The National Centre for Radio Astrophysics

The National Centre for Radio Astrophysics
Tata Institute of Fundamental Research, Pune University Campus, Pune – 411007 http://www.ncra.tifr.res.in
Welcome to the National Centre for Radio Astrophysics (NCRA-TIFR)!
Research areas at NCRA-TIFR range from solar physics to the most distant galaxies, as
well as fundamental physics. Of course, research here is centred on low frequency radio
astronomy, an area where the Giant Metrewave Radio Telescope (GMRT), built and
operated by us, is the biggest and most sensitive radio interferometer in the world. We
also built, and now operate, the Ooty Radio Telescope (ORT). Our research areas are
listed below, and are also described in more detail in the following pages.
Faculty members:
Research Areas and Facilities:
● Poonam Chandra
● Solar Physics
● Jayaram N. Chengalur
● Pulsars and Transients
● Tirthankar Roy Choudhury
● The Milky Way
● Swarna Kanti Ghosh
● The Interstellar Medium
● Yashwant Gupta
● Nearby Galaxies
● C. H. Ishwara-Chandra
● Active Galaxies and Clusters
● Bhal Chandra Joshi
● High Redshift Galaxies
● Nissim Kanekar
● The Epoch of Reionization
● Nimisha Kantharia
● Fundamental Constant Evolution
● Dharam Vir Lal
● Extragalactic Deep Fields
● P. K. Manoharan
● The TIFR GMRT Sky Survey
● Dipanjan Mitra
● Cosmology
● Divya Oberoi
● The Giant Metrewave Radio Telescope
● Subhashis Roy
● The Ooty Radio Telescope
● D. J. Saikia (on leave)
● Astronomical instrumentation
● Sandeep Sirothia
● Yogesh Wadadekar
Top & middle: MWA 1-second 150 MHz image of the active
Sun, showing rapid variations. Bottom panel: MWA 150MHz (left) and SOHO extreme ultraviolet (right) images of
the quiescent Sun (Oberoi et al. 2011).
GMRT 330 MHz image of possibly the youngest
supernova remnant (SNR) in the Milky Way (Roy et al.
2013). The SNR shell traces shocks caused in the
interstellar medium by the explosion.
The Sun and the Heliosphere: (P. K. Manoharan, Divya Oberoi)
Radio waves provide a view of the Sun that is very different from that at other wavelengths. Lowfrequency solar radio emission varies rapidly in time, frequency and spatial location; this variability has
long posed a challenge to solar studies. Besides using the GMRT, NCRA-TIFR researchers are involved
in mapping the Sun with the Murchison Widefield Array (MWA), a new radio telescope in Australia. The
unique high fidelity imaging capability of the MWA over short time intervals and narrow frequency
widths allows it to track changes in the solar emission across time, frequency and morphology.
A constant stream of charged and magnetized plasma flows out from the upper solar atmosphere. As
radio waves from distant sources traverse this inhomogeneous and turbulent “solar wind”, their wave
fronts get distorted. For compact sources, this leads to the phenomenon of Interplanetary Scintillation
(IPS), analogous to the optical twinkling of stars. IPS provides an excellent remote sensing probe for the
heliosphere, and the ORT has played a pivotal role in the development and application of IPS techniques.
IPS monitoring with the ORT is being used to provide insight into solar activity, including solar bursts,
coronal mass ejections, and solar-wind driven magnetic storms that affect the near-Earth environment.
Galactic Astronomy: (Poonam Chandra, Jayaram Chengalur, Swarna Kanti Ghosh, Nissim Kanekar,
Nimisha Kantharia, Subhashis Roy)
An active area of research at NCRA-TIFR is the centre of the Milky Way which is believed to be a very
compact, massive black hole. Research is also carried out to understand objects like novae, which are
bright explosions of compact stars. An ongoing programme to search for supernova remnants has led to
the discovery of one of the youngest known remnants. Attempts are also under way to understand
acceleration mechanisms in such remnants, via a combination of radio and gamma-ray studies. Radio
imaging is also being used to study the magnetospheres of massive stars in the Galaxy. Detailed studies
of the gaseous medium between the stars, known as the interstellar medium (ISM), constitute another
large area of research. The ISM consists of various phases, ionized, atomic and molecular, at different
temperatures, pressures and densities, and NCRA-TIFR astronomers use different spectral lines to probe
conditions in the different phases, and the existence and nature of the equilibrium between them.
GMRT radio and FERMI gamma-ray
pulses from an eclipsing black-widow
pulsar, newly detected with GMRT
(Bhattacharyya et al. 2013).
Dwarf galaxies from the Faint Irregular Galaxies GMRT
Survey (Begum et al. 2008). HI 21cm emission maps are
shown in the lower panel and a velocity field (left) and an HI
21cm spectrum (right) in the top panel.
Pulsars and Transients: (Poonam Chandra, Yashwant Gupta, Bhal Chandra Joshi, Nimisha Kantharia,
Dipanjan Mitra)
Pulsars are rapidly rotating neutron stars that emit beams of radio radiation from their magnetic poles, at
low frequencies, ideally suited for the ORT and the GMRT. NCRA-TIFR members are involved in blind
and targeted searches that have already resulted in a number of discoveries of new and interesting
pulsars. Other research areas, aimed at understanding the origin of pulsar radio emission, include pulsar
timing studies, studies of their emission properties such as the evolution of pulse profiles, nulling and
mode changing phenomena, as well as scattering and dispersion of pulsar signals by the ISM. Theoretical
attempts are also being made to find evolutionary pathways linking different classes of neutron stars.
Transients are astronomical objects that show sudden, dramatic changes in their intensity on short
timescales, ranging from seconds to days. The new capabilities of the GMRT correlator are being used to
search for new types of transients. Multi-waveband studies of supernovae and gamma ray bursts are also
being carried out, to better understand their environments. Radio and X-ray observations of supernovae
have been used to trace the density and temperature of the surrounding medium, along with the shock
conditions that accompany such events.
