2022

Abstract: We perform general-relativistic magnetohydrodynamic (GRMHD) simulations around the Kerr black hole. The goal is to understand the physics around them and extract physical information from astrophysical observations. Accordingly, our modeling is targeted to bridge the astrophysical theories and observations. The focus will be on our recent results from our simulations in the context of BH-XRBs and AGNs. Simplified assumptions in the thermodynamics of the electrons limit the potential and relevance of GRMHD simulations in the broader range of astrophysical applications. Keeping that in mind, we developed a self-consistent formalism for electron thermodynamics in GRMHD by incorporating heating, radiative cooling, and coupling between electrons and protons. In the talk, I will explain the properties of the accretion flow around black holes in the single-temperature and two-temperature paradigms. I will also explain the formation of relativistic jets and disc-wind driven by the Blandford & Znajek and Blandford & Payne mechanisms from the thin accretion disc.

Abstract: Fast radio bursts (FRBs) are one of the greatest unsolved mysteries in modern astronomy. Though a plethora of models has been proposed to explain FRBs, the origin of these extremely energetic millisecond-duration radio pulses remains a topic of great debate, owing to the paucity of well-localized FRBs. One of the promising methods to narrow down their origins is by identifying their hosts and/or multiwavelength counterparts. Unfortunately, due to the limited sensitivity of telescopes, multi-wavelength follow-up is most promising for local Universe FRBs (distance < 100 Mpc). The Canadian Hydrogen Intensity Mapping Experiment (CHIME)/FRB project has been detecting FRBs since July 2018, and many of them have sufficiently low dispersion measure (DM) suggesting a nearby origin. Even better, the localization of low-DM FRBs to a few arcminute precision using the CHIME/FRB baseband system can result in a reliable host association for nearby FRBs. In this talk, I will report on the CHIME/FRB discoveries of six local Universe FRBs. Multi-wavelength follow-ups of these local Universe FRBs will enable more stringent limits to be placed on high energy counterparts than for more distant FRBs, which are the majority of the FRBs localized to a host galaxy to date. Finally, I will also discuss the constraints we derived using these localized nearby FRBs on different proposed progenitor models, FRB energy function and Milky Way halo contribution.

Abstract: The Advanced LIGO and VIRGO observatories have detected around a hundred gravitational-wave signals from merging compact binaries in their first three observing runs. Gravitational wave astronomy has transitioned from a field with notable single detections to a stage in which large catalogues of events enable a systematic survey of the population of merging compact binary sources in the Universe. These detections were only possible due to sophisticated analyses of noisy strain data that were historically conducted within the collaboration. We developed an entirely independent analysis of LIGO data that improved its reach by rigorously accounting for inherent systematics, and thereby identified new binary black hole mergers within. In this talk, I will provide a birds-eye view of the detection process, and discuss the implications of these detections for models of binary black hole formation.

Abstract: Spiral galaxies are collections of stars and interstellar medium (ISM) with a morphology of a disc residing in a dark matter halo. They are dynamically active and form the playground for the ISM for star formation. Various factors like large-scale distribution in dark matter halo, differential galactic rotation, interaction with satellite galaxies, star formation and feedback, give rise to different types of density and velocity structures in the ISM. The interstellar medium acts as a compressible fluid with turbulent flows. Turbulence gives rise to scale-invariant random density and velocity structures. These structures play an active role in regulating star formation, by enhancing gravitational clustering and adding to thermal pressure gradients. These structures are traced in various recent observations and some of their statistical properties like the density power spectrum, velocity dispersion and its radial variation are well established. However, the dynamic correlation between these structures and their generation mechanism is yet to be understood.  In this talk, I will discuss our attempt to probe these structures and their dynamics in nearby spiral galaxies. We enhance the measurement of the HI column density power spectrum and the first time estimation of the line of sight velocity power spectrum for external spiral galaxies. These measurements let us infer the generating mechanism of coherent structures at ~10 kpc scales and comment on the energy they cascade to the star formation scales. 

