IIA Events List

The Einstein-Hilbert action is the simplest possible generally covariant action for gravity. While Einstein's equations conform with all observational data, theoretical considerations suggest the possibility to add higher curvature correction terms to Einstein's theory. In this seminar, I will discuss the various issues related to such higher curvature terms, particularly causality and stability.

Abstract : The advent of special relativity resulted from intense scientific thinking spanning several decades. The history of relativity begins with the strive to understand motion, inertia, and light, which finally led to the Einsteinian revolution in 1905. In this seminar, I attempt to summarize the rich history of the theory of special relativity; the emphasis would be to discuss the contributions of several physicists and mathematicians and the uniqueness of the approach taken by Einstein.

In galaxy formation on the scale of the large scale structure (>~ 10 Mpc), baryons follow dark matter, the dominant matter component in the Universe. On smaller scales, this breaks down - important baryonic processes such as radiative cooling and energy injection by supernovae and active galactic nuclei (AGNs) become important. After giving a broad overview, I'll describe our recent work [1] (arXiv:2401.00446) on the interaction of a conical AGN jet with a clumpy medium. I'll present the physics of jet collimation by the pressure of the surrounding cocoon and how the anisotropic cocoon becomes an isotropic outflow in the presence of a sufficiently
clumpy medium. I'll end with the implications of this work for Fermi/eRosita Bubbles in the center of our Milky Way, Seyfert jets, and jets/bubbles in galaxy clusters.

High-tea at auditorium Lounge - 03:00 PM

Abstract: Matt is an Engineer & Senior Business Developer in TNOs Space Science Division, Delft, The Netherlands. He will give an overview of TNO's different astronomy technology programs, including adaptive mirrors, laser launch telescopes for AO, large precision optical mounts, and segmented active/adaptive thin shell mirrors.

The process of star formation and evolution continues to fascinate astronomers while remaining a complex puzzle. Resolving binary star systems is crucial for advancing our understanding of them, which allows for precise mass measurements. While radio wavelengths have achieved remarkable resolutions, optical wavelengths pose significant challenges due to the need for longer baselines and sensitive apertures. Traditional amplitude interferometry, effective in the visible spectrum, needs help to maintain wavelength coherency over the observational setup. In the 1950s, Robert Hanbury Brown and Richard Q. Twiss observed correlation phenomena in intensity fluctuations, leading to the discovery of the Hanbury-Brown and Twiss (HBT) effect. Intensity Interferometry (II) and Speckle Interferometry, based on the HBT effect, overcome the limitations of amplitude interferometry by measuring the correlation of photon flux rather than wavelength. This approach makes Speckle interferometry less susceptible to issues like atmospheric turbulence. However, II also has fewer issues with mirror optical quality and longer baselines. Imaging Atmospheric Cherenkov Telescopes (IACTs) such as HESS, MAGIC, and VERITAS show promise for implementing II, particularly during moonlit periods. This presentation will explain the HBT effect using a lab experiment and share results from II simulations applied to binary star systems. These simulations accurately estimate key parameters, including star’s radii, limb-darkening coefficients, and orbital characteristics for both close and wide binary systems. This talk aims to provide an engaging understanding of how II resolves binary star systems and enhances our understanding of the stars.

Research Supervisor: Dr. Piyali chatterjee and Dr. Dipankar Banerjee

Nominal supervisor: Prof. Rajeev Kumar Jain

Abstract: Solar flares and Coronal Mass Ejections (CMEs) are among the most energetic and powerful events in the solar system. Solar flares cause sudden bursts of electromagnetic energy, while CMEs involve the expulsion of magnetized plasma clouds and high-energy particles from the Sun. Both phenomena can significantly disrupt space

weather on Earth, making their study crucial for better understanding and mitigating their effects.

Our research aims to distinguish between eruptive and confined solar flares by analyzing changes in magnetic patterns at the photosphere. We examined 26 eruptive and 11 confined major solar flares using SHARP vector-magnetograms from NASA's Helioseismic and Magnetic Imager (HMI) and Atmospheric Imaging Assembly (AIA), and compared these observations with data from two synthetic flares modelled in a δ-sunspot simulation. During flares, we observed a rapid increase in the horizontal magnetic field along the polarity inversion line (PIL) and a significant change in the downward-`directed Lorentz force. Importantly, all confined flares exhibited a total Lorentz force change below a certain threshold, helping to distinguish them from eruptive flares. Additionally, the change in total Lorentz force depends on the reconnection height in the solar corona at flare onset, shedding light on the propagation of force that leads to change in photospheric magnetogram.

