2009 (IPP)

Permanent URI for this collectionhttp://localhost:4000/handle/11007/332

Browse

Now showing 1 - 20 of 45

Non-stationary de Sitter cosmological models
(2009-01-01) Ibohal, Ng.
In this note it is proposed a class of non-stationary de Sitter, rotating and non- rotating, solutions of Einstein’s ﬁeld equations with a cosmological term of variable function Λ∗(u). It is found that the space-time of the rotating non-stationary de Sitter model is an algebraically special in the Petrov classiﬁcation of gravitational ﬁeld with a null vector, which is geodesic, shear free, expanding as well as non-zero twist. However, that of the non-rotating non-stationary model is conformally ﬂat with non-empty space.
Tensor-to-scalar ratio in punctuated inflation
(2009-01-01) Jain, Rajeev Kumar; Chingangbam, Pravabati; Sriramkumar, L.
Recently, we have shown that scalar spectra with lower power on large scales and certain other features naturally occur in punctuated inﬂation, i.e. the scenario wherein a brief period of rapid roll is sandwiched between two stages of slow roll inﬂation. Such spectra gain importance due to the fact that they can lead to a better ﬁt of the observed Cosmic Microwave Background (CMB) anisotropies, when compared to the conventional, featureless, power law spectrum. In this paper, with examples from the canonical scalar ﬁeld as well as the tachyonic models, we illustrate that, in punctuated inﬂation, a drop in the scalar power on large scales is always accompanied by a rise in the tensor power and, hence, an even more pronounced increase in the tensor-to-scalar ratio ron these scales. Interestingly, we ﬁnd that r actually exceeds well beyond unity over a small range of scales. To our knowledge, this work presents for the ﬁrst time, examples of single scalar ﬁeld inﬂationary models wherein r ≫ 1. This feature opens up interesting possibilities. For instance, we show that the rise in r on large scales translates to a rapid increase in the angular power spectrum, CBB ℓ , of the B-mode polarization of the CMB at the low multipoles. We discuss the observational implications of these results.
Model Independent Foreground Power Spectrum Estimation using WMAP 5-year Data
(2009-01-01) Ghosh, Tuhin; Saha, Rajib; Jain, Pankaj; et al.
In this paper, we propose & implement on WMAP 5-year data, a model independent approach of foreground power spectrum estimation for multifrequency observations of CMB experiments. Recently a model independent approach of CMB power spectrum estimation was proposed by Saha et al. 2006. This methodology demonstrates that CMB power spectrum can be reliably estimated solely from WMAP data without assuming any template models for the foreground components. In the current paper, we extend this work to estimate the galactic foreground power spectrum using the WMAP 5 year maps following a self contained analysis. We apply the model independent method in harmonic basis to estimate the foreground power spectrum and frequency dependence of combined foregrounds. We also study the behaviour of synchrotron spectral index variation over diﬀerent regions of the sky. We compare our results with those obtained from MEM foreground maps which are formed in pixel space. We ﬁnd that relative to our model independent estimates MEM maps overestimates the foreground power close to galactic plane and underestimates it at high latitudes.
Metal-Enriched Plasma in Protogalactic Halos: A Survey of N V Absorption in High-z Damped & Sub-Damped Lyman-alpha Systems
(2009-01-01) Andrew, Fox; Prochaska, Jason X.; Ledoux, C´edric; et al.
We continue our recent work to characterize the plasma content of high-redshift damped and sub-damped Lyman-α systems (DLAs/sub-DLAs), which represent multi-phase gaseous (proto)galactic disks and halos seen toward a background source. We survey Nv absorption in a sample of 91 DLAs and 18 sub-DLAs in the redshift range 1.67−2.3. The Nv and Civ component b-value distributions in DLAs are statistically similar, but the median b(Nv) of 18 kms −1 is narrower than the median b(Ovi) of ≈25 kms −1 . Some ≈20% of the Nv components have b<10 kms −1 and thus arise in warm, photoionized plasma at log (T/K)<4.92; local sources of ionizing radiation (as opposed to the extragalactic background) are required to keep the cloud sizes physically reasonable. The nature of the remaining ≈80% of (broad) Nv components is unclear; models of radiatively-cooling collisionally-ionized plasma at log (T/K)=5.2–5.4 are fairly successful in reproducing the observed integrated high-ion column density ratios and the component line widths, but we cannot rule out photoionization by local sources. Finally, we identify several unusual DLAs with extremely low metallicity (<0.01 solar) but strong high-ion absorption [logN(Nv)>14 or logN(Ovi)>14.2], which present challenges to either galactic inﬂow or outﬂow models.
Why do we live in four dimension?