Normal Galaxies: (Jayaram Chengalur, Nimisha Kantharia, Dipanjan Mitra, Subhashis Roy, Yogesh
Wadadekar)
“Normal” galaxies are quiescent systems that do not produce extremely energetic emission. In fact, the
Milky Way is a good example of a normal galaxy! The formation and evolution of these galaxies in the
Universe remains an open area in cosmology. Some of the topics of research at NCRA-TIFR include
morphological evolution of galaxies, high-resolution studies of the radio-far infrared correlation in
galaxies, the magnetic field and diffusion of cosmic rays in nearby galaxies, radio continuum and neutral
hydrogen studies of dwarf galaxies and compact galaxy groups, as well as studies of the disk-halo
connection in galaxies.
The radio galaxy 3C449, as seen at X-ray (blue) and radio (red and
green) frequencies. (Lal et al. 2013).
The distribution of neutral hydrogen in the
EoR, as predicted by simulations.
Active Galaxies and Clusters: (C. H. Ishwara-Chandra, Dharam Vir Lal, D. J. Saikia)
These are galaxies where extremely energetic phenomena take place, driven by activity around the supermassive black holes at their centres. These result in the emission of enormous amounts of radiation in
various wavebands, including the radio, making them observable out to very large distances in the
Universe. Such objects are also often found in clusters of galaxies. The identification and detailed study
of Compact Steep Spectrum and Gigahertz Peaked Spectrum sources, which constitute a significant
fraction of such bright radio sources but are not well understood, are carried out at NCRA-TIFR.
Research is also carried out on studies of giant radio sources, recurrent activity in radio galaxies, the
interaction between radio plasma and the inter-cluster and intra-group media, radio halos and relics.
Searches are also being carried out for radio galaxies at high redshifts, and efforts are under way to
model their gaseous environments, at all redshifts.
High Redshift Galaxies: (Jayaram Chengalur, Nissim Kanekar, D. J. Saikia, Yogesh Wadadekar)
The absorption of radio waves from background quasars by gas in intervening galaxies helps us to trace
the neutral hydrogen distribution of the distant Universe, as well as physical conditions in the absorbing
galaxies. Understanding conditions in such absorption-selected high-redshift galaxies is another area of
research at NCRA-TIFR. Besides absorption studies to measure the gas temperature and metallicity,
radio and optical imaging observations of the absorbing galaxies are being carried out, to determine their
typical size and mass, and the evolution of these properties with redshift. Multi-wavelength maging
studies using optical, infrared and radio data are also being carried out to detect and characterize
emission-selected galaxies at high redshifts. At even higher redshifts, in the epoch of reionization,
searches for emission from molecular and ionized gas are being carried out in the so-called Lyman-alpha
emitters and Lyman-break galaxies, to probe physical conditions in the earliest galaxies in the Universe.
The Epoch of Reionization: (Tirthankar Roy Choudhury)
The Epoch of Reionization (EoR) is the last phase transition in the Universe, during which the intergalactic medium moves from being predominantly neutral to being predominantly ionized. It has long
been known that the EoR provides an outstanding probe of cosmology; detecting redshifted HI 21cm
emission from neutral hydrogen in the EoR is being attempted at a number of telescopes around the
world, including the GMRT. Work is also being done at NCRA-TIFR on theoretical modelling of the
EoR, which probes the formation of the first stars in the early Universe. Simulations of the HI 21cm
emission signal from neutral hydrogen at different cosmic times are also being carried out.
A TGSS GMRT 150 MHz image of a typical
extragalactic field (Sirothia et al.).
Hydroxyl (OH) lines at z=0.247: Tentative evidence for
fundamental constant evolution (Kanekar et al. 2010).
Fundamental Constant Evolution: (Jayaram Chengalur, Nissim Kanekar)
A generic prediction of higher-dimensional theories that attempt to unify the standard model of particle
physics and general relativity is that low energy fundamental constants like the fine structure constant
should evolve with time. Astronomical studies allow one to probe such evolution over cosmological
timescales, and to thus test the validity of the standard model over billions of years. Researchers at
NCRA-TIFR use accurate measurements of the redshifts of atomic and molecular radio spectral lines,
from neutral hydrogen, hydroxyl, ammonia, methanol, etc, to carry out amongst the most accurate tests
of cosmological changes in the fine structure constant and the proton-electron mass ratio.
Extragalactic Deep Fields: (C. H. Ishwara-Chandra, Sandeep Sirothia, Yogesh Wadadekar)
An important area of astronomical research in recent years has been the use of deep multi-wavelength
studies of specific extragalactic fields to study in detail how galaxies and their stars and gas evolve
through the age of the Universe. Radio and infrared imaging is especially important in this area, because
most actively star-forming galaxies are obscured by dust and are hence not visible in optical images.
Researchers at NCRA-TIFR use deep radio images of such extra-galactic fields to address a number of
issues, including quantifying the number of sources of different types as a function of source luminosity
and redshift, finding massive radio galaxies at high redshifts via their ultra-steep radio spectra,
distinguishing between star-forming and active galaxies, etc.
The TIFR-GMRT Sky Survey: (C. H. Ishwara-Chandra, Nimisha Kantharia, Sandeep Sirothia)
Systematic surveys that map large areas of the sky with high sensitivity in a uniform manner are a major
programme at most observatories. Such surveys are expected to detect objects over a wide range of
distances, from objects in the Milky Way to objects at cosmological distances. The TIFR-GMRT Sky
Survey (TGSS) is an all-sky GMRT radio continuum survey at 150 MHz, covering about 37,000 square
degrees of the sky north of declination -55 degrees, at an angular resolution of about 20”. The TGSS is
more than 4 times better in both sensitivity and angular resolution than existing surveys at this frequency.
When complete, the survey is expected to detect more than 2 million sources and to serve as a major
database for multi-wavelength astronomy, yielding many new and interesting discoveries.
One of the thirty antennas of the GMRT at Narayangaon, 80 km from Pune,
towering over a group of students.
