The National Centre for Radio AstrophysicsTata 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:● Poonam Chandra

● Jayaram N. Chengalur

● Tirthankar Roy Choudhury

● Swarna Kanti Ghosh

● Yashwant Gupta

● C. H. Ishwara-Chandra

● Bhal Chandra Joshi

● Nissim Kanekar

● Nimisha Kantharia

● Dharam Vir Lal

● P. K. Manoharan

● Dipanjan Mitra

● Divya Oberoi

● Subhashis Roy

● D. J. Saikia (on leave)

● Sandeep Sirothia

● Yogesh Wadadekar

Research Areas and Facilities:● Solar Physics

● Pulsars and Transients

● The Milky Way

● The Interstellar Medium

● Nearby Galaxies

● Active Galaxies and Clusters

● High Redshift Galaxies

● The Epoch of Reionization

● Fundamental Constant Evolution

● Extragalactic Deep Fields

● The TIFR GMRT Sky Survey

● Cosmology

● The Giant Metrewave Radio Telescope

● The Ooty Radio Telescope

● Astronomical instrumentation

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 verycompact, massive black hole. Research is also carried out to understand objects like novae, which arebright explosions of compact stars. An ongoing programme to search for supernova remnants has led tothe discovery of one of the youngest known remnants. Attempts are also under way to understandacceleration mechanisms in such remnants, via a combination of radio and gamma-ray studies. Radioimaging is also being used to study the magnetospheres of massive stars in the Galaxy. Detailed studiesof the gaseous medium between the stars, known as the interstellar medium (ISM), constitute anotherlarge area of research. The ISM consists of various phases, ionized, atomic and molecular, at differenttemperatures, pressures and densities, and NCRA-TIFR astronomers use different spectral lines to probeconditions in the different phases, and the existence and nature of the equilibrium between them.

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. Low-frequency 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. Theunique 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.

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.

Top & middle: MWA 1-second 150 MHz image of the activeSun, showing rapid variations. Bottom panel: MWA 150-MHz (left) and SOHO extreme ultraviolet (right) images of the quiescent Sun (Oberoi et al. 2011).

Normal Galaxies: (Jayaram Chengalur, Nimisha Kantharia, Dipanjan Mitra, Subhashis Roy, YogeshWadadekar)

“Normal” galaxies are quiescent systems that do not produce extremely energetic emission. In fact, theMilky Way is a good example of a normal galaxy! The formation and evolution of these galaxies in theUniverse remains an open area in cosmology. Some of the topics of research at NCRA-TIFR includemorphological evolution of galaxies, high-resolution studies of the radio-far infrared correlation ingalaxies, the magnetic field and diffusion of cosmic rays in nearby galaxies, radio continuum and neutralhydrogen studies of dwarf galaxies and compact galaxy groups, as well as studies of the disk-haloconnection in galaxies.

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, atlow frequencies, ideally suited for the ORT and the GMRT. NCRA-TIFR members are involved in blindand targeted searches that have already resulted in a number of discoveries of new and interestingpulsars. Other research areas, aimed at understanding the origin of pulsar radio emission, include pulsartiming studies, studies of their emission properties such as the evolution of pulse profiles, nulling andmode changing phenomena, as well as scattering and dispersion of pulsar signals by the ISM. Theoreticalattempts 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 shorttimescales, ranging from seconds to days. The new capabilities of the GMRT correlator are being used tosearch for new types of transients. Multi-waveband studies of supernovae and gamma ray bursts are alsobeing carried out, to better understand their environments. Radio and X-ray observations of supernovaehave been used to trace the density and temperature of the surrounding medium, along with the shockconditions that accompany such events.

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 HI21cm spectrum (right) in the top panel.

GMRT radio and FERMI gamma-ray pulses from an eclipsing black-widow pulsar, newly detected with GMRT (Bhattacharyya et al. 2013).

