BSc -Physics -1988- SIES College of Arts Science and Commerce, Mumbai (University of Mumbai)
MSc-Physics-1990-University of Mumbai
PhD-Physics-Neutron Star Astrophysics -2000-Indian Institute of Astrophysics (Bangalore University)
RESEARCH, TEACHING, or OTHER INTERESTS
Physics and Astronomy
16
Scopus Publications
Scopus Publications
Optical Characterization of Chalcone-Doped PMMA Thin Films for Photonic Applications Using Spectroscopic Technique of Drop Casting Method Kalpana Sharma, Raveendra Melavanki, Basappa C. Yallur, Raviraj Kusanur, N. R. Patil, Vikas M. Shelar, Diksha Singh, Jones Rosario, Qurban Hussaini, Mohan. A, Arun Varma Thampan Macromolecular Symposia, 2020 Abstract Thin films of PMMA (polymethyl methacrylate) and chalcone prepared via drop casting method is reported here. The newly synthesized chalcone derivative namely 1‐(3‐methoxy‐phenyl)‐3‐(4‐nitro‐phenyl)‐propanone (C2) is used to dope the films with different concentrations. The prepared films are transparent so they are studied for applications in photonics. Optical characterization of the samples is done through different spectroscopy techniques. Absorption spectra of the samples is obtained using UV–vis spectrophotometer. Other significant optical parameters such as refractive index, extinction coefficient, and band gap energies are calculated from the absorption spectra. The doping effect is observed on emission spectra that is obtained using fluorescence spectrophotometer. Also, FTIR spectra of the doped films is obtained and compared with the pure compound to notice changes in the peak values and intensity of the peaks. The present work is done to study the effect of doping on optical properties and figure out the application of samples in photonics.
Two Coexisting Families of Compact Stars: Observational Implications for Millisecond Pulsars Sudip Bhattacharyya, Ignazio Bombaci, Domenico Logoteta, Arun V. Thampan Astrophysical Journal, 2017 It is usually thought that a single equation of state (EoS) model “correctly” represents cores of all compact stars. Here we emphasize that two families of compact stars, viz., neutron stars and strange stars, can coexist in nature, and that neutron stars can get converted to strange stars through the nucleation process of quark matter in the stellar center. From our fully general relativistic numerical computations of the structures of fast-spinning compact stars, known as millisecond pulsars, we find that such a stellar conversion causes a simultaneous spin-up and decrease in gravitational mass of these stars. This is a new type of millisecond pulsar evolution through a new mechanism, which gives rise to relatively lower mass compact stars with higher spin rates. This could have an implication for the observed mass and spin distributions of millisecond pulsars. Such a stellar conversion can also rescue some massive, spin-supported millisecond pulsars from collapsing into black holes. Besides, we extend the concept of critical mass for the neutron star sequence to the case of fast-spinning neutron stars, and point out that neutron star EoS models cannot be ruled out by the stellar mass measurement alone. Finally, we emphasize the additional complexity for constraining EoS models, for example, by stellar radius measurements using X-ray observations, if two families of compact stars coexist.
Fast spinning strange stars: Possible ways to constrain interacting quark matter parameters Sudip Bhattacharyya, Ignazio Bombaci, Domenico Logoteta, Arun V. Thampan Monthly Notices of the Royal Astronomical Society, 2016 For a set of equation of state (EoS) models involving interacting strange quark matter, characterized by an effective bag constant (B eff ) and a perturbative quantum chromodynamics corrections term (a 4 ), we construct fully general relativistic equilibrium sequences of rapidly spinning strange stars for the first time. Computation of such sequences is important to study millisecond pulsars and other fast spinning compact stars. Our EoS models can support a gravitational mass (M G ) and a spin frequency () at least up to 3.0 M and 1250 Hz, respectively, and hence are fully consistent with measured M G and values. This paper reports the effects of B eff and a 4 on measurable compact star properties, which could be useful to find possible ways to constrain these fundamental quark matter parameters, within the ambit of our EoS models. We confirm that a lower B eff allows a higher mass. Besides, for known M G and , measurable parameters, such as stellar radius, radius-to-mass ratio and moment of inertia, increase with the decrease of B eff . Our calculations also show that a 4 significantly affects the stellar rest mass and the total stellar binding energy. As a result, a 4 can have signatures in evolutions of both accreting and non-accreting compact stars, and the observed distribution of stellar mass and spin and other source parameters. Finally, we compute the parameter values of two important pulsars, PSR J1614-2230 and PSR J1748-2446ad, which may have implications to probe their evolutionary histories, and for constraining EoS models.