The Giant Metrewave Radio Telescope:
With thirty antennas, each of diameter 45 metres, spread out over a maximum distance of 25 km, the
GMRT is the biggest and most sensitive radio interferometer in the world at low frequencies, < 1 GHz. It
is used for scientific observations by astronomers around the world, via competitive selection of
observing proposals. In order to retain its premier status in the world over the next decade, much activity
is currently under way to upgrade the GMRT, including the building of new low-frequency radio
receivers and a new correlator, besides upgrading most of the electronics and the telescope control
system. The sensitivity of the upgraded GMRT will have increased by a factor of at least 3 at all
frequencies. Studies are also in progress to further upgrade the GMRT, by increasing both the number of
antennas and the maximum antenna separation.
The Ooty Radio Telescope:
Over the last decade, the ORT has been mainly used for studies of Inter-Planetary Scintillation, providing
an important probe of solar activity and space weather studies. In collaboration with the Raman Research
Institute, the ORT's analog and digital electronics are being upgraded, to provide a wide field of view and
improved sensitivity. In addition, a new pulsar backend has been installed. The versatile upgraded ORT
will allow a number of studies requiring high sensitivity, such as accurate pulsar observations, searches
for neutral hydrogen at high redshifts, searches for transients, solar and space weather studies, etc.
Astronomical Instrumentation: (Jayaram Chengalur, Swarna Kanti Ghosh, Yashwant Gupta, Bhal
Chandra Joshi, P. K. Manoharan)
The success of a radio observatory rests heavily on its ability to work at the frontiers of technology to
develop cutting-edge software and hardware instrumentation to detect and process weak radio
astronomical signals. Areas of research and development at NCRA-TIFR include wideband antenna feed
elements, sensitive front-end analog electronics with high dynamic range, new signal transport systems,
and back-end receiver systems combining hardware and software technologies. Of particular interest is
the development of flexible software- and hardware-based back-ends, an area where NCRA-TIFR has
played, and continues to play, a pioneering role. New modes include flexible post-processing of voltage
signals, mitigation of terrestrial interference, the detection of transients, new correlation modes for
interferometry, etc, which have greatly enhanced the capabilities of both the GMRT and the ORT.
Research areas of NCRA-TIFR Faculty members
Poonam Chandra
Supernovae and Gamma Ray bursts:
Transient objects, such as supernovae and gamma-ray bursts (GRBs), represent the most energetic
explosions in the Universe. A supernova, with an explosion energy 1E+51 ergs, often briefly
outshines an entire galaxy before fading from view over several weeks to months. GRBs are flashes
of gamma rays lasting from a few milliseconds to a few minutes. Supernovae and GRB explosions are
linked to the end stages of massive stars. Thus the major thrust in this direction is towards
understanding stellar deaths involving massive progenitors. The study of environments of the
progenitor stars that lead to supernovae and GRB explosions is the main focus of my research, which
mainly uses the radio and X-ray bands. I use the Giant Metrewave Radio Telescope (GMRT) and the
Very Large Array (VLA), for radio measurements and the Swift-XRT, Chandra and XMM-Newton
telescopes for the X-ray observations.
Radio magnetospheres of massive stars:
Hot, massive OB stars are the energetic and chemical engines of galaxies. The mass loss, combined
with rapid, sometimes near-critical stellar rotation, can exert a strong, even dominant influence on the
formation and evolution of such massive stars, and on their demise as supernovae or GRB-producing
hypernovae. But recent advances in observation and theory indicate a third agent - magnetic fields can also play a key role. Recent systematic investigations, such as the Magnetism in Massive Stars
(MiMeS) survey (Wade et al. 2012), have revealed strong, ordered (typically dipolar) magnetic fields
in a growing subset of massive stars. The channelling and confinement of an outflowing stellar wind
by the star's magnetic field leads to the formation of a shock-heated magnetosphere which can emit in
multiple wavebands. We are carrying out a systematic radio study with the GMRT and the VLA in
order to understand complex magnetosphere properties of massive stars and their winds.
Acceleration mechanisms in supernova remnants:
Recent observations of young supernova remnants (SNRs) in the gamma-ray domains have raised
several questions and triggered numerous theoretical investigations such as, when do the particle
energies reach maximum, during the free-expansion phase or during the Sedov stage? How do cosmic
rays escape from a SNR, what is the dynamics of escape, i.e. how the maximum energy evolves with
time? What is the primary particle population producing the gamma-ray emission? The first two
questions are intimately connected with the intensity of the magnetic field hence with the maximum
acceleration energies which are constrained by radiative losses and synchrotron radiation and hence
by radio emission. The third one can be traced efficiently thanks to the detection of gamma rays in the
high energy range with the Fermi-LAT or in the very-high-energy energy range with HESS. Multiwavelength data, and especially radio and gamma-ray data, are thus crucial to understand the nature
of these efficient particle accelerators in our Galaxy. We are investigating how young core-collapse
supernova shocks accelerate Cosmic Rays (CR -- electrons or protons) to very high energies and how
the acceleration efficiency evolve as the SN ages and moves to the supernova remnant stage.
Jayaram N. Chengalur
My main research focus is extragalactic astronomy, particularly studies of nearby dwarf irregular
galaxies, neutral hydrogen (HI) absorption in very high redshift galaxies (the so-called “damped
Lyman alpha” systems), and, in general, studies of the evolution of the neutral hydrogen content of the
Universe.
Dwarf galaxies:
The dwarf irregular galaxies that our group studies are 1000 to 10000 times less massive than our own
galaxy, the Milky Way. These galaxies are interesting in not only in their own right, but also in the
context of hierarchical galaxy formation models in which large galaxies like our own form by the
hierarchical merger of smaller progenitors. In this picture, the very small "dwarf" galaxies in the local
universe are the survivors of this merger process, and are possible analogs of the galaxies in the early
universe. Detailed studies of these galaxies hence provide insights and constraints on galaxy formation
models. Particular topics that our group has been investigating include the distribution and total dark
matter content of, as well as the processes that govern the conversion of gas into stars in, some of the
smallest known galaxies. Most of this work is in the context of a survey of HI 21cm emission from a
large sample (~ 75 objects) of nearby, extremely faint, dwarf irregular galaxies, viz. the Faint Irregular
Galaxies GMRT Survey (FIGGS).
