2024

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.

Abstract: The missing mass hypothesis is one of the fundamental challenges in physics today. The Lambda-Cold Dark Matter (LCDM) cosmological model advocates that our Universe consists of particles, termed dark matter (DM), which interact only via gravity. Modified Newtonian Dynamics (MOND), proposed by M. Milgrom in 1983 in an attempt to circumvent the dark matter paradigm. It postulates that systems that have acceleration, a, less than the fundamental acceleration constant a0, should exhibit deviations from Newtonian dynamics, thus offering an alternative explanation for the missing mass problem. Galaxies fall in the regime of low accelerations, a < a0, making them ideal testing grounds for this theory. MOND as a paradigm has modified gravity versions (AQUAL and QUMOND) and a modified inertia version. Many numerical solvers have implemented the modified gravity versions of MOND, thus allowing one to test MOND via numerical simulations. In this talk, I will briefly introduce MOND, its basic tenets, predictions, and tensions. Most importantly, I will present results from our work on simulations of disc galaxies, ultra-diffuse galaxies in galaxy clusters in MOND, and few other simulations, which have been used to test MOND. 

Abstract: Magnetic fields permeate the entire universe, extending from the smallest to the largest observable length scales. A popular explanation for the origin of the magnetic fields observed in galaxies, clusters of galaxies, and the intergalactic medium is that seed fields generated due to quantum fluctuations in the primordial universe are amplified later by astrophysical processes. According to the standard paradigm of magnetogenesis, the seed magnetic fields on cosmological scales are generated during inflation by breaking the conformal invariance of the standard electromagnetic action. This is usually achieved through a non-conformal coupling of the electromagnetic field to the scalar field that drives inflation. I will begin the talk with a brief introduction to the essential idea of inflation. Thereafter, I will highlight the challenges in generating primordial magnetic fields during inflation in scenarios involving departure from slow roll in single field models. I will also discuss how we circumvent those challenges with suitable construction of coupling functions in different scenarios. Further, I will discuss the scenario of pure ultra slow roll inflation and show that scale invariant magnetic fields can be obtained in such situations with the aid of a non-conformal coupling function that depends on the kinetic energy of the inflaton. Apart from the power spectrum, an important probe of the primordial magnetic fields is the three-point function, specifically, the cross-correlation between the curvature perturbation and the magnetic field. We calculate the three-point cross-correlation between the curvature perturbations and the magnetic fields in pure ultra slow roll inflation for the new choice of the non-conformal coupling function. I will compare the results we obtain for these three point functions in case of slow roll and ultra slow roll scenarios, particularly emphasizing on the squeezed limit.

Abstract: The presence of a strong, large-scale magnetic field in an accretion flow leads to the extraction of the rotational energy of the black hole through the Blandford-Znajek (BZ) process, believed to power relativistic jets in various astrophysical sources. I will present the results of 3D GRMHD simulations of a highly magnetized cold thin disk surrounding a black hole, exploring the extraction of its rotational energy through the BZ process. Our findings reveal a weaker dependence of magnetic flux on black hole spin in a thin cold disk compared to hot accretion flows, and a significant fraction of extracted energy is potentially channelled into winds or disk radiation rather than the jet. I will highlight the implications of our results for understanding X-ray corona formation, black hole spin measurements, and interpreting transient phenomena, and discuss how strong magnetic fields enhance disk radiative efficiency.

Abstract: The formation and evolution of the massive disk galaxies in the nearby universe (z<0.1) represents an important open question in the present-day galactic astrophysics research. One class of such galaxies are the giant low surface brightness galaxies, e.g, Malin1 ,UGC1378, UGC 1382 etc, that are observed to show a complex morphology- a central high surface brightness stellar disk (HSB) surrounded by an extended low surface brightness (LSB) disk. The extended LSB disk could form due to external accretion or by secular evolution processes. In this talk, I will discuss our exploration with IllustrisTNG50 simulation data to identify and study various properties of such double-exponential, massive disk galaxies in the stellar mass range >=1011 solar mass. The structural properties of such galaxies are obtained by 2D GALFIT modeling. The radial scale length of the LSB disks are found to lie in the range of ~10-30 Kpc, in agreement with observations. The specific star-formation properties of the above double-disk galaxies are studied to understand their distribution from blue star-forming to red quenched region. Finally, we study such galaxies in the Baryonic Tully-Fisher relation. Our theoretical exploration will be potentially useful in exploring such galaxies in the upcoming deep observed data such as from LSST.

Abstract: Galaxy formation and evolution is tied to the physical state of gas in the circumgalactic medium (CGM) and its interface with the intergalactic medium (IGM), which is determined by the complex interplay between inflows from the IGM and galaxy feedback. Therefore, a comprehensive understanding of the physical conditions of gas within and surrounding galaxies is of paramount importance to understanding the physical processes that regulate galaxy formation and evolution. Numerous efforts to trace the diffuse gas seen as an absorption line in the background quasar spectra have revealed that intervening metal absorbers arise from multiple pathways, including gas inflows and outflows, the intragroup medium, and cool stripped gas from environmental processes. In particular, MgII absorbers, which trace cool, 10^4K, metal-rich gas, are frequently observed across a wide range of impact parameters, up to 200 kpc. However, the notion that the absorption is caused by galaxies at close impact parameters remains viable because it is highly challenging to find such faint galaxies in the glare of a bright background quasar. I will discuss the possible origin of intervening metal absorbers, the distribution of gas in the circumgalactic medium, and how it relates to the absorber properties in general over a wide range of redshifts and stellar masses.