Simultaneously, we investigated the evolution of reconnection flux during CME eruptions using a realistic 3D magneto-hydrodynamic (MHD) model. Our simulations start from an isothermal atmosphere with a potential arcade topology for the ambient magnetic field. The arcade topology mimics the region below coronal streamers. We continuously inject a highly twisted flux rope into the coronal domain between the footpoints of the parallel arcade.

This process causes the surrounding magnetic field to stretch and compress, triggering magnetic reconnection and leading to the eventual expulsion of the flux rope due to ideal MHD instabilities like torus instability and/or helical kink instability. Using a novel approach to directly calculate the reconnection flux during eruptions, we compared our simulations with data from HMI, AIA, and Solar TErrestrial RElations Observatory (STEREO). We found that variations in reconnection flux are crucial for CME development in the lower corona, with the speed of the flux rope closely correlated to these flux variations, showing different patterns before and after eruptions.

Our study provides a near-realistic simulation of solar eruptions, offering valuable insights into the complex dynamics of CME initiation and progression. By better understanding the interactions between magnetic reconnection, Lorentz forces, and flux rope evolution, we can improve our ability to predict and mitigate the impacts of these powerful space weather events, ultimately enhancing our preparedness for such solar phenomena.

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: We present findings on the fastest coronal mass ejection (CME) observed during the ascending phase of solar cycle 25. This event occurred on September 5, 2022, as Parker Solar Probe (PSP) traversedthrough the CME during its 13th perihelion encounter. The CME exhibited an exceptional velocity of approximately 2500 km/s, categorizing it within the rare 0.15% of extreme-speed CMEs documented in the CDAW catalog. Positioned at a distance of around 13 solar radii (Rsun), PSP not only passed through the CME but also encountered its substructures and an associated current sheet on September 6 within this perihelion period. In-situ measurements within the current sheet region revealed distinctive features, including magnetic field reversal, elevated temperatures, and particle velocities consistent with analytical solutions of magnetic reconnection. Correlating these measurements with remote sensing data from Solar Orbiter indicates an ongoing reconnection as PSP traverses the current sheet. This marks it to be the first in-situ observation of an ongoing reconnection linked to an eruption in the solar corona.

Abstract: 'Bending Light' recounts the expedition to Western Australia in 1922 led by an international team of astronomers to photograph the solar eclipse and prove Einstein's theory of general relativity. Through conversations with leading experts, and the discovery of rare archival imagery, science journalist Bob McDonald illuminates how this significant event continues to support contemporary breakthroughs in the fields of astronomy and astrophysics.

Note: This documentary features our Kodaikanal Solar Observatory too. As you know that KSO is celebrating its 125th year of existence this year. Therefore, it is a great occasion to screen this documentary at IIA.

High Tea at 10:30 AM (First floor lounge)

Abstract: The formation and evolution of spiral arms in low surface brightness galaxies (LSBs) are not well-understood. We study the dynamics of spiral arms in two prototypical LSBs, F568-VI and F568-01, using both analytical models and N-body + hydrodynamical simulations. We first consider the disk as a 2-component system of gravitationally-coupled stars and gas in the force field of a \emph{spherical} dark matter halo, subjected to local, non-axisymmetric perturbations. However, no local spirals are formed. We next assume the disk to be a 1-component system of stars in the net gravitational potential of a galaxy with a spherical dark matter halo perturbed by a global m=2 instability. In this case, the growth time for spiral formation was low, equal to 0.78 and 0.96 Gyrs respectively, corresponding to a few dynamical times of the galaxies. Finally, we simulate the LSBs using the N-body + hydrodynamical simulation code RAMSES. Our results show that a quadrupolar field associated with an oblate halo with an axial ratio of 0.7 is necessary to drive a long-lived global spiral in the LSB disks. Further, feedback corresponding to a supernova mass fraction of ~ 0.05 is essential to comply with the observed stellar surface density. The simulated spirals survive for about ten dynamical times and the average pattern speed lies between 10 - 15 kms-1kpc-1. The spiral arm thus formed is therefore a transient global pattern driven by the tidal field of the oblate dark matter halo.

Abstract: The adaptive optics (AO) technology unfolds the capabilities of ground-based telescopes and opens new horizons. A cost-effective and resilient laser-guided single conjugated adaptive optics system offers the opportunity to a significant number of existing 1-4 meter telescopes. This opens the door for medium-sized telescopes to pursue cutting-edge scientific research that may be challenging to achieve with larger observatories due to a scarcity of time for various survey observations. The medium-class telescope, equipped with adaptive optics along with advanced image processing, provides unprecedented image quality in terms of spatial resolution (sub-arcsec), and high cadence with high SNR. Here, we present a robust Rayleigh scattered laser-guided single conjugated adaptive optics system called SALTO, which was designed, built, and tested in the Belgian countryside on a 1-meter class telescope and an automated robotic adaptive optics system called iRobo-AO for IUCAA 2m telescope. These projects aim to demonstrate the possibility of rejuvenating the scientific goals of medium-class telescopes with AO technology, as well as to enable optical telecommunication from relatively poor observing sites. This talk discusses the overall study of the design of the AO system, from the optics to the control system. It also includes a description of the integration and calibration of SALTO. It concludes with the presentation of successful on-sky results at 1.55µm under 2-3'' seeing.

