IUCAA Preprints

Permanent URI for this communityhttp://localhost:4000/handle/11007/149

Browse

Now showing 1 - 20 of 810

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).
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.
5D radiating black holes in einstein-yang-mills-gauss-bonnet gravity
(2011-08-25) Ghosh, S. G.
We derive nonstatic spherically symmetric solutions of a null uid, in ve dimension (5D), to Einstein-Yang-Mills (EYM) equations with the coupling of Gauss-Bonnet (GB) combination of quadratic curvature terms, namely, 5D-EYMGB radiating black hole solution. It is shown that, in the limit, we can recover known radiating black hole solutions. The spherically symmetric known 5D static black hole solutions are also retrieved. The e ect of the GB term and Yang-Mills (YM) gauge charge on the structure and location of horizons, of the 5D radiating black hole, is also discussed.
About the Dunham coefficients Y20 and Y11 for diatomic molecules
(2001-01-12) Sharma, A. K.; Chandra, S.
For a diatomic molecule, when the Dunham coefficients Y20 is negative, all fea- tures of potential-energy curve can be reproduced. When Y20 is positive; it is not possible to reproduce all features of the potential-energy curve. However, turning points for some lower vibrational states can be obtained.
Absorption of Electro-magnetic Waves in a Magnetized Medium
(2000-07-04) Ganguly, Avijit K.; Konar, Sushan
In continuation to our earlier work, in which the structure of the vacuum polarisation tensor in a medium was analysed in presence of a background electro-magnetic ﬁeld, we discuss the absorptive part of the vacuum polarization tensor. Using the real time formalism of ﬁnite temperature ﬁeld theory we calculate the absorptive part of 1-loop vacuum polarisation tensor in the weak ﬁeld limit (eB < m2 ). Estimates of the absorption probability are also made for diﬀerent physical conditions of the background medium.
Accelerated expansion of the universe driven by tachyonic matter
(2011-07-06) Padmanabhan, T.
It is an accepted practice in cosmology to invoke a scalar ﬁeld with potential V (φ) when observed evolution of the universe cannot be reconciled with theoretical prejudices. Since one function-degree- of-freedom in the expansion factor a(t) can be traded oﬀ for the function V (φ), it is always possible to ﬁnd a scalar ﬁeld potential which will reproduce a given evolution. I provide a recipe for determining V (φ) from a(t) in two cases: (i) Normal scalar ﬁeld with Lagrangian L = (1/2)∂aφ∂aφ−V (φ) used in quintessence/dark energy models; (ii) A tachyonic ﬁeld with Lagrangian L = −V (φ)[1−∂aφ∂aφ] 1/2 , motivated by recent string theoretic results. In the latter case, it is possible to have accelerated expansion of the universe during the late phase in certain cases. This suggests a string theory based nterpretation of the current phase of the universe with tachyonic condensate acting as eﬀective cosmological constant.
Accretion powered spherical wind in general relativity
(2001-04-14) Das, Tapas K.
Using full general relativistic calculations, we investigate the possibility of generation of mass outﬂow from spherical accretion onto non-rotating black holes. Introducing a relativistic hadronic-pressure-supported steady, standing, spherically-symmetric shock surface around a Schwarzschild black hole as the eﬀective physical barrier that may be responsible for the generation of spherical wind, we calculate the mass outﬂow rate R ˙ m in terms of three accretion parameters and one outﬂow parameter by simultaneously solving the set of general relativistic hydrodynamic equations describing spherically symmetric, transonic, polytropic accretion and wind around a Schwarzschild black hole. Not only do we provide a suﬃciently plausible estimation of R ˙ m, we also successfully study the dependence and variation of this rate on various physical parameters governing the ﬂow. Our calculation indicates that independent of initial boundary conditions, the baryonic matter content of this shock-generated wind always correlates with post-shock ﬂow temperature.
Acoustic perturbations on steady spherical accretion in Schwarzschild geometry
(2007-06-10) Naskar, Tapan; Ray, Arnab K.
The stationary background ﬂow in the spherically symmetric infall of a compressible ﬂuid, coupled to the space-time deﬁned by the static chwarzschild metric, has been subjected to linearized acoustic perturbations. The perturbative procedure is based on the continuity condition and it shows that the coupling of the ﬂow with the geometry of space-time brings about greater stability for the ﬂow, to the extent that the amplitude of the perturbation, treated as a standing wave, decays in time, as opposed to the amplitude remaining constant in the Newtonian limit. In ualitative terms this situation simulates the effect of a dissipative mechanism in the classical Bondi accretion ﬂow, deﬁned in the Newtonian construct of space and time. As a result of this approach it becomes impossible to deﬁne an acoustic metric for a conserved spherically symmetric ﬂow, described within the framework of Schwarzschild geometry. In keeping with this view, the perturbation, considered eparately as a high-frequency travelling wave, also has its amplitude reduced.
