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1.
Nissim Kanekar Jayaram N. Chengalur 《Monthly notices of the Royal Astronomical Society》2001,325(2):631-635
We report multi-epoch Giant Metrewave Radio Telescope (GMRT) H i observations of the z = 0.3127 damped absorber towards the quasar PKS 1127−145, which reveal variability in both the absorption profile and the flux of the background source, over a time-scale of a few days.
The observed variations cannot be explained by simple interstellar scintillation (ISS) models where there are only one or two scintillating components and all of the ISS occurs in the Galaxy. More complicated models, where either there are more scintillating components or some of the ISS occurs in the interstellar medium of the z =0.3127 absorber, may be acceptable. However, the variability can probably be best explained in models incorporating motion (on sub-VLBI scales) of a component of the background continuum source, with or without some ISS.
All models producing the variable 21-cm absorption profile require small-scale variations in the 21-cm optical depth of the absorber. The length-scale for the opacity variations is ∼0.1 pc in pure superluminal motion models, and ∼10 pc in pure ISS models. Models involving subluminal motion, combined with scintillation of the moving component, require opacity variations on far smaller scales of ∼ 10–100 au . 相似文献
The observed variations cannot be explained by simple interstellar scintillation (ISS) models where there are only one or two scintillating components and all of the ISS occurs in the Galaxy. More complicated models, where either there are more scintillating components or some of the ISS occurs in the interstellar medium of the z =0.3127 absorber, may be acceptable. However, the variability can probably be best explained in models incorporating motion (on sub-VLBI scales) of a component of the background continuum source, with or without some ISS.
All models producing the variable 21-cm absorption profile require small-scale variations in the 21-cm optical depth of the absorber. The length-scale for the opacity variations is ∼0.1 pc in pure superluminal motion models, and ∼10 pc in pure ISS models. Models involving subluminal motion, combined with scintillation of the moving component, require opacity variations on far smaller scales of ∼ 10–100 au . 相似文献
2.
Rekhesh Mohan K. S. Dwarakanath G. Srinivasan Jayaram N. Chengalur 《Journal of Astrophysics and Astronomy》2001,22(1):35-50
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. 相似文献
3.
The earliest decipherable record of the Dharwar tectonic province is left in the 3.3 Ga old gneissic pebbles in some conglomerates
of the Dharwar Group, in addition to the 3.3–3.4 Ga old gneisses in some areas. A sialic crust as the basement for Dharwar
sedimentation is also indicated by the presence of quartz schists and quartzites throughout the Dharwar succession. Clean
quartzites and orthoquartzite-carbonate association in the lower part of the Dharwar sequence point to relatively stable platform
and shelf conditions. This is succeeded by sedimentation in a rapidly subsiding trough as indicated by the turbidite-volcanic
rock association. Although conglomerates in some places point to an erosional surface at the contact between the gneisses
and the Dharwar supracrustal rocks, extensive remobilization of the basement during the deformation of the cover rocks has
largely blurred this interface. This has also resulted in accordant style and sequence of structures in the basement and cover
rocks in a major part of the Dharwar tectonic province. Isoclinal folds with attendant axial planar schistosity, coaxial open
folds, followed in turn by non-coaxial upright folds on axial planes striking nearly N-S, are decipherable both in the “basement”
gneisses and the schistose cover rocks. The imprint of this sequence of superposed deformation is registered in some of the
charnockitic terranes also, particularly in the Biligirirangan Hills, Shivasamudram and Arakalgud areas. The Closepet Granite,
with alignment of feldspar megacrysts parallel to the axial planes of the latest folds in the adjacent schistose rocks, together
with discrete veins of Closepet Granite affinity emplaced parallel to the axial planes of late folds in the Peninsular Gneiss
enclaves, suggest that this granite is late-tectonic with reference to the last deformation in the Dharwar tectonic province.
Enclaves of tonalite and migmatized amphibolite a few metres across, with a fabric athwart to and overprinted by the earliest
structures traceable in the supracrustal rocks as well as in a major part of the Peninsular Gneiss, point to at least one
deformation, an episode of migmatization and one metamorphic event preceding the first folding in the Dharwar sequence. This
record of pre-Dharwar deformation and metamorphism is corroborated also by the pebbles of gneisses and schists in the conglomerates
of the Dharwar Group.
Volcanic rocks within the Dharwar succession as well as some of the components of the Peninsular Gneiss give ages of about
3.0 Ga. A still younger age of about 2.6 Ga is recorded in some volcanic rocks of the Dharwar sequence, a part of the Peninsular
Gneiss, Closepet Granite and some charnockites. These, together with the 3.3 Ga old gneisses and 3.4 Ga old ages of zircons
in some charnockites, furnish evidence for three major thermal events during the 700 million year history of the Archaean
Dharwar tectonic province. 相似文献
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5.
Sk. Saiyad Ali Somnath Bharadwaj Jayaram N. Chengalur 《Monthly notices of the Royal Astronomical Society》2008,385(4):2166-2174
Foreground subtraction is the biggest challenge for future redshifted 21-cm observations to probe reionization. We use a short Giant Meter Wave Radio Telescope (GMRT) observation at 153 MHz to characterize the statistical properties of the background radiation across ∼1° to subarcmin angular scales, and across a frequency band of 5 MHz with 62.5 kHz resolution. The statistic we use is the visibility correlation function, or equivalently the angular power spectrum C l . We present the results obtained from using relatively unsophisticated, conventional data calibration procedures. We find that even fairly simple-minded calibration allows one to estimate the visibility correlation function at a given frequency V 2 ( U , 0) . From our observations, we find that V 2 ( U , 0) is consistent with foreground model predictions at all angular scales except the largest ones probed by our observations where the model predictions are somewhat in excess. On the other hand, the visibility correlation between different frequencies κ( U , Δν) seems to be much more sensitive to calibration errors. We find a rapid decline in κ( U , Δν) , in contrast with the prediction of less than 1 per cent variation across 2.5 MHz. In this case, however, it seems likely that a substantial part of the discrepancy may be due to limitations of data reduction procedures. 相似文献
6.
