K. S. DWARAKANATH
Articles written in Journal of Astrophysics and Astronomy
Volume 2 Issue 3 September 1981 pp 339-347
The structure of the Cygnus loop at 34.5 MHz
Ch. V. Sastry K. S. Dwarakanath R. K. Shevgaonkar
We have observed the large supernova remnant Cygnus Loop at 34.5 MHz with the low frequency radio telescope at Gauribi-danur, India. A radio map of the region with a resolution of 26 arcmin × 40 arcmin (α × δ) is presented. The integrated flux density of the Cygnus Loop at this frequency is 1245 ± 195 Jy. The radio fluxes of different parts of the nebula at this frequency were also measured and used to construct their spectra. It is found that the spectrum of the region associated with the optical nebulosity NGC 6992/5 is not flat at low frequencies, and also exhibits a break at a frequency around 400 MHz. The spectrum of the region associated with NGC 6960 also shows a break but around 1000 MHz, while the spectrum of the region associated with NGC 6974 is straight in the entire frequency range 25 to 5000 MHz. The implication of these results on the basis of existing theories of the origin of radio emission from supernova remnants is discussed.
Volume 3 Issue 2 June 1982 pp 207-216
Observations of the supernova remnants HB 9 and 1C 443 at 34.5MHz
K. S. Dwarakanath R. K. Shevgaonkar Ch. V. Sastry
We have observed the extended supernova remnants HB 9 (G 160.5 + 2.8) and IC 443 (G 189.1 + 2.9) at 34.5 MHz with a resolution of 26 arcmin × 40 arcmin. A map of HB 9 is presented. The integrated flux density of HB 9 at 34.5 MHz is 750 ± 150 Jy. The spectral index in the frequency range from 34.5 MHz to 2700 MHz is found to be constant (- 0.58 ± 0.06) without any spectral break such as was reported earlier by Willis (1973). There is no significant variation of the spectral index across the remnant. The integrated flux density of IC 443 at 34.5 MHz is 440 ± 88 Jy. The spectral index in the frequency range from 20 MHz to 10700 MHz is - 0.36 ± 0.04. The reduction in flux at very low frequencies (10 MHz) is attributable to free-free absorption in the interstellar medium and/or in the H II region S 249.
Volume 3 Issue 3 September 1982 pp 351-361
A new look at the birthrate of supernova remnan
G. Srinivasan K. S. Dwarakanath
We have reanalysed a homogeneous catalogue of shell-type supernova remnants and we find that the radio data are consistent with a birthrate of one in 22±3 yr. Our approach is based on the secular decrease of surface brightness of the historical remnants whose ages are precisely known. The abovementioned birthrate is significantly higher than most previous estimates which range from one in 50–150 yr, and is consistent with the supernova rate in our galaxy derived from historical observations, as well as with recent estimates of the pulsar birthrate.
Volume 5 Issue 4 December 1984 pp 403-423
On the supernova remnants produced by pulsars
G. Srinivasan D. Bhattacharya K. S. Dwarakanath
We conclude that pulsar-driven supernova remnants (SNRs) are extremely rare objects. Indeed an analysis of the known sample of plerions suggests a very low birthrate ∼ 1 in 240 years. Long-lived and bright plerions like the Crab nebula are likely to be produced only when the pulsar has an initial period ∼ 10–20 milliseconds and a field ∼ 1012 G. Such pulsars inside rapidly expanding shell remnants should also produce detectable plerions. The extreme rarity of SNRs with such hybrid morphology leads us to conclude that these pulsars must have been born with an initial period larger than ∼ 35–70 milliseconds.
