LATEST PROJECTS
Project | 01
Photometric Reverberation Mapping with Vera Rubin Observatory
Developing a pipeline for time-delay measurements in the Vera Rubin Observatory Legacy of Space and Time (LSST) photometric channels suitable for typical AGN (continuum + line emission components) and realistic prediction of the number of quasars for which the delay measurement can be performed with reasonable accuracy.
Check our Astronarium interview on how to make use of quasars for cosmological applications using the Vera C. Rubin Observatory
Check our predictions for time-delay measurements in active galactic nuclei with the Vera Rubin Observatory (on behalf of the AGN Science Collaboration)
Check our comprehensive review on the super-Eddington AGNs and how they can be used as `standardizable' candles to study the expansion of our Universe.
Project | 02
Modeling continuum anisotropy and super Eddington accreting quasar spectra
Modeling the broad-band spectral energy distribution (SED) effective in ionizing the gas-rich BLR is key to understanding the various radiative processes at play and their importance that eventually leads to the emission of emission lines from diverse physical conditions. In this work, we focus on a long-standing issue pertaining to the anisotropic continuum radiation from the very centers (few 10-100 gravitational radii) of these active galaxies. The anisotropic emission is a direct consequence of the development of a funnel-like structure at regions very close to the black hole due to a marked increase in the accretion rates.
This study allows us to look at the diversity of the Type-1 active galactic nuclei (AGNs) in the context of the main sequence of quasars, locate the super Eddington sources along the sequence, and constrain the physical conditions of their line-emitting BLR. This feat will eventually allow us to use the fascinating super Eddington quasars as probes to understand better the cosmological state of our Universe.
I wrote an article illustrating a new approach to connect the Eigenvector 1 to radius-luminosity relation through photoionization modeling
Project | 03
The origin of iron in Narrow-Line Seyfert 1 galaxies
Characterize Narrow Line Seyfert Type-1 (NLS1s) by studying their stellar populations and gas kinematics in the vicinity of their central engines, establish an evolutionary connection between the more massive and older Seyfert 1s and these smaller and younger NLS1s; and, model their spectral energy distribution (SED) using photoionization to constrain the physical parameters of the broad-line region. Through spectroscopic and radiative transfer modeling, I aim to unravel the many faces of the Quasar Main Sequence.
Check out one of my colloquia where I describe at length this research and its findings
Our results got published on the cover of Astronomical Notes (January 2022)
Project | 04
Taming the derivative: diagnostics of the continuum and Hβ emission in a prototypical Population B active galaxy NGC5548
NGC 5548 has been hailed as an archetypical type-1 active galactic nuclei (AGN) and serves as a valuable laboratory to study the long-term variation of its broad-line region (BLR). In this work, we demonstrate the connection between the continuum variability in the optical regime and the corresponding Hβ response to it, in order to realize the increase, albeit with a gradual saturation, in the Hβ emitting luminosity with increasing AGN continuum. This effect is also known as the Pronik-Chuvaev effect after the authors who first demonstrated this effect using long-term monitoring of another well-studied Type-1 AGN - Mrk 6. We employ a homogeneous, multi-component spectral fitting procedure over a broad range of spectral epochs that is then used to create the continuum and Hβ light curves. We attempt to recover this Hβ emission trend as a response to a significant continuum flux increase using CLOUDY photoionization simulations and employ a suite of broad-band spectral energy distributions for this source. We are successful in recovering the observed shallowing of the Hβ emission with respect to the rising AGN continuum and provide constraints on the local BLR densities and the location of the Hβ emitting BLR which agrees with the Hβ time-lags reported from the long-term reverberation mapping monitoring.
Project | 05
Multi-parameter visualization of the Quasar Main Sequence
(A) We devise a novel technique to understand the complexity within the Quasar Main Sequence by creating photoionization-based multi-dimensional maps as a function of 8 key parameters (black hole mass, Eddington ratio, the width of the emission line, cloud density, metallicity, turbulence and the shape of the ionizing continuum). This allows us to constrain the observed sources and estimate their inclination with respect to the observer with high accuracy; (B) To facilitate the use of the results from this massive suite of simulation (~40 million models, over 10 TB library of spectral lines), we intend to make our database public for future collaborative efforts.
My most-cited paper `The Quasar Main Sequence Explained by the Combination of Eddington Ratio, Metallicity, and Orientation' published in the Astrophysical Journal
Check our latest work where we discover a chemical clock using the relative abundances of Calcium and Iron in a sample of Type-1 AGNs
Project | 06
The interplay between the Fe+ and Ca+ species in the Broad-line Region
Resolving the complexity of Fe II species in quasar spectra has been an ongoing work for over 40 years. First identified and reported for the prototypical Narrow-line Seyfert 1 galaxy, I Zw 1, the study has made a niche of its own in the field of AGN research. A major part of the puzzle is lent by the sheer number of spectral lines in Fe II that span across a wide energy range (from UV to NIR). This extended emission seen in the spectra mimics a continuum of sorts, thus the telltale term pseudocontinuum. Gaining knowledge from past studies and our own, in this study we search for a reliable proxy for Fe II. This proxy, Ca II, is a much simpler ionic species that is characterized by its triplet in the near-infrared part of an AGN spectrum. The analogous line excitation mechanisms (dominated by the Lyα fluorescence and collisional excitation) for the production of these two species are confirmed by the tight correlation between the respective line strengths that we observe from our up-to-date collection of coincident measurements in the optical and NIR, and re-affirmed by our photoionization models.
Additionally, our models constrain the physical parameters, such as the required level of ionization and the density of the medium (i.e. the broad-line region) that contain these ionic species, hinting also at the cloud’s composition and structure (Panda et al. 2020b; Panda 2020). This study reveals the utility of the Ca II as a proxy for Fe II in ways more than one, primarily, establishing a new radius-luminosity relation and in quasar main sequence studies.
Check out one of my colloquia where I describe at length this research and its findings