Abstract: According to the theory of cosmic inflation, all the structures observed in our Universe (clusters of galaxies, CMB anisotropy etc.) are of quantum-mechanical origin. The fact that the inflationary predictions match the astrophysical observations is an indirect justification of this scenario. In this talk, I discuss whether we could obtain a direct proof of the quantum nature of the primordial fluctuations.

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Abstract: Changing-look AGNs (CLAGNs) switch between type 1 and type 2 state in a timescale of months to years. This rapid variability cannot be explained by unified model of AGNs which has been used to explain all the AGN phenomenon in the last 40 years. Here we discuss our current understanding and future prospects of CLAGNs.

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Abstract: The observations through very long baseline interferometry (VLBI) by the Event Horizon Telescope (EHT) collaboration has provided a new window of probing the strong gravity regime of black holes via black hole imaging. However, practical constraints such as sparse coverage of the observational Fourier domain, scattering effects of Earth’s atmosphere and maximum permissible baseline lengths due to the size of the Earth, have led to proposed ground-based and space-based missions extending the EHT, namely the next generation EHT (ngEHT) and the black hole explorer (BHEX) mission respectively. In this talk, I shall present my work done with collaborators (Daniel Palumbo, Michael Johnson and Shep Doeleman) on the ability of these missions to probe, through polarimetric signatures, the black hole’s photon’s ring, a characteristic consequence of General Relativity on strong lensing. Through geometric modelling of the photon ring’s signature in the Fourier domain, exact expressions will be presented for the point in the Fourier domain where the photon ring signal begins to dominate. These formulae are then compared with the full suite of general relativistic magnetohydrodynamic (GRMHD) simulations of M87* at 230 GHz. The subsequent inferences for spin measurement, discriminating magnetic field morphologies will be discussed along with peculiar signatures in circular polarisation for specific models that always seem to be photon ring dominated. Lastly, the ability of ngEHT and BHEX to observe these signals is quantified by interfacing M87*’s polarimetric best-bet models with instrumentation considerations of thermal noise and antenna diameter requirements for these missions.

Abstract: Magnetic reconnection is a fundamental process in plasmas where magnetic field lines tear and reconnect leading to conversion of magnetic energy to plasma kinetic energy in natural systems like solar flares, coronal mass ejection, and substorms in earth’s magnetosphere. We present laboratory measurements showing the 2-D structure of energy conversion during magnetic reconnection for the general case where the reconnection proceeds in the presence of a finite guide field (GF) such that the magnetic field lines meet at an angle less than 180°. The experiments showed that the guide field reconnection develops different electric field structure than anti-parallel reconnection with various regions developing electric fields either predominantly parallel or perpendicular to the magnetic field. We find that the electrons are energized by the parallel electric field in two regions of the reconnection layer, in the electron diffusion region (EDR) and outside EDR near the low density separatrices. The energy deposition on ions is driven by the perpendicular electric field in the vicinity of the high density separatrices where electrons work against the electric field. The perpendicular electric field is sufficiently strong to ballistically energize the ions. An energy balance calculation shows that about 40% of the magnetic energy is converted into particle energy, 2/3rd of which is transferred to ions and 1/3rd to electrons. Additional measurements showed that part of the energy deposited on electrons and ions causes heating. The results have implications for understanding magnetic reconnection in space plasmas by providing a comprehensive picture of where particle energization occurs within reconnection regions.

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Abstract: Stellar bars are ubiquitous in disc galaxies (including the Milky Way) in the Local Universe, with about two-thirds of them harbouring a stellar bar. Bars are present in high redshift (z ~1) disc galaxies as well. Recent JWST observations further revealed the presence of conspicuous stellar bars even at a higher redshift (z ~ 3). At these high redshifts, the galactic discs are known to be thick, kinematically hot (and turbulent), and more gas rich. A consensus of whether these bars are tidally-induced or formed due to the internal gravitational instability is still largely missing. In this talk, I will present results regarding the bar formation scenario in the presence of (kinematically-hot) thick discs using a suite of N-body models of (kinematically cold) thin and (kinematically hot) thick discs. I will further discuss the physical processes involved behind different bar formation scenarios as well as how the thick disc mass fraction impacts the properties and morphology of the resulting stellar bar. In addition, I will briefly mention the robustness of different bar instability criteria when applied to this suite of simulated barred galaxies.

Abstract: Recent results of the event horizon-scale images of M87* and Sagittarius A* from the Event Horizon Telescope Collaboration show that strong magnetic fields are likely present around the central black holes in these sources. Magnetically arrested disks (MADs), the end stage of magnetic flux saturation around black holes, are especially rich in horizon-scale physics due to the presence of powerful jets and magnetic flux eruptions that provide significant feedback on the accretion mechanism. I will provide an overview of our current knowledge about the magnetic field evolution in numerical simulations of accreting black holes, focusing on relativistic jet launching, black hole-ISM feedback, and black hole imaging of MADs.