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Abstract: POLIX is the main scientific payload on XPoSat (X-ray Polarimeter Satellite), a dedicated X-ray polarimetry mission, launched on 01st January 2024. POLIX is sensitive in the X-ray energy range of 8-30 keV. Measuring polarization in celestial X-ray sources is a new frontier in X-ray astronomy, providing crucial insights into emission mechanisms in X-ray sources. POLIX is based on the anisotropic Thomson scattering of polarized X-rays from a low atomic mass scatterer (Beryllium) and their subsequent detection in X-ray proportional counters. During this talk, I will discuss some instrument development aspects for the POLIX payload and some of the key scientific prospects for POLIX. In particular, I will discuss interesting science for the Ultraluminous X-ray sources (ULXs), their X-ray timing and spectral characteristics and possible X-ray polarization measurement for these sources.

Abstract: Standard stellar evolutionary theory explains the evolution of the majority of low-mass stars. However, some stars show anomalies like rapid rotation, infrared excess, and unusual surface chemical composition during their evolution. Our extensive studies made significant progress in understanding anomalous high Lithium and extreme deficiency of carbon in red giants, which were nagging problems in stellar astrophysics for many decades, thanks to asteroseismology and large-scale spectroscopic surveys. In this talk, I will discuss the observational results from the above studies that constrain non-standard processes during the evolution of giants from red giant branch to red clump phase, binary mass transfer, and merger scenarios of star-planets/star-white dwarf systems.

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Abstract: Type I supernovae (SNe) are, in general, deprived of Hydrogen in their early spectral sequence. Type Ia SNe, a subtype of type I SNe, result from thermonuclear explosion of white dwarf. I will talk about type Iax SNe, a rare subclass of type Ia SNe. These are low luminous, less energetic cousins of type Ia SNe and heterogeneous in nature. Because of the low energy budget of type Iax SNe, several progenitor systems have been proposed based on photometric and spectroscopic features. I will discuss a few promising progenitor channels for these events which are supported by observations. Type Ib SNe are another subtype of type I SNe with He signatures in their spectral time series. They are supposed to be originating from explosion of massive stars, stripped off their outer hydrogen envelope. I will also discuss their optical nature and most plausible progenitor channel.

Abstract: The Dark Energy Survey Instrument (DESI) is a Stage-IV large spectroscopic survey aimed to observe more than ~40 million optical spectra of galaxies, stars, and quasars by 2026. In June 2023, the early data release (EDR), comprising 1 percent of the survey data, was released, containing ~2 million spectra. The first major data release (DR1) is scheduled for early next year, which will be the most extensive spectroscopic dataset to date, comprising over ~20 million spectra. In this talk, I will provide an overview of the current status of the DESI survey, including the first cosmological results based on the EDR and DR1, as well as prospects for its future extensions. Additionally, I will also present how DESI can provide deep insights into the properties of metals in the Universe using quasar absorption lines.

Abstract: Fast radio bursts (FRBs) are among the newest tools in observational astrophysicists’ repertoire to study ionized gas. Their unique, millisecond-duration radio signal is subject to propagation effects in the intervening plasma. One such effect is the plasma dispersion of FRB pulses. FRB dispersion measures (DMs) quantify the net free electron column density through the sightline. FRB DMs can be precisely measured (~0.1%) and thus are sensitive to the most diffuse plasma in the intergalactic medium (IGM) that traditional probes have found challenging to illuminate. This ability to provide novel constraints on plasma has motivated studies of the circumgalactic medium (CGM) of galaxies intersecting FRB sightlines and the cosmic web filaments of the IGM. In my talk, I will highlight some of the work done leveraging FRBs and introduce the FLIMFLAM survey. FLIMFLAM is an ongoing spectroscopic endeavor to map foreground matter density along ~30 FRB sightlines. Its ultimate aim is to produce statistical constraints on key parameters describing matter distribution in the universe, including the fractions of ionized baryons residing in the diffuse IGM and the virialized gas of halos. With the first data release already published earlier this year, I shall discuss our results and ongoing work for the second data release. I will end with the prospects for the near future with FRB foreground mapping along a few hundred FRB sightlines in the era of large spectroscopic surveys such as DESI and 4MOST.

Abstract: We use the anti-de Sitter/conformal field theory (AdS/CFT) correspondence to find the least bounce action in an AdS false vacuum state, i.e., the most probable decay process of the metastable AdS vacuum within the Euclidean formalism by Callan and Coleman. It was shown that the O(4) symmetric bounce solution leads to the action minimum in the absence of gravity, but it is non-trivial in the presence of gravity. The AdS/CFT duality is used to evade the difficulties particular to a metastable gravitational system. To this end, we show that the Fubini bounce solution in CFT, corresponding to the Coleman de Luccia (CdL) bounce in AdS, gives the least action among all finite bounce solutions in a conformal scalar field theory. Thus, we prove that the CdL action is the least action among all possible large and thin-wall configurations under certain conditions.

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.