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Research Areas
 
Theoretical Astrophysics

Cosmology
The origin of magnetic fields in the universe remains a mystery. It is possible that they are of primordial origin and therefore affects cosmological observables, such as CMBR (Cosmic Microwave Background Radiation) anisotropies and large-scale structure formation. Cosmology has turned into a precision science in the past twenty years and observations can now potentially reveal the nature of these fields. The effects of magnetic fields on different physical processes, structures and signals are being studied. Some of these are cosmological gravitational lensing, Lyman-alpha clouds, neutral hydrogen signals from the epoch of reionization and the early formation of molecular hydrogen. A recent major result in this field has been a dating of the formation epoch of cold dark matter in the universe as having occurred before a redshift of z > 106. Other fascinating questions being addressed in this field include factoring in primordial magnetic fields as possible sources of density perturbation in the early universe. It has been shown that magnetic fields as small as 0.1nG may have a significant impact on reionization history. This year, data from the Murchison Widefield Array observations have also been analysed for detection of redshifted 21-cm from the Epoch of Reionization (EoR); additionally, a new method has been proposed for the detection of this signal using drift scan strategy.

Cosmic rays, gamma rays and neutrinos
Highly energetic particles (protons, nuclei, photons and neutrinos) detected on Earth are accelerated in galactic and extragalactic environments. Theoretical modelling of the origin, production and propagation of these high-energy particles is used to interpret observations from gamma ray detectors, such as Fermi LAT, HESS, MAGIC, AGILE and the IceCube neutrino telescope. The applications of these models include galactic sources, extragalactic sources like gamma ray bursts, and the emission mechanisms and environment in active galactic nuclei. During the period 2014-2015, several intriguing questions such as the origin of X-ray and Gamma ray emission in the large scale jet of quasar 3C 273, the origin of antiproton flux in the interactions of cosmic ray protons and nuclei with cold protons in Galactic sources to explain the PAMELA experiment results, and the reason behind the observed spectral cut-off in Gamma ray spectra from novae as detected by Fermi LAT, have been investigated and solutions proposed as research outcomes.

Galaxies and their surroundings
The interaction between galaxies and their surroundings plays an important role in the evolution of galaxies and the intergalactic medium. The interactions involve the flow of gas acted upon by radiation pressure, cosmic rays and the galaxy's gravitational field. These complex processes are studied using both analytical methods and hydrodynamic simulations. Of interest are violent gaseous outflows from galaxies. Research by the Astronomy and Astrophysics group addresses their origin, evolution, feedback effects on the parent galaxy, and transport of heavy elements such as carbon, oxygen to the outer parts of the galaxy and beyond. Recent research in this area is addressing problems such as the Sunyaev-Zeldovich distortion of the cosmic microwave background caused by the large amount of hot gas in the halos of massive galaxies, merging and collisions of multiple supernovae---exploding together at a single epoch or staggered in time---and the conditions under which they can give rise to galactic outflows, interaction of these supernovae driven outflows from Milky Way type galaxies with the hot halo gas and the propagation dynamics and escape of ionizing photons from disk galaxies consistent with observed local UV background radiation and re-ionization of the universe.

Stellar dynamics in galactic nuclei
The nuclei of galaxies have dense clusters of stars orbiting supermassive black holes (BH); two nearby examples are the Galactic centre and the nucleus of the Andromeda galaxy. Gravitational force between the bodies mainly governs the structure, dynamics and evolution of the stellar system. Both dynamical and statistical mechanical methods are being used to formulate a theory of the long-term evolution of these Keplerian stellar systems with long-range interactions. Phenomena studied include instabilities, relaxation to equilibrium and fuelling of the BH, which contributes to its mass and spin.

Exoplanetary Dynamics
Most exoplanetary systems in binary stars are of S-type, consisting of one or more planets orbiting a primary star with a wide binary stellar companion. The planetary eccentricities and mutual inclinations can be large, and may be due to gravitational forcing by the binary companion star. However, the role of a wide binary star on planetary evolution remains largely unexplored. RRI has worked on presenting a mechanism that has the potential to explain several observed features of exoplanetary statistics.

