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A. V. Thampan

Publications and source records attributed to A. V. Thampan.

9 recordsLinked to original sources
arXiv2004

Differentially rotating magnetised neutron stars: production of toroidal magnetic fields

We initiate numerical studies of differentially rotating magnetised (proto) neutron stars by studying - through construction from first principles - the coupling between an assumed differential rotation and an impressed magnetic field. For a perfect incompressible, homogeneous, non-dissipative fluid sphere immersed in an ambient plasma, we solve the (coupled) azimuthal components of the Navier-Stokes equation and the Maxwell induction equation. The assumed time--independent poloidal field lines get dragged by the rotating fluid and produce toroidal magnetic fields. Surface magnetic fields take away energy redistributing the angular momentum to produce rigid rotation along poloidal field lines. Due to absence of viscous dissipation, sustained torsional oscillations are set up within the star. However, the perpetual oscillations of neighbouring `closed' field lines get increasingly out of phase with time, leading to structure build up as in Liu & Shapiro (2004) implying the importance of taking into account diffusion (Spruit 1999) for realistic modeling.

astro-ph↗
arXiv2003

The Hartle-Thorne circular geodesics

The Hartle-Thorne metric is an exact solution of vacuum Einstein field equations that describes the exterior of any slowly and rigidly rotating, stationary and axially symmetric body. The metric is given with accuracy up to the second order terms in the body's angular momentum, and first order in its quadrupole moment. We give, with the same accuracy, analytic formulae for circular geodesics in the Hartle-Thorne metrics. They describe angular velocity, angular momentum, energy, epicyclic frequencies, shear, vorticity and Fermi-Walker precession. These quantities are relevant to several astrophysical phenomena, in particular to the observed high frequency, kilohertz Quasi Periodic Oscillations (kHz QPOs) in the X-ray luminosity from black hole and neutron star sources. It is believed that kHz QPO data may be used to test the strong field regime of Einstein's general relativity, and the physics of super-dense matter of which neutron stars are made of.

gr-qc↗
arXiv2002

Holonomy invariance, orbital resonances, and kilohertz QPOs

Quantized orbital structures are typical for many aspects of classical gravity (Newton's as well as Einstein's). The astronomical phenomenon of orbital resonances is a well-known example. Recently, Rothman, Ellis and Murugan (2001) discussed quantized orbital structures in the novel context of a holonomy invariance of parallel transport in Schwarzschild geometry. We present here yet another example of quantization of orbits, reflecting both orbital resonances and holonomy invariance. This strong-gravity effect may already have been directly observed as the puzzling kilohertz quasi-periodic oscillations (QPOs) in the X-ray emission from a few accreting galactic black holes and several neutron stars.

gr-qc↗
arXiv2000

Temperature Profiles of Accretion Disks around Rapidly Rotating Neutron Stars in General Relativity and Implications for Cygnus X-2

We calculate the temperature profiles of (thin) accretion disks around rapidly rotating neutron stars (with low surface magnetic fields), taking into account the full effects of general relativity. We then consider a model for the spectrum of the X-ray emission from the disk, parameterized by the mass accretion rate, the color temperature and the rotation rate of the neutron star. We derive constraints on these parameters for the X-ray source Cygnus X-2 using the estimates of the maximum temperature in the disk along with the disk and boundary layer luminosities, using the spectrum inferred from the EXOSAT data. Our calculations suggest that the neutron star in Cygnus X-2 rotates close to the centrifugal mass-shed limit. Possible constraints on the neutron star equation of state are also discussed.

astro-ph↗
arXiv2000

General relativistic spectra of accretion disks around rotating neutron stars

General relativistic spectra from accretion disks around rotating neutron stars in the appropriate space-time geometry for several different equation of state, spin rates and mass of the compact object have been computed. The analysis involves the computation of the relativistically corrected radial temperature profiles and the effect of Doppler and gravitational red-shifts on the spectra. Light bending effects have been omitted for simplicity. The relativistic spectrum is compared with the Newtonian one and it is shown that the difference between the two is primarily due to the different radial temperature profile for the relativistic and Newtonian disk solutions. To facilitate direct comparison with observations, a simple empirical function has been presented which describes the numerically computed relativistic spectra well. This empirical function (which has three parameters including normalization) also describes the Newtonian spectrum adequately. Thus the function can in principle be used to distinguish between the two. In particular, the best fit value of one of the parameters ($β$-parameter) $\approx 0.4$ for the Newtonian case, while it ranges from 0.1 to 0.35 for relativistic case depending upon the inclination angle, EOS, spin rate and mass of the neutron star. Constraining this parameter by fits to future observational data of X-ray binaries will indicate the effect of strong gravity in the observed spectrum.

astro-ph↗
arXiv2000

Rapidly rotating strange stars for a new equation of state of strange quark matter

For a new equation of state of strange quark matter, we construct equilibrium sequences of rapidly rotating strange stars in general relativity. The sequences are the normal and supramassive evolutionary sequences of constant rest mass. We also calculate equilibrium sequences for a constant value of $Ω$ corresponding to the most rapidly rotating pulsar PSR 1937 + 21. In addition to this, we calculate the radius of the marginally stable orbit and its dependence on $Ω$, relevant for modeling of kilo-Hertz quasi-periodic oscillations in X-ray binaries.

astro-ph↗
arXiv1999

Implications of kHz Quasi-Periodic Brightness Oscillations in X--Ray Binaries for Neutron Star Structure

Kilohertz quasi-periodic oscillations (QPOs) in low-mass X-ray binaries (LMXBs) may represent the orbital frequencies of the innermost Keplerian orbits around accreting neutron stars. Attempts have recently been made to derive constraints on the mass and the equation of state of the neutron star, by identifying the highest observed QPO frequency with the Keplerian frequency at the marginally stable orbit given by the general theory of relativity. These estimates have either neglected the effect of neutron star rotation or used an approximate treatment of rotation in general relativity. We rederive these constraints using a fully general relativistic formalism including the effect of rapid rotation. We also present constraints corresponding to the case where the innermost stable orbit touches the stellar surface.

astro-ph↗
arXiv1998

Equilibrium Sequences of Rotating Neutron Stars for New Microscopic Equations of State

For four newly suggested microscopic equations of state of neutron star matter, we construct equilibrium sequences of rapidly rotating neutron stars in general relativity. The sequences are the normal and supramassive evolutionary sequences of constant rest mass. We find that for these equations of state the maximum (gravitational) mass rotating models occur (in central density and rotation rate $Ω$) before the maximum--$Ω$ models. We calculate equilibrium sequences for a constant value of $Ω$ corresponding to the most rapidly rotating pulsar PSR 1937+21. Also calculated is the radius of the marginally stable orbit and its dependence on $Ω$.

astro-ph↗