Neutral hydrogen content of the Universe:
The redshift evolution of the gas content of galaxies is being studied using deep HI 21cm emission
observations of field and galaxy clusters at redshifts between ~0.2 and ~0.4. These observations
constrain the evolution of the HI content of galaxies as well as the effect of the cluster environment on
the gas content. At these redshifts, the emission from the individual galaxies is too faint to detect;
instead, the average emission is measured by stacking together the spectra of all the known galaxies in
the observed field. The detection of emission at a redshift of ~ 0.4 represents the highest redshift at
which there is a direct constraint on the gas associated with star-forming galaxies.
HI-21cm absorption studies:
At still higher redshifts, observations of HI 21cm absorption raising in damped-Lyman alpha systems
help us understand the physical conditions in the gas in these systems. Our observations indicate that
the spin temperature of the hydrogen in damped Lyman-alpha systems is significantly larger than that
typical of large spiral galaxies like the Milky Way. Comparisons between the observed redshifts of
different spectral lines in these systems also allows us to constrain the variation of fundamental
constants like the fine structure constant and the ratio of the proton mass to the electron mass with
cosmological time.
Neutral hydrogen on large scales:
A new experiment is also being planned to study the large scale neutral hydrogen correlation function
at a redshift of ~ 3 using the upgraded Ooty Radio Telescope. This is a large collaboration involving
astronomers from the Raman Research Institute, the Indian Institute for Science Education and
Research, Mohali, and the Indian Institute of Technology, Kharagpur.
Tirthankar Roy Choudhury
Cosmological Reionization:
In the framework of the hot big bang model, our Universe is expected to become almost neutral around
400,000 years after the big bang. On the other hand, we know from observations of quasar absorption
spectra that the same Universe has become highly ionized by the time it is one Gigayear old. This
implies that the Universe must have been "reionized" sometime in between.
As per our current understanding, the reionization of the Universe is driven by radiation from first
luminous sources (galaxies/stars). Studying reionization will thus tell us how the first stars formed
and how different they were from stars we see around us. In addition, reionization is extremely
important for studying cosmology in general as the details of it affects determination of cosmological
parameters from observations.
I have been involved in theoretical modelling of the process of reionization, and comparing these
models with different observations. I am also interested in developing numerical simulations to
generate realistic maps of HI 21cm emission arising from the hyperfine transition of neutral hydrogen,
that might be observed with current and upcoming radio telescopes such as the GMRT, the LOw
Frequency ARray (LOFAR) and the Square Kilometer Array (SKA).
C. H. Ishwara-Chandra
Search for high-redshift radio galaxies using observations with the GMRT:
Despite several decades of efforts only one radio galaxy is known at redshift > 5 (discovered in 1999),
though there are close to 50 high-redshift radio galaxies (HzRGs) with z > 3. Nearly all of them have
been found using the redshift-spectral index correlation. We have started a programme with the Giant
Metrewave Radio Telescope (GMRT) to exploit this correlation at flux density levels of about 10 to
100 times deeper than the known HzRGs. In this programme, we have obtained deep, high resolution
radio observations at 150 MHz with GMRT for several DEEP fields which are well studied at higher
radio frequencies and in other bands of the electromagnetic spectrum, with an aim to detect candidate
high redshift radio galaxies. From the deep 150 MHz observations of the LBDS-Lynx field, along
with available auxiliary data, we have found about 150 radio sources with spectra steeper than 1.
About two-third of these are not detected in Sloan Digital Sky Survey, and are hence strong candidates
for high-redshift radio galaxies. These need to be further explored with deep infra-red imaging and
optical spectroscopy to get the redshift. Work on other deep fields is now in progress.
Swarna Kanti Ghosh
My primary research interests lie in the interstellar medium (ISM), especially star-forming regions,
and in astronomical instrumentation. These are described in more detail below:
The Interstellar Medium:
My research broadly aims to understand star formation activity in the Milky Way by probing
conditions in massive star forming regions as well as young stellar objects. Different research projects
include
(1) Studying the structure of the ISM around massive star-forming regions and photo-dissociation
regions through mapping of emission in the 158 micron [CII] line, the polycyclic aromatic
hydrocarbon (PAH) lines, and the dust continuum. The PAH studies are especially interesting as they
are the main source of gas heating in photo-dissociation regions and the atomic ISM, via photo-electric
emission.
(2) Multi-wavelength monitoring of outbursts in young stellar objects to study episodic accretion on
these systems.
(3) Detailed studies of the post-outburst phase of McNeil's nebula, an exciting pre-main sequence star
which has recently undergone an eruption, illuminating the cocoon of gas and dust that surrounds it.
This rare event allows a probe of the late stages of the evolution of a star onto the main sequence.
(4) Multi-wavelength investigations of the morphology, physical environment, stellar contents and star
formation activity in Galactic star-forming regions, probing the distribution of young stellar objects,
their evolutionary sequence, star formation scenarios, etc.
Astronomical Instrumentation:
My other main research interest lies in astronomical instrumentation, especially the following projects:
(1) The Ultra-Violet Imaging Telescope (UVIT), for the Indian multi-wavelength mission ASTROSAT
(to be launched by ISRO), is currently under development. It consists of three imagers, in the FarUltra-Violet (FUV: 130 - 180 nm), the Near-Ultra-Violet (NUV: 200 - 300 nm) and the Visible (VIS:
320-550nm) bands. UVIT can image the sky simultaneously in the above three channels with a field of
view of ~28 arcminutes and an angular resolution better than 1.8". In addition, gratings are available in
the FUV and the NUV channels for slitless low-resolution spectroscopy.
