Research

The innermost regions of planetary systems host Ultra Short Period (USP) planets - exoplanets orbiting their stars in less than 24 hours, whose origins remain poorly understood (for analogy, we can think of the innermost planet in our solar system: Mercury takes 88 days to complete one single revolution around the sun).  Among the processes shaping planetary architectures, disk-driven migration, resonant interactions, and tidal evolution play a central role in transporting planets from their birth locations to extremely close-in orbits. However, the pathway by which planets survive migration and avoid engulfment by the host star is still unclear. With recent advances in combining hydrodynamic prescriptions and long-term N-body simulations, there is a possibility to understand planetary evolution in a unified, physically consistent framework. In our research, we simulate the coupled effects of migration, resonance formation and disruption, and tidal circularisation to track planetary systems from formation to final architecture, and compare the resulting populations with observations from the space-based telescopes: Kepler and TESS.

Massive contact binaries play a crucial role in shaping both the present-day properties and long-term evolution of galaxies. Quantifying their evolutionary pathways is essential to determine what fraction of such systems merge during the main-sequence phase and what fraction survive to form compact objects. The dominant mechanism driving these mergers is angular momentum loss in tidally synchronized, short-period systems. Our research aims to constrain the evolutionary trajectories of massive contact binaries through a combination of spectrophotometric observations and theoretical simulations. In particular, signatures of internal mixing will be investigated using high-resolution spectroscopy, while photometric period variations will be used to estimate angular momentum loss rates. Together merger timescales and the eventual fate of these systems can be robustly determined.

X-ray polarization provides a unique diagnostic tool to probe the complex physical processes and geometries in X-ray binary systems. In these systems, matter accreted from a donor star onto a compact object (Neutron star or Black hole) produces high-energy radiation, whose emission mechanisms and scattering environments imprint characteristic polarization signatures. Our research focuses on understanding the physical processes in X-ray binary systems using X-ray polarization measurement techniques and incorporating data from the Imaging X-ray Polarimetry Explorer (IXPE) along with complementary observations from missions such as XMM-Newton, Chandra X-ray Observatory, NICER, NuSTAR, XSPECT, and HXMT. By analyzing X-ray polarimetric and multi-mission data, we aim to address key research questions, including the geometry of the inner accretion flow, the structure of the corona, the origin and properties of relativistic jets, and flux-dependent polarization in these systems.

The increasing number of artificial satellites and space debris in Low Earth Orbit (LEO) has made precise orbit prediction and tracking essential for space situational awareness, satellite safety, and national security. Ground-based optical tracking systems provide a passive and cost-effective method for monitoring LEO objects; however, their performance strongly depends on the accuracy of orbital predictions used to guide telescope pointing and tracking. Incorporating a high-fidelity physics-based orbital propagator HPOP with assistance from a recurrent neural network algorithm in our research, we are developing an advanced orbit prediction framework designed to support high-accuracy optical tracking of LEO objects at altitudes between 500 and 1500 km, with a target positional accuracy of meter-scale. The resulting hybrid framework is expected to demonstrate the potential of combining physics-based orbit propagation with data-driven models to significantly improve prediction accuracy for next-generation optical space surveillance systems.

In this project, we are working on the ongoing Chandra Non-nuclear Source Characterisation (CNSC) survey, as part of the continued LeMMING legacy survey for nearby AGN, using the most sensitive X-ray (Chandra) telescope and the radio telescope array, e-MERLIN, UK, simultaneously.

Our survey encompasses all off-nuclear X-ray sources observed in 211

LeMMINGs galaxy fields using Advanced CCD Imaging Spectrometer (ACIS), which provides a comprehensive collection of detailed X-ray spectral and temporal properties, as well as corresponding optical counterparts. With stringent source selection criteria, our CNSC comprises a total of 1388 off-nuclear X-ray sources that were not studied, and their nature has yet to be identified in detail before. For each individual source, we provide detailed characteristics such as the long-term (> 10 years) X-ray light curve, the evolution of X-ray hardness, and the evolution of best-fit spectral parameters for different sources. We have identified several supernova remnants with high and low-mass X-ray binaries in our sample. While the classification is still preliminary and requires multiwavelength coverage in optical and infrared, the CNSC represents a significan advancement in understanding the nature of off-nuclear X-ray sources in nearby Galaxies.