(2009-01-01) Dadhich, Naresh
We perceive the dimension of physical spacetime we live in through physical ex- periments and hence it is pertinent to probe the dimension in which the fundamental physical forces exist and act? In this context we shall investigate the two classical ﬁelds of gravitation and electromagnetism and argue that four dimension is necessary for spacetime but may not be suﬃcient. Some motivation for higher dimension would also be discussed.
Assumptions of the primordial spectrum and cosmological parameter estimation
(2009-01-01) Shafieloo, Arman; Souradeep, Tarun
The observables of the perturbed universe, CMB anisotropy and large structures, depend on a set of cosmological parameters, as well as, the assumed nature of primordial perturbations. In particular, the shape of the primordial power spectrum (PPS) is, at best, a well motivated assumption. It is known that the assumed functional form of the PPS in cosmological parameter estimation can aﬀect the best ﬁt parameters and their relative conﬁdence limits. In this letter, we demonstrate that a speciﬁc assumed form actually drives the best ﬁt parameters into distinct basins of likelihood in the space of cosmological parameters where the likelihood resists improvement via modiﬁcations to the PPS. The regions where considerably better likelihoods are obtained allowing free form PPS lie outside these basins. In the absence of a preferred model of inﬂation, this raises a concern that current cosmological parameters estimates are strongly prejudiced by the assumed form of PPS. Our results strongly motivate approaches toward simultaneous estimation of the cosmological parameters and the shape of the primordial spectrum from upcoming cosmological data. It is equally important for theorists to keep an open mind towards early universe scenarios that produce features in the PPS.
On distribution of CMEs speed in solar cycle 23
(2009-02-01) Mittal, Nishant; Sharma, Joginder; Tomar, Vivek; et al.
We have analyzed the data for more than 12900 CMEs which were obtained by SOHO/LASCO during the period of 1996-2007. The online CME catalogue contains all major CMEs detected by LASCO C2 and C3 coronagraphs. Basically we determine the CME speeds from the linear and quadratic fits to the height-time measurements. It is found that linear (constant speed) fit is preferable for 90% of the CMEs. The distribution of speeds of CMEs in solar cycle 23 is presented along with those obtained by others. As expected, the speeds decrease in the decay phase of the cycle 23. There is an unusual drop in speed in the year 2001 and an abnormal increase in speed in the year 2003 due to the high concentration of CMEs, X-class soft X-ray flares, solar energetic particle (SEP) events and interplanetary shocks observed during October-November period called Halloween events.
Diffuse bubble-like radio-halo emission in MRC 0116+111: Imprint of AGN feedback in a distant cluster of galaxies
(2009-02-01) Bagchi, Joydeep
We report the discovery of a luminous, mini radio halo of ∼240 kpc dimension at the center of a distant cluster of galaxies at redshift z = 0.131. Our optical and multi-wavelength GMRT and VLA observations reveal a highly unusual structure showing a twin bubble-like diﬀuse radio halo surrounding a cluster of bright elliptical galaxies; very similar to the large-scale radio structure of M87, the dominant galaxy in Virgo cluster. It presents an excellent opportunity to understand the energetics and the dynamical evolution of such radio jet inﬂated plasma bubbles in the hot cluster atmosphere.
Models of high redshift luminosity functions and galactic outflows: The dependence on halo mass function
(2009-02-01) Samui, Saumyadip; Subramanian, Kandaswamy; Srianand, R.
The form of the halo mass function is a basic ingredient in any semi-analytical galaxy formation model. We study the existing forms of the mass functions in the literature and compare their predictions for semi-analytical galaxy formation models. Two methods are used in the literature to compute the net formation rate of halos, one by simply taking the derivative of the halo mass function and the other using the prescription due to Sasaki (1994). For the historically used Press-Schechter (PS) mass function, we compare various model predictions, using these two methods. However, as the Sasaki formalism cannot be easily generalized for other mass functions, we use the derivative while comparing model predictions of di erent mass functions. We show that the reionization history and UV luminosity function of Lyman break galaxies (LBGs) predicted by the PS mass function di ers from those using any other existing mass function, like Sheth-Tormen (ST) mass func- tion. In particular the reionization e ciency of molecular cooled halos has to be substantially reduced when one uses the ST and other mass functions obtained from the simulation instead of the PS mass function. Using 2 - minimization, we nd that the observed UV luminosity functions of LBGs at 3:0 z 7:4 are better reproduced by models using the ST mass function compared to models that use the PS mass function. On the other hand, the volume lling factor of the metals expelled from the galaxies through super- novae driven out ows di ers very little between models with di erent mass functions. It depends on the way we treat merging out ows. We also show that the porosity weighted average quantities related to the out ow are not very sensitive to the di erences in the halo mass function
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).