A seemingly different but still interesting dynamical effect in the galaxy’s disc is the bending waves. These arise due to interaction with satellite galaxies and tidal effects amongst many others.  Observationally the bending waves are traced as corrugation in edge-on discs for density and in the face on stellar discs for velocity mostly in optical or infrared. In the later part of the talk,  I will discuss our investigation of large-scale corrugation in gas density and velocity using 21-cm observations of several spiral galaxies. Here we find that the disk is vertically perturbed at the stellar extent, where lower multipole bending waves are more frequent. 

Abstract: Following recent work [Zhang, Duval, Gibbons and Horvathy (PRD, 2017)], there has been growing interest in understanding memory effects through the study of geodesic motion. One can, in principle, arrive at  a class of memory effects (displacement and velocity memory) by  solving the geodesic equation or the equation of geodesic deviation. Another route to memory (also termed as B-memory) involves the study of geodesic congruences by utilising the Raychaudhuri equation. In this talk, we will provide an overview of our recent work on such diverse aspects of memory in the context of exact, radiative solutions in General Relativity and modified theories of gravity.

Abstract: Massive stars end their lives as one of the most exotic explosions in the cosmos, i.e. supernovae. There are various kinds of supernovae based on the diverse evolution of the progenitor stars. However, mapping the various kinds of supernovae to their progenitors is a big mystery in stellar evolution and the archival survey data are limited to nearby supernovae. A unique way to trace the evolution is by studying the interaction of the supernovae shock with the slow moving immediate circumstellar medium, which is formed due to the mass loss rate of the progenitor star and hence carried the foot prints of the progenitor.  It effectively works as time machine. Since the CSM interaction mainly manifests in radio and X-ray bands, they provide a unique input to unravel the progenitor stars of supernovae. In this talk, I present the study of some supernovae from sub-GHz frequencies (using GMRT) upto X-ray energies.  Our study  allow us to observe these explosions for several years to decades probing  the conditions of the star in its early nucleosynthesis stages to moments before explosion. 

Abstract: The number of exoplanets that have been discovered has crossed a staggering number of 5000. In this talk I will show the astonishing variety of exoplanets that have been discovered. I will discuss how we study exoplanet atmospheres using the combination of observations from telescopes, planetary atmosphere models, retrieval techniques and thereby characterise them.  I will show what we have discovered in various exoplanet atmospheres in the recent years using the Hubble Space Telescope (HST) and the Very Large Telescope (VLT), and the challenges we encounter while characterising exoplanet atmospheres. I will discuss about the capabilities of the recently launched James Webb Space Telescope (JWST) and show our most recent discovery using JWST early release science program.  Finally, I will discuss where we are heading in our quest to demystify these far away worlds and in our search for life in the Universe. 

Abstract: we develop a model formalism to study the structure of a relativistic, viscous, optically thin, advective accretion flow around a rotating black hole in presence of radiative coolings. While doing this, we adopt a recently developed effective potential to mimic the spacetime geometry around the rotating black holes. We solve the governing equations to obtain the shock-induced global accretion solutions in terms of flow parameters. Using shock properties, we compute the quasi-periodic oscillation (QPO) frequency ($\nu_{\rm QPO}$) of the post-shock matter (equivalently post-shock corona) pragmatically, when the shock front exhibits quasi-periodic variations. We also calculate the luminosity of the entire disc for these shock solutions. Employing our results, we find that the present formalism is potentially promising to account the observed $\nu_{\rm QPO}$ and bolometric luminosity of a ULX source IC 342 X-1.