The inner 30 light-minute radii of accreting supermassive black holes are revealed mostly in UV and X-rays. Among the few fascinating events we observe using UV/X-ray satellites, X-ray reverberation is one where X-ray flashes occur in a region as close as ten light-minutes away from the supermassive black hole and are reflected in the accretion disc before reaching the observer. However, such an echoed light is delayed due to the bending caused by the extreme gravity of the black hole. This delay encodes crucial information about the geometry, inclination, and physical conditions of the innermost accretion flow. It also provides a unique probe of strong-field gravity, allowing us to test predictions of general relativity in regimes inaccessible elsewhere and to map the spatial extent of the X-ray emitting corona. Due to a dramatic breakthrough in developing a fully relativistic, time-dependent, ray-tracing disc reflection model, a more realistic quantitative analysis of X-ray reverberation is now possible. In this research, we compute the delay between the direct and reflected light as a function of photon energy, model the X-ray spectrum and use the fitted parameters to perform a simulation that agrees with the observed energy-dependent delay spectra, thereby constraining black hole spin, corona geometry, and disc ionization structure. We further explore how variations in accretion rate and coronal height influence the lag-energy and lag-frequency spectra, enabling a comprehensive interpretation of timing and spectral signatures observed in active galactic nuclei.

Reverberation mapping across X-ray, UV, and optical bands has emerged as a powerful technique to probe the innermost structure of Seyfert galaxies hosting accreting supermassive black holes. Rapid X-ray variability originating in a compact corona irradiates the surrounding accretion disc, producing delayed echoes in softer X-rays, UV, and optical wavelengths. These interband time delays trace light-travel distances, enabling constraints on the geometry, size, and stratification of the disc–corona system. In particular, X-ray reverberation reveals relativistic effects within a few gravitational radii, while UV/optical lags map larger disc scales and thermal reprocessing.

Recent advances in high-cadence, long, multiwavelength monitoring campaign now allow joint spectral–timing analyses with unprecedented precision. By modeling energy-dependent lags and continuum reprocessing, one can infer black hole spin, coronal height, and disc ionization structure. Deviations from standard thin-disc predictions, often observed in UV/optical lags, suggest complex temperature profiles or additional reprocessing regions. This multi-band reverberation framework thus provides a unified view of accretion physics, bridging strong-gravity effects near the event horizon with large-scale disc emission in Seyfert galaxies.

Black hole X-ray binaries (BHXRBs) exhibit dramatic spectral and timing evolution as they transition between the spectrally hard, powerlaw-dominated state and spectrally-soft blackbody-dominated state over timescales of days to weeks, accomanied by Radio jet ejections. In the hard state, the X-ray spectrum is dominated by a power-law component (photon index 1.5–1.7) with a high-energy cutoff (~50–150 keV), and strong aperiodic variability is observed with fractional rms amplitudes of 20–40%. The power density spectrum (PDS) shows broad-band noise and quasi-periodic oscillations (QPOs) at ~0.1–10 Hz. During the transition, the spectrum softens (Index: 2.2–2.5), the disc blackbody component strengthens (kT ~ 0.5–1 keV), and the rms variability drops to <10%. Spectro-timing analysis reveals energy-dependent time lags, with hard photons lagging soft photons by ~10–100 ms at frequencies of 0.1–10 Hz, consistent with inward propagation of mass accretion rate fluctuations. Phase lags and coherence functions constrain the coupling between the disc and corona. In the soft state, variability is suppressed (rms <5%) and QPOs weaken or disappear. Modeling these observables with propagating fluctuation and Comptonization frameworks enables constraints on the truncation radius (tens to a few gravitational radii) and coronal geometry, providing a quantitative picture of accretion flow evolution during state transitions.