Browsing by Author "et al."

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21-cm absorbers at intermediate redshifts
(2009-02-01) Gupta, N.; Srianand, R.; Petitjean, Patrick; et al.
Damped Lyman-α systems (DLAs) seen in the spectra of high-z QSOs allow us to probe the physical conditions in protogalaxies. Our understanding of physical conditions in DLAs at high-z is primarily based on the absorption lines of H2 molecules and ﬁne-structure transitions. Another important way of probing the thermal state of interstellar medium in these systems is by studying the 21-cm absorption in the spectra of background quasars. Here we report the main results of our GMRT survey to search for 21-cm absorption in a representative and unbiased sample of 35 DLA candidates at 1.10≤z≤1.45. Our sample of DLA candidates is drawn from the strong Mg ii systems in SDSS DR5 and has resulted in discovery of 9 new 21-cm absorbers. Prior to our survey only one 21-cm absorber was known in the redshift range: 0.7≤z≤2. This survey has allowed us to investigate the dependence of detectability of 21-cm absorption on the properties of UV absorption lines detected in SDSS spectra and estimate the number per unit redshift of 21-cm absorbers. Our GMRT survey provides a representative sample of systems that can be used in combination with various follow-up observations: (1) for investigating the physical conditions in the absorbing gas using spin temperature (TS) measurements, (2) for investigating the eﬀect of metallicity and dust content on the detectability of 21-cm absorption, (3) for studying the morphology of the absorbing gas and (4) for probing the time evolution of various fundamental constants. Results from the ﬁrst phase of our survey are presented in Gupta et al. (2007). Detailed description of the entire sample and results from the survey are presented in Gupta et al. (2009).
3C 273 : A gravitationally lensed quasar?
(EDP Sciences, 1984-05-30) Chitre, S. M.; Narasimha, D.; Narlikar, J. V.; et al.
It is suggested that the abnormal brightness of the quasar 3C 273, coupled with the observations of superluminal motions in its radio core B as well as the misalignment of its VLBI jet with the optical jet, can be understood by postulating a faint spheroidal lensing galaxy located about halfway along the line of sight to the quasar. The lens model also helps to understand why the quasar is found slightly off-centre with respect to its nebulosity. The probability for such a lens system to arise for 3C 273 by chance is shown to be no less than that computed on the basis of the relativistic beaming hypothesis. Further tests of the lens model are discussed.
3D Automated Classification Scheme for the TAUVEX data pipeline
(2007-11-28) Bora, Archana; Gupta, Ranjan; Singh, Harinder P.; et al.
In order to develop a pipeline for automated classi cation of stars to be observed by the TAUVEX ultraviolet space Telescope, we employ an arti cial neural network (ANN) technique for classifying stars by using synthetic spectra in the UV region from 1250 A to 3220 A as the training set and International Ultraviolet Explorer (IUE) low resolution spectra as the test set. Both the data sets have been pre-processed to mimic the observations of the TAUVEX ultraviolet imager. We have successfully classi ed 229 stars from the IUE low resolution catalog to within 3-4 spectral sub-class using two di erent simulated training spectra, the TAUVEX spectra of 286 spectral types and UVBLUE spectra of 277 spectral types. Further, we have also been able to obtain the colour excess (i.e. E(B-V) in magnitude units) or the interstellar reddening for those IUE spectra which have known reddening to an accuracy of better than 0.1 magnitudes. It has been shown that even with the limitation of data from just photometric bands, ANNs have not only classi ed the stars, but also provided satisfactory estimates for interstellar extinction. The ANN based classi cation scheme has been successfully tested on the simulated TAUVEX data pipeline. It is expected that the same technique can be employed for data validation in the ultraviolet from the virtual observatories. Finally, the interstellar extinction estimated by applying the ANNs on the TAUVEX data base would provide an extensive extinction map for our galaxy and which could in turn be modeled for the dust distribution in the galaxy.