Abstract: The ability of AGN feedback to self-regulate in massive galaxies depends critically on environmental factors like the depth of the potential well and the pressure of the surrounding circumgalactic medium (CGM). I have carried out high resolution 3D hydrodynamic simulations exploring the dependence of AGN feedback in galaxies on those environmental factors with a range of halo masses. These simulations also include in situ star formation and stellar feedback along with feedback from massive galaxy’s old stellar population. Our simulations show that this feedback mechanism is tightly self-regulating in a massive galaxy with a deep central potential and low CGM pressure, permitting only small amounts of multiphase gas to accumulate and allowing no star formation. In a similar mass galaxy with shallower central potential and greater CGM pressure, the feedback mechanism is more episodic, producing extended multiphase gas and allowing small rates of star formation. I will also discuss as “how does kinetic AGN feedback with a strong momentum flux interacts with the CGM?” Our analysis shows that large scale CGM circulation plays an important role in reconfiguring the galactic atmosphere and regulating the atmosphere’s central entropy level. Finally, I will discuss some of the latest work regarding the cool-core cycles in most massive galaxy clusters with large SMBH mass. 

Abstract: Energetic charged particles (Cosmic rays) are one of the main contributors to the nonthermal energies in the universe. Although diffusive shock acceleration (DSA) is the promising mechanism for particle acceleration at shocks, whether the processes that promote particles to DSA act similarly for electrons and protons/ions is still not well understood. In this talk, I will present our ongoing efforts using ab-initio kinetic simulations to solve the problem and discuss under which conditions electrons and protons participate in the DSA. I will also outline how the energetic particles modify the initial magnetic field by producing different plasma instabilities. Finally, I will discuss the implications of these results to astrophysical sources from planetary bow shocks to galaxy clusters.

Abstract: The Hydrogen Intensity and Real-time Analysis eXperiment (HIRAX) is an upcoming radio interferometer array with an initial 256-element array to be deployed at the South African Radio Astronomy Observatory (SARAO) Square Kilometer Array (SKA) site in South Africa. Through intensity mapping of the 21cm emission line of neutral hydrogen, HIRAX will provide a cosmological survey of the distribution of large-scale structure over the redshift range of 0.775 < z < 2.55 over ∼15,000 square degrees of the southern sky. With an initial overview of the scientific goals and the design of the telescopic system, I shall talk about forecasting constraints on the relevant cosmological parameters using the end-to-end cosmology simulations pipeline that incorporates beams from full EM simulations.

Abstract: One of the major aims of gravitational wave astronomy is to observationally test the Kerr nature of black holes. The strongest such test, with minimal additional assumptions, is provided by observations of multiple ringdown modes, also known as black hole spectroscopy. I will be discussing our recent study where we provide strong observational evidence of preferring two fundamental ringmodes over one for the gravitational wave merger event GW190521. The dominant mode is the l = m = 2 harmonic, and the sub-dominant mode corresponds to the l = m = 3 harmonic. We estimate the redshifted mass and dimensionless spin of the final black hole as ~ 330 solar mass and ~ 0.87 respectively. We also find that the final black hole is consistent with the no-hair theorem and constraint on the fractional deviation from the general relativity of the sub-dominant mode’s frequency to be < 20%.

