arXiv subjects

R. Samtaney

Publications and source records attributed to R. Samtaney.

8 recordsLinked to original sources

arXiv2020

The remnant of GW170817: a trapped neutron star with a hypermassive incompressible superfluid core

Our bimetric spacetime model of glitching pulsars is applied to the remnant of GW170817. Accordingly, pulsars are born with embryonic incompressible superconducting gluon-quark superfluid cores (SuSu-matter) that are embedded in Minkowski spacetime, whereas the ambient compressible and dissipative media (CDM) are imbedded in curved spacetime. As pulsars cool down, the equilibrium between both spacetime is altered, thereby triggering the well-observed glitch phenomena. Based thereon and assuming all neutron stars (NSs) to be born with the same initial mass of $M_{NS}(t=0) \approx 1.25\,\mathcal{M}_{\odot},$ we argue that the remnant of GW170817 should be a relatively faint NS with a hypermassive central core made of SuSu-matter. The effective mass and radius of the remnant are predicted to be $[2.8 \mathcal{M}_{\odot} < \mathcal{M}_{rem} \le 3.351 \mathcal{M}_{\odot}]$ and $R_{rem}=10.764$ km, whereas the mass of the enclosed SuSu-core is $\mathcal{M}_{core}=1.7 \mathcal{M}_{\odot}.$ Here, about $1/2~ \mathcal{M}_{core}$ is an energy enhancement triggered by the phase transition of the gluon-quark-plasma from the microscopic into macroscopic scale. The current compactness of the remnant is $\alpha_c = 0.918,$ but predicted to increase as the CDM and cools down, rendering the remnant an invisible dark energy object, and therefore to an excellent black hole candidate.

astro-ph.HE↗

arXiv2020

Richtmyer-Meshkov Instability of an Imploding Flow with a Two-Fluid Plasma Model

The two-fluid (ions and electrons) plasma Richtmyer-Meshkov instability of a cylindrical light/heavy density interface is numerically investigated without an initial magnetic field. Varying the Debye length scale, we examine the effects of the coupling between the electron and ion fluids. When the coupling becomes strong, the electrons are restricted to co-move with the ions and the resulting evolution is similar to the hydrodynamic neutral fluid case. The charge separation that occurs between the electrons and ions results in self-generated electromagnetic fields. We show that the Biermann battery effect dominates the generation of magnetic field when the coupling between the electrons and ions is weak. In addition to the Rayleigh-Tayler stabilization effect during flow deceleration, the interfaces are accelerated by the induced spatio-temporally varying Lorentz force. As a consequence, the perturbations develop into the Rayleigh-Taylor instability, leading to an enhancement of the perturbation amplitude compared with the hydrodynamic case.

physics.plasm-ph↗

arXiv2020

How massive are the superfluid cores in the Crab and Vela pulsars and why their glitch-events are accompanied with under and overshootings?

The Crab and Vela are well-studied glitching pulsars and the data obtained sofar should enable us to test the reliability of models of their internal structures. Very recently it was proposed that glitching pulsars are embedded in bimetric spacetime: their incompressible superfluid cores (SuSu-cores) are embedded in flat spacetime, whereas the ambient compressible and dissipative media are enclosed in Schwarzschild spacetime. In this letter we apply this model to the Crab and Vela pulsars and show that a newly born pulsar initially of $1.25\text{M}_{\odot}$ and an embryonic SuSu-core of $0.029\text{M}_{\odot}$ could evolve into a \ARZL{Crab-like}\ARZR pulsar after 1000 years and into a \ARZL{Vela-like}\ARZR pulsar 10000 years later to finally fade away as an invisible dark energy object after roughly 10 Myr. Based thereon we infer that the Crab and the Vela pulsars should have SuSu-cores of $0.15\text{M}_{\odot}$ and $0.55\text{M}_{\odot}$, respectively. Futhermore, the under- and overshootings phenomena observed to accompany the glitch events of the Vela pulsar are rather a common phenomenon of glitching pulsars that can be well-explained within the framework of bimetric spacetime.

astro-ph.HE↗

arXiv2019

Glitching pulsars: unraveling the interactions of general relativity with quantum fields in the strong field regimes