An analysis of the distribution of background star
(Mon. Not. R. Astron. Soc, 2002-02-02) Sen, A.K.; Mukai, T; Gupta, Ranjan; et al.
The polarization observed for stars background to dark clouds (Bok Globules) is often used as diagnostic to study the ongoing star formation processes in these clouds. Such polarization maps in the optical have been reported for eight nearby clouds CB3, CB25, CB39, CB52, CB54, CB58, CB62 and CB246 in one of our previous work (Sen et al 2000). With a view to understand the origin of this polarization, in the present work attempts are made to look for any possible relation between this observed polarization and other physical parameters in the cloud (like temperature, turbulence etc.). The observed polarization does not seem to be clearly related to the dust and gas temperatures (Td and Tg) in the cloud as expected from Davis-Greenstein grain alignment mechanism (Davis & Greenstein 1952). However, the average observed polarization (pav) appears to be related to the turbulence V (measured by 12CO line width) by the mathematical relation pav = 2.95 exp(−0.24 V ). The possible relation between the direction of polarization vector and other physical parameters are also discussed. For this analysis in addition to the data on above eight dark clouds, the data on CB4 (Kane et al. 1995) are also included for comparison. In order to study the spatial distribution of the degree of polarization and position angles across the different parts of the cloud a simple model is proposed, where the cloud has been assumed to be a simple dichroic polarizing sphere and the light from the background star first passes through the IS medium and then through the cloud, before reaching the observer. One finds this simple model can explain to a reasonable extent the observed spatial (radial) dependence of the value of p for two of the clouds (CB25 , CB39), but for rest of the clouds the model fails. However, through this model one can explain why the polarization (p) need not always increase with total extinction Av as one moves in the deeper interior part of the cloud.
Analysis of the distribution of background star polarization in dark clouds
(2005-04-01) Sen, A.K.; Mukai, T.; Gupta, Ranjan; et al.
The polarization observed for stars background to dark clouds (Bok Globules) is often used as diagnostic to study the ongoing star formation processes in these clouds. Such polarization maps in the optical have been reported for eight nearby clouds CB3, CB25, CB39, CB52, CB54, CB58, CB62 and CB246 in one of our previous work (Sen et al 2000). With a view to understand the origin of this polarization, in the present work attempts are made to look for any possible relation between this observed polarization and other physical parameters in the cloud (like temperature, turbulence etc.). The observed polarization does not seem to be clearly related to the dust and gas temperatures (Td and Tg) in the cloud as expected from Davis-Greenstein grain alignment mechanism (Davis & Greenstein 1952). However, the average observed polarization (pav) appears to be related to the turbulence ∆V (measured by 12CO line width) by the mathematical relation pav = 2.95 exp(−0.24∆V ). The possible relation between the direction of polarization vector and other physical parameters are also discussed. For this analysis in addition to the data on above eight dark clouds, the data on CB4 (Kane et al. 1995) are also included for comparison.
Analytic formulas for frequency and size dependence of absorption and scattering efficiencies of PAHs
(2011-11-22) Roy, Ashim K.; Sharma, S. K.; Gupta, Ranjan; et al.
Are these stratospheric nanoparticles bacteria?
(2004-03-27) Wainwright, M.; Wickramasinghe, N. C.; Narlikar, J. V.; et al.
Associated spectral and temporal state transition of the bright ULX NGC 1313 X-1
(2010-05-21) Dewangan, Gulab Chand; et al.