Abstract: The dynamics of merging black holes occur in the strong field limits of the general theory of relativity. Gravitational waves emitted during this process offer an unique opportunity to empirically assess whether our understanding of gravity still holds in this extreme regime. A merger leads to the formation of a distorted black hole that "rings" down as it settles into a final stable state. The gravitational waves emitted during this process is a crucial probe for exploring the strong-field gravity dynamics. I will briefly review conventional tests like black hole spectroscopy that probe the linear dynamics predicted using perturbation theory in this regime. Then, I will introduce a novel test—the amplitude-phase consistency test—designed to indirectly probe dynamics in the non-linear regime. Finally, I will explore the prospects and challenges associated with implementing this test using data from current and future gravitational wave detectors.

Abstract: Clusters of galaxies are the largest, most massive gravitationally bound objects in the Universe. They are also the most recent of the cosmic objects to form. According to the currently accepted models of cosmic structure formation, the number density distribution of these systems is very sensitive to the parameters describing the large-scale geometry and the expansion history of the universe. For this reason, galaxy clusters are regarded as important cosmological probes. However, to use clusters as precision probes of the cosmological parameters, we need to be able to "weigh them". To do so, and do so properly is challenging. Here I will describe our effort to get a handle on the various systematics and biases that can influence the outcome, and present our mass measurements for 50 galaxy clusters comprising the "Canadian Cluster Comparison Project" or CCCP sample and the LoCuSS Cluster Sample. Using clusters as cosmological probes, however, requires many more than 50-odd clusters with known masses but this is not feasible at the present. I will discuss our effort to identify and calibrate "easy-to-observe" proxies for the mass, focusing on the clusters' Compton Y-parameter. One remarkable upshot from this work is that cosmology suggested by clusters is in tension with Planck CMB results.

Abstract: The study of astrophysical phenomena within turbulent, multiphase environments presents unique challenges. Initially, the coexistence of turbulence and multiphase media may appear paradoxical, as turbulence tends to homogenize the gas, but recent research has illuminated the role of radiative cooling in addressing this issue. However, a critical yet unexplored aspect is the impact of magnetic fields. Magnetic fields can significantly alter the dynamics of turbulent multiphase media. This has implications for phenomena such as small-scale cold gas evolution, low-density gas surface brightness, and the broader baryon cycle. In this talk, I will introduce and share the journey of the field, while also sharing our latest findings from small-scale idealised simulations aimed at understanding the dynamics of multiphase gas. Such small-scale physics poses a substantial problem for cosmological simulations and similar theoretical studies are necessary for their inclusion into such large-scale simulations as sub-grid models.

Abstract: Supermassive black holes at the centres of merging galaxies are expected to form binary systems whose orbital motion generates gravitational waves. A cosmological population of such systems combine to build up a gravitational wave background (GWB). A significant detection of this GWB will provide the first stringent constraints on the dynamical evolution of supermassive black holes and their host galaxies while also providing a tantalising probe into the properties of the early Universe. Searches for the GWB have typically used sensitive radio telescopes around the world which observe an ensemble of extremely stable millisecond pulsars to probe the characteristics of the GWB signal. In this talk, I will discuss the first compelling evidence of the GWB seen by global pulsar timing array (PTA) collaborations as announced this year, its scientific impact, potential biases and the road ahead. Focussing on future advancements, I will talk about the powerful potential of a gamma-ray PTA and how it can improve our understanding of the astrophysical origins of the GWB.

Abstract: We will discuss interpretation of the nHz stochastic gravitational wave background (SGWB) seen by NANOGrav and other Pulsar Timing Array (PTA) Collaborations in the context of supermassive black hole (SMBH) binaries. The frequency spectrum of this stochastic background is predicted more precisely than its amplitude. We will discuss how Dark Matter friction can suppress the spectrum around nHz frequencies, where it is measured, allowing robust and significant bounds on the Dark Matter density, which, in turn, controls indirect detection signals from galactic centers. Next we will discuss alternative cosmological interpretations including cosmic strings, phase transitions, domain walls, primordial fluctuations and axion-like physics each of which may lead to Primordial Black Hole formation. 

Focussing on primordial black holes (PBHs) production in various cosmological scenarios involving single-field inflation, multiple fields, particularly the Curvaton model, as well as those based on the presence of remnants dominated by the false vacuum and show the PBH formation from these remnants including the contribution from the false vacuum and the bubble walls, during strong first-order phase transition by estimating the collapse using the hoop conjecture. Such PBH formations have associated Gravitational Waves from bubble collisions, the spectral shape of which is distinct from that of scalar-induced GW. Finally we will end by putting these comparative studies to test via We will discuss how well these different hypotheses fit the NANOGrav data, both in isolation and in combination with SMBH binaries, and address the questions: which interpretations fit the data best, and which are disfavoured. Finally we also discuss experimental signatures that can help discriminate between different sources of the PTA GW signals with complementary probes using CMB experiments and searches for light particles in DUNE, IceCUBE-Gen2, neutrinoless double beta decay, and forward physics facilities at the LHC like FASER nu, etc. along with Primordial Black Hole formation and its constraints.