Volume 11 Issue 3 September 1990 pp 311-322
A modified algorithm for CLEANing wide-field maps with extended structures
K. S. Dwarakanath A. A. Deshpande N. Udaya Shankar
A simple but effective modification to the conventional CLEAN algorithm is suggested. This modification ensures both stability and speed when CLEAN is applied to maps containing a mixture of point sources and extended structures. The method has been successfully applied to the recently-completed sky survey at 34.5 MHz (Dwarakanath & Udaya Shankar 1990). This survey was made using the Gauribidanur T array (GEETEE)1 in 1-D aperture synthesis mode. Since in this case the ‘dirty beam’ (point spread function) cannot be directly computed, a method to obtain this is discussed in detail. The results of this deconvolution procedure have been encouraging in terms of reduced computing time and improved dynamic range in our maps. This algorithm should find wider application in deconvolving maps which have both extended structures and point sources
Volume 11 Issue 3 September 1990 pp 323-410
A synthesis map of the sky at 34.5 MHz
K. S. Dwarakanath N. Udaya Shankar
This paper describes a wide-field survey made at 34.5 MHz using GEETEE,1 the low frequency telescope at Gauribidanur (latitude 13°36′12′′N). This telescope was used in the transit mode and by per forming 1-D synthesis along the north-south direction the entire observable sky was mapped in a single day. This minimized the problems that hinder wide-field low-frequency mapping. This survey covers the declination range of-50° to + 70° (- 33° to +61° without aliasing) and the complete 24 hours of right ascension. The synthesized beam has a resolution of 26′ x 42′ sec (δ
Volume 12 Issue 3 September 1991 pp 199-211
Low-frequency observations of the Vela supernova remnant and their implications
We have studied the Vela supernova remnant in the light of the 34.5 MHz observations made with the GEETEE low frequency array. The flux densities of Vela X and YZ at 34.5 MHz are estimated to be 1800 and 3900 Jy respectively. These values, along with those from earlier observations at higher frequencies, imply spectral indices (S∞Να) of-0.16 ± 0.02 for Vela X and -0.53 ± 0.03 for Vela YZ. This situation is further substantiated by the spectral-index distribution over the region obtained between 34.5 and 408 MHz.
The spectral-index estimates, along with other known characteristics, strengthen the earlier hypothesis that Vela X is a plerion, while Vela YZ is a typical shell-type supernova remnant. We discuss the implications of this result.
Volume 19 Issue 3-4 December 1998 pp 97-116
The interstellar clouds of adams and blaauw revisited: An HI absorption study-I
Jayadev Rajagopal G. Srinivasan K. S. Dwarakanath
This investigation is aimed at clarifying the nature of the interstellar gas seen in absorption against bright O and B stars. Towards this end we have obtained for the first time HI absorption spectra towards radio sources very close to the lines of sight towards twenty five bright stars previously studied. In this paper we describe the selection criteria, the details regarding our observations, and finally present the absorption spectra. In the accompanying paper we analyse the results and draw conclusions.
Volume 19 Issue 3-4 December 1998 pp 117-131
The interstellar clouds of adams and blaauw revisited: An HI absorption study-II
Jayadev Rajagopal G. Srinivasan K. S. Dwarakanath
In the preceding paper (Paper I), we presented HI absorption spectra towards radio sources very close to the lines of sight towards twenty five bright stars against which optical absorption spectra had been obtained earlier, In this paper we analyse the results and draw some conclusions.
To summarize briefly, in most cases we found HI absorption at velocities corresponding to the optical absorption features provided one restricted oneself to velocities ≲10 kms-1. At higher velocities we did not detect any HI absorption down to an optical depth limit of 0.1 (except in four cases which we attribute to gas in systematic motion rather than clouds in random motion). After discussing various scenarios, we suggest that this trend should perhaps be understood in terms of the high velocity interstellar clouds being accelerated, heated and ablated by expanding supernova remnants.