The Epoch of Reionization: (Tirthankar Roy Choudhury)

The Epoch of Reionization (EoR) is the last phase transition in the Universe, during which the inter-galactic medium moves from being predominantly neutral to being predominantly ionized. It has longbeen known that the EoR provides an outstanding probe of cosmology; detecting redshifted HI 21cmemission from neutral hydrogen in the EoR is being attempted at a number of telescopes around theworld, including the GMRT. Work is also being done at NCRA-TIFR on theoretical modelling of theEoR, which probes the formation of the first stars in the early Universe. Simulations of the HI 21cmemission signal from neutral hydrogen at different cosmic times are also being carried out.

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 tracethe neutral hydrogen distribution of the distant Universe, as well as physical conditions in the absorbinggalaxies. Understanding conditions in such absorption-selected high-redshift galaxies is another area ofresearch 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 theirtypical size and mass, and the evolution of these properties with redshift. Multi-wavelength magingstudies using optical, infrared and radio data are also being carried out to detect and characterizeemission-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-alphaemitters and Lyman-break galaxies, to probe physical conditions in the earliest galaxies in the Universe.

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 super-massive black holes at their centres. These result in the emission of enormous amounts of radiation invarious wavebands, including the radio, making them observable out to very large distances in theUniverse. Such objects are also often found in clusters of galaxies. The identification and detailed studyof Compact Steep Spectrum and Gigahertz Peaked Spectrum sources, which constitute a significantfraction 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, theinteraction 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 tomodel their gaseous environments, at all redshifts.

The radio galaxy 3C449, as seen at X-ray (blue) and radio (red andgreen) frequencies. (Lal et al. 2013).

The distribution of neutral hydrogen in the EoR, as predicted by simulations.

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 majorprogramme at most observatories. Such surveys are expected to detect objects over a wide range ofdistances, from objects in the Milky Way to objects at cosmological distances. The TIFR-GMRT SkySurvey (TGSS) is an all-sky GMRT radio continuum survey at 150 MHz, covering about 37,000 squaredegrees of the sky north of declination -55 degrees, at an angular resolution of about 20”. The TGSS ismore 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 majordatabase for multi-wavelength astronomy, yielding many new and interesting discoveries.

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-wavelengthstudies of specific extragalactic fields to study in detail how galaxies and their stars and gas evolvethrough the age of the Universe. Radio and infrared imaging is especially important in this area, becausemost 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 ofissues, including quantifying the number of sources of different types as a function of source luminosityand redshift, finding massive radio galaxies at high redshifts via their ultra-steep radio spectra,distinguishing between star-forming and active galaxies, etc.

Fundamental Constant Evolution: (Jayaram Chengalur, Nissim Kanekar)

A generic prediction of higher-dimensional theories that attempt to unify the standard model of particlephysics and general relativity is that low energy fundamental constants like the fine structure constantshould evolve with time. Astronomical studies allow one to probe such evolution over cosmologicaltimescales, and to thus test the validity of the standard model over billions of years. Researchers atNCRA-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 testsof cosmological changes in the fine structure constant and the proton-electron mass ratio.

A TGSS GMRT 150 MHz image of a typicalextragalactic field (Sirothia et al.).

Hydroxyl (OH) lines at z=0.247: Tentative evidence for fundamental constant evolution (Kanekar et al. 2010).

Astronomical Instrumentation: (Jayaram Chengalur, Swarna Kanti Ghosh, Yashwant Gupta, BhalChandra Joshi, P. K. Manoharan)

The success of a radio observatory rests heavily on its ability to work at the frontiers of technology todevelop cutting-edge software and hardware instrumentation to detect and process weak radioastronomical signals. Areas of research and development at NCRA-TIFR include wideband antenna feedelements, 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 isthe development of flexible software- and hardware-based back-ends, an area where NCRA-TIFR hasplayed, and continues to play, a pioneering role. New modes include flexible post-processing of voltagesignals, mitigation of terrestrial interference, the detection of transients, new correlation modes forinterferometry, etc, which have greatly enhanced the capabilities of both the GMRT and the ORT.