Abstract: The coronal magnetic field, the ultimate driver of space weather, plays an essential role in the formation, evolution, and dynamics of the small and large-scale structures in the solar corona. These structures may lead to gigantic explosions in the solar atmosphere in the form of large-scale eruptions, such as coronal mass ejections (CMEs), which may severely impact near-Earth space. CMEs can reach Earth within several hours to days, and depending on the orientation of its internal magnetic field; they can interact with the Earth’s magnetosphere causing severe geomagnetic storms. Moreover, the shocks generated by CMEs can accelerate the energetic particles leading to highly energetic solar radiation storms. These extreme space weather conditions may damage satellite operations and Earth’s communication and navigation system. Therefore, studying such violent solar eruptions is crucial to understand their consequences on space weather. CMEs are often accompanied by radio emissions, which provide access to observations of the related solar, heliospheric, and ionospheric space weather phenomena. Radio techniques can provide early signatures of particle acceleration associated with solar flares and CMEs, which give insights into CME initialisation and eruption processes (1) These observational techniques also serve as a powerful tool to constrain the coronal and heliospheric models. With state-of-the-art radio instruments such as LOw-Frequency ARray (LOFAR) and legacy instruments such as Nançay Radio Heliograph (NRH), it has now been possible to study these bursts and their structures in great spectral, temporal and spatial resolutions (2) Using these observations and time-dependent data-driven numerical modelling of active region magnetic fields(3), we study the formation and eruption of the coronal flux-ropes leading to CME eruptions. We estimate various properties of the CME flux-rope and compare them with the associated multi-wavelength ground- and space-based observations. In this talk, I will highlight the radio techniques to constrain the initial CME properties close to the Sun and the numerical modelling approach to understanding the initiation and evolution of large-scale solar eruptions.

Abstract: Despite being challenging to detect, the ≥10^6 K hot circumgalactic medium (CGM) is a treasure trove of galaxy evolution. By probing the hot CGM of the Milky Way (MW) using X-ray absorption lines, we have discovered a super-virial 10^7 K phase coexisting with the well-known virialized 10^6 K phase, featuring non-solar abundance ratios of light elements, α-enhancement, and non-thermal line broadening. I have also detected this super-virial phase of MW CGM in X-ray emission analyses. Detection of these surprising properties of the CGM along multiple directions in the sky suggests a strong connection between the hot CGM and past Galactic outflow(s). Observations of MW-like galaxies complement our observations of the Milky Way. We have discovered the hot CGM emission of an MW-mass galaxy NGC 3221 that is extended (≥150 kpc) and is massive enough to account for its missing baryons. The CGM is not isothermal, with the CGM within 100 kpc of NGC 3221 being super-virial, and fainter along the minor axis than the global average. These results, at par with our findings in the Milky Way, compel us to rethink the impact of galactic feedback on the hot CGM of star-forming galaxies without an active nucleus.

Abstract: The absorption features in the spectra of distant quasar by intergalactic neutral hydrogen is one of the most sensitive observations that put strong constraints on the end stages of reionization. The main challenge lies in observing the high redshift quasar and modeling the patchy reionization in cosmological simulations. In this talk, we present the measurement of the photo-ionization rate ($\Gamma_{\rm HI}$),  the mean free path ($\lambda_{\rm mfp,HI}$) and the neutral fraction ($f_{\rm HI}$)  of hydrogen at redshift $4.85<z<6.05$ from a large sample of unprecedented quality quasar absorption spectra observed using XShooter (VLT) and ESI (Keck) instruments. The fluctuations in the ionizing radiation field are modeled by post-processing simulations from the Sherwood suite using our new code ``Extended reionization based on Code for Ionization and Temperature Evolution'' (Ex-CITE).  Our measurements suggest that the hydrogen reionization is completed by $z \sim 5.3$. Our simulations show that the neutral islands persist down to $z \sim 5.3$ giving rise to long dark troughs seen in the observed spectra. Our simulation that matches the effective optical depth distribution also reproduces other statistics of the the Ly$\alpha$ forest at $z>5$ suggesting the robustness and accuracy of the measured parameters. Finally we will discuss the implication  of these measurements to our current cycle-2 JWST proposal and to future E-ELT / TMT observations.

Abstract: Black holes are black, but material captured by their immense gravity forming a disk-like structure can be heated to extremely high temperatures as it approaches inward, resulting in powerful X-ray emission. The infalling material occasionally produces powerful jets from the inner region of the disk. The phenomenology of X-ray observations of these objects is very rich and has been exhaustively studied. To date, however, there is little agreement on the physics behind these accretion states. To go beyond the standard model of a stable accretion flow, with no corona and only a parametric viscosity of unknown origin, it is necessary to rely on a promising physical solution. In this talk, I will discuss how the spectral and temporal properties during the outburst phase of the black holes change and the role of cooling, viscosity, and jets/outflows in triggering them. In addition, I will also present some of my recent results on understanding the powering mechanism of ultraluminous X-ray sources, changing look phenomena in active galactic nuclei, and simulations of the Fermi Bubbles. Finally, I shall briefly talk about the imaging of galaxy clusters in X-rays, which I am currently working on.

