2008 (IPP)

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APSIS - an Artificial Planetary System in Space to probe extra-dimensional gravity and MOND
(2008-01) Sahni, Varun; Shtanov, Yuri
A proposal is made to test Newton’s inverse-square law using the perihelion shift of test masses (planets) in free fall within a spacecraft located at the Earth–Sun L2 point. Such an Artiﬁcial Planetary System In Space (APSIS) will operate in a dragfree environment with controlled experimental conditions and minimal interference from terrestrial sources of contamination. We demonstrate that such a space experiment can probe the presence of a ‘hidden’ ﬁfth dimension on the scale of a micron, if the perihelion shift of a ‘planet’ can be measured to sub-arc-second accuracy. Some suggestions for spacecraft design are made.
Automated classification of sloan digital sky survey (SDSS) stellar spectra using artificial neural networks
(2008-04) Bazarghan, M.; Gupta, Ranjan
Automated techniques have been developed to automate the process of classiﬁcation of objects or their analysis. The large datasets provided by upcoming spectroscopic surveys with dedicated telescopes urges scientists to use these automated techniques for analysis of such large datasets which are now available to the community. Sloan Digital Sky Survey (SDSS) is one of such surveys releasing massive datasets. We use Probabilistic Neural Network (PNN) for automatic classiﬁcation of about 5000 SDSS spectra into 158 spectral type of a reference library ranging from O type to M type stars.
Complex light-curve of the afterglow of GRB071010A
(2008-04) Covino, S.; Avanzo, P. D.
We present and discuss the results of an extensive observational campaign devoted to GRB071010A, a long-duration gamma-ray burst detected by the Swift satellite. This event was followed for almost a month in the optical/near-infrared (NIR) with various telescopes starting from about 2min after the high-energy event. Swift -XRT observations started only later at about 0.4 d. The light-curve evolution allows us to single out an initial rising phase with a maximum at about 7min, possibly the afterglow onset in the context of the standard reball model, which is then followed by a smooth decay interrupted by a sharp rebrightening at about 0.6 d. The rebrightening was visible in both the optical/NIR and X-rays and can be interpreted as an episode of discrete energy injection, although various alternatives are possible. A steepening of the afterglow light curve is recorded at about 1 d. The entire evolution of the optical/NIR afterglow is consistent with being achromatic.
Constrained semi-analytical models of Galactic outflows
(2008-01) Samui, Saumyadip; Subramanian, Kandaswamy; Srianand, R.
We present semi-analytic models of galactic outﬂows that are constrained by available ob- servations on high redshift star formation and reionization. Galactic outﬂows are modeled in a manner akin to models of stellar wind blown bubbles. Large scale outﬂows can generically escape from low mass halos (M . 109 M ) for a wide range of model parameters while this is not the case in high mass halos (M & 1011 M ). The ﬂow generically accelerates within the halo virial radius, then starts to decelerate, and traverses well into the intergalactic medium (IGM), before freezing to the Hubble ﬂow. The acceleration phase can result in shell fragmentation due to the Rayleigh-Taylor instability, although the ﬁnal outﬂow radius is not signiﬁcantly altered. The gas phase metallicity of the outﬂow and within the galaxy are computed assuming uniform instantaneous mixing. Ionization states of different metal species are calculated and used to examine the detectability of metal lines from the outﬂows. The global inﬂuence of galactic outﬂows is also investigated using porosity weighted averages and probability density functions of various physical quantities. Models with only atomic cooled halos signiﬁcantly ﬁll the IGM at z 3 with metals (with 2:5 & [Z=Z ] & 3:7), the actual extent depending on the efﬁciency of winds, the initial mass function (IMF) and the fractional mass that goes through star formation. The reionization history has a signiﬁcant effect on the volume ﬁlling factor, due to radiative feedback. In these models, a large fraction of outﬂows at z 3 are supersonic, hot (T 105K) and have low density, making metal lines difﬁcult to detect. They may also result in signiﬁcant perturbations in the IGM gas on scales probed by the Lyman- forest. On the contrary, models including molecular cooled halos with a normal mode of star formation can potentially volume ﬁll the universe at z 8 without drastic dynamic effects on the IGM, thereby setting up a possible metallicity ﬂoor ( 4:0 [Z=Z ] 3:6). The order unity ﬂuctuations at z 8 that becomes the mildly non-linear ﬂuctuations traced by Lyman- forest at z < 4 will then have this metallicity. Interestingly, molecular cooled halos with a “top-heavy” mode of star formation are not very successful in establishing the metallicity ﬂoor because of the additional radiative feedback, that they induce.