We present the first set of polarimetric images made with the GMRT. We find that the instrumental polarization leakage at
the GMRT varies with frequency. It is possible to calibrate these terms to better than 1% accuracy, making it feasible to
study sources that are polarized at the few per cent level. We present 610 MHz polarization images of two extended FR-II radio
galaxies, viz. 3C 79 and 3C 265. We present high resolution polarization images of these two sources and also find that the
polarization fractions of the two sources as seen at the GMRT are consistent with those reported by Conway & Strom (1984). 相似文献
7.
Observed oceanic response to tropical cyclone Jal from a moored buoy in the south-western Bay of Bengal 总被引：1，自引：1，他引：0
M. S. Girishkumar K. Suprit Jayaram Chiranjivi T. V. S. Udaya Bhaskar M. Ravichandran R. Venkat Shesu E. Pattabhi Rama Rao 《Ocean Dynamics》2014,64(3):325-335
Upper oceanographic and surface meteorological time-series observations from a moored buoy located at 9.98°N, 88°E in the south-western Bay of Bengal (BoB) were used to quantify variability in upper ocean, forced by a tropical cyclone (TC) Jal during November 2010. Before the passage of TC Jal, salinity and temperature profiles showed a typical BoB post-monsoon structure with relatively warm (30 °C) and low-saline (32.8 psu) waters in the upper 30- to 40-m layer, and relatively cooler and higher salinity (35 psu) waters below. After the passage of cyclone, an abrupt increase of 1 psu (decrease of 1 °C) in salinity (temperature) in the near-surface layers (up to 40-m depth) was observed from buoy measurements, which persisted up to 10–12 days during the relaxation stage of cyclone. Mixed layer heat budget analysis showed that vertical processes are the dominant contributors towards the observed cooling. The net surface heat flux and horizontal advection together contributed approximately 33 % of observed cooling, during TC Jal forced stage. Analysis showed the existence of strong inertial oscillation in the thermocline region and currents with periodicity of ～2.8 days. During the relaxation stage of the cyclone, upward movement of thermocline in near-inertial frequencies played significant role in mixed layer temperature and salinity variability, by much freer turbulent exchange between the mixed layer and thermocline. 相似文献
8.
Probabilistic seismic risk assessment for spatially distributed lifelines is less straightforward than for individual structures. While procedures such as the ‘PEER framework’ have been developed for risk assessment of individual structures, these are not easily applicable to distributed lifeline systems, due to difficulties in describing ground‐motion intensity (e.g. spectral acceleration) over a region (in contrast to ground‐motion intensity at a single site, which is easily quantified using Probabilistic Seismic Hazard Analysis), and since the link between the ground‐motion intensities and lifeline performance is usually not available in closed form. As a result, Monte Carlo simulation (MCS) and its variants are well suited for characterizing ground motions and computing resulting losses to lifelines. This paper proposes a simulation‐based framework for developing a small but stochastically representative catalog of earthquake ground‐motion intensity maps that can be used for lifeline risk assessment. In this framework, Importance Sampling is used to preferentially sample ‘important’ ground‐motion intensity maps, and K‐Means Clustering is used to identify and combine redundant maps in order to obtain a small catalog. The effects of sampling and clustering are accounted for through a weighting on each remaining map, so that the resulting catalog is still a probabilistically correct representation. The feasibility of the proposed simulation framework is illustrated by using it to assess the seismic risk of a simplified model of the San Francisco Bay Area transportation network. A catalog of just 150 intensity maps is generated to represent hazard at 1038 sites from 10 regional fault segments causing earthquakes with magnitudes between five and eight. The risk estimates obtained using these maps are consistent with those obtained using conventional MCS utilizing many orders of magnitudes more ground‐motion intensity maps. Therefore, the proposed technique can be used to drastically reduce the computational expense of a simulation‐based risk assessment, without compromising the accuracy of the risk estimates. This will facilitate computationally intensive risk analysis of systems such as transportation networks. Finally, the study shows that the uncertainties in the ground‐motion intensities and the spatial correlations between ground‐motion intensities at various sites must be modeled in order to obtain unbiased estimates of lifeline risk. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
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10.
Prasun Dutta Ayesha Begum Somnath Bharadwaj Jayaram N. Chengalur 《Monthly notices of the Royal Astronomical Society》2009,398(2):887-897
We estimate the power spectrum of H i intensity fluctuations for a sample of eight galaxies (seven dwarf and one spiral). The power spectrum can be fitted to a power-law for six of these galaxies, indicating turbulence is operational. The estimated best-fitting value for the slope ranges from ∼−1.5 (AND IV, NGC 628, UGC 4459 and GR 8) to ∼−2.6 (DDO 210 and NGC 3741). We interpret this bi-modality as being due to having effectively 2D turbulence on length-scales much larger than the scale-height of the galaxy disc and 3D otherwise. This allows us to use the estimated slope to set bounds on the scale-heights of the face-on galaxies in our sample. We also find that the power-law slope remains constant as we increase the channel thickness for all these galaxies, suggesting that the fluctuations in H i intensity are due to density fluctuations and not velocity fluctuations, or that the slope of the velocity structure function is ∼0. Finally, for the four galaxies with '2D turbulence' we find that the slope α correlates with the star formation rate (SFR) per unit area, with larger SFRs leading to steeper power laws. Given our small sample size, this result needs to be confirmed with a larger sample. 相似文献