Abstract: Last three decades of observational Exoplanet research has completely transformed our understanding about planets as well as their formation environment. This revolution in our understanding is in many ways thanks to the modern ultra-stable spectrographs. This talk is the story of how we built two extreme precision radial velocity spectrographs, HPF (in near-infrared) and NEID (in optical) for two US telescopes at PennState. After a brief overview of the main scientific results from these two instruments, I shall also talk about a new kind of multi object, optical to near infrared spectrograph we are currently building in India to conduct world's largest survey of the protoplantery disc systems.

Abstract: Our motion through the Universe generates a dipole in the temperature anisotropies of the Cosmic Microwave Background (CMB) and also in the angular distribution of sources. If the cosmological principle is valid, these two dipoles are directly linked, such that the amplitude of one determines that of the other. However, it is a longstanding problem that number counts of radio sources and of quasars at low and intermediate redshifts exhibit a dipole that is well aligned with that of the CMB but with about twice the expected amplitude, leading to a tension reaching up to 4.9σ. In this talk, I revisit the theoretical derivation of the dipole in the sources number counts, explicitly accounting for the redshift evolution of the population of sources. If the spectral index and magnification bias of the sources vary with redshift, the standard theoretical description of the dipole may be inaccurate. I will provide an alternative expression which does not depend on the spectral index, but instead on the time evolution of the population of sources. I will then determine the values that this evolution rate should have in order to remove the tension with the CMB dipole.

Abstract: Relativistic jets from AGNs are an important driver of feedback in galaxies with an active black hole. They impact the nearby environment over different physical scales during their lifetime, with varying effects. They first interact with the host galaxy's ISM before breaking out to larger scales, significantly affecting the galaxy's morphology and evolution. I shall present the results of our recent 3D relativistic (magneto) hydrodynamic simulations, performed on scales of several kpc, of AGN jets interacting with the ambient ISM and CGM. The young relativistic jets initially couple strongly with the turbulent ISM, before breaking out to larger scales. I will subsequently present the results of a new hybrid fluid+particle scheme to model the spectral and spatial evolution of non-thermal electrons in jets. This allows us to present a more realistic description of synchrotron emitting particles. 

Abstract: Minor merger of galaxies are common during the evolutionary phase of galaxies. Also, about two-third of the disk galaxies in the local Universe host bars. This large abundance of stellar bars in disk galaxies and the relatively larger frequency of occurrence of minor merger events raises an important question - what happens to a stellar bar when the host galaxy experiences a minor merger event with a satellite galaxy? The exact dynamical role of minor mergers on the final fate of a stellar bar remains to be explored in full details, particularly so when the satellite ultimately plunges into the host galaxy and the host galaxy readjusts after the merger is completed. In this talk, I will discuss the dynamical impact of a minor merger (mass ratio 1:10) event on the final fate of a stellar bar in the post-merger remnant. I will also discuss the underlying physical mechanisms causing the bar weakening, and the possibility of minor merger of galaxies as an avenue for bar weakening in disk galaxies.