Abstract: A Bayesian method is used in this extensive work to generate a large set of minimally constrained equations of state (EOSs) for matters in neutron stars (NS). These EOSs are analyzed for their correlations with key NS properties, such as the tidal deformability, radius, and maximum mass, within the mass range of 1.2−2 solar mass. The observed connections between the pressure of β-equilibrated matter and the properties of NSs at different densities offer significant insights into the behaviour of NS matter in a nearly model-independent manner. The study also examines the influence of various factors on the correlation of symmetry energy parameters, such as slope and curvature parameters at saturation density (ρ_0 = 0.16 fm^−3 ), with the tidal deformability and radius of NSs. This study investigates the robustness of the observed correlations by considering the distributions and interdependence of symmetry energy parameters. Furthermore, the utilization of Principal Component Analysis (PCA) is employed to unveil the complicated relationship between various nuclear matter parameters and properties of NSs. This analysis highlights the importance of employing multivariate analysis techniques in order to comprehend the variety in tidal deformability and radius observed across distinct masses of NS. This comprehensive study aims to establish a connection between the parameters of nuclear matter and the properties of NSs, providing significant insights into the behaviour of NS matter across different circumstances.

Abstract: In standard cosmology, it is postulated that the Universe underwent an accelerated period in its initial phase. This brief episode of rapid expansion is referred to as inflation. The inflationary paradigm provides a simple and elegant mechanism for the origin of perturbations in the early universe. After inflation, curvature perturbation leads to the inhomogeneities in matter distribution, which are amplified by gravitational instability and become observable structures in the universe like galaxies and clusters of galaxies. The inflationary expansion is usually driven with the aid of one or more scalar fields. While the classical component of the scalar fields is supposed to drive the rapid expansion, it is the quantum fluctuations associated with the scalar fields that are supposed to be responsible for the primordial perturbations. The quantum fluctuations are expected to grow and turn into classical perturbations during the later stages of inflation. In this talk, I will discuss the evolution of the quantum state of the perturbations in single and multi-field models of inflation. I will utilize measures such as squeezing, entanglement entropy or quantum discord to track the evolution of the quantum state. 

Abstract: Fast Radio Bursts (FRBs) are one of the super-energetic radio pulsed signals with a short (< 1 sec) time duration. In recent years, numerous theoretical explanations for the origin of FRBs have been proposed. However, even with exotic physics, models have been unable to universally explain the properties of these events, such as peak flux and pulse width. In this study, we present a novel model that explains the origin of FRBs of GHz frequency radio waves. The model has three ingredients: compact object, progenitor with very strong effective magnetic field strength, and GHz frequency gravitational waves (GWs). Due to the Gertsenshtein-Zel'dovich effect, when GWs pass through the magnetosphere of such compact objects, their energy is converted into electromagnetic waves. This conversion produces bursts of electromagnetic waves in the GHz range, leading to FRBs. Therefore, we infer that millisecond pulsars may be the origin of FRBs. Further, our model offers a novel perspective on the indirect detection of GWs at high-frequency beyond detection capabilities.

Abstract: Gravitational waves are a new observational probe that is bringing new insights about the cosmos. I will discuss how this avenue can explore new frontiers that can play a vital role in mapping the history of the Universe. I will show some latest findings from the current gravitational wave observations and discuss the future scope of gravitational waves in discovering uncharted territories in astrophysics, cosmology, and fundamental physics in synergy with other cosmic messengers. 

Abstract: The solar atmosphere is a complex and dynamic region that consists of several layers, including the photosphere, chromosphere, transition region, and corona. These layers are interconnected and magnetically coupled. MHD waves transfer mass and energy between different layers of the solar atmosphere. These waves are often excited by the turbulent convective motion of the plasma in the photosphere consisting of disordered plasma motions across a wide variety of lengthscales and/or timescales. Vortices or rotational motions are omnipresent in turbulent flows, and the solar surface is no exception. Vortices are known to perturb magnetic flux footprints anchored at the solar surface and excite torsional Alfven waves. These waves travel high in the atmosphere and potentially heat the plasma. Thus, investigating kinetic vortices and associated magnetic perturbations is essential to probe their role in the excitation of Alfven waves. We compare the distribution of vortices for three different magnetic regions, viz., Quiet Sun, Weak Plage and Strong Plage, using the realistic three-dimensional radiation-MHD code, MURaM. The spatial scales of vortices at different heights, their origin at Intergranular lanes and the opposite sense of rotation between velocity and magnetic vortices are verified, which validates the Alfv ́enic nature of chromospheric vortices. By examining power spectra of horizontal velocity at various layers, we conjecture that vortex interaction leads to energy transfer to smaller-scale vortices and contributes to chromospheric turbulence. Although photospheric kinetic vortices show similar properties in all magnetic configurations, the associated kinetic and magnetic vortices in the chromosphere are highly correlated for the Quiet Sun configuration compared to the Plage regions.

Abstract: We investigate the photon analog of Fermi acceleration where a photon scatters with shearing layers of relativistic plasma and produces power-law-shaped spectra at high energies. It is an alternative to existing explanations of power law spectra such as synchrotron process or inverse Comptonization. Among several potential applications of this phenomenon, I will describe two examples in this talk. (i) We explain the high energy spectra of Gamma-ray bursts (GRBs). (ii) we demonstrate that the Compton scattering of photons with shearing plasma leads to a natural explanation for well-observed phenomena of Limb brightening in the blazar jet base.