Turbulent magnetic fields
The ionized gas in astrophysical bodies (stars, galaxies) is often differentially rotating and magnetized, with the magnetic field generated by electric currents in the highly conducting gas. Not only is the magnetic field strongly coupled to the gas, but it also acts back on the gas through Lorentz forces. Astronomical observations raise questions concerning the origin and growth of magnetic fields on large scales (dynamo action), and the organization of the field on small scales (turbulence). Some problems of interest are (a) dynamo action in turbulent shear flows, and (b) magneto-hydrodynamic turbulence.

Observational Astronomy

Diffuse Radio Emission from Galaxy Clusters
Galaxy Clusters are some of the largest gravitationally bound structures in the Universe. As many as hundreds of galaxies can be bound in a volume with linear extents of a few million light years. The space between the galaxies, the Intra Cluster Medium (ICM), is often home to sources of bright diffuse radio emission. This radio emission is known to be of non-thermal origin and is due to relativistic electrons radiating in the cluster magnetic field. Observations and interpretations of the origin of diffuse radio emission from galaxy clusters has implications on understanding of large scale magnetic fields in clusters, cluster mergers, and cluster evolution. These observations are being carried out using a variety of radio telescopes like the Giant Meterwave Radio Telescope, the Very Large Array and the Murchison Widefield Array, which was built by RRI and international partners in a collaborative project. Archival X-ray observations are used along with these radio observations to understand these intriguing sources of radio emission. An interesting example of a galaxy cluster, which is known to host a double radio relic and has been reported to contain a faint radio halo and bridge, is Abell 3667. The origin of radio haloes, relics and bridges is still under contention although galaxy cluster merger is acknowledged to be an important factor. During the period 2014-2015, Murchison Widefield Array observations of the Abell 3667 have been analysed by the A&A group at RRI.

Pulsars
Pulsars are rapidly spinning high-magnetic-field neutron stars that originate in supernovae. They emit pulsed radio waves, X-rays and Gamma rays. Pulsar emissions provide information about their structure, the high-energy processes in the magnetosphere, their interactions with their environment and can also be used as a test platform for General Relativity. Millisecond pulsars are one of the most accurate clocks in the Universe. RRI conducts active research on pulsars and develops radio receivers and X-ray detectors that can help us understand their nature. Studies on timing and spectral characterisation, orbital evolution, apsidal advance and orbital reprocessing of emitted X-rays are being conducted on pulsars of various types. Detection of off-pulse radio emission from pulsars and studies of the dependence of the shape of the emission beam on emission height and distance from the magnetic pole, which in turn affects the intrinsic circular polarization in pulsar emission, is being pursued.

Lifecycles of radio galaxies
A particular type of galaxy called the restarting radio galaxy was first discovered by astronomers of the Institute and has been their research interest for two decades. It is believed that activity in the nucleus of the galaxy had stopped and then restarted again afresh in multiple epochs. Using increasingly sensitive and high resolution radio telescopes it has now become possible to address questions like why the central engine ceased activity, how long it was dead before activity restarted, what mechanisms triggered a rebirth and so on. Astronomers at RRI are working on these questions together with other international collaborators.

Radio Galaxy Morphologies
Amongst the population of radio galaxies it is the X-shaped radio galaxy in particular, with its two typical lobes accompanied by two others shaped like the letter X, which is currently being investigated by some members of the Astronomy group at RRI. How the two additional lobes are generated is debatable and is of importance to understand the coalescence of supermassive black holes and generation of the gravitational wave background. In the X-shaped radio galaxy, two lobes appear to be active while the other two are relics or spillovers of plasma from the main axis. Research is on at RRI to test the various existing theories on the shape and structure of X-shaped radio galaxies. During the period 2014-2015, work was done to characterize the morphologies of radio galaxies in a unique sample of 52 radio galaxies selected for their low axial ratios. Based on the image analysis, three main types of ‘distortions’ to radio galaxy morphologies were isolated. A fraction of these source morphologies are suggested as generated by binary black hole mergers, which has been used to constrain the expectation for the gravitational wave background arising from coalescence of supermassive black holes.