Cosmic acceleration and extra dimensions
(2008-11) Sahni, Varun; Shtanov, Yuri
Brane cosmology presents many interesting possibilities including: phantom acceleration (w < −1), self-acceleration, uniﬁcation of dark energy with inﬂation, transient acceleration, loitering cosmology, new singularities at which the Hubble parameter remains ﬁnite, cosmicmimicry, etc. The existence of a time-like extra dimension can result in a singularity-free cyclic cosmology
Detailed study of the GRB 030329 radio afterglow deep into the non-relativistic phase
(2008-01) Horst, A. J. van der; Kamble, A.; Resmi, L.; et al.
Radio afterglows ever, GRB 030329, at late times when the jet is non- relativistic. Aims. We determine the physical parameters of the blast wave and its surroundings, in particular the index of the electron energy distribution, the energy of the blast wave, and the density (structure) of the circumburst medium. We then compare our results with those from image size measurements. Methods. We observed the GRB030329 radio afterglow with the Westerbork Synthesis Radio Telescope and the Giant Metrewave Radio Telescope at frequencies from 325 MHz to 8.4 GHz, spanning a time range of 268-1128 days after the burst. We modeled all the available radio data and derived the physical parameters. Results. The index of the electron energy distribution is p = 2.1, the circumburst medium is homogeneous, and the transition to the non-relativistic phase happens at tNR ∼ 80 days. The energy of the blast wave and density of the surrounding medium are comparable to previous ﬁndings. Conclusions. Our ﬁndings indicate that the blast wave is roughly spherical at tNR, and they agree with the implications from the VLBI studies of image size evolution. It is not clear from the presented dataset whether we have seen emission from the counter jet or not. We predict that the Low Frequency Array will be able to observe the afterglow of GRB030329 and many other radio afterglows, constraining the physics of the blast wave during its non-relativistic phase even further
Detection of the 2175\AA~ extinction feature and 21-cm absorption in two MgII systems at z~1.3
(2008-09) Srianand, R.
We have discovered two dusty intervening Mg ii absorption systems at z ∼ 1.3 in the Sloan Digital Sky Survey (SDSS) database. The overall spectra of both QSOs are red (u-K>4.5 mag) and are well modelled by the composite QSO spectrum reddened by the extinction curve from the Large Magellanic Cloud(LMC2) Supershell redshifted to the rest-frame of the Mg ii systems. In particular, we detect clearly the presence of the UV extinction bump at λrest ∼ 2175 ˚ A. Absorption lines of weak transitions like Si iiλ1808, Cr iiλ2056, Cr ii+Zn iiλ2062, Mn iiλ2594, Ca iiλ3934 and Ti iiλ1910 from these systems are detected even in the low signal-to-noise ratio and low resolution SDSS spectra, suggesting high column densities of these species. The depletion pattern inferred from these absorption lines is consistent with that seen in the cold neutral medium of the LMC. Using the LMC AV vs. N(H i) relationship we derive N(H i)∼ 6×1021 cm−2 in both systems. Metallicities are close to solar. Giant Metrewave Radio Telescope (GMRT) observations of these two relatively weak radio loud QSOs (fν ∼ 50 mJy) resulted in the detection of 21-cm absorption in both cases.We show that the spin temperature of the gas is of the order of or smaller than 500 K. These systems provide a unique opportunity to search for molecules and diﬀuse interstellar bands at z > 1.
Dielectronic recombination and stability of warm gas in active galactic nuclei
(2008-09) Chakravorty, Susmita; Kembhavi, A.K.