Abstract: Fast Radio Bursts (FRBs) are millisecond-timescale radio transients originating from cosmological distances (~Gpc) that have been discovered a little more than a decade ago. At these distances, they have to be a trillion times more luminous than the brightest radio pulses observed from Galactic pulsars. The engine and emission mechanism that can produce such luminosities is still unknown despite ~80 different theories. Over the past few years, the Canadian Hydrogen Intensity Mapping Experiment (CHIME) FRB backend has detected hundreds of FRBs including a dozen repeating FRBs and a few of the nearest FRB sources. The repeating nature of these FRBs, allows for precise localization with radio interferometers and a detailed study of their environment and nature with multi-wavelength observations. I will introduce the broad questions about the nature of FRBs and their promise as tools for cosmology. I will discuss recent results from the CHIME/FRB backend and the inferences that we can draw about the origins of FRBs. Apart from radio observations, I will discuss X-ray and optical studies of FRB locations and the search for prompt counterparts of FRBs. I will finish by discussing the ongoing efforts of the CHIME/FRB collaboration to build outrigger telescopes for FRB localization and indigenous plans for identifying the brightest and rarest FRBs using an ultra-wide field of view telescope.

Abstract: One of the most exciting developments in astronomy and astrophysics is the discovery of planets around stars, other than our own Sun, termed as “Exoplanets.” The discoveries of these exoplanets have revealed an astonishing diversity in their physical characteristics - masses, temperatures, radii, orbital properties, and host stars. Exoplanets known today range from super Jupiters to Earth-size rocky planets over a wide range of temperatures, including several in the habitable zones of their host stars. In this talk, I will discuss how these planets form, why they are so different from those in our solar system, and which ingredients are available to build them? I will also discuss how we study the atmospheres of these exoplanets using the combination of ground and space-based observations and physicochemical models and thereby characterize them. Finally, I will discuss the near future of this emerging frontier in the context of significant advances expected from facilities such as JWST, ARIEL, and large ground-based telescopes. 

Abstract: The collimated and relativistic flow of plasma originating from the central black hole forms an ideal laboratory to understand multi-messenger astronomy. Due to the non-linear coupling of myriad physical processes in these jets, studying them becomes a formidable challenge both observationally and theoretically. In particular, from the numerical perspective, such large kpc to Mpc scales posit difficulties in our understanding due to their inherent nature of scale disparity. In this talk, I shall provide an overview of the work done within my group at IIT Indore focussing on bridging this divide in length scales via coupling micro-physical sub-grid processes with macro-physical dynamical and emission signatures. This talk will comprise of recent results that will decipher the AGN jets from their launching to the feedback at large kilo-parsec scales.  I will showcase our unique approach of handling multiple particle acceleration processes and its implications on synthetic emission from AGN jets at various lengths scales. Further, I shall discuss the utility of such numerical simulations in complementing the low-frequency radio surveys along with high-energy X-ray and gamma-ray emission. Finally, I shall also discuss some open problems and how they are essential to unravel the enigma of AGN jets. 

Abstract: Gas in the central regions of cool core clusters has a short radiative cooling time (<1 Gyr). However, observations do not show cooling flows in most central galaxies of such clusters. The Active Galactic Nuclei (AGN) feedback model is an attractive solution to this problem in the intracluster medium (ICM). Turbulence driven by AGN jets can transfer energy from the jet inflated bubbles to the ambient intracluster medium (ICM) by turbulent mixing and turbulent dissipation and prevent its catastrophic cooling. However, current X-ray observations lack the spectral resolution to directly measure turbulent velocities in the hot ICM. Using our idealised simulations, we have studied the effects of the amount of turbulent heating and the different types of turbulence driving on the statistical properties of the ICM gas, such as temperature, pressure and density distribution, velocity structure of hot (10^7 K gas) and cold (10^4 K) phases, etc. We use our results to constrain different indirect observational techniques for estimating turbulent velocities of the hot phase, such as Halpha filament (10^4 K) velocities and fluctuations in the X-ray brightness and the thermal Sunyaev-Zeldovich effect, which are dependent on the density and pressure fluctuations, respectively.