Stellar mass black hole X-ray binaries exhibit X-ray spectral states which also have distinct and characteristic temporal properties. These states are believed to correspond to diﬀerent accretion disc geometries. We present analysis of two XMM-Newton observations of the Ultra-Luminous X-ray source (ULX) NGC 1313 X-1, which reveal that the system was in two diﬀerent spectral states. While spectral variations have been observed in this source before, this data provides clear evidence that the spectral states also have distinct temporal properties. With a count rate of ~ 1.5 counts/s and a fractional variability amplitude of ~ 15%, the ULX was in a high ﬂux and strongly variable state in March 2006. In October 2006, the count rate of the ULX had reduced by a factor of ~ 2 and the spectral shape was distinctly diﬀerent with the presence of a soft component. No strong variability was detected during this low ﬂux state with an upper limit on the amplitude < 3%. Moreover, the spectral properties of the two states implies that the accretion disc geometry was diﬀerent for them. The low ﬂux state is consistent with a model where a standard accretion disc is truncated at a ra- dius of ~ 17 Schwarzschild radius around a ~ 200M⊙ black hole. The inner hot region Comptonizes photons from the outer disc to give the primary spectral component. The spectrum of the high ﬂux state is not compatible with such a geometry. Instead, it is consistent with a model where a hot corona covers a cold accretion disc and Comptonizes the disc photons. The variability as a function of energy is also shown to be consistent with the corona model. Despite these broad analogies with Galactic black hole systems, the spectral nature of the ULX is distinct in having a lower temperature (~ 2 keV) of the hot Comptonizing plasma and higher optical depth (~ 15) than what is observed for the Galactic ones.
Associations between quasi-stellar objects and galaxies
(American Astronomical Society, 1990-11-12) Burbidge, G.; Hewitt, A.; Narlikar, J. V.; et al.
Astronomical Data Management
(2007-01-01) Norris, Ray; Andernach, Heinz; Eichhorn, Guenther; et al.
We present a summary of the major contributions to the Special Session on Data Management held at the IAU General Assembly in Prague in 2006. While recent years have seen enormous improvements in access to astronomical data, and the Virtual Observatory aims to provide astronomers with seamless access to on-line resources, more attention needs to be paid to ensuring the quality and completeness of those resources. For example, data produced by telescopes are not always made available to the astronomical community, and new instruments are sometimes designed and built with insuﬃcient planning for data management, while older but valuable legacy data often remain undigitised. Data and results published in journals do not always appear in the data centres, and astronomers in developing countries sometimes have inadequate access to on-line resources. To address these issues, an “Astronomers Data Manifesto” has been formulated with the aim of initiating a discussion that will lead to the development of a “code of best practice” in astronomical data management.
Automated Classification of 2000 Bright IRAS Sources
(2011-07-06) Gupta, Ranjan; Singh, Harinder P.; Volk, K.; et al.
An Artiﬁcial Neural Network (ANN) scheme has been employed that uses a supervised back-propagation algorithm to classify 2000 bright sources from the Calgary database of IRAS (Infrared Astronomical Satellite) spectra in the region 8µm to 23µm. The database has been classiﬁed into 17 predeﬁned classes based on the spectral morphology. We have been able to classify over 80 percent of the sources correctly in the ﬁrst instance. The speed and robustness of the scheme will allow us to classify the whole of the LRS database, containing more that 50,000 sources, in the near future.
Automated star-galaxy segregation using spectral and integrated band data for TAUVEX/ASTROSAT satellite data pipeline
(2009-04-01) Bora, Archana; Harinder, P.; Gupta, Ranjan; et al.
We employ an Artiﬁcial Neural Network (ANN) based technique to develop a pipeline for automated segregation of stars from the galaxies to be observed by Tel-Aviv University Ultra-Violet Experiment (TAUVEX). We use synthetic spectra of stars from UVBLUE library and selected International Ultraviolet Explorer (IUE) low resolution spectra for galaxies in the ultraviolet (UV) region from 1250 to 3220Å as the training set and IUE low-resolution spectra for both the stars and the galaxies as the test set. All the data sets have been pre-processed to get band integrated ﬂuxes so as to mimic the observations of the TAUVEX UV imager. We also perform the ANN based segregation scheme using the full length spectral features (which will also be useful for the ASTROSAT mission). Our results suggest that, in the case of the non-availability of full spectral features, the limited band integrated features can be used to segregate the two classes of objects; although the band data classiﬁcation is less accurate than the full spectral data classiﬁcation.