Volume 22 Issue 1 March 2001 pp 1-8
GMRT detection of HI 21 cm-line absorption from the peculiar galaxy in Abell 2125
Using the recently completed Giant Meterwave Radio Telescope, we have detected the HI 21 cm-line absorption from the peculiar galaxy C153 in the galaxy cluster Abell 2125. The HI absorption is at a redshift of 0.2533, with a peak optical depth of 0.36. The full width at half minimum of the absorption line is 100 km s−1. The estimated column density of atomic Hydrogen is 0.7×1022(T
Volume 22 Issue 1 March 2001 pp 35-50
GMRT observations of interstellar clouds in the 21cm line of atomic hydrogen
Rekhesh Mohan K. S. Dwarakanath G. Srinivasan Jayaram N. Chengalur
Nearby interstellar clouds with high (|ν|≥10km s−1) random velocities although easily detected in NaI and CaII lines have hitherto not been detected (in emission or absorption) in the HI 21cm line. We describe here deep Giant Metrewave Radio Telescope (GMRT) HI absorption observations toward radio sources with small angular separation from bright O and B stars whose spectra reveal the presence of intervening high random velocity CaII absorbing clouds. In 5 out of the 14 directions searched we detect HI 21cm absorption features from these clouds. The mean optical depth of these detections is ∼0.09 and FWHM is ∼10km s−1, consistent with absorption arising from CNM clouds.
Volume 24 Issue 1-2 March 2003 pp 37-43
GMRT detection of HI 21 cm associated absorption towards the
C. H. Ishwara-Chandra K. S. Dwarakanath K. R. Anantharamaiah
We report the GMRT detection of associated HI 21 cm-line absorption in the
Volume 25 Issue 3-4 September 2004 pp 129-141
On the origin of the wide HI absorption line towards Sgr A*
K. S. Dwarakanath W. M. Goss J. H. Zhao C. C. Lang
We have imaged a region of ∼ 5′ extent surrounding Sgr A* in the HI 21 cm-line absorption using the Very Large Array. A Gaussian decomposition of the optical depth spectra at positions within ∼ 2′ (∼ 5 pc at 8.5 kpc) of Sgr A* detects a wide line underlying the many narrow absorption lines. The wide line has a mean peak optical depth of 0.32 ± 0.12 centered at a mean velocity of V1sr = −4 ± 15 km s{−1}. The mean full width at half maximum is 119 ± 42 km s−1. Such a wide line is absent in the spectra at positions beyond ∼ 2′ from Sgr A*. The position-velocity diagrams in optical depth reveal that the wide line originates in various components of the circumnuclear disk (radius ∼ 1.3′ ) surrounding Sgr A*. These components contribute to the optical depth of the wide line in different velocity ranges. The position-velocity diagrams do not reveal any diffuse feature which could be attributed to a large number of HI clouds along the line of sight to Sgr A*. Consequently, the wide line has no implications either to a global population of shocked HI clouds in the Galaxy or to the energetics of the interstellar medium as was earlier thought.
Volume 25 Issue 3-4 September 2004 pp 143-183
A high galactic latitude HI 21 cm-line absorption survey using the GMRT: I. Observations and spectra
Rekhesh Mohan K. S. Dwarakanath G. Srinivasan
We have used the Giant Meterwave Radio Telescope (GMRT) to measure the Galactic HI 21-cm line absorption towards 102 extragalactic radio continuum sources, located at high (
Volume 25 Issue 3-4 September 2004 pp 185-201
Rekhesh Mohan K. S. Dwarakanath G. Srinivasan
We have carried out a sensitive high-latitude (
Volume 26 Issue 1 March 2005 pp 1-70
GMRT
The GMRT
Volume 26 Issue 1 March 2005 pp 71-87
The
The HI content of galaxies in the Eridanus group is studied using the GMRT observations and the HIPASS data. A significant HI deficiency up to a factor of 2–3 is observed in galaxies in the high galaxy density regions. The HI deficiency in galaxies is observed to be directly correlated to the local projected galaxy density, and inversely correlated to the line-of-sight radial velocity. Furthermore, galaxies with larger optical diameters are predominantly in the lower galaxy density regions. It is suggested that the HI deficiency in Eridanus is due to tidal interactions. In some galaxies, evidences of tidal interactions are seen. An important implication is that significant evolution of galaxies can take place in the group environment. In the hierarchical way of formation of clusters via mergers of groups, a fraction of the observed HI deficiency in clusters could have originated in groups. The co-existence of S0s and severely HI deficient galaxies in the Eridanus group suggests that tidal interaction is likely to be an effective mechanism for transforming spirals to S0s.