The Giant Metrewave Radio Telescope:

With thirty antennas, each of diameter 45 metres, spread out over a maximum distance of 25 km, theGMRT is the biggest and most sensitive radio interferometer in the world at low frequencies, < 1 GHz. Itis used for scientific observations by astronomers around the world, via competitive selection ofobserving proposals. In order to retain its premier status in the world over the next decade, much activityis currently under way to upgrade the GMRT, including the building of new low-frequency radioreceivers and a new correlator, besides upgrading most of the electronics and the telescope controlsystem. The sensitivity of the upgraded GMRT will have increased by a factor of at least 3 at allfrequencies. Studies are also in progress to further upgrade the GMRT, by increasing both the number ofantennas and the maximum antenna separation.

One of the thirty antennas of the GMRT at Narayangaon, 80 km from Pune, towering over a group of students.

The Ooty Radio Telescope:

Over the last decade, the ORT has been mainly used for studies of Inter-Planetary Scintillation, providingan important probe of solar activity and space weather studies. In collaboration with the Raman ResearchInstitute, the ORT's analog and digital electronics are being upgraded, to provide a wide field of view andimproved sensitivity. In addition, a new pulsar backend has been installed. The versatile upgraded ORTwill allow a number of studies requiring high sensitivity, such as accurate pulsar observations, searchesfor neutral hydrogen at high redshifts, searches for transients, solar and space weather studies, etc.

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 energeticexplosions in the Universe. A supernova, with an explosion energy 1E+51 ergs, often brieflyoutshines an entire galaxy before fading from view over several weeks to months. GRBs are flashesof gamma rays lasting from a few milliseconds to a few minutes. Supernovae and GRB explosions arelinked to the end stages of massive stars. Thus the major thrust in this direction is towardsunderstanding stellar deaths involving massive progenitors. The study of environments of theprogenitor stars that lead to supernovae and GRB explosions is the main focus of my research, whichmainly uses the radio and X-ray bands. I use the Giant Metrewave Radio Telescope (GMRT) and theVery Large Array (VLA), for radio measurements and the Swift-XRT, Chandra and XMM-Newtontelescopes 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, combinedwith rapid, sometimes near-critical stellar rotation, can exert a strong, even dominant influence on theformation and evolution of such massive stars, and on their demise as supernovae or GRB-producinghypernovae. 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 fieldsin a growing subset of massive stars. The channelling and confinement of an outflowing stellar windby the star's magnetic field leads to the formation of a shock-heated magnetosphere which can emit inmultiple wavebands. We are carrying out a systematic radio study with the GMRT and the VLA inorder 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 raisedseveral questions and triggered numerous theoretical investigations such as, when do the particleenergies reach maximum, during the free-expansion phase or during the Sedov stage? How do cosmicrays escape from a SNR, what is the dynamics of escape, i.e. how the maximum energy evolves withtime? What is the primary particle population producing the gamma-ray emission? The first twoquestions are intimately connected with the intensity of the magnetic field hence with the maximumacceleration energies which are constrained by radiative losses and synchrotron radiation and henceby radio emission. The third one can be traced efficiently thanks to the detection of gamma rays in thehigh energy range with the Fermi-LAT or in the very-high-energy energy range with HESS. Multi-wavelength data, and especially radio and gamma-ray data, are thus crucial to understand the natureof these efficient particle accelerators in our Galaxy. We are investigating how young core-collapsesupernova shocks accelerate Cosmic Rays (CR -- electrons or protons) to very high energies and howthe 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 irregulargalaxies, neutral hydrogen (HI) absorption in very high redshift galaxies (the so-called “dampedLyman alpha” systems), and, in general, studies of the evolution of the neutral hydrogen content of theUniverse.