Axisymmetric black hole accretion in the Kerr metric as an autonomous dynamical system
(2007-02-07) Goswami, Sanghamitra; Khan, Saba Nashreen; Ray, Arnab K.; et al.
In a stationary, general relativistic, axisymmetric, inviscid and rotational accretion ﬂow, described within the Kerr geometric framework, transonicity has been examined by setting up the governing equations of the ﬂow as a ﬁrst-order autonomous dynamical system. The consequent linearised analysis of the critical points of the ﬂow leads to a comprehensive mathematical prescription for classifying these points, showing that the only possibilities are saddle points and centre-type points for all ranges of values of the ﬁxed ﬂow parameters. The spin parameter of the black hole inﬂuences the multitransonic character of the ﬂow, as well as some of its speciﬁc critical properties. The special case of a ﬂow in the space-time of a non-rotating black hole, characterised by the Schwarzschild metric, has also been studied for comparison and the conclusions are compatible with what has been seen for the Kerr geometric case.
Bacillus aerius sp. nov., Bacillus aerophilus sp. nov., Bacillus stratosphericus sp. nov. and Bacillus altitudinis sp. nov., isolated from cryogenic tubes used for collecting air samples from high altitudes
(Society for General Microbiology Journals, 2006-11-24) Shivaji, S.; Chaturvedi, Preeti; Suresh, K.; et al.
Four novel bacterial strains were isolated from cryogenic tubes used to collect air samples at altitudes of 24, 28 and 41 km. The four strains, 24KT, 28KT, 41KF2aT and 41KF2bT, were identified as members of the genus Bacillus. Phylogenetic analysis based on 16S rRNA gene sequences indicated that three of the strains, 24KT, 28KT and 41KF2aT, are very similar to one another (>98 % sequence similarity) and show a similarity of 98–99 % with Bacillus licheniformis and 98 % with Bacillus sonorensis. DNA–DNA hybridization studies showed that strains 24KT, 28KT and 41KF2aT exhibit <70 % similarity with each other and with B. licheniformis and B. sonorensis. Differences in phenotypic and chemotaxonomic characteristics between the novel strains and B. licheniformis and B. sonorensis further confirmed that these three isolates are representatives of three separate novel species. Strain 41KF2bT showed 100 % 16S rRNA gene sequence similarity to Bacillus pumilus, but differed from its nearest phylogenetic neighbour in a number of phenotypic and chemotaxonomic characteristics and showed only 55 % DNA–DNA relatedness. Therefore, the four isolates represent four novel species for which the names Bacillus aerius sp. nov. (type strain, 24KT=MTCC 7303T=JCM 13348T), Bacillus aerophilus sp. nov. (type strain, 28KT=MTCC 7304T=JCM 13347T), Bacillus stratosphericus sp. nov. (type strain, 41KF2aT=MTCC 7305T=JCM 13349T) and Bacillus altitudinis sp. nov. (type strain, 41KF2bT=MTCC 7306T=JCM 13350T) are proposed.
Balloon experiment to detect microorganisms in the outer space
(Kluwer Academic Publishers, 2003-03-30) Narlikar, J. V.; Lloyd, David; Wickramasinghe, N. C.; et al.
The results of biological studies of a cryosampler flown with a balloon, in which air samples were collected at altitudes ranging from 20 to 41 km, well above the Tropopause over Hyderabad, are described. In the analysis carried out in Cardiff, voltage-sensitive dyes that could detect the presence of viable cells were used on these air-amples. Clumps of viable cells were found to be present in samples collected at all the altitudes. The images obtained from electron microscopy are consistent with the above finding. Reference is also made to a not her paper presented at this conference describing the identification of bacterial species in the sample carried out in Sheffield. Counter arguments are discussed against the criticism that the detected cells and microorganisms (in the samples collected above the local tropopause at 16 km) are due toterrestrial contamination.