Active Region Moss: Doppler Shifts from Hinode/EIS Observations
(2012-04-04) Tripathi, Durgesh; Mason, Helen E.; Kelimchuk, Jams A
Studying the Doppler shifts and the temperature dependence of Doppler shifts in moss regions can help us understand the heating processes in the core of the ac- tive regions. In this paper we have used an active region observation recorded by the Extreme-ultraviolet Imaging Spectrometer (EIS) onboard Hinode on 12-Dec- 2007 to measure the Doppler shifts in the moss regions. We have distinguished the moss regions from the rest of the active region by de ning a low density cut-o as derived by Tripathi et al. (2010). We have carried out a very careful analy- sis of the EIS wavelength calibration based on the method described in Young, O'Dwyer and Mason (2012). For spectral lines having maximum sensitivity be- tween log T = 5:85 and log T = 6:25 K, we nd that the velocity distribution peaks at around 0 km s􀀀1 with an estimated error of 4􀀀5 km s􀀀1. The width of the distribution decreases with temperature. The mean of the distribution shows a blue shift which increases with increasing temperature and the distribution also shows asymmetries towards blue-shift. Comparing these results with observables predicted from di erent coronal heating models, we nd that these results are consistent with both steady and impulsive heating scenarios. However, the fact that there are a signi cant number of pixels showing velocity amplitudes that exceed the uncertainty of 5 km s􀀀1 is suggestive of impulsive heating. Clearly, further observational constraints are needed to distinguish between these two heating scenarios.
Adaptive filtering techniques for gravitational wave interferometric data : Removing long-term sinusoidal disturbances and oscillatory transients
(2000-02-12) Chassande-Mottin, E.; Dhurandhar, Sanjeev
We propose an adaptive denoising scheme for poorly modeled non-Gaussian features in the gravitational wave interferometric data. Preliminary tests on real data show encouraging results
Adaptive filtering techniques for gravitational wave interferometric data : Removing long-term sinusoidal disturbances and oscillatory transients
(2000-04-04) Chassande-Mottin, E.; Dhurandhar, Sanjeev
It is known by the experience gained from the gravitational wave detector proto-types that the interferometric output sig- nal will be corrupted by a signiﬁcant amount of non-Gaussian noise, large part of it being essentially composed of long-term sinusoids with slowly varying envelope (such as violin res- onances in the suspensions, or main power harmonics) and short-term ringdown noise (which may emanate from servo control systems, electronics in a non-linear state, etc.). Since non-Gaussian noise components make the detection and esti- mation of the gravitational wave signature more diﬃcult, a de- noising algorithm based on adaptive ﬁltering techniques (LMS methods) is proposed to separate and extract them from the stationary and Gaussian background noise. The strength of the method is that it does not require any precise model on the observed data : the signals are distinguished on the basis of their autocorrelation time. We believe that the robustness and simplicity of this method make it useful for data prepa- ration and for the understanding of the ﬁrst interferometric data. We present the detailed structure of the algorithm and its application to both simulated data and real data from the LIGO 40meter proto-type.
AIGO: a southern hemisphere detector for the worldwide array of ground based interferometric gravitational wave detectors
(2009-10-01) Dhurandhar, Sanjeev; Souradeep, Tarun; Coward, D
This paper describes the proposed AlGO detector for the worldwide array of interferometric gravitational wave detectors. The first part of the paper summarises the benefits that AlGO provides to the worldwide array of detectors. The second part gives a technical description of the detector, which will follow closely the Advanced LIGO design. Possible technical variations in the design are discussed.
Algebraic approach to time-delay data analysis for LISA
(2001-12-20) Dhurandhar, Sanjeev; Nayak, K. R.; Vinet, J-Y.
Cancellation of laser frequency noise in interferometers is crucial for attaining the requisite sensitivity of the triangular 3-spacecraft LISA conﬁguration. Raw laser noise is several orders of magnitude above the other noises and thus it is essential to bring it down to the level of other noises such as shot, acceleration, etc. Since it is impossible to maintain equal distances between spacecrafts, laser noise cancellation must be achieved by appropriately combining the six beams with appropriate time-delays. It has been shown in several recent papers that such combinations are possible. In this paper, we present a rigorous and systematic formalism based on algebraic geometrical methods involving computational commutative algebra, which generates in principle all the data combinations cancelling the laser frequency noise. The relevant data combinations form the ﬁrst module of syzygies, as it is called in the literature of algebraic geometry. The module is over a polynomial ring in three variables, the three variables corresponding to the three time-delays around the LISA triangle. Speciﬁcally, we list several sets of generators for the module whose linear combinations with polynomial coeﬃcients generate the entire module. We ﬁnd that this formalism can also be extended in a straight forward way to cancel Doppler shifts due to optical bench motions. The two modules are infact isomorphic. We use our formalism to obtain the transfer functions for the six beams and for the generators. We speciﬁcally investigate monochromatic gravitational wave sources in the LISA band and carry out the maximisiation over linear combinations of the generators of the signal-to-noise ratios with the frequency and source direction angles as parameters.