Volume 26 Issue 1 March 2005 pp 89-102
Radio continuum and far-infrared emission from the galaxies in the Eridanus group
The Eridanus galaxies follow the well-known radio—FIR correlation. The majority (70%) of these galaxies have their star formation rates below that of the Milky Way. The galaxies that have a significant excess of radio emission are identified as low luminosity AGNs based on their radio morphologies obtained from the GMRT observations. There are no powerful AGNs (
Volume 27 Issue 1 March 2006 pp 7-23
The Tully-Fisher relations of the Eridanus group of galaxies
The Tully-Fisher (TF) or the luminosity-linewidth relations of the galaxies in the Eridanus group are constructed using the HI rotation curves and the luminosities in the optical and in the near-infrared bands. The slopes of the TF relations (absolute magnitude
Volume 32 Issue 4 December 2011 pp 425-426
K. S. Dwarakanath Lawrence Rudnick N. Udaya Shankar Tiziana Venturi
Volume 32 Issue 4 December 2011 pp 529-532
Discovery of a Giant Radio Halo in a Massive Merging Cluster at 𝑧 = 0.443
K. S. Dwarakanath Siddharth Malu Ruta Kale
We have discovered a giant radio halo in the massive merging cluster MACSJ0417.5-1154. This cluster, at a redshift of 0.443, is one of the most X-ray luminous galaxy cluster in the MAssive Cluster Survey (MACS) with an X-ray luminosity in the 0.1–2.4 keV band of 2.9 × 1045 erg s-1. Recent observations from GMRT at 230 and 610 MHz have revealed a radio halo of ∼ 1.2 × 0.3 Mpc2 in extent. This halo is elongated along the North-West, similar to the morphology of the X-ray emission from Chandra. The 1400 MHz radio luminosity (𝐿r) of the halo is ∼ 2 × 1025 W Hz-1, in good agreement with the value expected from the 𝐿x - 𝐿r correlation for cluster halos.
Volume 32 Issue 4 December 2011 pp 533-536
Double Relics in the Outskirts of A3376: Accretion Flows Meet Merger Shocks?
Ruta Kale K. S. Dwarakanath Joydeep Bagchi Surajit Paul
The case of spectacular ring-like double radio relics in the merging, rich galaxy cluster A3376 is of great interest to study non-thermal phenomena at cluster outskirts.We present the first low frequency (330 and 150 MHz) images of the double relics using the GMRT. With our GMRT 330 MHz map and the VLA 1400 MHz map (Bagchi
Volume 32 Issue 4 December 2011 pp 537-538
A Zoo of Radio Relics: Cluster Cores to Filaments
Radio relics in galaxy clusters can be electrons accelerated at cluster merger shocks or adiabatically compressed fossil radio cocoons or dying radio galaxies. The spectral evolution of radio relics is affected by the surrounding thermal plasma. We present a low frequency study of three radio relics representing environments of dense cluster core (A4038), cluster outskirts (A1664) and filaments (A786). The properties of the relics are found to be consistent with the effect of confinement by external medium if the effects of projection are ignored.
