Day 1 (May 6, 2021)

White dwarfs are observed to have a wide range of magnetic fields from less than 1kG to more than 1GG. The strongest magnetic fields, particularly when coupled with rapid rotation, can significantly increase the maximum mass beyond the Chandrasekhar limit. Misaligned magnetic and spin axes can make them visible to future gravitational wave detectors. We take a look at the origin of magnetic fields in white dwarfs, paying particular homage to collaborative work with Chitre on the evolution of magnetic fields and angular momentum in stars in general. We again draw attention to the fact that white dwarfs in isolated wide binary systems never have high magnetic fields, which are most commonly found in the magnetic cataclysmic variables, while the highest fields are found in single stars. We deduce that strong magnetic fields are intimately tied to duplicity and consider how a common envelope mechanism for orbital shrinkage may be responsible.

Abstract: The solar neutrino problem was the long standing issue of the observed shortfall of the neutrinos from the sun with reference to the predictions of the Standard Solar Model (SSM) calculations. These experiments indicated a lower flux of the Boron neutrinos coming from the Sun. Neutrino Oscillation in which one type of neutrino gets converted to another type was proposed as a plausible solution. However, the question whether any alteration of the prediction in fluxes can explain the results better was also being discussed. In this context, it was interesting to note that a seismic model for the Sun calculated using the accurate helioseismic data predicts a lower Boron neutrino flux as compared to the standard solar model (SSM). Can such a scenario fit the data better? I will discuss this aspect and show that even in such a model invoking neutrino oscillation was necessary. I will also compare the oscillation parameters obtained using both the models. Later developments and the current status of the solar neutrino problem will also be highlighted.

Abstract: When a compact stellar object like a neutron star closely approaches a normal star, tidal oscillations are set up in the latter. The energy for these oscillations comes from the orbital energy of the system. In the right circumstances, the tidal energy is large enough for the the orbital energy to become negative, in which case a gravitationally bound binary system is formed. The binary can evolve over a period of time into an X-ray binary. In the 1970s This mechanism was proposed as the origin of the startlingly large number of X-ray binaries in globular clusters. I will describe work on such formation done by A. Ray, A. K. Kembhavi and H.M. Antia and briefly summarise the current situation.

Fore-wake excitations in the form of solitons and shocks ahead of a moving object in a fluid is a spectacular phenomenon that has often been observed in front of fast-moving ships and has been widely studied in hydrodynamics. Can a similar phenomenon occur in plasmas in front of a moving charged object? I will discuss this question, that has surprisingly not received much attention in plasma physics, and present our consolidated findings on this phenomenon based on laboratory experimental studies, theoretical modelling as well as fluid simulation and molecular dynamic investigations. The relevance of such structures in gaining understanding of wave excitations occurring during the interaction of the solar wind with the earth and the moon will be discussed. Some possible practical applications of such precursors e.g. in the detection of charged space debris in the ionosphere will be pointed out.

Partially Ionised plasma is a multispecies system of electrically charged and neutral particles. The simplest system consists of electrons, ions and neutrals. The three species of particles can be described as three fluids, each governed by its own laws of mass and momentum conservation along with the total energy conservation. Combined with the electrodynamics, the system offers a rich variety of nonlinear physical phenomena especially modulating the evolution of the magnetic field. The collisional nature brings in the multifluid effects even in the much simplified description in the form of a weakly Ionised plasma. In this talk after giving a few examples of partially Ionised astrophysical Plasmas, I shall briefly outline the mathematical formulation of a weakly Ionised plasma. I shall then go on to emphasize the role of the nonlinearities on diffusion, waves, current sheet formation and the dynamo mechanism.

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GMRT has produced the highest sensitivity (rms ~ 5 mJy/beam), highest angular resolution (b~25"), low frequency all sky survey at 150 MHz, namey the TIFR GMRT SKy Survey. It makes the data a gold-mine for searching steep-spectrum high-redshift radio galaxies although they will be structurally unresolved. Alternatively, it is also the gold-mine for discovering the faint-fuzzy, steep-spectral relic radio emission from nearby galaxies to AGNs up to the "quasar era" or "cosmic noon" (z~ 1- 2). Relic lobes of typical large (~300 kpc) radio galaxies can be resolved by higher resolution data of TGSS as compared to NVSS (b=45") from VLA. Finding a sample of such faint fuzzy radio emission blobs beside optical/IR galaxies would serve as possible "smoking gun evidence" of AGN-feedback that has led to decline of star formation rate density in our Universe since this cosmic noon. Citizen Science research is naturally the best approach to spot such rare faint fuzzy emission from heterogeneous multi-wavelength data and a modified approach, incorporating face-to-face training to online interaction, was adopted to interpret complicated angular-scale sensitive radio interferometric data. Several new objects discovered by #RADatHomeIndia modified citizen science research collaboratory which has been supported by two dozen research and educational institutes all over India, will be presented. The preliminary discoveries from TGSS data have also been followed up with the upgraded GMRT. Our results showing exotic Speca-like spiral-host radio galaxies, episodic radio galaxies, radio jet-external galaxy interaction (similar to Minkowski Objects), cluster radio relics imaged with unprecedented resolution and sensitivity will be presented. Studies of star formation history and AGN jet activity, as in cosmic leaf blower galaxy NGC3801, have been highlighted as one of the eight science goal icons of the Square Kilometre Array (SKA) mega project and thus RAD@home citizen science research naturally prepares the next generation for the SKA era.

Abstract: The extraordinary circumstances engendering the author’s well-cited publication co-authored with Prof. Chitre, and the lightening pace of its completion are recounted. It introduced a novel idea of “jet-shell interaction” in radio galaxies, which has continued to be applied over the past 4 decades to explain some of the striking morphological peculiarities of radio galaxies. The paper is also an exemplary case of the proverbial “strike when the iron is hot”.

Abstract: Incontrovertible evidence suggests that supermassive black holes (SMBHs) of mass greater than billion solar masses, in some of the galaxies, had formed when the universe was just a billion years old. Formation of SMBHs so early in the evolution of the universe is a veritable conundrum. In this study, we propose to use the gravitational lensing of the explosive end of Population III stars (in the redshift range 10 - 30) due to the massive cores in the centres of the early galaxies. In particular, if SMBHs were formed due to the evolution of Bose-Einstein condensates of axion-like particles (ALPs), there would be very specific signatures in the gravitational lensing events. Future telescopes like the TMT or James Webb space telescope are likely to catch such events.

Abstract: Gravitational lensing occurs when the gravitational field of a massive object en route, like a star, galaxy or cluster of galaxies, bends the path of light rays emitted by a distant source. Manifestation of multiple images with characteristic distortion but identical intrinsic properties are the main indicators of lensing. Gravitational lensing is one of the most bias free racers of mass in compact objects. Galaxies, Galaxy Clusters and Stars can act as efficient lenses for well-aligned background sources, each having it's own signatures. Multiple images and Einstein Rings produced by the lensing action of galaxies at intermediate redshifts have been well-studied. Rich, compact galaxy clusters at redshifts upwards of 0.2 have enough surface mass density (projected on the plane of the sky) to formmultiple images of aligned background sources or highly magnified and distorted arc-like images. By analysing the multiple images or giant arcs formed near the cluster centre as well as tracing the weak distortion of background galaxies within a few arcminute radius, the mass profile of the cluster lens can be determined. Stars passing through the line of sight of a compact source like quasar can produce variation in the radiation flux we receive from the sourceat time scale of days to months, resulting in characteristic Microlens light curve.

Professor Chitre did pioneering work in all the above three fields. In this talk, I shall explain these concepts in a simple language.