Abstract: Understanding the intergalactic medium is essential for comprehending galaxy evolution and structure formation. While our theoretical understanding of the high-redshift intergalactic medium (z>2) aligns well with observations, the low-redshift intergalactic medium (z<1) presents significant challenges. Observations reveal that over 30% of the gas predicted by the standard model of the Universe remains unaccounted for, and the distribution of Doppler widths in the low-redshift Lyman alpha forest eludes accurate reproduction in all existing simulations. There are still unexplored periods spanning 5 to 10 billion years of cosmic time where measurements of the UV ionizing background and the thermal state of the intergalactic medium are lacking. Additionally, the impact of galaxy formation feedback on the intergalactic medium, particularly at low redshifts, cannot be ignored. In this talk, the speaker will address these pressing issues, focusing on new measurements of the thermal state of the intergalactic medium that suggests something is missing in either simulations or theoretical understanding of the intergalactic medium.

Abstract: While nuclei lighter than iron are fused over the course of typical stellar evolution, almost half of the elements heavier than iron are created through the rapid neutron capture process (r-process). These nuclei are thought to be produced in magnetized outflows from neutron-rich explosive events including compact mergers and core-collapse supernovae. In this talk, I will discuss the potential of neutrino-driven winds from strongly magnetized and rapidly rotating protomagnetars as plausible sites for r-process nucleosynthesis. As heavy nuclei can eventually produce ultra-high energy cosmic rays, we examine the acceleration and survival conditions for these nuclei. We also explore the propagation of these jets within Wolf-Rayet stars and blue/red supergiants. In particular, we analyze the criteria for a successful jet breakout, maximum energy deposited into the cocoon and structural stability of these magnetized jets. We show that high-energy neutrinos can be produced for extended progenitors like blue/red supergiants and estimate the detectability of these neutrinos with IceCube-Gen2.

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Abstract: Massive O and B stars release a significant portion, approximately 50%, of their original mass into space through stellar winds throughout their lifespan. This continuous mass loss plays a crucial role in shaping their evolution and the turbulence within nearby interstellar gas clouds. However, simulating magnetic massive stars remains challenging due to limitations in the employed simulation tools. The recent development of the Riemann-Geomesh MHD code allows for the handling of high rotation rates and large magnetic tilt angles, facilitating a comprehensive analysis of stellar winds and the X-ray emission from magnetic massive stars.

The initial set of simulations utilizes an isothermal MHD approach to investigate the overall dynamics of magnetically channeled winds under varying conditions, including different magnetic strengths, magnetic tilt angles and high rotation rates. These simulations are run until a quasi-steady state is reached, exhibiting the dynamics of magnetic channeling and centrifugal breakout events in the mass outflow. The mass outflow is confined by the magnetic field loops that form the closed magnetosphere of the star. The cataloged results provide valuable perspective on the variations in the rates of angular momentum loss and mass loss across different configurations of rotation rate, magnetic field strength, and large magnetic tilt angles.

Additionally, a second set of simulations focuses specifically on studying the high-temperature X-ray emission from θ1 Orionis C, a magnetic massive star. These simulations incorporate the energy equation and cooling terms, enabling accurate modeling of the formation and subsequent cooling of dense shock regions. Remarkably, the simulated data consistently reproduces the observed features, such as the peaks and positive slope in the X-ray emission measure distribution of θ1 Orionis C, as obtained from Chandra data. Furthermore, the total X-ray luminosity obtained from the simulated data aligns well with the observed range of θ1 Orionis C.

Abstract: Universe has several poorly constrained periods over its evolution. Formation of first stars and galaxies, followed by reionization of the intergalactic medium is one such epoch. Often referred to as "cosmic dawn", observing this period is extremely challenging due to the faint nature of the signals originating from it. The redshifted 21-cm line from neutral hydrogen offers one such possibility to observe the cosmic dawn and extract information about the nature of first stars and galaxies. In this talk, I will discuss our attempts at detecting the 21-cm signal from cosmic dawn. I will elaborate the challenges involved in detecting faint cosmological signals, and how our in-house designed experiments address those challenges. I will finally discuss the recent results from our observations, and explore how 21-cm signal can also be employed to probe other cosmic mysteries at different redshifts. 

Abstract: This work focuses on the gravitational interactions of astrophysical systems. In particular,  we focus on the triple system dynamics, including mildly hierarchical three body secular dynamics, as well as precession induced resonances of binaries under the perturbation of a third companion. We apply our theoretical investigations of these physical processes to wide-orbit planetary systems and black hole binaries embedded in AGN disks. More specifically, we consider the secular dynamics of a test particle in a mildly-hierarchical configuration. We find the limit within which the secular approximation is reliable, present resonances and chaotic regions using surface of sections, and characterize regions of phase space that allow large eccentricity and inclination variations. Finally, we apply the secular results to the outer solar system. We focus on the distribution of extreme trans-neptunian objects (eTNOs) under the perturbation of a possible outer planet (Planet-9), and find that in addition to a low inclination Planet-9, a polar or a counter-orbiting one could also produce pericenter clustering of eTNOs, while the polar one leads to a wider spread of eTNO inclinations.

