eddington accretion rate. The magnetic field at the outer zone of the advective flow is more than the corresponding Eddington limit. Such field becomes dynamically dominant near the BH through continuous accretion process due to flux freezing, but maintaining its Eddington limit. This unique field configurations enhance the synchrotron, and synchrotron self-Comptonization process to achieve very large luminosity. Through the same mechanism, our solutions for supermassive BHs can explain the unusual large luminosity of ultra-luminous quasars.

astro-ph.HE↗