X-ray binaries
Compact stars (neutron stars and black holes) create some of the most extreme conditions in the universe. Their high-energy emission, mostly X-rays, can be accessed using space X-ray observatories. Astronomers at RRI use observations made using a range of international space observatories to investigate topics such as orbital evolution of binary X-ray stars and orbital glitches, quasi-periodic oscillations in X-ray pulsars, self absorption in X-ray pulsars and dips in the pulse profiles, cyclotron absorption lines in X-ray pulsars, neutron star magnetic field, optical reprocessing and temperature measurements of thermonuclear X-ray bursts, variations in intensity and pulsation characteristics during eclipse/partial eclipses. Recently Suzaku studies of a pulsar that belongs to the class of highly absorbed supergiant high-mass X-ray binary (HMXB), which is characterized by a very high column density of absorbing matter, have been undertaken by this group. The upcoming Indian multi-wavelength astronomy satellite ASTROSAT and an Indian X-ray polarimeter POLIX that is being built at the Raman Research Institute will enhance X-ray binary research at RRI substantially.

Giant Radio Galaxies
Giant radio galaxies (GRGs) typically have lobes or plumes extending to megaparsec scales implying a timescale for growth of the order of tens to hundreds of millions of years. A major research project carried out and completed in the last year has been regarding GRGs and their unique relationship with their environments, both at local and global scales. This work on GRGs and their environments is aimed at the larger goal of using GRGs to reveal the nature of the elusive warm-hot intergalactic medium (WHIM) in their vicinity. The comprehensive study undertaken by the RRI group in conjunction with universities in Australia has led to a possible characterization of the conditions under which they grow to giant sizes.

Radio Telescopes and Astronomy


Members of the Astronomy & Astrophysics group along with the Radio Astronomy Laboratory of the Institute are involved in projects concerning various telescopes throughout the world. Questions like radio signatures from the Epoch of Reionization and transient radio phenomena, particularly fast radio transients, have changed the frequency range of interest for many next generation radio telescopes to less than a GHz, with significantly improved sensitivity, larger bandwidths and wide fields of view. Modern radio telescopes are now realized as phased arrays, or Aperture Arrays, where the number of beams depends only on the available processing power and signal transport, unlike earlier radio telescopes that were based on large reflectors with a single or small number of feeds. The Murchison Widefield Array (MWA) and an Indian Sky Watch Network (SWAN) are some examples of recent radio telescope projects in which astronomers at RRI have been actively engaged.

Murchison Widefield Array (MWA)
The MWA radio telescope or the Murchison Widefield Array radio telescope located in Murchison Shire in the Australian outback is an array of antennas arranged as square ‘tiles’ consisting of a total of 2048 dual-polarization wide-band ‘bow-tie shaped’ antennas that operate in the frequency range 80-330 MHz. They are arranged as 128 square ‘tiles’ each having 16 pairs of antennas. The antenna distribution is designed for precision imaging of a wide field of several hundred square degrees of the sky at any instant and over a wide frequency band. The antennas are connected to digital receivers which process the data before transmitting it via high-speed fibre optic cables to a centralized imaging system located 800 kilometres away at Perth. The digital receivers that take the signals from the antennas and perform complex high-speed signal processing of the data prior to transmission to the central processing unit, which computes the imaging information, were designed and built at RRI. RRI along with Harvard and MIT in the US as well as institutions in Australia and New Zealand was involved in the successful installing and commissioning of the telescope. The construction and commissioning of the MWA was completed in mid 2013.
The MWA has already begun gathering weak radio signals from deep space that are being analysed by astronomers at RRI and in the US and Australia. The data is expected to provide an insight into the dramatic evolution experienced by primordial cosmic gas as the first stars and galaxies formed in the early universe. That is a challenging goal and continued progress has been made towards this key science goal in the year gone by, with critical participation by RRI. Besides this, MWA data is helping study structure of the intergalactic gas in our Milky Way galaxy and galaxies and clusters of galaxies beyond, and the influence of the Sun on inter-planetary weather close to Earth, and numerous collaborative publications have appeared during the year gone by.