Beyond mildly hierarchical triple dynamics, we also propose a novel pathway through which compact binaries could merge due to eccentricity excitation, including in a near coplanar configuration. Mechanisms have been proposed to enhance the merger rate of stellar mass black hole binaries, such as the Von Zeipel-Lidov-Kozai mechanism (vZLK). However, high inclinations are required in order to greatly excite the eccentricity and to reduce the merger time through vZLK. Specifically, a compact binary migrating in an AGN disk could be captured in a precession-induced resonance, when the apsidial and nodal precession rates of the binary are commensurable to the orbital period around the supermassive black hole. We find 8 such resonances upto quardupole order of the Hamiltonian.  We show that if a binary is captured in these resonances and is migrating towards the companion, it can experience large eccentricity and inclination variations. Eccentricity is excited when the binary sweeps through the resonance which happens only when it migrates on a timescale 10-100 times the libration timescale of the resonance. Libration timescale decreases as the mass of the disk increases. The eccentricity excitation of the binary can reduce the merger timescale by a factor up to $10^{3−5}$.

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Abstract: PRATUSH -- Probing ReionizATion of the Universe using Signal from Hydrogen -- is a proposed cosmology experiment to detect the global red-shifted 21-cm signal from the Cosmic Dawn and Epoch of Reionization (CD/EoR). PRATUSH orbiting the Moon will seek to precisely measure the low-frequency radio sky-spectrum over 40 to 200 MHz. The scientific observations would be made in the radio-quiet region when in the farside of the Moon, and the data would be transmitted back to Earth when in the near-side. PRATUSH was proposed to the Indian Space Research Organization (ISRO) during a call for proposals in the announcement of opportunity for science payloads in 2018. PRATUSH is in the pre-project studies phase. In this talk, I will discuss the motivation for PRATUSH, its current status and future plans.

Abstract: Role of magnetic fields (B-fields) and turbulence in star formation is still under debate but considering the magnetised nature of molecular clouds, we expect B-fields to have a significant impact on this process. Observations of molecular clouds in different environments using different astronomical techniques helps in understanding the various important aspects of star formation. I mostly worked towards mapping B-fields in nearby low-mass star forming regions in different environments viz. molecular clouds in isolation and HII regions at their different evolutionary stages using multi band polarization observations. This was done using optical, near-IR, and sub-mm polarization observations to map B-fields from pc to sub-pc scales. In this talk, I will briefly present my work on B-fields in various Galactic regions i.e. cores, filaments, and HII regions.  We know that grain alignment is the key to understand interstellar dust polarization. Therefore, I will mostly emphasise on my recently accomplished work on investigation of grain alignment efficiency in PDRs using dust polarization measurements.

Abstract: The “quasar mode” (or radiative mode) of AGN feedback, operated through outflows, plays an essential role in the evolution of galaxies. Quasar outflows are detected as blue-shifted broad absorption lines in the UV/optical spectra of quasars. Thanks to the Sloan digital sky survey, ~100,000 broad absorption line quasars are available now for ensemble statistical studies. This rich dataset has also enabled us to identify some peculiar cases of these sources. In this talk, I will review the current understanding of the quasar outflows and present our recent efforts to understand (i) the nature of these sources and (ii) the primary driver for the variability in the absorption lines.

Abstract: Recent coordinated observations and interpretations of disparate "messenger" signals from GW170817-GRB170817A-EM170817 have inaugurated the era of multi-messenger transient gravitational wave (GW) astronomy. I will argue that the bright blazar OJ 287 should allow us to pursue persistent multi-messenger GW astronomy during the era of Square Kilometer Array. This is mainly due to the several successful multi-wavelength observational campaigns that allowed us to argue for the presence of a spinning supermassive black hole binary that spirals in due to the emission of nano-Hertz GWs in the central engine of a unique blazar OJ287. Our on-going efforts, relevant to both the Event Horizon Telescope consortium and the International Pulsar Timing Array consortium which aims to detect GWs from such massive BH systems in the coming years, will be also listed.

Abstract: Coronal Mass Ejections (CMEs) are the most energetic and episodic expulsions of magnetized plasma from the Sun. In the talk, I will begin with the current status of the problem and specific challenges holding back progress in better understanding the evolution of CMEs in the heliosphere. I will highlight our efforts in implementing the three-dimensional reconstruction techniques, primarily using wide field-of-view imager's observations combined with in situ observations and modeling. Our study shows that the kinematics of CMEs can change significantly in the interplanetary medium due to the interaction of multiple CMEs and the substantial aerodynamic drag they experience. The talk will also briefly cover my recent attempts to use the estimated kinematics of the CMEs as inputs to an analytical model to probe the thermodynamic evolution of CMEs at distances much closer to the Sun that have yet to be accessed by in situ spacecraft. The talk's end segment will show a glimpse of solar variability due to episodic CMEs and the ubiquitous quasi-study solar wind over the last two decades. Finally, I will conclude with the lesson learned from our studies and further improvements required in CME research, which have much wider application across astrophysics and the search for habitable exoplanets.