Dwarf galaxies:

The dwarf irregular galaxies that our group studies are 1000 to 10000 times less massive than our owngalaxy, the Milky Way. These galaxies are interesting in not only in their own right, but also in thecontext of hierarchical galaxy formation models in which large galaxies like our own form by thehierarchical merger of smaller progenitors. In this picture, the very small "dwarf" galaxies in the localuniverse are the survivors of this merger process, and are possible analogs of the galaxies in the earlyuniverse. Detailed studies of these galaxies hence provide insights and constraints on galaxy formationmodels. Particular topics that our group has been investigating include the distribution and total darkmatter content of, as well as the processes that govern the conversion of gas into stars in, some of thesmallest known galaxies. Most of this work is in the context of a survey of HI 21cm emission from alarge sample (~ 75 objects) of nearby, extremely faint, dwarf irregular galaxies, viz. the Faint IrregularGalaxies 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 emissionobservations of field and galaxy clusters at redshifts between ~0.2 and ~0.4. These observationsconstrain the evolution of the HI content of galaxies as well as the effect of the cluster environment onthe 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 inthe observed field. The detection of emission at a redshift of ~ 0.4 represents the highest redshift atwhich 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 systemshelp us understand the physical conditions in the gas in these systems. Our observations indicate thatthe spin temperature of the hydrogen in damped Lyman-alpha systems is significantly larger than thattypical of large spiral galaxies like the Milky Way. Comparisons between the observed redshifts ofdifferent spectral lines in these systems also allows us to constrain the variation of fundamentalconstants like the fine structure constant and the ratio of the proton mass to the electron mass withcosmological time.

Neutral hydrogen on large scales:

A new experiment is also being planned to study the large scale neutral hydrogen correlation functionat a redshift of ~ 3 using the upgraded Ooty Radio Telescope. This is a large collaboration involvingastronomers from the Raman Research Institute, the Indian Institute for Science Education andResearch, 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 around400,000 years after the big bang. On the other hand, we know from observations of quasar absorptionspectra that the same Universe has become highly ionized by the time it is one Gigayear old. Thisimplies 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 firstluminous sources (galaxies/stars). Studying reionization will thus tell us how the first stars formedand how different they were from stars we see around us. In addition, reionization is extremelyimportant for studying cosmology in general as the details of it affects determination of cosmologicalparameters from observations.

I have been involved in theoretical modelling of the process of reionization, and comparing thesemodels with different observations. I am also interested in developing numerical simulations togenerate 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 LOwFrequency 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 havebeen found using the redshift-spectral index correlation. We have started a programme with the GiantMetrewave Radio Telescope (GMRT) to exploit this correlation at flux density levels of about 10 to100 times deeper than the known HzRGs. In this programme, we have obtained deep, high resolutionradio observations at 150 MHz with GMRT for several DEEP fields which are well studied at higherradio frequencies and in other bands of the electromagnetic spectrum, with an aim to detect candidatehigh redshift radio galaxies. From the deep 150 MHz observations of the LBDS-Lynx field, alongwith 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 candidatesfor high-redshift radio galaxies. These need to be further explored with deep infra-red imaging andoptical 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 probingconditions in massive star forming regions as well as young stellar objects. Different research projectsinclude

(1) Studying the structure of the ISM around massive star-forming regions and photo-dissociationregions through mapping of emission in the 158 micron [CII] line, the polycyclic aromatichydrocarbon (PAH) lines, and the dust continuum. The PAH studies are especially interesting as theyare the main source of gas heating in photo-dissociation regions and the atomic ISM, via photo-electricemission.

(2) Multi-wavelength monitoring of outbursts in young stellar objects to study episodic accretion onthese systems.

(3) Detailed studies of the post-outburst phase of McNeil's nebula, an exciting pre-main sequence starwhich 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 starformation 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 Far-Ultra-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 ofview of ~28 arcminutes and an angular resolution better than 1.8". In addition, gratings are available inthe FUV and the NUV channels for slitless low-resolution spectroscopy.