(2) The Infrared Spectroscopic Imaging Survey (IRSIS) payload, targeted for the Small Satellite
Mission of ISRO, aims to carry out low resolution (R~100) spectroscopic measurements in the
wavelength range 1.7 to 6.4 micron with seamless coverage, covering a large fraction (~ 50%) of the
sky (including the Galactic Plane), with good sensitivity. The primary science goals of this project
include: (i) Discovery & classification of Brown Dwarfs and M-L-T Dwarfs, probing the faint end of
the stellar Initial Mass Function; (ii) Large scale mapping in the emission features of large organic
molecules, via a Galactic Plane survey; (iii) Probing minor bodies of the Solar System, including
Asteroids, Comets, and Inter-Planetary Dust; and (iv) a Galactic Bulge survey to study Asymptotic
Giant Branch, Red-Super-Giant and Carbon-rich stars.
Yashwant Gupta
My research areas cover different aspects of the study of pulsars -- rapidly rotating compact neutron
stars that emit intense beams of radio emission. Areas of interest range from detailed studies of their
emission process, searching for and finding new pulsars, timing studies to understand their dynamics,
and using them as a probe to study the interstellar medium. The other main area of interest is
development of new instrumentation and signal processing techniques for radio astronomy.
Searching for pulsars: Though more than 2000 pulsars have been found by astronomers so far, there
are many more waiting to be discovered, including some that could be much more interesting and
exotic than the ones found so far. In addition to blind searches that target large areas of the sky in a
uniform manner using sensitive telescopes like the GMRT, I have been involved in targeted searches in
specific locations such as supernova remnants, globular clusters and compact sources of high energy
emission that are likely to harbour neutron stars. Some very unique and interesting pulsars have been
discovered in these searches and I continue to be involved in further explorations of this kind.
Timing studies of pulsars: Once a new pulsar is discovered, a host of interesting new things can be
learnt about it (and its environment) by a careful study of the time of arrival of the pulses, over long
durations of time, spanning weeks to months to years. I continue to be involved in several such studies
that have (a) revealed irregularities in rotation (called glitches) in young neutron stars, and (b) been
used to infer the nature of the orbit for neutron stars in binary systems, in addition to determining basic
parameters like accurate values for the period and its derivative with time. Another interesting area of
my research is the study of timing noise -- the residual random behaviour seen in timing data when all
known effects have been modelled.
Understanding radio emission properties: Working out exactly how and why radio pulsars shine
remains one of the biggest unsolved problems in the field. Some of my research work revolves around
efforts to try and understand better the location and distribution of emission regions in the
magnetosphere of a neutron star, aided and abetted by high quality single frequency and simultaneous
multi-frequency observations of phenomena such as drifting subpulses, using the GMRT. Even as we
improve our understanding of such issues, there is much to learn and look forward to in this exciting
area of work.
Probing the interstellar medium using pulsars: Due to the fact that pulsars are extremely compact
objects and emit narrow duty pulses, they form excellent probes of several properties of the interstellar
medium (ISM). My interest here ranges from a more detailed understanding of the distribution of the
ionised plasma of the ISM, to using interstellar scintillations as a probe to resolve the very compact
emission regions of pulsars.
Instrumentation for radio astronomy: The development of next generation instrumentation for
radio telescopes is an area of keen interest for me. In particular, my emphasis is on digital back-end
systems which can play an important role in expanding the capabilities and versatility of a radio
telescope, enabling new science to be carried out. I have been actively involved in 3 generations of
back-ends for the GMRT, starting with a completely hardware-based implementation to a softwarebased approach using general purpose CPUs, and now to using accelerated computing with GPUs for
the back-end for the upgraded GMRT.
Bhal Chandra Joshi
Pulsar studies:
A variety of investigation of pulsed radio emission from neutron stars, using the ORT and the GMRT
as well as observations at optical and high energies, are carried out. Blind and targeted searches of
pulsars, using the unique capabilities of GMRT, are ongoing with a view to increase the sample of
radio emitting pulsars. The newly discovered pulsars with the GMRT are followed up to determine
precise timing solution to learn about the neutron star characteristics. Together with the imaging
observations of neutron star environments with the GMRT, such observations are useful to constrain
physics of pulsar wind and the resultant nebula. New pulsar discoveries have also uncovered new
classes of neutron stars, such as intermittent pulsars and highly magnetized neutron stars along with
exotic binary and millisecond pulsars.
Single pulse emission studies:
Another kind of study involves single pulse emission from radio pulsars, where a wide variety of
single pulse phenomena such as pulse nulling, drifting and resultant mode-changing are observed. The
GMRT and the ORT have been very useful in providing long simultaneous multi-frequency single
pulse observations, which are useful in constraining the pulsar magnetospheric physics and beam
geometry. Extreme forms of unexplained emission in single pulses, such as isolated radio bursts from
rotating radio transients (RRATs) and fast radio bursts (FRBs), are also studied as part of this
programme. Both telescopes have the capability to carry out low frequency studies of the pulsed
emission, which gets dispersed and scatter-broadened by the interstellar medium. Multiple frequency
observations in these studies are useful in constraining the structure and the dynamics of the
interstellar medium.
Precision timing studies:
Both the GMRT and the ORT are capable of high time resolution observations of pulse emission with
precision timing. Observations of millisecond pulsars, which are very stable clocks, are therefore
useful in constraining relativity theories and provide an ensemble of clocks, which could be used as a
long baseline detector for stochastic gravitational background at nanohertz frequencies. Studies
contributing to an international effort to detect such a background using these pulsar timing arrays is
also currently ongoing. Both experimental and theoretical work in this direction is currently under way
at NCRA-TIFR.
Nissim Kanekar
My main research interests are in the areas of galaxy formation and evolution, tests of fundamental
physics, and the interstellar medium of galaxies. Individual areas are described in more detail below:
Fundamental constant evolution:
Astronomical studies provide the only avenue to test for changes in the fundamental constants of
physics, such as the fine structure constant and the proton-electron mass ratio, over cosmological
timescales, billions of years. My research in this field involves using radio spectroscopy of highredshift galaxies to accurately measure the redshifts of different types of spectral lines (e.g. the HI
21cm hyperfine line, ammonia inversion lines, hydroxyl Lambda-doubled lines, carbon monoxide
rotational lines, etc) to test for changes in the fundamental constants. I also work on devising new
techniques for this purpose, based on theoretical studies of new molecules.
