2001 (IPP)

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On some transonic aspects of general relativistic spherical accretion onto schwarzschild black holes
(2001-01-02) Das, Tapas K.
The equations governing general relativistic, spherically symmetric, hydrodynamic accretion of polytropic ﬂuid onto black holes are solved in Schwarzschild metric to investigate some of the transonic properties of the ﬂow. Only stationary solutions are discussed. For such accretion, it has been shown that real physical sonic points may form even for ﬂow with γ < 4 3 or γ > 5 3 . Behaviour of some ﬂow variables in the close vicinity of the event horizon are studied as a function of speciﬁc energy and polytropic index of the ﬂow.
Magnetic helicity in stellar dynamos : new numerical experiments
(2001-01-04) Axel, Brandenburg; Wolfgang, Dobler; Subramanian, Kandaswamy
The theory of large scale dynamos is reviewed with particular emphasis on the magnetic helicity constraint in the presence of closed and open boundaries. In the presence of closed or periodic boundaries, helical dynamos respond to the helicity constraint by developing small scale separation in the kinematic regime, and by showing long time scales in the nonlinear regime where the scale separation has grown to the maximum possible value. A resistively limited evolution towards saturation is also found at intermediate scales before the largest scale of the system is reached. Larger aspect ratios can give rise to different structures of the mean ﬁeld which are obtained at early times, but the ﬁnal saturation ﬁeld strength is still decreasing with decreasing resistivity. In the presence of shear, cyclic magnetic ﬁelds are found whose period is increasing with decreasing resistivity, but the saturation energy of the mean ﬁeld is in strong super-equipartition with the turbulent energy. It is shown that artiﬁcially induced losses of small scale ﬁeld of opposite sign of magnetic helicity as the large scale ﬁeld can, at least in principle, accelerate the production of large scale (poloidal) ﬁeld. Based on mean ﬁeld models with an outer potential ﬁeld boundary condition in spherical geometry, we verify that the sign of the magnetic helicity ﬂux from the large scale ﬁeld agrees with the sign of α. For solar parameters, typical magnetic helicity ﬂuxes lie around 1047 Mx2 per cycle.
Non-conformally flat bulk spacetime and the 3-brane world
(2001-01-05) Singh, Parampreet; Dadhich, Naresh
We show that for non-conformally ﬂat bulk spacetime, there exist no bound modes for zero mass graviton on the 3-brane. The brane world model is therefore unstable for the bulk spacetime being diﬀerent from the conformally ﬂat anti - de Sitter space.
BeppoSAX observation of the rich cluster of galaxies Abell 85
(2001-01-06) Lima Neto, G. B.; Pislar, V.; Bagchi, Joydeep
We report the observation of the Intra-Cluster Medium (ICM) of Abell 85 by the X-ray satellite Bep- poSAX. We have both analysed the spectrum obtained in the central 8 arcmin circular region centred on the Very Steep Spectrum Radio Source (VSSRS) and the spec- tra from a number of sub-regions. Analysis of the spec- tra allowed us to independently obtain new estimates of the temperature, metallicity and line-of-sight hydro- gen density column, both globally (T = 6.6 ± 0.3 keV, Z = 0.38 ± 0.06Z⊙ and NH = 5.5+0.9 −0.71020cm−2) and lo- cally. These measures are in good agreement with previ- ous measures based on ROSAT and ASCA data. In the region of the VSRSS, we have tried to disentangle the thermal from the non-thermal X-ray emission. Although we could not do this unambiguously, we have nonethe- less estimated the extended magnetic ﬁeld using the radio spectrum available for this region.We obtain a lower limit intensity of 0.9µG, consistent with our previous estimate. We also derive α-elements/iron abundance ratios that turn out to be higher than 1. Such a result tends to support the burst model for elliptical galaxies, where a strong galac- tic wind develops early in the galaxy history and type II supernovae (SN) may have the main role in the enrich- ment of the ICM. A two-temperature ICM model was ﬁt- ted in the central region yielding a main component with roughly the mean cluster temperature and a cooler com- ponent with temperature less than 0.1 keV.
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.
On the mass of a uniform density star in higher dimensions
(2001-01-14) Paul, B. C.
Within the framework of higher dimensions the mass of a uniform density star is evaluated. The four-dimensional upper bound for the mass-to-radius ratio obtained by Schwarzschild is generalized within the framework of higher- dimensional spacetime. It is found that the analogue upper bound for the mass- to-radius ratio in higher dimensions tends to increase at ﬁrst as the number of dimensions of spacetime increases, it attains amaximumat nine dimensions and thereafter decreases. It is found that D = 4 is the lowest number of spacetime dimensions for which the mass-to-radius ratio of a uniform density star can be derived.