Gauribadanur Radio Telescope GRT)
This decametre wave radio telescope at Gauribadanur is operated in collaboration with the Indian Institute of Astrophysics. Operating at 34.5 MHz, this meridian-transit instrument has some tracking capabilities. It consists of 1000 fat dipoles arranged in the form of a letter 'T'. The bandwidth is about 10 MHz centred at 32 MHz, while maximum effective collecting area is about 18,000 square metres. The N-S arm of the beam can be tilted within a declination range of -45° to 75° while the E-W beam can be tilted in Hour Angle within 10° around the meridian enabling tracking for a minimum of 42 min. Members of the astronomy group currently study emissions from pulsars and build models for the emission regions at the polar caps of pulsars. They infer the astrophysics of this phenomenon using the GRT.

X-ray polarimeter (POLIX)
Anisotropy in the Thomson scattering of polarised X-rays is used to measure the degree and angle of linear polarisation of cosmic X-rays. Several types of cosmic X-ray sources are expected to have some linear polarisation, with very strong polarisation in some sources like accretion powered X-ray Pulsars and Blazars. Important breakthroughs may be made in some key scientific aspects of these sources from their X-ray polarisation measurements. In spite of this, X-ray polarisation measurement is an as yet unexplored area. To date, only one experiment for X-ray polarization measurement has been done and the Crab Nebula is the only source with a definite measurement of its polarisation. RRI astronomers are developing methods of detecting polarized X-rays from celestial bodies and building X-ray instruments for detecting these polarized X-rays. For this, the development of an X-ray polarimeter sensitive in the range of 5-30 keV is underway, and is well along the road towards being launched into space in collaboration with ISRO. A laboratory model has been built and tested successfully. The design and fabrication of the engineering model is now under progress. Developmental work for the polarimeter that has been completed in the last year include calibration of the collimator, fabrication of space qualified front end electronics, processing electronics, housekeeping electronics and the design of a photo-electron polarimeter.
Based on the development that has been achieved in this project and the research potential, a committee has been constituted by the Programme Director of the Small Satellite Programme at ISRO to configure a small satellite mission.

Apart from POLIX, preliminary investigations for development of a X-ray pulsar based interplanetary navigation system are also underway. The advantages of such an instrument over currently used techniques would be its position independence and autonomous operation. Stability profiles of two sources are now being observed extensively for this purpose.

ASTROSAT
This ISRO satellite mission for multi-wavelength astronomy is designed for studies of Neutron Star X-ray binaries, AGNs (Active Galactic Nuclei), clusters of galaxies, stellar coronae, sky surveys etc. RRI has been working with two teams at ISAC, Bangalore and SAC, Ahmedabad on the Large Area X-ray Proportional Counter (LAXPC), one of the ASTROSAT payloads, which is to be used for X-ray timing and low-resolution spectral studies over a broad energy band. LAXPC is designed to have a large photon collection area enabling detailed studies of high-energy features in X-ray light curves of bright and medium intensity point X-ray sources. Work has been done on the timing and spectral calibration of the Astrosat-LAXPC instrument and a software design for LAXPC data reduction is currently under development. The main goal of the timing qualification and calibration work is to minimize systematic uncertainties in the timing response of LAXPC detectors and the processing electronics.

15-Metre Fan-Beam Telescope
Members of the Astronomy & Astrophysics group at RRI have proposed two new optical designs for radio telescopes that use a Fan Beam Telescope (FBT) as a component. Currently, RRI is engaged in building a Fan Beam Telescope - Low Frequency (FBT-LF) with the purpose of demonstrating its functioning and the expected benefits of the new designs. Once commissioned, it will pave the way for the realization of the above two optical designs. For this purpose, a Ku-band receiver chain is also being built, including low-noise amplifiers, covering the 7-14 GHz range. Recently, another novel method has been proposed that enables single dishes to add short-spacing data to synthesis instruments. This method also allows for accurate measurement of surface deviations of antennas. Recent work in this has presented a scheme that uses two FBTs in cross configuration. This can form two orthogonal fan beams on the sky, which may be cross-correlated to make a large instantaneous map. This set-up has been named “An efficient linear array imager for Radio Astronomy”. Another device designed to make correlation measurements is a “One-element interferometer” where Ryle’s phase switching technique is adapted to a single dish with either coherent or incoherent receivers.