Galaxy evolution:
In galaxy evolution, my research has focussed on understanding the nature of and evolution of highredshift gas-rich galaxies that are detected due to their strong absorption of the light of background
quasars. I use radio absorption studies of such galaxies to study the evolution of physical conditions in
their interstellar media. We have shown that atomic gas in these galaxies is predominantly warm at
high redshifts, apparently due to their low metallicity and a paucity of cooling routes. We have also
come up with a new method to image such galaxies, based on quasar sightlines with two absorbers,
and using the higher-redshift absorber as a ''blocking filter'' to blank out the background quasar at
certain wavelengths, so that one can then image the lower redshift galaxy. We are currently carrying
out imaging and spectroscopic studies of a large sample of such galaxies with the Keck Telescope and
the Hubble Space Telescope, as well as radio spectroscopic imaging of low-redshift absorbers, to
determine their size, mass, etc.
The Interstellar Medium:
My work on the interstellar medium of the Milky Way has aimed to understand the distribution of gas
between different temperature phases, and specifically whether gas exists in the ``unstable''
temperature phase, ~ 1000 K. Our studies have found evidence for significant amounts of gas in this
unstable phase, which is not expected in standard three-phase models of the ISM. We are currently
attempting to carry out a self-consistent modelling of hydrogen absorption and emission spectra, to
obtain a better understanding of physical conditions in the ISM.
Nimisha Kantharia
Star forming Galaxies:
Galaxies evolve in isolation or in denser environments such as small groups or large clusters. As
expected, the evolution of these galaxies in terms of star formation rate, gas content and other internal
properties will depend on the environment. In the denser environments, the galaxies are subject to tidal
interactions due to the presence of other nearby galaxies which can modify star-forming properties and
to hydrodynamic processes due to intra-cluster gas like ram pressure stripping and viscous stripping
which can lead to gas depletion. On the other hand, when galaxies evolve in isolation, they are only
subject to internal perturbations. Thus, it is important to study both (1) the physical processes that act
on these galaxies in dense environments and (2) the evolutionary paths of galaxies in both
environments. This is particularly important for small groups since more than 60% of galaxies are
believed to evolve in small groups. At GMRT, research includes study of star-forming galaxies ranging
from small to large spirals evolving in a range of environments in the near Universe. Research is
actively being pursued on normal disk galaxies, low surface brightness galaxies and HII galaxies in
terms of understanding their low radio frequency spectral energy distribution, magnetic field
distribution, halo emission, atomic gas distribution and kinematics, interactions and star formation
properties in different environments. Research also involves combining radio results with results from
other wavebands and studying the global properties of these galaxies.
Novae at GMRT frequencies:
It is believed that about half the stars in the Milky Way are found in multiple or binary systems with the
number being higher for massive OB stars and lower for low mass red dwarf stars. Stars evolving in
binaries will follow a distinct evolutionary track from those evolving as singles. There are binary
systems which consist of a white dwarf and a red giant or main sequence star as the companion. In
several such binaries, the white dwarf accretes matter which overflows the Roche lobe of the
companion star and the material accumulates on the surface of the white dwarf. A cataclysmic
thermonuclear reaction is ignited in this material when sufficient material is deposited on the surface
leading to the ejection of mass and energy into the surrounding medium. These cataclysmic systems
known as novae brighten by several magnitudes in the optical within a short interval and are observed
to emit in the entire electromagnetic spectrum due to a range of physical phenomena. The fast-moving
ejecta from the explosion set up shocks in the interstellar medium as they encounter the ambient
material, and this leads to the emission of synchrotron radiation which can be ideally observed at
GMRT frequencies. There is a class of novae known as recurrent novae (e.g RS Ophiuchi) where
outbursts recur on timescales of a few years and these systems are believed to harbour a white dwarf
whose mass is close to the Chandrasekhar limit. These, then, are strong contenders for progenitor
systems of Type 1a supernovae. Studying the evolution of the radio synchrotron emission in successive
recurrent nova outbursts can shed light on the evolution of these intriguing systems, the progenitor
scenario and the possibility of them evolving into Type 1a supernovae, in addition to shock physics.
The GMRT is used to study and model the early light curves and the longer term evolution of the
synchrotron emission. There are also works on studying the early evolution of supernovae including
Type 1a supernovae and gamma ray bursts.
Probing ionised regions using radio recombination lines:
The ultraviolet radiation from stars ionises their surrounding gaseous regions containing hydrogen,
helium or carbon. Subsequently an equilibrium is established wherein the ionisation is balanced by
recombination. These regions around stars are called Stromgren spheres. There are also regions in the
Milky Way where hydrogen, carbon or helium can be ionized by the interstellar radiation field. In all
these regions, electrons can recombine to excited quantum levels in the atom and cascade down to
lower levels giving rise to recombination lines at radio frequencies. This spectral line emission can be
mapped from the ionized regions and physical properties such as temperature, electron density and
size can be modelled. Moreover, the distribution of this gas and its kinematics can also be studied
using these lines. These results are combined with other diagnostics from the same gas to better
constrain the physical parameter space. Thus, studies of HII regions around stars and the photodissociation regions using these spectral lines are carried out at NCRA-TIFR.
Dharam Vir Lal
My research interests are focussed on improving our understanding of physical conditions in extragalactic radio sources, by modelling the gaseous environments of radio galaxies in both the early and
the late Universe. This work is based on data from the Chandra Space Observatory, the Giant
Metrewave Radio Telescope, and other ground- and space-based observatories. We are also interested
in issues relating radio interferometry and the effects of the atmosphere, specifically, the ionosphere,
on radio observations. A brief description of two of these research areas is given below:
Inverse-Compton emission from high-redshift Active Galactic Nuclei:
High-redshift radio galaxies (HzRGs) are excellent beacons for pin-pointing the most massive objects
in the early universe, including galaxies, super-massive black holes (SMBHs), or galaxy clusters. The
strongest constraint on the high-redshift evolution of SMBHs comes from the observation of powerful
HzRGs. The luminosities of these sources imply that SMBHs of mass comparable to a billion solar
masses were already in place when the universe was only 1–3 Gyr old. To grow from seed fluctuations
up to such a high mass requires an almost continuous accretion of gas. Therefore, to understand the
evolution of the first SMBHs in the first pre-galactic radio sources and their impact on the reionization
of the universe, it is important to understand the balance in the energy budget between mechanical and
radiative power at these high redshifts.