Volume 37 Issue 4 December 2016 Article ID 0031 Review
Clusters of Galaxies and the Cosmic Web with Square Kilometre Array
Ruta Kale K. S. Dwarakanath Dharam Vir Lal Joydeep Bagchi Surajit Paul Siddharth Malu Abhirup Datta Viral Parekh Prateek Sharma Mamta Pandey-Pommier
The intra-cluster and inter-galactic media that pervade the large scale structure of the Universe are known to be magnetized at sub-micro Gauss to micro Gauss levels and to contain cosmic rays. The acceleration of cosmic rays and their evolution along with that of magnetic fields in these media is still not well understood. Diffuse radio sources of synchrotron origin associated with the Intra-Cluster Medium (ICM) such as radio halos, relics and mini-halos are direct probes of the underlying mechanisms of cosmic ray acceleration. Observations with radio telescopes such as the Giant Metrewave Radio Telescope, the Very Large Array and the Westerbork Synthesis Radio Telescope have led to the discoveries of about 80 such sources and allowed detailed studies in the frequency range 0.15–1.4 GHz of a few. These studies have revealed scaling relations between the thermal and non-thermal properties of clusters and favour the role of shocks in the formation of radio relics and of turbulent re-acceleration in the formation of radio halos and mini-halos. The radio halos are known to occur in merging clusters and mini-halos are detected in about half of the cool-core clusters. Due to the limitations of current radio telescopes, low mass galaxy clusters and galaxy groups remain unexplored as they are expected to contain much weaker radio sources. Distinguishing between the primary and the secondary models of cosmic ray acceleration mechanisms requires spectral measurements over a wide range of radio frequencies and with high sensitivity. Simulations have also predicted weak diffuse radio sources associated with filaments connecting galaxy clusters. The Square Kilometre Array (SKA) is a next generation radio telescope that will operate in the frequency range of 0.05–20 GHz with unprecedented sensitivities and resolutions. The expected detection limits of SKA will reveal a few hundred to thousand new radio halos, relics and mini-halos providing the first large and comprehensive samples for their study. The wide frequency coverage along with sensitivity to extended structures will be able to constrain the cosmic ray acceleration mechanisms. The higher frequency (>5 GHz) observations will be able to use the Sunyaev–Zel’dovich effect to probe the ICM pressure in addition to tracers such as lobes of head–tail radio sources. The SKA also opens prospects to detect the ‘off-state’ or the lowest level of radio emission from the ICM predicted by the hadronic models and the turbulent re-acceleration models.
Volume 38 Issue 3 September 2017 Article ID 0036 Editorial
Dipankar Bhattacharya K. S. Dwarakanath Sushan Konar
Volume 42 All articles Published: 24 July 2021 Article ID 0085 SCIENCE RESULTS
M. DAS J. YADAV N. PATRA K. S. DWARAKANATH S. S. MCGAUGH J. SCHOMBERT P. T. RAHNA J. MURTHY
Wepresent a UV study of 3 extended UV (XUV) galaxies that we have observed with the UVIT and the GMRT. XUV galaxies show filamentary or diffuse star formation well beyond their optical disks, in regions where the disk surface density lies below the threshold for star formation. GALEX observations found that surprisingly 30% of all the nearby spiral galaxies have XUV disks. The XUV galaxies can be broadly classified as Type 1 and Type 2 XUV disks. The Type 1 XUV disks have star formation that is linked to that in their main disk, and the UV emission appears as extended, filamentary spiral arms. The UV luminosity is associated with compact star forming regions along the extended spiral arms. The star formation is probably driven by slow gas accretion from nearby galaxies or the intergalactic medium (IGM). But the Type 2 XUV disks have starformation associated with an outer low luminosity stellar disk that is often truncated near the optical radius of the galaxy. The nature of the stellar disks in Type 2 XUV disks are similar to that of the diffuse stellar disks of lowsurface brightness galaxies. The star formation in Type 2 XUV disks is thought to be due to rapid gas accretion or gas infall from nearby high velocity clouds (HVCs), interacting galaxies or the IGM. In this paper, we investigate the star formation properties of the XUV regions of two Type 2 galaxies and one mixed XUV type galaxy and compare them with the neutral hydrogen (HI) emisison in their disks.We present preliminary results of our UVIT (FUV and NUV) observations of NGC 2541, NGC 5832 and ESO406-042, as well as GMRTobservations of their HI emission. We describe the UV emission morphology, estimate the star formation rates and compare it with the HI distribution in these Type 2 and mixed XUV galaxies.
Volume 44, 2023
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