Cosmic Radio Background Distortions
DISTORTION (Detection of Spectral signaTures of cOsmic baRyon evoluTION) is a collective within the Astronomy & Astrophysics group, working together towards the goal of building precise spectral radiometers to detect weak spectral distortions in the Cosmic Radio and Microwave Backgrounds arising from the Epochs of Recombination all the way to Reionization. Detection of signals from neutral hydrogen atoms at the time the first luminous objects in the universe were born is challenging. Detection helps the astronomer to decipher the physical processes during the Dark Ages and subsequent EoR, which are necessary to understand the formation of first stars and galaxies as well as the evolution of the diffuse intervening medium to its present state. The global or all-sky 21 cm from cosmological reionization is a spectral distortion of 10's to at most about 100 mK signal and this is present in the cosmic radio background in the frequency range 30-200 MHz as a trace additive component. Towards detecting this signal researchers at RRI are working on the experiment SARAS (Shaped Antenna measurement of the background RAdio Spectrum) where they are continually designing, deploying, trialling and improving a spectral radiometer system consisting of a frequency independent antenna, self-calibratable receivers and a broadband precision digital spectrometer. The goal is to make observations not limited by sensitivity to the systematics of the receiver used and also to the man-made Radio Frequency Interference environment.
All of the bound-bound transitions in hydrogen and helium during cosmological recombination result in spectral features in the spectrum of the relict radiation and these redshift with cosmological expansion to appear at cm and mm wavelengths today. These additive photons ought to appear as 'tiny' spectral structures in the CMB spectrum, as predicted from our understanding of the physics of recombination. The Array of Precision Spectrometers for the Epoch of RecombinAtion - APSERa - is a venture to detect these recombination lines from the Epoch of Cosmological Recombination. In 2014-15 RRI has carried out a feasibility study for an experimental detection of these recombination lines. Researchers at RRI have proposed an algorithm to detect these tiny fluctuations buried in the sky spectrum by modelling the sky radiation emanating from Galactic and extragalactic sources. In this algorithm, for the first time, they have introduced the concept of Maximally Smooth functions that is capable of describing the foreground spectrum and distinguishing the complex signal of interest.

SWAN: (Indian Sky Watch Network)
RRI has proposed the design, development and use of a wideband Sky Watch Array Network across India with the following objectives –
a) To facilitate and conduct searches and studies of fast (typically of sub-second duration) and slow transient radio radiation originating from astronomical sources.
b) To facilitate and conduct high angular resolution imaging of discrete galactic and extragalactic sources at low radio frequencies.
c) To train, involve and provide hands-on experience to a large number of undergraduate and postgraduate students in all aspects of the SWAN, through their direct and active participation starting from the design stage to research using the array network.
The proposed competitive coordinated network with nominally 1000 sq. m array area at each location and operation spanning a decade in frequency over 50-500 MHz will be developed in three phases.
In the first phase the plan is for a moderate setup to be attempted for realizing and demonstrating the essential features. During the year 2014-15 the focus has been on realizing a setup with 8 independent stations operating over a common narrow band, using available hardware from previous projects (a GBT receiver, MWA tiles and beam-formers). Once this setup is fully developed and tested at Gauribidanur field station, relocation of the system at 8 different sites will be considered. Test observations will include targeted searches for radio transients in a few select directions of specific astronomical interest, and coordinated observations with the GMRT.
The implicit aim is to initiate a collective effort to develop the SWAN with as many of the 40+ technology and science institutes, such as the IITs, NITs, IISERs & NISERs, as well as many universities across India. To prepare for student involvement, appropriate schools/workshops (in co-ordination with existing programs) are being arranged to formally introduce under-graduate and master’s students to radio astronomy, basic concepts and advanced topics/techniques, over a few weeks each year, along with hands-on experience with systems – the first such school was conducted in 2015. Active participation from students in all aspects of the SWAN, including studies of astronomical sources, will be sought and explicitly encouraged/supported.

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