Morphology of (head-tail) radio galaxies as tracers of cluster potential:
Head-tail sources are characterized by a head identified with the optical galaxy and two trails of radio
emission sweeping back from the head. The long tails of these galaxies carry the imprint of relative
motion between the non-thermal plasma and the ambient hot gas. Fortunately, the jets survive the
encounter with the ICM, with possible shocks leading to the formation of the long tails, and
specifically they seem to be devoid of the growth of Kelvin-Helmholtz instabilities. Hence, in the
parlance of the field, they reflect the weather conditions of the ICM, which allows one to make
quantitative statements about their dynamics and energetics. Such observations can potentially reveal
details of cluster mergers such as subsonic/transonic bulk flows, shocks and turbulence.
P. K. Manoharan
I am actively involved in research on solar and interplanetary physics. We have developed a unique
method to determine the speed and other physical properties of the solar wind using interplanetary
scintillation (IPS) measurements from a single radio telescope. Based on IPS measurements from the
Ooty Radio Telescope (ORT), several important studies leading to new results pertaining to the solar
wind Space Weather processes in the inner heliosphere and their effects have been made by our group.
Our single-station technique to estimate the solar wind speed is being used at several international
observatories.
The Solar Wind:
Our studies, especially based on ORT scintillation measurements, have been able to explain the threedimensional structure of solar wind density turbulence and its changes with solar cycles at the crucial
heliocentric distance range above the solar wind acceleration region, for both low-speed and highspeed winds. A recent study, based on ORT IPS measurements, showed a clear evidence for a steady
decline in density turbulence (and hence, mass flux) of the solar wind from solar cycles 22 to 24,
indicating that the Sun may be heading towards a deep minimum in the solar activity cycle.
Coronal Mass Ejections:
We were also the first to explain the formation of the twisted magnetic loop system (which was later
named ‘sigmoid’) at the sites of flares or coronal mass ejections (CMEs), as observed at X-ray and
radio wavelengths. Several studies by our group, based on multi-wavelength solar and interplanetary
observations, have been useful to understand fundamental issues regarding the physics of magnetic
reconnection and the associated initiation of CMEs and particle acceleration processes in the low
corona and in the near-Sun region. The propagation of CMEs (e.g., magnetic flux-rope structures) and
their 3-D evolution have clarified the aerodynamical drag experienced by CMEs in the inner
heliosphere. Furthermore, shocks produced by fast CMEs have been identified in IPS images,
suggesting the sources of solar energetic particles and their energy spectra, transport variability, and
acceleration mechanism. These studies have also been extremely useful to pinpoint the arrival of
CMEs, and to identified the interplanetary conditions that drive geomagnetic activities and storms at
the near-Earth environment.
Dipanjan Mitra
Pulsars are highly magnetized fast rotating neutron stars (a highly dense star with a radius of about 10
km comprising mostly of neutrons), capable of emitting beams of electromagnetic radiation. As the
beam crosses the observer in earth a pulse of emission is seen. The pulsar taps its own rotational
energy to produce the EM radiation which is seen across the electromagnetic spectrum from radio to
gamma rays. However we do not know how the strong electric and magnetic field are oriented around
the neutron star and how they accelerate charged particles to generate the radiation. The physical
mechanism of how pulsars shine is still unknown, even after 45 years of its discovery and is one of the
most challenging problem in astrophysics.
My research focuses on understanding the radio emission mechanism in pulsars. The Radio emission
form a tiny fraction of the total energy released by pulsars during spin down. The majority of this
energy is lost as high energy X-ray and gamma-ray radiation and as a wind of relativistically charged
particles flowing out into the ambient interstellar medium. The radio emission however is unique as it
has a very high brightness temperatures (or an equivalent blackbody temperature from a thermal
source) of about 1027 K. The extreme nature of the emission is apparent when one realizes that a spatial
region of only about 500 meters is capable of generating emission having such high equivalent
blackbody temperature (one can compare this with the Sun which is a 5780 K blackbody and has a
radius of 695,500 km). The pulsar radio emission hence has a non-thermal origin, and is commonly
termed as a coherent emission mechanism. The emission is generated in regions of ultra-strong
magnetic and electric fields where energetic photons splits into an electron and positron pair through a
process of magnetic pair creation and can be accelerated to relativistic speeds. It is believed that
growth of plasma instabilities in the electron positron plasma leads to the pulsar radio emission,
although a self-consistent theory is yet to be found.
My research comprises of observational, phenomenological and theoretical studies that assist to
unravel the coherent emission mechanism in pulsars (Refer to this article for open problems that I am
interested in pulsars: http://arxiv.org/pdf/1304.1980v1.pdf) More specifically the area of my study
includes:
(1) Use of radio observatories like GMRT, Arecibo, WSRT, LOFAR etc to study radio emission
properties such as pulsar polarization, off-pulse emission, pulsar drifting, moding, nulling. In-depth
understanding of these properties provide constraint to the pulsar emission theories.
(2) Use of simultaneous X-ray (using XMM satellite) and radio studies to understand the global
structure of the pulsar magnetosphere.
(3) Understanding theoretically how plasma waves are genee and propagate in the pulsar
magnetosphere.
My additional research interests include understanding the ISM using pulsar and probes and studying
cosmic ray propagation process in grand design spiral galaxies.
Divya Oberoi
My research has focussed on solar physics and interferometry techniques in the past few years. The
bulk of my research work tends to be at the intersection of science and numerical analysis or
techniques, or is about harnessing the recent developments in technology and computing for meeting
science goals which have remained elusive using the earlier generation of instrumentation and
computing resources. Trying to extract the most information from the available data has been an
enduring theme of my work. Along the frequency axis, my research interests have so far typically been
at the low radio frequency end.