(2) The Infrared Spectroscopic Imaging Survey (IRSIS) payload, targeted for the Small SatelliteMission of ISRO, aims to carry out low resolution (R~100) spectroscopic measurements in thewavelength range 1.7 to 6.4 micron with seamless coverage, covering a large fraction (~ 50%) of thesky (including the Galactic Plane), with good sensitivity. The primary science goals of this projectinclude: (i) Discovery & classification of Brown Dwarfs and M-L-T Dwarfs, probing the faint end ofthe stellar Initial Mass Function; (ii) Large scale mapping in the emission features of large organicmolecules, via a Galactic Plane survey; (iii) Probing minor bodies of the Solar System, includingAsteroids, Comets, and Inter-Planetary Dust; and (iv) a Galactic Bulge survey to study AsymptoticGiant 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 inspecific 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 studiesthat 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 basicparameters 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 interstellarmedium (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 software-based 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 GMRTas well as observations at optical and high energies, are carried out. Blind and targeted searches ofpulsars, using the unique capabilities of GMRT, are ongoing with a view to increase the sample ofradio emitting pulsars. The newly discovered pulsars with the GMRT are followed up to determineprecise timing solution to learn about the neutron star characteristics. Together with the imagingobservations of neutron star environments with the GMRT, such observations are useful to constrainphysics of pulsar wind and the resultant nebula. New pulsar discoveries have also uncovered newclasses of neutron stars, such as intermittent pulsars and highly magnetized neutron stars along withexotic binary and millisecond pulsars.

Single pulse emission studies:

Another kind of study involves single pulse emission from radio pulsars, where a wide variety ofsingle pulse phenomena such as pulse nulling, drifting and resultant mode-changing are observed. TheGMRT and the ORT have been very useful in providing long simultaneous multi-frequency singlepulse observations, which are useful in constraining the pulsar magnetospheric physics and beamgeometry. Extreme forms of unexplained emission in single pulses, such as isolated radio bursts fromrotating radio transients (RRATs) and fast radio bursts (FRBs), are also studied as part of thisprogramme. Both telescopes have the capability to carry out low frequency studies of the pulsedemission, which gets dispersed and scatter-broadened by the interstellar medium. Multiple frequencyobservations in these studies are useful in constraining the structure and the dynamics of theinterstellar medium.

Precision timing studies:

Both the GMRT and the ORT are capable of high time resolution observations of pulse emission withprecision timing. Observations of millisecond pulsars, which are very stable clocks, are thereforeuseful in constraining relativity theories and provide an ensemble of clocks, which could be used as along baseline detector for stochastic gravitational background at nanohertz frequencies. Studiescontributing to an international effort to detect such a background using these pulsar timing arrays isalso currently ongoing. Both experimental and theoretical work in this direction is currently under wayat NCRA-TIFR.

Nissim Kanekar

My main research interests are in the areas of galaxy formation and evolution, tests of fundamentalphysics, 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 ofphysics, such as the fine structure constant and the proton-electron mass ratio, over cosmologicaltimescales, billions of years. My research in this field involves using radio spectroscopy of high-redshift galaxies to accurately measure the redshifts of different types of spectral lines (e.g. the HI21cm hyperfine line, ammonia inversion lines, hydroxyl Lambda-doubled lines, carbon monoxiderotational lines, etc) to test for changes in the fundamental constants. I also work on devising newtechniques 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 high-redshift gas-rich galaxies that are detected due to their strong absorption of the light of backgroundquasars. I use radio absorption studies of such galaxies to study the evolution of physical conditions intheir interstellar media. We have shown that atomic gas in these galaxies is predominantly warm athigh redshifts, apparently due to their low metallicity and a paucity of cooling routes. We have alsocome 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 atcertain wavelengths, so that one can then image the lower redshift galaxy. We are currently carryingout imaging and spectroscopic studies of a large sample of such galaxies with the Keck Telescope andthe Hubble Space Telescope, as well as radio spectroscopic imaging of low-redshift absorbers, todetermine their size, mass, etc.

The Interstellar Medium:

