The Speakers

The changing dynamics of the solar interior

Helioseismic data have allowed us to probe the internal dynamics of the Sun in great detail. We now know how the interior of the Sun rotates and have also learned of other north-south (meridional) and east-west (zonal) flows. We now have helioseismic data over two solar-cycle, and these have revealed that internal solar dynamics changes as a solar cycle progresses. The changes in Cycle 24 however, are quite different from those in Cycle ~23. In this talk, I shall summarize what we have learned about solar dynamics over the last 25 years and describe some of the features that are still unknown.

The Syzygy between Hilbert and Gravitational Wave Detectors in Space

The Nobel Prize in physics in 2017 was awarded for the development of the LIGO detectors and for the observation of gravitational waves (GW). Several ground-based detectors around the world are in various stages of development including LIGO-India. However, there are astrophysically interesting GW sources, such as supermassive black-holes, which emit GW below 10 mHz. These lie below the band-width of ground-based detectors because it is impossible to screen off noise (for instance, gravity-gradient noise) below few Hz. The solution is to build a detector in space.

The Laser Interferometric Space Antenna (LISA) - a ESA-NASA project is an instrument for observing low frequency GW. LISA and the ground based detectors complement each other in an essential way, just as various astronomies such as the optical, radio, etc. complement each other. The LISA configuration forms an unequal arm interferometer in the shape of a giant triangle which is almost equilateral.

A major noise source is the laser frequency noise which arises due to phase fluctuations in the laser. Amongst the important noise sources, laser phase noise is expected to be several orders of magnitude larger than other noises in the instrument. Thus, cancelling the laser frequency noise is vital for LISA to reach the requisite sensitivity. There are six elementary data streams which are formed by going clockwise and anti-clockwise around the LISA triangle. In time-delay interferometry the data streams are combined with appropriate time delays in order to cancel the laser frequency noise. This scheme can be interestingly translated to an algebraic-geometric problem posed by David Hilbert in 1890. The laser noise cancelling data combinations can be expressed as six-tuple polynomial vectors which form a module over a polynomial ring, well known in the literature, as the first module of syzygies. Gr¨obner basis methods are used to find the generators of the module and which generate all the noise cancelling data combinations. The talk will briefly mention the salient features of this approach and also the recent approaches involving matrix representation theory.

The solar neutrino problem in the light of helioseismically constrained fluxes

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.

Gravitational lensing as a probe to understand supermassive black holes

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.

Imaging the fluid dynamics of the solar interior

Prof. Chitre has made notable contributions to the inference of solar internal flows and dynamics using various methods of helioseismology. The area continues to be very promising, especially with the advent of high-quality modern space- and ground-based solar observations. In this talk, I will describe the significant advances that we have made using the technique of normal-mode coupling, which holds great promise for the detection of novel physics in the Sun. The range of seismic inferences is broad, covering temporally stationary axisymmetric flows such as rotation and meridional circulation to non-axisymmetric time-varying phenomena such as Rossby waves and convection and I will present some exciting results in these areas.

Kelvin-like wakes in a charged fluid

One of the spectacular sights in a lake is a wake formed past a moving duck or a boat. Owing to the mathematical analogy between shallow water waves and ion-acoustic waves, we discover the analogue of Kelvin wakes created as an object moves past a plasma. Although not universal as the Kelvin wake, there still is a range of Mach numbers where they occur.

Tidal Capture Binaries

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 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.

From a flash of idea to publication: A magical experience!

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”.

Nonlinearities in Partially Ionised Plasmas

Partially Ionised plasma is a multi-species 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 multi-fluid 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.

On the (low) cost of speculation in astronomy

Surface Signatures of Subsurface Magnetic Fields:

Written on the Stellar Atmospheric Waves

Internal dynamo-generated magnetic fields in the stars are inaccessible to direct observations, thus deterring our understanding of their origin and evolution in time. A tool that has proved very useful in analyzing the stellar interiors, called asteroseismology, however, has not yet been able to provide details of internal stellar magnetic fields. Here, we investigate the signatures of subsurface magnetic fields in the dispersion relations of acoustic waves (trapped near the stellar surface).