Abstract: Astrophysical observations in the last three decades have told us that we live in a preposterous universe described by the  standard LCDM model consisting of  70% dark energy, 25% cold dark matter and 5% ordinary baryons. We then discuss  how galaxy clusters  can help us understand the Physics of the above dark sector. In particular we shall focus on two conundrums within the standard LCDM model:  Radial Acceleration relation and observations of constant dark matter surface density and how galaxy cluster observations can shed light on whether these observations can be explained within the standard LCDM model.

Abstract: The massive stars that reionized the Universe (reionizers) were short-lived and the galaxies in which they formed likely merged and disappeared. However, the fossils of those stars and their low-mass siblings might still be around us today and it is a challenge to identify them. If identified, they can provide key insights to the nature of early galaxies in which they formed. Naturally, the population of metal poor stars in our Galaxy is a primary target of investigation and a considerable progress has been made in recent years at the observational front. We use the Eagle cosmological simulation to compare the abundance of the siblings and descendants of reionizers with the observed metal poor stars in the Milky Way. We find that most of them live in high mass galaxies, and in the Milky-Way like galaxies their distribution is spatially more extended than that of normal stars. Furthermore, we find that a subset of metal poor stars known as the carbon enhanced metal poor (CEMP) stars, intriguingly carry signatures of the first few generations of stars in the Universe. The origin of CEMP stars and in particular that of their subtypes (e.g. CEMP-no, -s) is under debate. We have identified the CEMP population in the simulation and investigated the physics behind their origin and that of their sub-groups. We find that the galaxy building processes such as the feedback plays a crucial role in the formation of CEMP stars by creating a poorly mixed interstellar medium. In this scenario, various sub-groups of CEMP-stars form during different temporal stages of the star formation in first galaxies. Our investigation leads to the conclusion that the CEMP-no stars are the siblings of the stars that reionized the Universe.

Abstract: We study self-gravitating bosonic systems, candidates for dark-matter halos, by carrying out a suite of direct numerical simulations designed to investigate the formation of finite-temperature, compact objects in the three-dimensional (3D) Fourier-truncated Gross-Pitaevskii-Poisson equation (GPPE). This truncation allows us to explore the collapse and fluctuations of compact objects and show the following: (i) The statistically steady state of the GPPE, in the large-time limit and for the system sizes we study, can also be obtained efficiently by tuning the temperature in an auxiliary stochastic Ginzburg-Landau-Poisson equation. (ii) Over a wide range of model parameters, this system undergoes a thermally driven first-order transition from a collapsed, compact, Bose-Einstein condensate to a tenuous Bose gas (that is not gravitationally condensed). (iii) By a suitable choice of initial conditions in the GPPE, we also obtain a binary condensate that comprises a pair of collapsed objects rotating around their center of mass. (iv) We use a generalised GPPE to study the collapse of an axion star. (v) By introducing a solid-crust potential and rotation in the GPPE, we develop a minimal model for pulsars and their glitches.

Abstract: Gravitational waves (GW) can probe various epochs in the early Universe. In this talk, I will discuss the production of GW in a particular model of magnetic field generation during inflation. In this model, we require a low energy scale for inflation and reheating (reheating temperature, T_R < 10^4 GeV) and have a blue spectrum of electromagnetic fields (EM), which peaks around the horizon scale of reheating. The anisotropic stress associated with these EM fields naturally sources the production of a stochastic background of GW with frequencies in the range of tens of nano Hertz to milli Hertz. These two extremes of the range can be probed respectively by pulsar timing arrays experiments and the upcoming Laser Interferometric Space Array. We also perform three-dimensional direct numerical simulations to study GW production. We notice some new features in the GW spectrum compared to earlier analytical work. I will also discuss those.