My work on the interstellar medium of the Milky Way has aimed to understand the distribution of gasbetween 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 thisunstable phase, which is not expected in standard three-phase models of the ISM. We are currentlyattempting to carry out a self-consistent modelling of hydrogen absorption and emission spectra, toobtain 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. Asexpected, the evolution of these galaxies in terms of star formation rate, gas content and other internalproperties will depend on the environment. In the denser environments, the galaxies are subject to tidalinteractions due to the presence of other nearby galaxies which can modify star-forming properties andto hydrodynamic processes due to intra-cluster gas like ram pressure stripping and viscous strippingwhich can lead to gas depletion. On the other hand, when galaxies evolve in isolation, they are onlysubject to internal perturbations. Thus, it is important to study both (1) the physical processes that acton these galaxies in dense environments and (2) the evolutionary paths of galaxies in bothenvironments. This is particularly important for small groups since more than 60% of galaxies arebelieved to evolve in small groups. At GMRT, research includes study of star-forming galaxies rangingfrom small to large spirals evolving in a range of environments in the near Universe. Research isactively being pursued on normal disk galaxies, low surface brightness galaxies and HII galaxies interms of understanding their low radio frequency spectral energy distribution, magnetic fielddistribution, halo emission, atomic gas distribution and kinematics, interactions and star formationproperties in different environments. Research also involves combining radio results with results fromother 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 thenumber being higher for massive OB stars and lower for low mass red dwarf stars. Stars evolving inbinaries will follow a distinct evolutionary track from those evolving as singles. There are binarysystems which consist of a white dwarf and a red giant or main sequence star as the companion. Inseveral such binaries, the white dwarf accretes matter which overflows the Roche lobe of thecompanion star and the material accumulates on the surface of the white dwarf. A cataclysmicthermonuclear reaction is ignited in this material when sufficient material is deposited on the surfaceleading to the ejection of mass and energy into the surrounding medium. These cataclysmic systemsknown as novae brighten by several magnitudes in the optical within a short interval and are observedto emit in the entire electromagnetic spectrum due to a range of physical phenomena. The fast-movingejecta from the explosion set up shocks in the interstellar medium as they encounter the ambientmaterial, and this leads to the emission of synchrotron radiation which can be ideally observed atGMRT frequencies. There is a class of novae known as recurrent novae (e.g RS Ophiuchi) whereoutbursts recur on timescales of a few years and these systems are believed to harbour a white dwarfwhose mass is close to the Chandrasekhar limit. These, then, are strong contenders for progenitorsystems of Type 1a supernovae. Studying the evolution of the radio synchrotron emission in successiverecurrent nova outbursts can shed light on the evolution of these intriguing systems, the progenitorscenario 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 thesynchrotron emission. There are also works on studying the early evolution of supernovae includingType 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 theMilky Way where hydrogen, carbon or helium can be ionized by the interstellar radiation field. In allthese regions, electrons can recombine to excited quantum levels in the atom and cascade down tolower levels giving rise to recombination lines at radio frequencies. This spectral line emission can bemapped from the ionized regions and physical properties such as temperature, electron density andsize can be modelled. Moreover, the distribution of this gas and its kinematics can also be studiedusing these lines. These results are combined with other diagnostics from the same gas to betterconstrain the physical parameter space. Thus, studies of HII regions around stars and the photo-dissociation 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 extra-galactic radio sources, by modelling the gaseous environments of radio galaxies in both the early andthe late Universe. This work is based on data from the Chandra Space Observatory, the GiantMetrewave Radio Telescope, and other ground- and space-based observatories. We are also interestedin 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 objectsin the early universe, including galaxies, super-massive black holes (SMBHs), or galaxy clusters. Thestrongest constraint on the high-redshift evolution of SMBHs comes from the observation of powerfulHzRGs. The luminosities of these sources imply that SMBHs of mass comparable to a billion solarmasses were already in place when the universe was only 1–3 Gyr old. To grow from seed fluctuationsup to such a high mass requires an almost continuous accretion of gas. Therefore, to understand theevolution of the first SMBHs in the first pre-galactic radio sources and their impact on the reionizationof the universe, it is important to understand the balance in the energy budget between mechanical andradiative 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 radioemission sweeping back from the head. The long tails of these galaxies carry the imprint of relativemotion between the non-thermal plasma and the ambient hot gas. Fortunately, the jets survive theencounter with the ICM, with possible shocks leading to the formation of the long tails, andspecifically they seem to be devoid of the growth of Kelvin-Helmholtz instabilities. Hence, in theparlance of the field, they reflect the weather conditions of the ICM, which allows one to makequantitative statements about their dynamics and energetics. Such observations can potentially revealdetails 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 uniquemethod to determine the speed and other physical properties of the solar wind using interplanetaryscintillation (IPS) measurements from a single radio telescope. Based on IPS measurements from theOoty Radio Telescope (ORT), several important studies leading to new results pertaining to the solarwind 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 internationalobservatories.