We first begin with an isothermal, stratified atmosphere, permeated by a non-uniform (exponential function of the vertical coordinate) horizontal magnetic field (arXiv:1812.06947). We solve the problem exactly. Next we consider a more realistic polytropic atmosphere. We calculate the dispersion relation numerically and perturbatively. We show that the presence of a horizontal magnetic field breaks the symmetry of rings of constant frequencies over the horizontal wavenumbers. Such asymmetry arising from the magnetic fields eludes the standard helioseismologywith its present resolution. Our results hint that internal stellar magnetic fields might be possible to infer based upon stellar surface oscillations.

Gravitational Lensing

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 form multiple 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 source at 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.

Cahn-Hilliard-Navier-Stokes Turbulence: An Introduction

I will begin with examples of binary- and ternary-fluid flows and of turbulence in such flows. I will then introduce the Cahn-Hilliard-Navier-Stokes (CHNS) equations for such binary and ternary fluids. I will then give some illustrative results from (a) our direct numerical simulations of such flows and (b) a regularity criterion for the solutions of the three-dimensional CHNS equations.

This talk is based on studies that I have carried out with: Nadia Bihari Padhan, Department of Physics, I.I.Sc., Bangalore; Nairita Pal, Los Alamos National Laboratory, USA; Prasad Perlekar, TIFR, Hyderabad; John D. Gibbon, Department of Mathematics, Imperial College, London; Anupam Gupta, IIT, Hyderabad. I would like to acknowledge support from CSIR, DST, UGC (India) and SERC (IISc).

Gravitational lenses in galaxy groups

Abstract: Multiply-imaged quasar images reveal the mass of the lensing galaxy, and often the geometry of the images indicates that contribution of the galaxy group to which the lens belongs. This technique can be used to find galaxy groups at high/intermediate redshift.

Recurrent jet activity in galaxies

There has been clear observational evidence that jet activity in active galactic nuclei (AGN), which is believed to be due to accretion onto a supermassive black hole, could be episodic over a wide range of time scales. The most striking examples of such episodic or recurrent nuclear activity are the double-double or triple-double radio galaxies with distinct pairs of radio lobes on opposite sides of the nucleus of the parent optical object, reflecting the different cycles of nuclear jet activity. Estimates of the ages of the different pairs of lobes may be used to constrain the time scales of recurrent jet activity. Episodic nuclear activity may also be probed via multiwavelength studies. This includes radio sources in clusters of galaxies as well as an old electron population revealed via inverse-Compton scattering of cosmic

microwave background photons with low-energy electrons. We present an overview of recurrent jet activity in AGN, possible models to understand this phenomenon, and suggestions for further work.

Testing cosmic ray acceleration in the laboratory

In recent years it has become possible using high power lasers to simulate interesting astrophysical processes in the laboratory, e.g. we have observed amplification of seed magnetic fields by plasma turbulence generated by colliding laser-produced plasma flows. Such turbulent, magnetized plasmas drive MHD instabilities that are seen to energise electrons above the thermal background, thus demonstrating an injection mechanism for cosmic ray acceleration. Our ongoing experiment is studying whether 2nd-order stochastic Fermi acceleration can indeed occur in such environments. We cannot yet create a universe in the laboratory - but a supernova seems possible!

Nonlinear Fore-wake Excitations in Space and Laboratory Plasmas

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.

Galactic magnetism: from a battery to the turbulent Dynamo

The origin and maintenance of coherent magnetic fields in galaxies is reviewed. We begin with an interesting baroclynic battery which can generate seed magnetic fields, worked out in collaboration with Prof. Chitre. These seed fields need to be further amplified and maintained by a dynamo to explain observed magnetic fields in galaxies. Basic ideas behind large-scale turbulent dynamos are discussed with particular emphasis on current theoretical challenges and their possible resolution.

Black hole Galaxy Coevolution study using DAE GMRT and UGC startup #RADatHomeIndia for Citizen Science Research

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.

Role of ambipolar diffusion in the solar atmosphere

The existence of magnetic field dependent ambipolar diffusion inside a weakly ionized plasma was first proposed in 1955-1956 by Piddington, Cowling, Mestel and Spitzer [1-3]. Today this process is widely accepted to be of crucial importance in galactic molecular clouds and interstellar medium. We will revisit the potential role played by the ambipolar diffusion in the atmospheres of cool stars such as our Sun. In particular, we will explore its influence on the surface dynamo and magnetic reconnection process in the photosphere and lower chromosphere.