Abstract: Extreme mass ratio inspirals (EMRIs) will be important sources for future space-based gravitational-wave detectors. In recent work, tidal resonances in binary orbital evolution induced by the tidal field of nearby stars or black holes have been identified as being potentially significant in the context of extreme mass-ratio inspirals. These resonances occur when the three orbital frequencies describing the orbit are commensurate. During the resonance, the orbital parameters of the small body experience a ‘jump’ leading to a shift in the phase of the gravitational waveform. While the ‘instantaneous’ effect of a tidal resonance is small, its effect on the accumulated phase of the gravitational waveform of an EMRI system can be significant due to its many cycles in band. In my talk, I will summarise how common and important such resonances are over the orbital parameter space and estimate their detectability for LISA.

Abstract: Solar spicules are plasma jets that are observed in the dynamic interface region between the visible solar surface and the hot corona. At any given time, it is estimated that about three million spicules are present, making a visual impression of burning savanna over the whole solar disk. Due to their ubiquitous nature, they are believed to transport momentum to the solar wind and non-thermal energy to heat the solar corona. We find an intriguing parallel between the simulated spicular forest in a solar-like atmosphere  and the numerous jets of polymeric fluids when both are subjected to harmonic forcing. In a radiative magnetohydrodynamic numerical simulation with sub-surface convection, solar global surface oscillations are excited similarly to those harmonic vibrations. We also observe a forest of jets in the laboratory experiment when a petri-dish half-filled with a viscoelastic (polymeric) fluid is vibrated using a subwoofer speaker cone ( formally known as “Faraday excitation”). Likewise, in nature, the alligators’ bellowing and water jet formation is a stunning example of such excitation at large amplitudes. Fascinated by the visual similarity between these highly non-linear astrophysical and terrestrial systems, we further explore the mathematical and phenomenological commonalities using MHD simulations and insightful yet simple laboratory experiments. We finally report four sufficient conditions to form a forest of jets on the Sun as well as polymeric fluids in the laboratory.

Abstract: Standard model of cosmology (ΛCDM model) mainly suffers from two drawbacks, first one is the fine tuning problem and second one is a cosmic-coincidence problem. In this standard model of cosmology, Λ represents the cosmological constant and CDM denotes the cold-dark matter. Another important downside of the Λ-CDM model from the observational perspective is the discrepancy between the present observed value of Hubble’s constant and with predicted value of Hubble’s constant from theory. These fundamental discrepancies motivate us to study different kinds of cosmological models based on the coupled field-fluid sector. Based on these above considerations, we can build a theoretical framework for coupled field-fluid sector. Where field sector is made of a non-canonical scalar field (k-essence sector) and the fluid sector is composed of pressureless dust. The nonminimal coupling term is introduced at the Lagrangian level. We employ the variational approach with respect to independent variables that produce modified k-essence field equations and the Friedmann equations. We have analyzed the coupled field-fluid framework explicitly using the dynamical system technique considering two forms of the potential; one is constant and other is inverse power-law type. After examining these models it is seen that both models are capable of producing accelerating attractor solutions satisfying adiabatic sound speed conditions.

Abstract: Asymptotic giant branch (AGB) stars drive the chemical evolution of galaxies — the gas and dust ejected in slow winds from these objects seeds the formation of the next generation of stars. Over the past two decades, advanced space-based infrared facilities have allowed us to identify and characterise the properties of AGB stars in galaxies in the Local Group. Studies of Galactic AGB stars, however, are impeded by the foreground extinction and confusion in the Galactic Plane, which complicate the distance estimation.

In this talk, I will present preliminary results from the ongoing Nearby Evolved Stars Survey (NESS; https://evolvedstars.space), which is collecting sub-millimetre data for a volume-complete sample of ~850 nearby (<3 kpc) dusty evolved-star candidates. As the sample is well-studied, there is an abundance of observation over the entire range of relevant wavelengths (ultraviolet to radio) for these stars; the NESS team is using open science and reproducible techniques in order to exploit these archival observations in a systematic fashion -- the code used for data retrieval, reduction, and analysis will be made publicly available to avoid duplication of effort, which is critical in this "data deluge" era. These characteristics will make NESS one of the go-to databases for evolved stars over the next decade.