The radio Sun is very dynamic, especially at metre wavelengths. Its emission can not only change
rapidly in time, it also has very strong spectral features and the morphology of the emission changes
with both time and frequency. So, to study the radio Sun, one essentially needs a video camera which
can simultaneously make independent movies at multiple different radio frequencies.
The above requirements have posed a tough challenge for traditional radio interferometers. Recently,
with a new generation of instruments, like the Murchison Widefield Array (MWA), now becoming
available, this is changing. The MWA offers unprecedented capabilities for high dynamic range, high
fidelity spectroscopic imaging with good time and frequency resolution over a wide band, and useful
angular resolution to explore many interesting problems in solar physics which could be addressed
only in a limited manner earlier due to lack of suitable data. These include:
1. Imaging studies of the origin and evolution of different types of solar bursts, especially type II and
type III bursts.
2. Reliable studies of low radio frequency solar flux, spectral index and its variations over small and
long time scales. These will in turn help understand scattering processes and micro turbulence in the
corona.
3. A truly exciting possibility is about looking for missing contributions to the famous and long
standing coronal heating problem. I have been a part of the design, construction and commissioning
team of the MWA from its inception, and play a leading role for solar science with this instrument.
Subhashis Roy
My research interests lie in several fields involving the interstellar medium (ISM) of the central region
of the Milky Way, supernova remnants in the Milky Way, and magnetic fields in our galaxy and nearby
galaxies. More detail on each research area is provided below:
The interstellar medium and magnetic fields near the centre of the Milky Way:
The central kilo-parsec region of the Milky Way harbours a variety of activity unique to that region.
Because of a significantly increased gravitation potential, the gaseous medium in the central few
hundred pcs is characterised by high density, large velocity dispersions, comparatively higher
temperatures and magnetic fields. My multi-frequency GMRT observations led to the detection of the
central compact radio source (Sgr A*) for the first time at 610 MHz . From observations at 240 and
150 MHz, I have identified a large number (~62) of compact extragalactic radio sources. The ionised
gas in the region scatters electromagnetic radiation passing through it and the amount of scattering
gives information on the electron density and its fluctuation in the region. I have measured the angular
sizes of the background sources at 150, 240 and 1400 MHz and showed for the first time presence of an
enhanced scattering screen within a degree from the Galactic Centre. We have also shown that the
fluctuating component of the magnetic field in the Galactic Centre region is ~ 20 micro-Gauss. In this
turbulent region, the systematic field will have similar or lower magnitude, implying that its strength is
relatively low, tens of micro-gauss, significantly lower than earlier estimates of milli-Gauss fields.
Missing supernova remnants near the Galactic Centre:
Due to the central turbulent environment in our Galaxy, heavier stars are preferentially formed close to
the centre of the Milky Way. Also, our line of sight through this region passes through the longest path
length in the Galaxy, which maximises the probability of finding a supernova remnant (SNR). In the
Milky Way, the number of expected SNRs is more than 1000, but only about 270 have been discovered.
Most of the missing SNRs are believed to be concentrated in the inner Galaxy where identification
becomes difficult due to increased source density and confusion (for larger remnants), while the
smaller remnants are missed due to finite angular resolutions of surveys. The GMRT offers the highest
sensitivity and angular resolution at metre wavelengths. From our search, we have confirmed the
nature of 5 candidate SNRs in the region out of 7 observed systems. From one of these, I identified a
small shell-like structure, and re-observed this with the GMRT at 325 MHz and 1.4 GHz. We recently
confirmed this system as one of the youngest SNRs in the Milky Way, with an age of 100 to 500 years.
Magnetic fields in galaxies:
We have studied magnetic fields in the disks of 6 nearby normal galaxies. Assuming equipartition of
energy between magnetic fields and cosmic ray particles, we estimated the field strength from the
intensity of synchrotron emission (emission from high-energy electrons gyrating in the magnetic
fields). We obtained field strengths of ~20-25 micro-Gauss at the galaxy centres, with a systematic
decrease to the outer parts, where the field strengths are ~10 micro-Gauss. We found that the energy
density in the magnetic fields and the gas are within a factor of two, indicating equipartition of energy
between them. We also showed that the slope of the radio flux density to the far infrared flux density
in these galaxies is the same at low and high frequencies in the ``arm'' regions. However, the slope
steepens for the inter-arm regions, when observations are done at high frequencies (> 1 GHz). This is
due to the longer propagation length of the comparatively lower energy electrons radiating below 1
GHz, as compared to their high energy counterparts radiating at frequencies above 1 GHz.
Yogesh Wadadekar
Distant obscured galaxies with Herschel and GMRT:
A large number of star-forming and active galaxies are obscured by dust and are invisible in optical
surveys. To identify, characterise and understand the physical properties of such objects far-infrared
(FIR) and radio data are very useful. Our group has obtained FIR data from the HerMES survey on the
Herschel telescope with our own 325 MHz radio observations with GMRT. We have also obtained new
observations of the XMMLSS field (9 sq. degrees), the Lockman Hole (18 sq. degrees) and ELAIS-N1
(9 sq. degrees) with GMRT. Besides identifying distant obscured star forming galaxies, we are using
the data to identify distant radio galaxies (z>3) and study the radio-FIR correlation from normal
galaxies at redshifts lower than 1. We are also using stacking techniques to characterise the radio
properties of a variety of source populations, such as X-ray sources, normal galaxies, quasars, starforming galaxies, etc.
Formation of lenticular galaxies:
Over the last few years, we have used near-ultraviolet, optical and near-infrared observations of
lenticular galaxies to demonstrate that these objects belong to two subclasses, differentiated by stellar
mass or luminosity. More luminous lenticulars seem to have a predominantly old stellar population
like elliptical galaxies, while less luminous lenticulars have a stellar population with a wide variety of
ages like in spiral galaxies. A number of observational probes such as star-formation history, bar
fraction, bulge disk size correlations have been used to show that the formation history of more
luminous lenticulars is similar to that of ellipticals while less luminous lenticulars are essentially
spirals whose star formation has been quenched by environmental or secular effects.