On some properties of coronal mass ejections in solar cycle 23
(2009-03-01) Mittal, Nishant
We have investigated the some properties such as speed, apparent width, acceleration, latitude, mass and kinetic energy etc. of all types of CMEs observed during the period 1996-2007 by SOHO/LASCO covering the solar cycle 23. The results are in satisfactory agreement with previous investigations.
Studying the Imaging Characteristics of Ultra Violet Imaging Telescope (UVIT) through Numerical Simulations
(2009-03-01) Srivastava, Mudit K.; Prabhudesai, Swapnil M.; Tandon, S. N.
Ultra-Violet Imaging Telescope (UVIT) is one of the ﬁve payloads aboard the Indian Space Research Organization (ISRO)’s ASTROSAT space mission. The science objectives of UVIT are broad, extending from individual hot stars, star-forming regions to active galactic nuclei. Imaging performance of UVIT would depend on several factors in addition to the optics, e.g. resolution of the detectors, Satellite Drift and Jitter, image frame acquisition rate, sky background, source intensity etc. The use of intensiﬁed CMOS-imager based photon counting detectors in UVIT put their own complexity over reconstruction of the images. All these factors could lead to several systematic eﬀects in the reconstructed images.
CMB Polarization and Temperature Power Spectra Estimation using Linear Combination of WMAP 5-year Maps
(2009-03-01) Souradeep, Tarun; Jain, Pankaj; Saha, Rajib; et al.
We estimate CMB polarization and temperature power spectra using WMAP 5-year foreground contaminated maps. The power spectrum is estimated by using a model independent method, which does not utilize directly the diﬀuse foreground templates nor the detec tor noise model. The method essentially consists of two steps, (i) removal of diﬀuse foregrounds contamination by making linear combination of individual maps in harmonic space and (ii) cross-correlation of foreground cleaned maps to minimize detector noise bias. For temperature power spectrum we also estimate and subtract residual unre- solved point source contamination in the cross-power spectrum using the point source model provided by the WMAP science team. Our 1TT, TE and EE power spectra are in good agreement with the published results of the WMAP science team. The error bars on the polarization power spectra, however, turn out to be smaller in comparison to what is obtained by the WMAP science team. We perform detailed numerical simulations to test for bias in our procedure. We ﬁnd that the bias is small in all cases. A negative bias at low l in TT power spectrum has been pointed in an earlier publication. We ﬁnd that the bias corrected quadrupole power (l(l + 1)Cl/2π) is 532 µK2, approximately 2.5 times the estimate (213.4 µK2) made by the WMAP team.
Composite grains: Effects of porosity and inclusions on the 10 um silicate feature
(2009-03-01) Vaidya, D.B.; Gupta, Ranjan
We calculate the absorption eﬃciency of the composite grains, made up of host silicate spheroids and inclusions of ices/graphites/or voids, in the spectral region 7.0− 14.0µm The absorption eﬃciencies of the composite spheroidal grains for three axial ratios are computed using the discrete dipole approximation (DDA) as well as using the eﬀective medium approximation & T-Matrix (EMT-Tmatrix) approach. We study the absorption as a function of the volume fraction of the inclusions and porosity. In particular, we study the variation in the 10.0µm feature with the volume fraction of the inclusions and porosity. We then calculate the infrared ﬂuxes for these composite grains and compare the model curves with the average observed IRAS-LRS curve, obtained for several circumstellar dust shells around stars. These results on the composite grains show that the wavelength of the peak absorption shifts and the width of the 10.0µm feature varies with the variation in the volume fraction of the inclusions. The model curves for composite grains with axial ratios not very large (AR∼1.3) and volume fractions of inclusions with f=0.20, and dust temperature of about 250-300◦K , ﬁt the observed emission curves reasonably well.
Shear dynamo problem: Quasilinear kinematic theory
(2009-04-01) Sridhar, S.; Subramanian, Kandaswamy
Large–scale dynamo action due to turbulence in the presence of a linear shear ﬂow is studied. Our treatment is quasilinear and kinematic but is non perturbative in the shear strength. We derive the integro–differential equation for the evolution of the mean magnetic ﬁeld, by systematic use of the shearing coordinate transformation and the Galilean invariance of the linear shear ﬂow. For non helical turbulence the time evolution of the cross–shear components of the mean ﬁeld do not depend on any other components excepting themselves. This is valid for any Galilean–invariant velocity ﬁeld, independent of its dynamics. Hence the shear–current assisted dynamo is essentially absent, although large–scale non helical dynamo action is not ruled out.
Multi-wavelength observations of afterglow of GRB080319B and the modeling constraints
(2009-04-01) Pandey, S.B.; Castro-Tirado, A. J.; Jelinek, M.; et al.