How to distinguish a nearly flat Universe from a flat Universe using the orientation independence of a comoving standard ruler
(2001-01-20) Roukema, B. F.
Several recent observations using standard rulers and standard candles now suggest, either individually or in combination, that the Universe is close to ﬂat, i.e. that the curvature radius is about as large as the horizon radius (∼ 10h−1 Gpc) or larger. Here, a method of distinguishing an almost ﬂat universe from a precisely ﬂat universe using a single observational data set, without using any microwave background information, is presented. The method (i) assumes that a standard ruler should have no preferred orientation (radial versus tangential) to the observer, and (ii) requires that the (comoving) length of the standard ruler be known independently (e.g. from low redshift estimates). The claimed feature at ﬁxed comoving length in the power spectrum of density perturbations, detected among quasars, Lyman break galaxies or other high redshift objects, would provide an adequate standard candle to prove that the Universe is curved, if indeed it is curved. For example, a combined intrinsic and measurement uncertainty of 1% in the length of the standard ruler L applied at a redshift of z = 3 would distinguish an hyperbolic (Ωm = 0.2,ΩΛ = 0.7) or a spherical (Ωm = 0.4, ΩΛ = 0.7) universe from a ﬂat one to 1 − P > 95% conﬁdence
Heat flow in general relativity
(2001-02) Banerjee, A.
A few models of bounded sphere containing heat flux in the radial direction are constructed. These models satisfy the conditions of fit at their boundaries with vaidya's radiating metric. further the casual heat transport equation is considered in the background of one of the above metrics and it is found to be consistent with the inflationary expansion. Our case is more general than that discussed previously by maartens et al.
Near-solar metallicity damped lyman-alpha system toward the BAL quasar Tol 1037 - 2703
(2001-02-01) Srianand, R.; Petitjean, Patrick
We report the detection of a Broad Absorption Line (BAL) outﬂow in the spectrum of the zem (Mg ii) = 2.201 QSO Tol 1037−2703 with three main BALs at 36000, 25300 and 22300 km s −1 outﬂow velocities. Although the overall ﬂow is dominated by high ionization lines like N v and C iv , the gas of highest velocity shows absorption from Mg i, Mg ii and Fe ii. Covering factor arguments suggest that the absorbing complexes are physically associated with the QSO and have transverse dimensions smaller than that of the UV continuum emitting region (r < 0.1 pc). We show that the C iv absorption at zabs = 2.082 has a covering factor fc ∼ 0.86 and the absorption proﬁle has varied over the last four years. The detection of absorption from excited ﬁne structure levels of C ii and Si ii in narrow components embedded in the C iv trough reveals large density inhomogeneities. IR pumping is the most likely excitation process. The zabs = 2.139 system is a moderately damped Lyman-α system with log N(H i) ∼ 19.7. The weakness of the metal lines together with the high quality of the data make the metallicity measurements particularly reliable. The absolute metallicity is close to solar with [Zn/H] = −0.26. The α-chain elements have metallicities consistently solar (respectively +0.05, −0.02, −0.03 and −0.15 for [Mg/H], [Si/H], [P/H] and [S/H]) and iron peak elements are depleted by a factor of about two ([Fe/Zn], [Cr/Zn], [Mn/Zn] and [Ni/Zn] are equal to −0.39, −0.27, −0.49, −0.30). Lines from C i are detected but H2 is absent with a molecular to neutral hydrogen fraction less than 8×10−6 . From the ionization state of the gas, we argue that the system is situated ∼few Mpc away from the QSO. High metallicity and low nitrogen abundance, [N/Zn] = −1.40, favor the idea that metals have been released by massive stars during a starburst of less than 0.5 Gyr of age. Using the upper limit on the C i ∗ column density in two components, we obtain upper limits on the background temperature of 16.2 and 13.2 K respectively.
Near-infrared [Fe II] emission from supernova remnants and the supernova rate of starburst galaxies
(2001-02-01) Morel, T.; Doyon, R.; St-Louis, N.