Abstract:

TGlobular clusters (GCs) are the oldest stellar systems in the universe, even older than their parent galaxies. Studying them is of interest since they carry information on the early stages of the formation of the Galaxy. Two colour time-series photometry of GCs and proper motion analysis enable us to build accurate Colour- Magnitude diagrams and conjecture the structure and evolutionary time-scale of low- mass post-RGB stars. Pulsating stars are accurate indicators of mean cluster metallicity and distance. We have been studying the variable star populations contained in a sample of nearly 35 Galactic GCs for more than two decades, in collaboration with astronomers at the Indian Institute of Astrophysics. In this talk, I will briefly describe our approach strategies to the understanding of GC and the structure of the Horizontal Branch, and comment on how we pass from the counting of photons at the telescope, to the determination of physical parameters of astrophysical interest.

Title: The Formation and Evolution of Massive Galaxies in the Cosmos and their Circumgalactic Environment

Contrary to many stereotypes about massive galaxies, the observed systems are diverse in their star formation rates, kinematic properties, and morphologies. Studying how they evolve into and express such diverse characteristics is an important piece of the galaxy formation puzzle. Here, we focus on a subset of massive galaxies, the brightest group galaxies (BGGs). We use a high resolution cosmological suite of simulations based on the Romulus galaxy formation model, and compare simulated central galaxies in group-scale halos at 𝑧 = 0 to their observed counterparts. Since most galaxy formation models are calibrated using measures that are strongly influenced by the properties and evolution of "normal" Milky-Way like galaxies, this exercise is also an opportunity to test the limits of these models. The comparison encompasses the stellar mass-halo mass relation, various kinematic properties and scaling relations, morphologies, and the star formation rates. We find Romulus BGGs that are early-type S0 and elliptical galaxies as well as late-type disk galaxies; we find BGGs that are fast-rotators as well as slow-rotators; and we observe BGGs transforming from late-type to early-type following strong dynamical interactions with satellites. In short, we find that Romulus reproduces the full spectrum of diversity in the properties of the BGGs very well. Additionally, due to its superb mass and spatial resolution, Romulus also offers a unique window onto the joint evolution of the BGGs and the surrounding intragroup medium. With respect to the latter, we are able to observe the emergence of multiphase structure - in the form of cold clouds - in the intragroup medium. Groups also experience repeated AGN feedback episodes that drive large-scale collimated outflows into the IGrM. While the present resolution does not allow direct exploration of the coupling between the clouds and the AGN jets, we argue that the clouds will cause the SMBHs (and hence, the jets) to change direction every so often. Returning back to the BGGs, we find that early type galaxies can rejuvenate by growing disks, in agreement with recent observations. However, we also note a tendency towards lower than the observed fraction of quenched BGGs, with increasing halo mass. The problem appears to be due to decreasing effectiveness of AGN feedback with increasing halo mass. Examining some of the other galaxy formation models, we find that they too run into trouble on the same scale — but in an opposite sense. I will conclude by discussing what we are to make of this and what the path forward looks like.

High tea at the auditorium lounge at 03:00 PM

Abstract:

Compact object binaries represent extraordinary laboratories within the universe, characterized by some of the strongest gravitational regimes and interactions of high-temperature matter. X-ray observations of these systems offer valuable insights into their emission components, shedding light on the physical properties of compact objects owing to the proximity of the emission region to these objects. In this seminar, I will delve into some of the intriguing White Dwarf and Neutron Star binaries, exploring their spectral and timing properties. The initial segment of my presentation will focus on Nova Her 2021, renowned as one of the fastest-declining Novae in recent times. Using simultaneous X-ray and UV observations with AstroSat, we probe the rotation period and the spectral properties of the source as a function of the rotation of the white dwarf. Subsequently, I will delve into a peculiar persistent Neutron Star binary, GX 340+0, elucidating how its spectro-timing properties can provide invaluable insights into the evolution of the source. I will discuss the prospects of X-ray polarization observations of these sources and how they can reveal the insights into the geometry of the emission components.

Abstract:

The ages of astronomical objects, while not directly measurable, are of use in constructing chronologies, and the key to understanding origins. The ages of cool field dwarfs, although important in a Galactic context, are especially challenging to obtain. I will present a route to the ages of such objects that is called "Gyrochronology," one based on the measured rotation periods of such stars.