Ritaban Chatterjee

Assistant Professor

“Don't ask yourself what the world needs; ask yourself what makes you come alive. And then go and do that. Because what the world needs is people who have come alive”

--- Howard Thurman, Gil Bailie's Violence Unveiled, p. xv (Thanks to Max Tegmark at MIT from whom I first came to know this quote).

A supreme quality of teaching and top level research are symbiotic in nature. This is apparent at the majority of the premier research universities in the world where students get to learn and draw inspiration from the faculty members who are working on cutting-edge research while the faculty members rejuvenate their drive for research through teaching and spirited discussion with students.

Presidency University is trying to provide such an environment to its students and faculty.  I am glad to be a part of this endeavour.

My broad research interest is in astrophysics. More specifically I probe the physics of astronomical objects containing black holes, e.g., active galactic nuclei (AGN) and X-ray binaries, primarily by studying their time variability. Below I give a (relatively) non-technical description of my research. More technical details are in the section named Research / Administrative Experience.

All galaxies are believed to contain a super-massive black hole (having a million to a billion times the mass of the Sun) at their center. About 5% of these black holes are actively accreting mass from its surroundings and as a result producing enormous amount of electromagnetic radiation from gamma-ray to radio frequencies. These galaxies are called active galaxies and the bright central part of the active galaxies are called active galactic nucleus.

There are several reasons for which learning more about the nature of AGNs is important:

1. An active galactic nucleus (AGN) is brighter than the rest of the galaxy and sometimes ten thousand times as bright as an average galaxy. Being the most luminous long-lived class of objects in the sky that are more common at high redshifts, their light comes from a very young universe. They are an important tool for the astronomers to probe the distant and early universe.

2. The extremely high luminosities of AGNs are thought to be produced by the accretion of matter on to a supermassive black hole. Hence, AGN is a natural laboratory to test strong field General Relativity.

3. In many cases AGNs produce two oppositely directed jets of magnetized plasma moving at near-light speed that are equally luminous over a large range of wavelengths from radio to gamma rays. Jets are the brightest non-transient objects in the sky. The apparent brightness of jets is amplified by factors of few-10000 if they are pointed toward our line of sight. Hence we can observe them even if they are far away. In some cases, high-energy emission (X-rays, Gamma-rays) from the jets varies by a factor of a few in minutes. To explain how such large amount of emission is generated at such short-timescales is a challenge for the theories of acceleration of particles.

4. It has been established now that the stellar motion near the center of the galaxies is correlated to the mass of the central super-massive black hole (SMBH). In addition, it has been observed that kinetic energy of the jets affect its environment such as the intra-cluster medium. Hence, the formation and evolution of galaxies are influenced by the SMBH and the jets.

5. Considerable progress has been made in our understanding of the nature of active galactic nuclei (AGNs) over the last two decades. But how and why the jets form is not understood. Knowledge about this will be useful for many astrophysical systems containing a jet such as stellar mass black hole X-ray binaries, and young stellar objects as well as active galactic nuclei.

Due to their large distances, AGNs are not spatially resolved with current and near-future technologies except by radio interferometry. However, one of the defining properties of AGNs is time variability, that is, their brightness significantly varies with time due to changes in emission processes and internal dynamics. Hence, these systems can be probed by analyzing the change of its brightness with time at a given wave band (e.g. X-rays) as well as how the change at different wave bands (e.g. optical vs. gamma-rays) compare.

This analysis and its theoretical interpretation is the central theme of my research.


Presidency University,
86/1 College Street, Kolkata - 700073,
West Bengal, India

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Email: ritaban.physics at presiuniv.ac.in
alternate E-mail: rito.chat at gmail.com

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