Algorithm for optimally distributing quantized load on transputers with unequal speed: An application to the detection of gravitational wave signals from coalescing binaries
(2015-01-13) Pitre, Sangita. N.; Dhurandhar, S.V.
In a parallel computing system, we work with a network of a large number of processors wherein the performance characteristics each processer may have are different. This leads to a situation that when there is equal load on all the processer, some complete the job before the others. To make the optimum use of the available computing facility and optimise on time, it is necessary to balance the load on the processers according to there characteristics like speed etc. Here we present an algorithm to optimse to on ‘ time ‘ when difference processer have difference speed and the load is quantised in integral multiples of a given unit of load. The algorithm distribute the load in such a manner that all the processer work optimally and the processing time is minimal. The optimal distribution of the load is achived by employing the well known bisection technique for finding the rots of an equation. We discuss this algorithm in the context of our application for filtering the coalescing binary gravitational wave signals. Numerical result are finally discussed for the 64 transputer machine ( PARAM ) .
Analysis of low z absorbers in the QSO spectra
(2015-02-07) Srianand, R.
Analysis of Ly α absorption lines in the vicinity of QSOS
(2015-02-07) Srianand, R.; Khare, Pushpa
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.
Analysis of trapped surfaces in higher dimensional dust collapse
(2006-04-12) Patil, K. D.
In the present work we analyze the dynamics of apparent horizon in higher dimensional (HD) dust collapse. For this study we have taken in to consideration the two different types of dust models. We propose the concept of ‘trapped range’ of initial data in the different higher dimensional spacetimes. We show that ‘trapped range’ of initial data increases with the increase in dimensions of the spacetimes.
Angular momentum transport in quasi-Keplerian accretion disks
(2011-07-06) Subramanian, Prasad; Pujari, B. S.; Becker, Peter A.
We reexamine arguments advanced by Hayashi &Matsuda (2001), who claim that several simple, physically motivated deriva- tions based on mean free path theory for calculating the viscous torque in a quasi-Keplerian accretion disk yield results that are inconsistent with the generally accepted model. If correct, the ideas proposed by Hayashi & Matsuda would radically alter our understanding of the na- ture of the angular momentum transport in the disk, which is a central feature of accretion disk theory. However, in this paper we point out several fallacies in their arguments and show that there indeed exists a simple derivation based on mean free path theory that yields an expres- sion for the viscous torque that is proportional to the radial derivative of the angular velocity in the accretion disk, as expected. The deriva- tion is based on the analysis of the epicyclic motion of gas parcels in adjacent eddies in the disk.
Angular power spectrum of CMB anisotropy from WMAP
(2011-08-09) Souradeep, Tarun; Saha, Rajib; Jain, Pankaj
The remarkable improvement in the estimates of diﬀerent cosmological parameters in recent years has been largely spearheaded by accurate measurements of the angular power spectrum of Cosmic Microwave Background (CMB) radiation. This has required removal of foreground contamination as well as detector noise bias with reliability and precision. Recently, a novel model independent method for the estimation of CMB angular power spectrum from multi-frequency observations has been proposed and implemented on the ﬁrst year WMAP (WMAP-1) data by Saha et al. 2006. We review the results from WMAP-1 and also present the new angular power spectrum based on three years of the WMAP data (WMAP-3). Previous estimates have depended on foreground templates built using extraneous observational input to remove foreground contamination. This is the ﬁrst demonstration that the CMB angular spectrum can be reliably estimated with precision from a self contained analysis of the WMAP data. The primary product of WMAP are the observations of CMB in 10 independent diﬀerence assemblies (DA) distributed over 5 frequency bands that have uncorrelated noise. Our method utilizes maximum information available within WMAP data by linearly combining DA maps from diﬀerent frequencies to remove foregrounds and estimating the power spectrum from the 24 cross power spectra of clean maps that have independent noise. An important merit of the method is that the expected residual power from unresolved point sources is signiﬁcantly tempered to a constant offset at large multipoles (in contrast to the ∼ L² contribution expected from a Poisson distribution) leading to a small correction at large multipoles. Hence, the power spectrum estimates are less susceptible to uncertainties in the model of point sources.