My broad research interest is in the area of Astrophysics. I am particularly interested in
multi-wavelength time variability of Active Galactic Nuclei (AGNs). In an active galactic
nucleus (AGN), the central region of a galaxy is brighter than the rest of the galaxy and
sometimes ~104 times as bright as an average galaxy. The extremely high luminosities
of AGNs are thought to be produced by the accretion of matter on to a supermassive black
hole (SMBH, 106-109 solar mass). 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. 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. Understanding the structure and ongoing physical processes of AGNs
has important implications in cosmology, black hole physics, galaxy formation theory,
theories of acceleration of particles to high energies and other areas of astronomical
interest. Due to their large distances, AGNs are not spatially resolved with current
and near-future technologies except by radio interferometry. However, we can use time
variability, one of the defining properties of AGNs, to probe the location and physical
processes related to the emission.
I have been involved in the following projects:
1. Long Term Multi-wavelength Time Variability of Blazars and Radio Galaxies
I use extensive multi-frequency monitoring data of the blazars (a sub-class of AGNs
with prominent, highly relativistic jets) 3C 279 and PKS 1510-089 (over 10 years long)
and the radio galaxies 3C 120 and 3C 111 (~5 years) including well-sampled light
curves (radiative flux vs. time) at X-ray energies (2-10 keV), optical wavelengths
(R band), and radio frequencies (14.5 GHz and 37 GHz) as well as monthly images
obtained with the Very Long Baseline Array (VLBA) at 43 GHz that follow changes in
the emission structure of the jet on parsec scales.
The analysis involves power spectrum, correlation between different wavelengths, checking
significance of correlation using Monte-Carlo simulation etc. I am currently in the
process of improving and refining the code.
The detailed characterization of the the time variability of AGNs includes the
power spectral density (PSD) at all available wavelengths. The PSD corresponds
to the power in the variability of emission as a function of time scale. I compute
the PSD and its uncertainties using an adaptation of a method developed by Uttley
et al. I also employ the discrete cross-correlation function method (of Edelson and Krolik)
to find the correlation between variations at pairs of wavebands and use simulated light
curves based on the PSD to estimate the significance of such correlation coefficients.
I study the changes in the correlation function and the cross-frequency time delay over
the years, which are important for distinguishing between possible models.
2. Numerical Modeling of Emission from Relativistic Jets
To complement the above data analysis, I am also working on the theoretical modeling
of the emission from AGN jets. I model the time variable emission spectrum of an AGN jet
using a numerical simulation that includes conical geometry,
turbulent magnetic field and density, and energization of electrons due to a moving shock front.
Comparing the results of the simulation and the application of the above-mentioned statistical
procedures on the real data, I draw conclusions about the location of the emission regions of
these objects and identify the ongoing emission mechanisms and implications regarding the
physics of jets.
3. Connection between Accretion Disk and Jet in AGN
I have also worked on the accretion disk-jet connection in the radio-galaxy 3C 120.
The time variability of 3C 120 in X-rays and radio wavelengths and times of ejection of
superluminal knots gave a hint that there is some connection between the events in the jet
and those in the accretion disk. My detailed analysis using the pipeline confirms that in
3C120 the radiative state of the accretion disk plus corona system has a direct effect on
events in the jet. This is important in the context of the connection between AGNs and
Black Hole Binaries. We are currently working on suitable physical models for this
connection.
The above researches are funded by NASA and NSF.
Maintained by Ritaban Chatterjee. Last updated 12/09/08.