We present a modification of our previous model for the mechanisms underlying the glitch phenomena in pulsars and young neutron stars. Accordingly, pulsars are born with embryonic cores comprising of purely incompressible superconducting gluon-quark superfluid (henceforth SuSu-cores). As the ambient medium cools and spins down due to emission of magnetic dipole radiation, the mass and size of SuSu-cores are set to grow discretely with time, in accordance with the Onsager-Feynmann analysis of superfluidity. Presently, we propose that the spacetime embedding glitching pulsars is dynamical and of bimetric nature: inside SuSu-cores the spacetime must be flat, whereas the surrounding region, where the matter is compressible and dissipative, the spacetime is Schwarzschild. It is further proposed that the topological change of spacetime is derived by the strong nuclear force, whose operating length scales is found to increase with time to reach O(1) cm at the end of the luminous lifetimes of pulsars. The model presented here model is in line with the recent radio and GW observations of pulsars and NSs.

gr-qc↗

arXiv2019

Cross helicity sign reversals in the dissipative scales of magnetohydrodynamic turbulence

We perform direct numerical simulations of magnetohydrodynamic (MHD) turbulence with kinetic energy and cross helicity injections at large scales. We observe that the cross helicity changes sign as we go from large and intermediate scales to small scales. In addition, the magnetic reconnections are strongest at the regions where the cross helicity changes sign and becomes smallest in magnitude. Thus, our simulations provide an important window to explore the regions of magnetic reconnections in nonlinear MHD.

physics.plasm-ph↗

arXiv2018

Kinetic Turbulence in Astrophysical Plasmas: Waves and/or Structures?

The question of the relative importance of coherent structures and waves has for a long time attracted a great deal of interest in astrophysical plasma turbulence research, with a more recent focus on kinetic scale dynamics. Here we utilize high-resolution observational and simulation data to investigate the nature of waves and structures emerging in a weakly collisional, turbulent kinetic plasma. Observational results are based on in situ solar wind measurements from the Cluster and MMS spacecraft, and the simulation results are obtained from an externally driven, three-dimensional fully kinetic simulation. Using a set of novel diagnostic measures we show that both the large-amplitude structures and the lower-amplitude background fluctuations preserve linear features of kinetic Alfven waves to order unity. This quantitative evidence suggests that the kinetic turbulence cannot be described as a mixture of mutually exclusive waves and structures but may instead be pictured as an ensemble of localized, anisotropic wave packets or "eddies" of varying amplitudes, which preserve certain linear wave properties during their nonlinear evolution.

physics.plasm-ph↗

arXiv2011

Magnetic reconnection and stochastic plasmoid chains in high-Lundquist-number plasmas

A numerical study of magnetic reconnection in the large-Lundquist-number ($S$), plasmoid-dominated regime is carried out for $S$ up to $10^7$. The theoretical model of Uzdensky {\it et al.} [Phys. Rev. Lett. {\bf 105}, 235002 (2010)] is confirmed and partially amended. The normalized reconnection rate is $\normEeff\sim 0.02$ independently of $S$ for $S\gg10^4$. The plasmoid flux ($\Psi$) and half-width ($w_x$) distribution functions scale as $f(\Psi)\sim \Psi^{-2}$ and $f(w_x)\sim w_x^{-2}$. The joint distribution of $\Psi$ and $w_x$ shows that plasmoids populate a triangular region $w_x\gtrsim\Psi/B_0$, where $B_0$ is the reconnecting field. It is argued that this feature is due to plasmoid coalescence. Macroscopic "monster" plasmoids with $w_x\sim 10$% of the system size are shown to emerge in just a few Alfv\'en times, independently of $S$, suggesting that large disruptive events are an inevitable feature of large-$S$ reconnection.

astro-ph.SR↗

arXiv2009

Formation of Plasmoid Chains in Magnetic Reconnection

A detailed numerical study of magnetic reconnection in resistive MHD for very large, previously inaccessible, Lundquist numbers ($10^4\le S\le 10^8$) is reported. Large-aspect-ratio Sweet-Parker current sheets are shown to be unstable to super-Alfv\'enically fast formation of plasmoid (magnetic-island) chains. The plasmoid number scales as $S^{3/8}$ and the instability growth rate in the linear stage as $S^{1/4}$, in agreement with the theory by Loureiro et al. [Phys. Plasmas {\bf 14}, 100703 (2007)]. In the nonlinear regime, plasmoids continue to grow faster than they are ejected and completely disrupt the reconnection layer. These results suggest that high-Lundquist-number reconnection is inherently time-dependent and hence call for a substantial revision of the standard Sweet-Parker quasi-stationary picture for $S>10^4$.

astro-ph.SR↗