The Solar Wind:

Our studies, especially based on ORT scintillation measurements, have been able to explain the three-dimensional structure of solar wind density turbulence and its changes with solar cycles at the crucialheliocentric distance range above the solar wind acceleration region, for both low-speed and high-speed winds. A recent study, based on ORT IPS measurements, showed a clear evidence for a steadydecline 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 laternamed ‘sigmoid’) at the sites of flares or coronal mass ejections (CMEs), as observed at X-ray andradio wavelengths. Several studies by our group, based on multi-wavelength solar and interplanetaryobservations, have been useful to understand fundamental issues regarding the physics of magneticreconnection and the associated initiation of CMEs and particle acceleration processes in the lowcorona and in the near-Sun region. The propagation of CMEs (e.g., magnetic flux-rope structures) andtheir 3-D evolution have clarified the aerodynamical drag experienced by CMEs in the innerheliosphere. 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, andacceleration mechanism. These studies have also been extremely useful to pinpoint the arrival ofCMEs, and to identified the interplanetary conditions that drive geomagnetic activities and storms atthe near-Earth environment.

Dipanjan Mitra

Pulsars are highly magnetized fast rotating neutron stars (a highly dense star with a radius of about 10km 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 rotationalenergy to produce the EM radiation which is seen across the electromagnetic spectrum from radio togamma rays. However we do not know how the strong electric and magnetic field are oriented aroundthe neutron star and how they accelerate charged particles to generate the radiation. The physicalmechanism of how pulsars shine is still unknown, even after 45 years of its discovery and is one of themost challenging problem in astrophysics.

My research focuses on understanding the radio emission mechanism in pulsars. The Radio emissionform a tiny fraction of the total energy released by pulsars during spin down. The majority of thisenergy is lost as high energy X-ray and gamma-ray radiation and as a wind of relativistically chargedparticles flowing out into the ambient interstellar medium. The radio emission however is unique as ithas a very high brightness temperatures (or an equivalent blackbody temperature from a thermalsource) of about 1027 K. The extreme nature of the emission is apparent when one realizes that a spatialregion of only about 500 meters is capable of generating emission having such high equivalentblackbody temperature (one can compare this with the Sun which is a 5780 K blackbody and has aradius of 695,500 km). The pulsar radio emission hence has a non-thermal origin, and is commonlytermed as a coherent emission mechanism. The emission is generated in regions of ultra-strongmagnetic and electric fields where energetic photons splits into an electron and positron pair through aprocess of magnetic pair creation and can be accelerated to relativistic speeds. It is believed thatgrowth 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 tounravel the coherent emission mechanism in pulsars (Refer to this article for open problems that I aminterested in pulsars: http://arxiv.org/pdf/1304.1980v1.pdf) More specifically the area of my studyincludes:

(1) Use of radio observatories like GMRT, Arecibo, WSRT, LOFAR etc to study radio emissionproperties such as pulsar polarization, off-pulse emission, pulsar drifting, moding, nulling. In-depthunderstanding 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 globalstructure of the pulsar magnetosphere.

(3) Understanding theoretically how plasma waves are genee and propagate in the pulsarmagnetosphere.

My additional research interests include understanding the ISM using pulsar and probes and studyingcosmic 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. Thebulk of my research work tends to be at the intersection of science and numerical analysis ortechniques, or is about harnessing the recent developments in technology and computing for meetingscience goals which have remained elusive using the earlier generation of instrumentation andcomputing resources. Trying to extract the most information from the available data has been anenduring theme of my work. Along the frequency axis, my research interests have so far typically beenat the low radio frequency end.