In an eﬀort to better calibrate the supernova rate of starburst galaxies as deter- mined from near-infrared [Fe II] features, we report on a [Fe II] λ1.644 µm line-imaging survey of a sample of 42 optically-selected supernova remnants (SNRs) in M33. A wide range of [Fe II] luminosities are observed within our sample (from less than 6 to 695 L⊙). Our data suggest that the bright [Fe II] SNRs are entering the radiative phase and that the density of the local interstellar medium (ISM) largely controls the amount of [Fe II] emission. We derive the following relation between the [Fe II] λ1.644 µm line luminosity of radiative SNRs and the electronic density of the postshock gas, ne: L[Fe II] (L⊙) ≈ 1.1 ne (cm−3). We also ﬁnd a correlation in our data between L[Fe II] and the metallicity of the shock-heated gas, but the physical interpretation of this re- sult remains inconclusive, as our data also show a correlation between the metallicity and ne. The dramatically higher level of [Fe II] emission from SNRs in the central regions of starburst galaxies is most likely due to their dense environments, although metallicity eﬀects might also be important. The typical [Fe II]-emitting lifetime of a SNR in the central regions of starburst galaxies is found to be of the order of 104 yr. On the basis of these results, we provide a new empirical relation allowing the de- termination of the current supernova rate of starburst galaxies from their integrated near-infrared [Fe II] luminosity.
Inflation with oscillations
(2001-02-02) Sami, M.
In this paper we investigate the general features of ”Oscillatory Inﬂation”. In adiabatic approxi- mation , we derive a general formula for the number of e-foldings ˜ N which reduces to the standard expression in case of the slow role approximation and leads to the Damour-Mukhanov type expres- sion for the slowly varying adiabatic index. We apply our result to the logarithmic potential and arrive at a simple and elegant formula for the number of e-foldings.We evolve the ﬁeld equations numerically and observe a remarkable agreement with the analytical result.
Photon propagation in a magnetized medium
(2001-02-03) Konar, Sushan
Using the real time formalism of the ﬁnite temperature ﬁeld theory we calculate the 1-loop polarization tensor in the presence of a background magnetic ﬁeld in a medium. The expression is obtained to linear order in the background ﬁeld strength. We discuss the Faraday rotation as well as the photon absorption probabilities in this context.
Cosmic inventory of energy densities : issues and concerns
(2001-02-10) Padmanabhan, T.
The dynamics of our universe is characterised by the density parameters for cos- mological constant (ΩV ), nonbaryonic darkmatter(Ωwimp), radiation (ΩR) and baryons (ΩB). To these parameters — which describe the smooth background universe — one needs to add at least another dimensionless number (∼ 10−5) characterising the strength of primordial ﬂuctuations in the gravitational potential, in order to ensure formation of structures by gravi- tational instability. I discuss several issues related to the description of the universe in terms of these numbers and argue that we do not yet have a fundamental understanding of these issues
Anisotropy dissipation in brane-world inflation
(2001-02-14) Maartens, Roy; Sahni, Varun; Saini, Tarun Deep
We examine the behavior of an anisotropic brane-world in the presence of inﬂationary scalar ﬁelds. We show that, contrary to naive expectations, a large anisotropy does not adversely aﬀect inﬂation. On the contrary, a large initial anisotropy introduces more damping into the scalar ﬁeld equation of motion, resulting in greater inﬂation. The rapid decay of anisotropy in the brane-world signiﬁcantly increases the class of initial conditions from which the observed universe could have originated. This generalizes a similar result in general relativity. A unique feature of Bianchi I brane-world cosmology appears to be that for scalar ﬁelds with a large kinetic term the initial expansion of the universe is quasi-isotropic. The universe grows more anisotropic during an intermediate transient regime until anisotropy ﬁnally disappears during inﬂationary expansion.
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.
Consequences on variable /\ - models from distant type Ia supernovae and compact radio sources
(2001-02-20) Vishwakarma, R. G.
We study the magnitude-redshift relation for the Type Ia super- novae data and the angular size-redshift relation for the updated compact radio sources data (from Gurvits et al) by considering four variable Λ-models: Λ ∼ S−2 , Λ ∼ H2 , Λ ∼ ρ and Λ ∼ t −2 . It is found that all the variable Λ-models, as well as the constant Λ- Friedmann model, ﬁt the supernovae data equally well with χ2 /dof ≈ 1 and require non-zero, positive values of Λ and an accelerating expansion of the universe. The estimates of the density parameter for the variable Λ-models are found higher than those for the constant Λ-Friedmann model. From the compact radio sources data, it is found, by assuming the no- evolution hypothesis, that the Gurvits et al’ model (Friedmann model with Λ = 0) is not the best-ﬁtting model for the constant Λ case. The best-ﬁtting Friedmann model (with constant Λ) is found to be a low density, vacuum- dominated accelerating universe. The ﬁts of this data set to the (variable, as well as, constant Λ-) models are found very good with χ2 /dof ≈ 0.5 and require non-zero, positive values of Λ with either sign of the deceleration parameter. However, for realistic values of the matter density parameter, the only interesting solutions are (a) estimated from the supernovae data: the best-ﬁt solutions for the ﬂat models (including the constant Λ case); (b) estimated from the radio sources data: the global best-ﬁt solutions for the models Λ ∼ H2 and Λ ∼ ρ, the best-ﬁt solution for the ﬂat model with Λ = constant and the Gurvits et al’ model. It is noted that, as in the case of recent CMB analyses, the data sets seem to favour a spherical universe (k > 0).