Big bang continued
(Nature Publishing Group, 1992-05-28) Narlikar, J. V.; Arp, H. C.; Burbidge, G.; et al.
Binned cosmic microwave background anisotropy power spectra : Peak location
(2001-02-15) Podariu, Silviu; Souradeep, Tarun; Gott, J. Richard; et al.
We use weighted mean and median statistics techniques to combine individual cosmic microwave background (CMB) anisotropy detections and determine binned, multipole- space, CMB anisotropy power spectra. The resultant power spectra are peaked. The derived weighted-mean CMB anisotropy power spectrum is not a good representation of the individual measurements in a number of multipole-space bins, if the CMB anisotropy is Gaussian and correlations between individual measurements are small. This could mean that some observational error bars are underestimated, possibly as a consequence of undetected systematic eﬀects. Discarding the most discrepant 5% of the measure- ments alleviates but does not completely resolve this problem. The median-statistics power spectrum of this culled data set is not as constraining as the weighted-mean power spectrum. Nevertheless it indicates that there is more power at multipoles ℓ ∼ 150 − 250 than is expected in an open cold dark matter (CDM) model, and it is more consistent with a ﬂat CDM model. Unlike the weighted-mean power spectrum, the median-statistics power spectrum at ℓ ∼ 400 − 500 does not exclude a second peak in the ﬂat CDM model.
Bipolar Harmonic encoding of CMB correlation patterns
(2009-12-01) Joshi, Nidhi; Jhingan, S.; Souradeep, Tarun; et al.
Deviations from statistical isotropy can be modeled in various ways, for instance, anisotropic cosmological models (Bianchi models), compact topologies and presence of primordial magnetic ﬁeld. Signature of anisotropy manifests itself in CMB correlation patterns. Here we explore the symmetries of the correlation function and its implications on the observable measures constructed within the Bipolar harmonic formalism for these variety of models. Diﬀerent quantiﬁers within the Bipolar harmonic representation are used to distinguish between plausible models of breakdown of statistical isotropy and as a spectroscopic tool for discriminating between distinct cosmic topology.
C iv absorption in damped and sub-damped Lyman-alpha systems: Correlations with metallicity and implications for galactic winds at z~2-3
(2007-07-27) Fox, Andrew J.; Ledoux, C.; Petitjean, Patrick; et al.
We present a study of Civ absorption in a sample of 63 damped Lyman-α (DLA) systems and 11 sub-DLAs in the redshift range 1.75
CCD photometric study of the late type contact binary EK comae berenices
(2010-04-26) Deb, Sukanta; Singh, Harinder P.; Seshadri, T. R.; et al.
We present CCD photometric observations of the W UMa type contact binary EK Comae Berenices using the 2 metre telescope of IUCAA Girawali Observatory, India. The star was classiﬁed as a W UMa type binary of subtype-W by Samec et al. [20]. The new V band photometric observations of the star reveal that shape of the light curve has changed signiﬁcantly from the one observed by Samec et al. [20]. A detailed analysis of the light curve obtained from the high-precision CCD photometric observations of the star indicates that EK Comae Berenices is not a W-type but an A-type totally eclipsing W UMa contact binary. The photometric mass ratio is determined to be 0.349 ± 0.005. A temperature diﬀerence of ∆T = 141 ± 10 K between the components and an orbital inclination of i[°] = 89.800 ± 0.075 were obtained for the binary system. Absolute values of masses, radii and luminosities are estimated by means of the standard mass-luminosity relation for zero age main-sequence stars. The star shows O’Connell eﬀect, asymmetries in the light curve shape around the primary and secondary maximum. The observed O’Connell eﬀect is explained by the presence of a hot spot on the primary component.