We study the solution for a viscous accretion disc around a rotating compact/central object having a hard surface, i.e. a neutron star, a strange star or any other highly gravitating object. We choose a pseudo-Newtonian approach to describe the relativistic accretion disc. For this purpose, a new pseudo-Newtonian potential is established that is applicable to describing the relativistic properties of a star and its disc. As we know, the Hartle-Thorne metric can describe the geometry of a star as well as the space-time outside it, so we use this metric to establish our potential. Our potential reproduces the marginally stable orbit exactly as does general relativity. It also reproduces the marginally bound orbit and specific mechanical energy at the marginally stable orbit with at most 4 and 10 per cent error, respectively. Using this potential, we study the global parameter space of the accretion disc. Thus we find the physical parameter regime for which a stable accretion disc can be formed around a gravitating object with a hard surface. We also study how the fluid properties get changed with different rotations of the central star. We show that, with a change in rotation of the central object, the valid disc parameter region changes dramatically. We also show the effect of viscosity on the fluid properties of the disc. Subsequently, we give a theoretical prediction of kilohertz QPO frequencies, at least for one of a pair, for a fast-rotating compact object such as 4U 1636-53.

A self-powered system is very much essential aspect in the recent trend to improve the working efficiency of the portable and wearable devices. Here, we have reported a fully sustainable, self-poled, bio-compatible, and bio-waste based piezoelectric energy harvester which has been made of Pomelo Fruit Membrane (PFM). PFM based piezoelectric generator (PFMBPEG) could generate ~ 6.4 V output voltage and ~ 7.44 μA output current directly, only by finger tapping on the device and registers a power density of ~ 12 μW cm−2 whereas, the same piezoelectric generator can generate ~ 15 V output voltage, 130 μA output current, and power density of ~ 487.5 μW cm−2 by using a full wave rectifier. The sensitivity and energy harvesting competence of the generator have also been assessed by attaching this nanogenerator into various parts of human body (as energy sources) such as wrist, elbow, finger, throat, jaws, leg and putting the device into ultrasonic bath and in every case, it could successfully generate voltage. Therefore, this bio-waste based energy harvester can be used as a power source for the different potable and wearable electronic goods where a small amount of energy is required, specifically in the biomedical applications (i.e., health monitoring, power source for the implantable devices and so on). Finally, mechanical stability the developed piezoelectric generator has been evaluated by cyclic bending test and it has been observed that there is no significant deformation of the PFM film even after 100 cycles.

Isolated massive elliptical galaxies, or that are present at the center of cool-core clusters, are believed to be powered by hot gas accretion directly from their surrounding hot X-ray emitting gaseous medium. This leads to a giant Bondi-type spherical/quasispherical accretion flow onto their host SMBHs, with the accretion flow region extending well beyond the Bondi radius. In this work, we present a detailed study of Bondi-type spherical flow in the context of these massive ellipticals by incorporating the effect of entire gravitational potential of the host galaxy in the presence of cosmological constant Λ, considering a five-component galactic system (SMBH + stellar + dark matter + hot gas + Λ). The current work is an extension of Ghosh & Banik (2015), who studied only the cosmological aspect of the problem. The galactic contribution to the potential renders the (adiabatic) spherical flow to become multi-transonic in nature, with the flow topology and flow structure significantly deviating from that of classical Bondi solution. More notably, corresponding to moderate to higher values of galactic mass-to-light ratios, we obtain Rankine-Hugoniot shocks in spherical wind flows. Galactic potential enhances the Bondi accretion rate. Our study reveals that there is a strict lower limit of ambient temperature below which no Bondi accretion can be triggered; which is as high as ∼ 9 × 10 6 K for flows from hot ISM-phase, indicating that the hot phase tightly regulates the fueling of host nucleus. Our findings may have wider implications, particularly in the context of outflow/jet dynamics, and radio-AGN feedback, associated with these massive galaxies in the contemporary Universe.

The late time accelerated expansion of the Universe demands that even in local galactic-scales it is desirable to study astrophysical phenomena, particularly relativistic accretion related phenomena in massive galaxies or in galaxy mergers and the dynamics of the kiloparsecs-scale structure and beyond, in the local-galaxies in Schwarzschild-de Sitter (SDS) background, rather than in Schwarzschild or Newtonian paradigm. Owing to the complex and nonlinear character of the underlying magnetohydrodynamical equations in general relativistic (GR) regime, it is quite useful to have an Newtonian analogous potential containing all the important GR features that allows to treat the problem in Newtonian framework for study of accretion and its related processes. From the principle of conserved Hamiltonian of the test particle motion, here, a three dimensional Newtonian analogous potential has been obtained in spherical geometry corresponding to SDS/Schwarzschild anti-de Sitter (SADS) spacetime, that reproduces almost all of the GR features in its entirety with remarkable accuracy. The derived potential contains an explicit velocity dependent term of the test particle that renders an approximate relativistic modification of Newtonian like potential. The complete orbital dynamics around SDS geometry and the epicyclic frequency corresponding to SDS metric have been extensively studied in the Newtonian framework using the derived potential. Applying the derived analogous potential it is found that the current accepted value of Λ ∼ 10 −56 cm −2 moderately influences both sonic radius as well as Bondi accretion rate, especially for spherical accretion with smaller values of adiabatic constant and temperature, which might have interesting consequences on the stability of accretion disk in AGNs/radio galaxies.PACS numbers: 98.62. Mw, 98.62.Js, 98.80.Es, 95.30.Sf,

We prescribe a pseudo-Newtonian vector potential for studying accretion disks around Kerr black holes. The potential is useful for studying the inner properties of a disk not confined to the equatorial plane where general relativistic effects are indispensable. Therefore, we incorporate the essential properties of the metric at the inner radii through the pseudo-Newtonian potential derived from the general Kerr spacetime. The potential, reproducing most of the salient features of general relativity, is valid for the entire regime of the Kerr parameter. It reproduces the last stable circular orbit exactly as that in the Kerr geometry. It also reproduces the last bound orbit and energy at the last stable circular orbit with a maximum error $7% and $15%, respectively, up to an orbital inclination 30 .