How to avoid the ambiguity in applying the copernican principle for cosmic topology : Take the observational approach
(2001-03-01) Roukema, B. F.
It is often stated that homogeneity and isotropy of the Universe are assumptions of the almost Friedmann- Lemaˆıtre (FL) model (the hot big bang model), inspired from the Copernican Principle. However, only local homogeneity and isotropy are required by the model: multiply connected almost FL models are locally homogeneous and isotropic, but they can be globally anisotropic and/or globally inhomogeneous. Toy models are used here to show how global anisotropy and/or global inhomogeneity of an almost FL model could be shown directly in observations. This approach may avoid having to make any assumptions regarding global anisotropy and inhomogeneity.
Cosmic Microwave Background Anisotropy Measurement from Python V
(2001-03-01) Coble, Kim; Dodelson, S.; Dragovan, Mark; et al.
We analyze observations of the microwave sky made with the Python exper- iment in its ﬁfth year of operation at the Amundsen-Scott South Pole Station in Antarctica. After modeling the noise and constructing a map, we extract the cosmic signal from the data. We simultaneously estimate the angular power spectrum in eight bands ranging from large (ℓ ∼ 40) to small (ℓ ∼ 260) angular scales, with power detected in the ﬁrst six bands. There is a signiﬁcant rise in the power spectrum from large to smaller (ℓ ∼ 200) scales, consistent with that ex- pected from acoustic oscillations in the early Universe. We compare this Python V map to a map made from data taken in the third year of Python. Python III observations were made at a frequency of 90 GHz and covered a subset of the region of the sky covered by Python V observations, which were made at 40 GHz. Good agreement is obtained both visually (with a ﬁltered version of the map) and via a likelihood ratio test.
Pseudo-Schwarzschild Description of Transonic Spherical Accretion onto Compact Objects
(2001-03-02) Das, Tapas K.
A number of ‘modiﬁed’ Newtonian potentials of various forms are available in the literature which ac- curately approximate some general relativistic eﬀects important for studying accretion discs around a Schwarzschild black hole. Such potentials may be called ‘pseudo-Schwarzschild’ potentials because they nicely mimic the space-time around a non-rotating/slowly rotating compact object. In this paper, we examine the validity of the application of some of these potentials to study the spherically symmetric, transonic, hydrodynamic accretion onto a Schwarzschild black hole. By comparing the values of various dynamical and thermodynamic accretion parameters obtained for ﬂows using these potentials with full general relativistic calculations, we have shown that though the potentials discussed in this paper were originally proposed to mimic the relativistic eﬀects manifested in disc accretion, it is quite reasonable to use most of the potentials in studying various dynamical as well as thermodynamic quantities for spherical accre- tion to compromise between the ease of handling of a Newtonian description of gravity and the realistic situations described by complicated general relativistic calculations. Also we have shown that depending on the chosen regions of parameter space spanned by speciﬁc energy E and adiabatic index γ of the ﬂow, one potential may have more importance than another and we could identify which potential is the best approximation for full general relativistic ﬂow in Scwarzschild space-time for particular values of E and γ.
One parameter solution of spherically symmetric accretion in various pseudo-schwarzschild potentials
(2001-03-02) Sarkar, Aveek; Das, Tapas K.
In this paperwe have solved the hydrodynamic equations governing the spher- ically symmetric isothermal accretion (wind) onto (away from) compact ob- jects using various pseudo-Schwarzschild potentials.These solutions are es- sentially one parameter solutions in a sense that all relevant dynamical as well as thermodynamic quantities for such a ﬂow could be obtained (with the as- sumption of a one-temperature ﬂuid) if only one ﬂow parameter (temperature of the ﬂow T) is given. Also we have investigated the transonic behaviour of such a ﬂow and showed that for a given T, transitions from subsonic to the supersonic branch of accretion (wind) takes place at different locations de- pending on the potentials used to study the ﬂow and we have identiﬁed these transition zones for ﬂows in various such potentials.