The radio Sun is very dynamic, especially at metre wavelengths. Its emission can not only changerapidly in time, it also has very strong spectral features and the morphology of the emission changeswith both time and frequency. So, to study the radio Sun, one essentially needs a video camera whichcan 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 becomingavailable, this is changing. The MWA offers unprecedented capabilities for high dynamic range, highfidelity spectroscopic imaging with good time and frequency resolution over a wide band, and usefulangular resolution to explore many interesting problems in solar physics which could be addressedonly 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 andtype III bursts.

2. Reliable studies of low radio frequency solar flux, spectral index and its variations over small andlong time scales. These will in turn help understand scattering processes and micro turbulence in thecorona.

3. A truly exciting possibility is about looking for missing contributions to the famous and longstanding coronal heating problem. I have been a part of the design, construction and commissioningteam 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 regionof the Milky Way, supernova remnants in the Milky Way, and magnetic fields in our galaxy and nearbygalaxies. 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 fewhundred pcs is characterised by high density, large velocity dispersions, comparatively highertemperatures and magnetic fields. My multi-frequency GMRT observations led to the detection of thecentral compact radio source (Sgr A*) for the first time at 610 MHz . From observations at 240 and150 MHz, I have identified a large number (~62) of compact extragalactic radio sources. The ionisedgas in the region scatters electromagnetic radiation passing through it and the amount of scatteringgives information on the electron density and its fluctuation in the region. I have measured the angularsizes of the background sources at 150, 240 and 1400 MHz and showed for the first time presence of anenhanced scattering screen within a degree from the Galactic Centre. We have also shown that thefluctuating component of the magnetic field in the Galactic Centre region is ~ 20 micro-Gauss. In thisturbulent region, the systematic field will have similar or lower magnitude, implying that its strength isrelatively 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 tothe centre of the Milky Way. Also, our line of sight through this region passes through the longest pathlength in the Galaxy, which maximises the probability of finding a supernova remnant (SNR). In theMilky 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 identificationbecomes difficult due to increased source density and confusion (for larger remnants), while thesmaller remnants are missed due to finite angular resolutions of surveys. The GMRT offers the highestsensitivity and angular resolution at metre wavelengths. From our search, we have confirmed thenature of 5 candidate SNRs in the region out of 7 observed systems. From one of these, I identified asmall shell-like structure, and re-observed this with the GMRT at 325 MHz and 1.4 GHz. We recentlyconfirmed 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 ofenergy between magnetic fields and cosmic ray particles, we estimated the field strength from theintensity of synchrotron emission (emission from high-energy electrons gyrating in the magneticfields). We obtained field strengths of ~20-25 micro-Gauss at the galaxy centres, with a systematicdecrease to the outer parts, where the field strengths are ~10 micro-Gauss. We found that the energydensity in the magnetic fields and the gas are within a factor of two, indicating equipartition of energybetween them. We also showed that the slope of the radio flux density to the far infrared flux densityin these galaxies is the same at low and high frequencies in the ``arm'' regions. However, the slopesteepens for the inter-arm regions, when observations are done at high frequencies (> 1 GHz). This isdue to the longer propagation length of the comparatively lower energy electrons radiating below 1GHz, 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 opticalsurveys. 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 theHerschel telescope with our own 325 MHz radio observations with GMRT. We have also obtained newobservations 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 usingthe data to identify distant radio galaxies (z>3) and study the radio-FIR correlation from normalgalaxies at redshifts lower than 1. We are also using stacking techniques to characterise the radioproperties of a variety of source populations, such as X-ray sources, normal galaxies, quasars, star-forming galaxies, etc.

Formation of lenticular galaxies:

Over the last few years, we have used near-ultraviolet, optical and near-infrared observations oflenticular galaxies to demonstrate that these objects belong to two subclasses, differentiated by stellarmass or luminosity. More luminous lenticulars seem to have a predominantly old stellar populationlike elliptical galaxies, while less luminous lenticulars have a stellar population with a wide variety ofages like in spiral galaxies. A number of observational probes such as star-formation history, barfraction, bulge disk size correlations have been used to show that the formation history of moreluminous lenticulars is similar to that of ellipticals while less luminous lenticulars are essentiallyspirals whose star formation has been quenched by environmental or secular effects.