Abstract: Stars are born deeply ensconced in dense clouds of interstellar medium. Protostellar jets launched in the early evolutionary phase of these young stellar objects are targets of intense scrutiny, to understand the properties of the protostellar systems. We have investigated a few jets from massive embedded protostars through their interaction with the ambient medium, in radio and infrared wavebands and we trace emission from shocked regions where the jet impacts the medium. Our results from GMRT, India, at low radio frequencies have confirmed the presence of non-thermal synchrotron emission from these jets, unlike measurements at higher radio frequencies that usually trace thermal free-free emission. We assimilate the observed emission measurements within a numerical model that we have developed, which incorporate the effects of both thermal and non-thermal emission, to understand the properties of the jets and ambient medium in the vicinity.

Abstract: A central aspect of Astrobiology research concerns the delineation of life’s origin and its early evolution on our planet. This intriguing and complex story that is thought to have started several billions of years ago, continues to be a very fascinating scientific mystery. It involves characterising how the transition from chemistry to biology would have occurred on the early Earth. In this context, I will give an overview of the current understanding prevalent in the field while also sharing some of the contributions that the COoL lab@IISER Pune has made towards these attempts. Pertinently, discerning the transition from non-life to life also has important implications for understanding life’s presence/distribution in the Universe.

Abstract: 'Slow-roll' of the inflaton field during inflationary phase is an essential feature of the inflationary dynamics, as the inflationary phase can make a 'graceful exit' when the inflaton field ceases to slow-roll. The slow-rolling of the inflaton field demands the inflaton potential to be nearly flat. However, when the potential becomes extremely flat, such as near any inflection point, the dynamics of the inflaton deviates from slow-roll and enters a phase of so-called Ultra slow-roll. Having a phase of Ultra slow-roll during standard inflationary phase often turns out to be useful, because such an Ultra slow-roll phase can enhance the cosmological perturbations enough which can later collapse and give rise to Primordial Black Holes. Presently, Primordial Black Holes are considered as favoured candidates for the Dark Matter. 

  In this talk, we will ask the question: "What happens in Warm Inflation if the dynamics enters an extremely flat part of the potential?" Warm inflation is a variant inflationary scenario where a concurrent radiation bath is maintained throughout inflation due to the dissipation of the inflaton energy densities to the radiation bath. The system is studied under the assumption of near thermal equilibrium which is one of the essential features of Warm Inflation. We will discuss in this talk the challenges one faces in maintaining the thermal equilibrium of the system when it undergoes an evolution through an extremely flat region of the potential. Thus, realising an Ultra slow-roll phase while keeping the thermal equilibrium in Warm Inflation is a daunting task. We will then show that only a subclass of Warm Inflationary models can undergo Ultra slow-roll while maintaining the thermal equilibrium of the Warm Inflationary system. 

Abstract: Merging stars and compact objects offer extraordinary laboratories to study a wide array of physical phenomena like binary stellar evolution, dust formation, gravitational waves and the origins of the heaviest elements in the universe. Yet, many gaps persist in our understanding of the wide spectrum of outcomes resulting from these mergers, that range from peculiar, low-luminosity explosions to large-amplitude variable stars. In this talk, I will demonstrate how the landscape of mergers can be explored systematically using time-domain surveys. I will describe results from searches for merging massive stars, white dwarfs and neutron stars using the Zwicky Transient Facility (ZTF), Palomar Gattini IR (PGIR) and the newly commissioned Wide-Field Infrared Transient Explorer (WINTER) surveyors at the Palomar Observatory in California. For each class of mergers, I will discuss insights that these systematic samples provide about the formation, evolution and final fates of their progenitor binary systems. In closing, I will describe the exciting potential of the upcoming time-domain surveys in significantly improving our view of the merger landscape.

Abstract: Geodesy is the science of measuring Earth’s shape, size, orientation, and gravity in a 4-D (space-time) coordinate system. This field is at least 2500 years old and is an ally of astronomy. However, the major advances in Geodesy have been made in the last 4 decades after the advent of the satellite era. The most influential development occurred with the launch of GRACE satellite mission in 2002. The concept of this satellite mission is unique and distinctly different from any satellite mission. It provides changes in the gravitational field of the Earth at monthly scale, which has been used to study the mass transport in the Earth system. In this talk, I will shed light on GRACE satellite mission, its working principle, major breakthroughs, and its limitations. Towards the end, I will introduce a new emerging field, Relativistic Geodesy, and talk about opportunities and challenges.

Abstract: The Galactic Warm Ionized Medium (WIM) is one of the four atomic phases of the Galaxy, containing most of the ionized gas (and not HII regions).  The consensus view is that the WIM is powered by ionizing radiation leaking from HII regions. However, we lack clear understanding of how the ionizing radiation diffuses to great distances.  The traditional probes are H-alpha (degree scale; Fabry-Perot) and pulsar dispersion measure (point measurement). Fabry-Perot observations have been conducted in a few optical nebular and auroral lines of metals.  Here, I discuss the diagnostic power of  IFUs on large space- (JWST) & ground-based (Keck) optical telescopes that allow us to study the WIM on scales of arseconds. I will fortify this conclusion by presenting detections of the Galactic diffuse medium via mid-IR fine structure lines with the MIRI-MRS IFU on JWST. I will end the talk reviewing an old but important topic: the ionization fraction of H and He in the WIM.