[1] J. H. Piddington, Mon. Not. R. Astron. Soc. 114, 638 and 651 (1955).

[2] T. G. Cowling Mon. Not. R. Astron. Soc. 116, 114 (1956).

[3] L. Mestel and L. Spitzer, Mon. Not. R. Astron. Soc. 116, 503 (1956).

Gravitational memory and quantum entanglement

Abstract: Asymptotically flat spacetimes are known to possess an infinite number of symmetries known as the Bondi-Metzner-Sachs (BMS) supertranslations. These BMS symmetries were shown to be related, both, to the gravitational memory effect and Weinberg’s soft theorems, the significance of which was recently realised by Hawking et. al. who conjectured that applying these relations to an asymptotically flat spacetime with a black hole in the interior would imply the existence of an infinite number of soft hairs for the black hole. We discuss the effect of BMS symmetries on quantum entanglement and its implications in the context of the black hole information paradox.

We illustrate the gravitational memory effect for linear uniformly accelerated observers in a physical process involving a BMS shock-wave without planar/spherical symmetry. This classical memory is accompanied by a quantum memory that modulates the quantum entanglement between the opposing accelerated wedges in a flat spacetime. A corresponding phenomenon across the Schwarzschild black hole horizon suggests that the Negativity measure of entanglement between infalling and outgoing Hawking pair should be degraded due to an infalling BMS shockwave while there should be linear order generation of Negativity between two outgoing Hawking particles.

Cosmological simulations of galaxy formation:

Fluid mechanics at the largest scales

Cosmological simulations of galaxy formation solve for fluid mechanics at the largest possible scales. They have come a long way in their development and are now able to produce galaxy populations broadly consistent with observations. My talk will cover the basic physical principles that are involved in these simulations, including their numerical implementation which typically involves powerful supercomputing facilities. I'll highlight the successes of current state of the art simulations in explaining various aspects of observed galaxies. Finally, I'll also talk about inevitable limitations of these simulations in their current form and discuss possible directions for current and future endeavours.

Magnetic fields in the Milky Way from pulsar observations

Pulsars can act as an excellent probe of the Milky Way magnetic field. The average strength of the Galactic magnetic field component parallel to the line of sight can be estimated as 1.232 RM/DM, where RM and DM are the rotation and dispersion measure of the pulsar. However, this assumes that the thermal electron density and magnetic field of the interstellar medium are uncorrelated. Using numerical simulations and observations, we test the validity of this assumption. Based on magneto-hydro-dynamical simulations of driven turbulence, we show that the correlation between the thermal electron density and the small-scale magnetic field increases with increasing Mach number of the turbulence. We find that the assumption of uncorrelated thermal electron density and magnetic fields is valid only for subsonic and transonic flows, but for supersonic turbulence, the field strength can be severely overestimated by using 1.232 RM/DM. We then correlate existing pulsar observations from the Australia Telescope National Facility with regions of enhanced thermal electron density and magnetic fields probed by CO data of molecular clouds, magnetic fields from the Zeeman splitting of the 21 cm line, neutral hydrogen column density, and Halpha observations. Using these observational data, we show that the thermal electron density and magnetic fields are largely uncorrelated over kpc scales. Thus, we conclude that the relation 1.232 RM/DM provides a good estimate of the magnetic field on Galactic scales, but might break down on sub-kpc scales.

Radio emission from solar corona: present and future

The Sun is a laboratory to understand the plasma processes in a stellar environment. In the Sun, the outermost layer, the corona, shows a vast range of magnetic field and plasma density structures. Such variation provides contrasting plasma environments residing in the magnetic field topology, rooted in the photosphere. The coronal magnetic configuration is dynamic due to the changing photospheric magnetic flux, presence of MHD waves and interaction with neighbouring magnetic loops, some resulting in many transient phenomena like solar flares and coronal mass ejection. These phenomena lead to particle acceleration and heating of ambient plasma, i.e. significant w.r.t space weather. We capture them in various electromagnetic bands. In particular, at radio wavelengths from a few cm to few meters, the coronal plasma is optically thin, i.e. captures the detail of these process. With new generation instruments in radio wavelengths, we observe unprecedented and fine observations of the solar flares. In this presentation, I will review the new results, challenges and talk about future missions to understand the solar corona using radio wavelengths as a probe in conjunction with other wavebands.