Context. We present observations of the afterglow of GRB 080319B at optical, mm and radio frequencies from a few hours to 67 days after the burst. Aims. To understand the nature of this brightest explosion based on the observed properties and it’s comparison with the afterglow models. Methods. Present observations along with other published multi-wavelength data have been used to study the light-curves and spectral energy distributions of the burst afterglow. Results. Our results show that the observed features of the afterglow ﬁts equally good with the Inter Stellar Matter and the Stellar Wind density proﬁles of the circum-burst medium. In case of both density proﬁles, location of the maximum synchrotron frequency νm is below optical and the value of cooling break frequency νc is below X−rays, ∼ 104 s after the burst. Also, the derived value of the Lorentz factor at the time of naked eye brightness is ∼ 300 with the corresponding blast wave size of ∼ 1018 cm. Conclusions. The numerical ﬁt to the multi-wavelength afterglow data constraints the values of physical parameters and the emission mechanism of the burst.
Complete sample of 21-cm absorbers at z ~ 1.3: Giant Metrewave Radio Telescope Survey Using Mg II Systems
(2009-04-01) Gupta, N.; Srianand, R.; Petitjean, Patrick; et al.
We present the results of a systematic Giant Metrewave Radio Telescope (GMRT) survey of 21-cm absorption in a representative and unbiased sample of 35 strong Mg ii systems in the redshift range: zabs∼1.10−1.45, 33 of which have Wr ≥1˚ A. The survey using ∼400 hrs of telescope time has resulted in 9 new 21-cm detections and stringent 21-cm optical depth upper limits (median 3σ optical depth per 10 kms−1 of 0.017) for the remaining 26 systems. This is by far the largest number of 21-cm detections from any single survey of intervening absorbers. Prior to our survey no intervening 21-cm system was known in the above redshift range and only one system was known in the redshift range 0.7 ≤ z ≤ 1.5. We discuss the relation between the detectability of 21-cm absorption and various properties of UV absorption lines. We show that if Mg ii systems are selected with the following criteria, Mg ii doublet ratio ≤1.3 and Wr(Mg i)/Wr(Mg ii) ≥ 0.3, then a detection rate of 21-cm absorption up to 90% can be achieved. We estimate n21, the number per unit redshift of 21-cm absorbers with Wr(Mg ii) > Wo and integrated optical depth T21 > To and show that n21 decreases with increasing redshift. In particular, for Wo = 1.0 ˚ A and To > 0.3 km s−1, n21 falls by a factor 4 from < z > = 0.5 to < z > = 1.3. The evolution seems to be stronger for stronger Mg ii systems. Using a subsample of systems for which high frequency VLBA images are available, we show that the eﬀect is not related to the structure of the background radio sources and is most probably due to the evolution of the cold neutral medium ﬁlling factor in Mg ii systems. We ﬁnd no correlation between the velocity spread of the 21-cm absorption feature and Wr(Mg ii) at z ∼ 1.3
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.
Study of frequency and size distribution dependence of extinction for astronomical silicate and graphite grains
(2009-04-01) Roy, Ashim K.; Sharma, S. K.; Gupta, Ranjan
It is generally agreed that interstellar dust grains consist of two main components, namely, silicates and graphites. Some models, like MRN model, assume these grains to be homogeneous spheres following a power law size distribution. This paper presents, in the framework of Mie theory, a parametrization of extinction spectrum curves of the silicates and the graphites separately in terms of frequency and the minimum and maximum of sizes in the distribution. Analytic expressions in ultraviolet and farultraviolet are presented for both types of grains.The values of maximum and minimum sizes for which these equations are valid have been identiﬁed. These equations can be useful in a number of situations involving silicate and graphite grains
Primordial magnetic fields and formation of molecular hydrogen
(2009-04-01) Sethi, Shiv K.; Nath, B. B.; Subramanian, Kandaswamy; et al.
We study the implications of primordial magnetic ﬁelds for the thermal and ionization history of the post-recombination era. In particular we compute the eﬀects of dissi- pation of primordial magnetic ﬁelds owing to ambipolar diﬀusion and decaying tur- bulence in the intergalactic medium (IGM) and the collapsing halos and compute the eﬀects of the altered thermal and ionization history on the formation of molecular hy- drogen.We show that, for magnetic ﬁeld strengths in the range 2×10−10 G < ∼ B0 < ∼ 2× 10−9 G, the molecular hydrogen fraction in IGM and collapsing halo can increase by a factor 5 to 1000 over the case with no magnetic ﬁelds. We discuss the implication of the increased molecular hydrogen fraction on the radiative transfer of UV photons and the formation of ﬁrst structures in the universe