Holonomy invariance, orbital resonances and kilohertz QPOs M A Abramowicz, G J E Almergren, W Kluzniak, A V Thampan, F Wallinder Classical and Quantum Gravity, 2002 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.
General relativistic spectra of accretion discs around rapidly rotating neutron stars: Effect of light bending S. Bhattacharyya, D. Bhattacharya, A. V. Thampan Monthly Notices of the Royal Astronomical Society, 2001 We present computed spectra, as seen by a distant observer, from the accretion disc around a rapidly rotating neutron star. Our calculations are carried out in a fully general relativistic framework, with an exact treatment of rotation. We take into account the Doppler shift, gravitational redshift and light-bending effects in order to compute the observed spectrum. We find that light bending significantly modifies the high-energy part of the spectrum. Computed spectra for slowly rotating neutron stars are also presented. These results would be important for modelling the observed X-ray spectra of low-mass X-ray binaries containing fast-spinning neutron stars.
General relativistic spectra of accretion disks around rotating neutron stars Sudip Bhattacharyya, Ranjeev Misra, Arun V. Thampan Astrophysical Journal, 2001 General relativistic spectra from accretion disks around rotating neutron stars in the appropriate spacetime geometry for several different equations of state, spin rates, and masses 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 redshifts 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 a result of the different radial temperature profiles 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 $(\\beta-parameter)$ $B\\approx0.4$ for the Newtonian case, while it ranges from 0.1 to 0.35 for the relativistic case depending upon the inclination angle, equation of state (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.
Temperature profiles of accretion discs around rapidly rotating strange stars in general relativity: A comparison with neutron stars S. Bhattacharyya, A. V. Thampan, I. Bombaci Astronomy and Astrophysics, 2001 We compute the temperature profiles of accretion discs around rapidly rotating strange stars, using constant gravitational mass equilibrium sequences of these objects, considering the full effect of general relativity. Beyond a certain critical value of stellar angular momentum (J), we observe the radius (rorb) of the innermost stable circular orbit (ISCO) to increase with J (a property seen neither in rotating black holes nor in rotating neutron stars). The reason for this is traced to the crucial dependence of on the rate of change of the radial gradient of the Keplerian angular velocity at rorb with respect to J. The structure parameters and temperature profiles obtained are compared with those of neutron stars, as an attempt to provide signatures for distinguishing between the two. We show that when the full gamut of strange star equation of state models, with varying degrees of stiffness are considered, there exists a substantial overlap in properties of both neutron stars and strange stars. However, applying accretion disc model constraints to rule out stiff strange star equation of state models, we notice that neutron stars and strange stars exclusively occupy certain parameter spaces. This result implies the possibility of distinguishing these objects from each other by sensitive observations through future X-ray detectors.
Possible signatures for strange stars in stellar X-ray binaries Astronomy and Astrophysics, 2000
Temperature profiles of accretion disks around rapidly rotating neutron stars in general relativity and the implications for Cygnus X-2 Sudip Bhattacharyya, Arun V. Thampan, Ranjeev Misra, Bhaskar Datta Astrophysical Journal, 2000 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. Subject headings: X-rays:binaries-X-rays:spectra-stars:neutron-stars:rotation-Cygnus X-2 1.