Abstract: To understand black hole growth, we need to understand the physical processes that drive the accretion of the gas on to the black hole and how they evolve over time. Stellar-mass black holes in X-ray Binaries (XRBs) exhibit extreme spectral state transitions that occur on observable timescales and as a function of accretion rate. A comparison of spectral state changes between stellar and supermassive black holes can inform our understanding of AGN accretion. However, observable timescales for state transitions of AGN are typically not attainable, but can be explored with a large sample of AGN. Here, I will present our analysis of a sample of 3500+ AGN with simultaneous UV and X-ray observations from the XMM-Newton and Neil Gehrels Swift satellites, complemented with radio, optical and infrared data. Our results establish that AGN and XRBs display analogous spectral states, most notably linking the radio emission from the relativistic jet with the energetic emission from the X-ray corona and thermal emission from an accretion disc. I will highlight our unique result demonstrating how the AGN radio morphology correlates with the accretion state change, analogous to the presence of radio jets during a typical XRB outburst. Further, I will present the salient results of our analysis of how different AGN properties, viz. nuclear obscuration, Seyfert type, optical classification are correlated with their observed accretion states, along with their relation to the star formation and stellar masses of their host-galaxies, hinting at the co-evolution of AGN and host galaxy with feeding and feedback.

Abstract: The search for planets beyond our solar system is one of the biggest scientific quests at present. The first exoplanet detection by the radial velocity (RV) method nearly three decades back was awarded the Nobel prize in Physics. Since then, the number of exoplanets found has increased exponentially with nearly 5000 exoplanet detections till date. We have learnt that diverse planetary systems form with orbits, densities, and planetary atmospheres that are very different from the planets in our solar system. In my talk, I will discuss planet demographics across the stellar spectral range focusing on large planets around the solar-type stars and smaller planets around the late spectral type stars. Despite the advent of large telescopes and precise spectrographs, the detection of smaller planets is largely limited by the intrinsic stellar jitter. I will talk about the ways that we devise to disentangle this stellar jitter from the planet signature. I will finally highlight the role of small to medium-size telescopes for exoplanet characterization in the era of current and future space missions like TESS and PLATO.

Abstract: Energy transport across a wide range of dynamical scales in the intracluster medium (and generally in gaseous halos) is one of the most interesting topics in current research and future interest. Hot baryons, visible in the X-rays, need to be stably sustained against radiative cooling over a large inner fraction of the cluster virial radius. A historical motivation has been the lack of sufficient observed cold gas in the cluster cores that is expected in the absence of efficient heating. Quantitatively, there is enough energy from active galactic nuclei to solve the problem at the simplest level, but the complexity of how that energy flows around is not well understood. Multiple transport mechanisms are being actively discussed including long wavelength, nearly isotropic sound waves, anisotropic heat conduction only along local magnetic fields (depending on the local temperature gradient), generation and dissipation of volume-filling turbulence, etc. While sound waves and turbulence have been strong contenders, thermal conduction has been claimed to be further suppressed by gyro-scale whistlers that scatter thermal electrons efficiently in the weakly magnetised ICM. In the latter scenario, thermal instability domain may be enhanced leading to excess and/or smaller scale cold gas. This further implies that observations may need to account for excess cold/mixed phase gas. In my talk, I will discuss these topics of energy transport in the ICM and the consequences. 

Abstract: It is estimated that the solar surface is covered by millions of (almost ballistic) plasma jets at any given time that are believed to be conduits for transfer of mass and momentum especially to the fast solar wind in the coronal holes. About 2 decades ago it was conjectured that these jets are powered by the energy of the bubbling solar convection beneath. However, in later years the narrative has taken a hyperbole to instead include complex microphysics of the solar chromosphere in order explain a certain energetic variety of these spicules (a.k.a type-II) which are believed to be more important than their lethargic counterparts (type I). In our recent work, we combine solar observations, 2D numerical simulations and laboratory experiments to show that observed features of spicule jets can be clearly explained by the inclusion of realistic solar convection in presence of magnetic fields. We further find that our results contradict the existing paradigm of bi-modality (types I vs II) of solar spicules. Our 3D magneto-convection simulations also bring forth a surprising topological possibility- that spicules may not be conical 1D plasma needles dotting the solar surface but, rather like folds in the 2D plasma drapery embedded in the 3D solar atmosphere.

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Abstract: To understand the structuring and dynamics of the upper photosphere and the chromosphere of the Sun, we need to improve and extend the existing numerical radiation-magnetohydrodynamical (MHD) simulations. In the solar chromosphere, radiative energy transport is dominated by only the strongest spectral lines. For these lines, the approximation of local thermo-dynamic equilibrium (LTE) is known to be very inaccurate, and a state of equilibrium cannot be assumed in general. To calculate the radiative energy transport under these conditions, the population evolution equation must be evaluated including all time dependent terms. To this end, we have developed a non-LTE non-equilibrium radiative transfer (RT) module to the well-known MHD code MURaM. In this module we have developed a numerical method to solve the evolution equation for the atomic level populations in a time-implicit way, keeping all time dependent terms to first order. Our main non-equilibrium treatment is of the Hydrogen bound and free states. For the equation of state, to determine kinetic temperature, we treat the Hydrogen molecular evolution also in non-equilibrium. The other elements comprising the gas are treated in LTE. Finally, the pressure and the radiative flux divergence from the RT module are provided to the MHD equations, to evolve the MHD and the radiative quantities self-consistently and iteratively. The module is developed for one, two and three dimensions (1d,2d, 3d) but currently tested in 1d. In this talk, I will describe the method, discuss equilibrium solutions and show some results of the dynamic evolution.

Abstract: The energy produced by supernovae or supermassive black holes plays a major role in dictating the evolution of galaxies. The produced energy is directly responsible for heating the star-forming gas in the interstellar medium or stopping the gas to reach the galaxy thus suppressing overall star formation. The interaction between the produced energy and the interstellar medium is often mediated by bubbles and winds, such as the gamma-ray bubbles (known as the Fermi Bubbles) in our Galaxy. Understanding such processes require multi-wavelength observations and a detailed theoretical understanding of these bubbles and winds. In the talk, I will present some of my recent works to understand how bubbles and winds produced by supernovae and supermassive black holes interact with the interstellar medium and their observational signatures. I will also present my works on the state-of-the-art numerical simulations that have been successful to discover so far unknown properties in these systems and are bringing us much closer to bridging the gap between theory and observations in the interstellar medium.

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Abstract: Stellar feedback is one of the most important ingredients in the evolution of the interstellar medium (ISM). Massive stars inject immense amounts of energy into their surroundings through stellar winds (mechanical feedback) and through emitting energetic photons (radiative feedback). Mechanical energy input pushes the gas into different physical structures and the radiative energy input heats up the surrounding medium by ionizing and dissociating various species. Quantifying stellar feedback is crucial to understand its impact on various physical and chemical processes in the ISM. FEEDBACK is a recent observational survey performed using the SOFIA telescope to observe C+ in 11 sources of our Galaxy. I will present the first results of this survey emphasizing the need of multi-wavelength observations and machine learning to understand our Galaxy better.

Abstract: Metal-poor stars of $\lesssim 0.8 ~\mathrm{M}_\odot$ that have [Fe/H]~$\lesssim -2.5$ are thought to have formed within $\sim 1$ Gyr after the Big Bang. Because of their low mass, they have very long lifetimes and are still around today. The surface composition of these stars is a fossil record of interstellar gas in the early Galaxy from which they were formed and are crucial for studying the early Galactic and chemical evolution. Abundance patterns of elements observed in these stars provide a unique probe for studying the nucleosynthesis and constraining the nature of the first and early massive stars. Interestingly, elements heavier than Fe group such as Sr, Ba, and Pb, which are primarily produced by neutron capture processes, are ubiquitous in these stars. Furthermore, their abundance patterns show a large variation that seems to indicate that all types of neutron capture processes, rapid, intermediate, and slow, operated in the early Galaxy. The sites for neutron capture processes that can operate at such early times however are still a major puzzle. I will discuss the current status of the sites heavy element synthesis in the early Galaxy including recent results from my works.

Abstract: There are several classes of modified gravity theories whose extra degrees of freedom are not completely screened in the interiors of stellar and substellar objects. Such theories predict an altered hydrostatic equilibrium condition inside these objects. Moreover, the interior structures of these objects might have a small pressure anisotropy induced by several physical processes, including rotation and magnetic fields. All these effects, both individually and in conjunction with each other, induce changes in the predicted stellar observables. Such changes also have an impact on different phases of the stellar life cycle, starting from its birth to its death. The aim of this talk will be to understand these and predict new constraints using stellar physics within the framework of degenerate higher-order scalar-tensor theories beyond Horndeski.

Abstract: Magnetic braking, a mechanism with which angular momentum is lost from the system, governs the evolution of binary stars such as cataclysmic variables (CVs) and AM Canum Venaticorum (AM CVn) stars, as well as regulates the spin-down of single stars, like our Sun. However, a physical phenomenon governing such a braking mechanism is still not well understood. In this talk, I will present our physically motivated Double Dynamo formalism of magnetic braking, which explains the prominent features of CVs and AM CVns. In addition, I will touch upon how this model can be used to address the problem of stellar spin-down and its possible implications on habitability.

Abstract: Primordial non-Gaussianity arising from inflationary models is a unique probe of non-trivial dynamics of the inflaton field and its possible interactions with other fields. Often when examining and constraining the scalar non-Gaussianity arising from inflation, certain templates are adopted for the scalar non-Gaussianity parameter f_NL. The current constraints from cosmic microwave background (CMB) on such templates of f_NL provide weak bounds on their amplitudes. However, f_NL shall lead to indirect imprints on the scalar power spectrum through the higher-order loop-level contributions and we may expect effective constraints from them. In this talk, I shall discuss our recent examinations of such non-Gaussian imprints on observables namely, the angular spectrum of CMB and the spectral density of scalar-induced secondary gravitational waves. I shall present the interesting insights provided by this method, particularly for realistic models that do not conform to conventional templates of f_NL. I shall illustrate that the non-Gaussian contributions to observable spectra are of strengths comparable to the Gaussian contributions and sensitive to model parameters that are degenerate at the level of the Gaussian power spectrum. Finally, I shall conclude with an outlook of exciting possibilities of different implementations of this method.

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.