Understanding the thermal equilibrium (stability) curve may offer insights into the nature of the warm absorbers often found in active galactic nuclei. Its shape is determined by factors such as the spectrum of the ionizing continuum and the chemical composition of the gas. We ﬁnd that the stability curves obtained under the same set of the above-mentioned physical factors, but using recently derived dielectronic recombination rates, give signiﬁcantly different results, especially in the regions corresponding towarmabsorbers, leading to different physical predictions. Using the current rates we ﬁnd a larger probability of having a thermally stable warmabsorber at 105 Kthan previous predictions and also a greater possibility for itsmultiphase nature. The results obtained with the current dielectronic recombination rate coefﬁcients are more reliable because the warm absorber models along the stability curve have computed coefﬁcient values, whereas previous calculations relied on guessed averages for these because of a lack of available data.
Double inflation and the low CMB multipoles
(2008-09) Souradeep, Tarun
We investigate inﬂationary scenarios driven by a class of potentials which are similar in form to those that arise in certain minimal supersymmetric extensions of the standard model. We show that these potentials allow a period of fast roll sandwiched between two stages of slow roll inﬂation. We ﬁnd that the modes which exit the Hubble radius during the period of fast roll have lower power when comparedto the amplitude of the nearly scale invariant spectrum associated with the modes that leave during the second stage of slow roll inﬂation. We set the scales such that the drop in the scalar power spectrum occurs at a length scale that corresponds to the Hubble radius today—a feature that seems necessary to explain the lower power observed in the quadrupole moment of the Cosmic Microwave Background (CMB) anisotropies. We perform a Markov Chain Monte Carlo analysis to determine the values of the model parameters that provide the best ﬁt to the recent WMAP 5-year data for the CMB angular power spectrum. We ﬁnd that an inﬂationary spectrum with a suppression of power at large scales that we obtain leads to a much better ﬁt (with just one extra parameter, χ2 eﬀ improves by 6.62) of the observed data when compared to the best ﬁt reference ΛCDM model with a featureless, power law, primordial spectrum.
First detection of CO in a high-redshift damped Lyman - Alpha system
(2008-04) Srianand, R.
We present the ﬁrst detection of carbon monoxide (CO) in a damped Lyman-α system (DLA) at zabs =2.41837 toward SDSS J143912.04+111740.5. We also detected H2 and HD molecules. The measured total column densities (in log units) of H i, H2, and CO are 20.10±0.10, 19.38±0.10, and 13.89±0.02, respectively. The molecular fraction, f = 2N(H2)/(N(H i)+2N(H2)) = 0.27+0.10 −0.08, is the highest among all known DLAs. The abundances relative to solar of S, Zn, Si, and Fe are −0.03±0.12, +0.16±0.11, −0.86±0.11, and −1.32±0.11, respectively, indicating a high metal enrichment and a depletion pattern onto dust-grains similar to the cold ISM of our Galaxy. The measured N(CO)/N(H2) = 3×10−6 is much less than the conventional CO/H2 ratio used to convert the CO emission into gaseous mass but is consistent with what is measured along translucent sightlines in the Galaxy. The CO rotational excitation temperatures are higher than those measured in our Galactic ISM for similar kinetic temperature and density. Using the C i ﬁne structure absorption lines, we show that this is a consequence of the excitation being dominated by radiative pumping by the cosmic microwave background radiation (CMBR). From the CO excitation temperatures, we derive TCMBR = 9.15±0.72 K, while 9.315±0.007 K is expected from the hot big-bang theory. This is the most precise high-redshift measurement of TCMBR and the ﬁrst conﬁrmation of the theory using molecular transitions at high redshift.
Galactic dynamo action in presence of stochastic alpha and shear
(2008-10) Sur, Sharanya; Subramanian, Kandaswamy
Using a one-dimensional αω-dynamo model appropriate to galaxies, we study the possibility of dynamo action driven by a stochastic alpha effect and shear. To determine the ﬁeld evolution, one needs to examine a large number of different realizations of the stochastic component of α. The net growth or decay of the ﬁeld depends not only on the dynamo parameters but also on the particular realization, the correlation time of the stochastic α compared to turbulent diffusion timescale and the time over which the system is evolved. For dynamos where both a coherent and ﬂuctuating α are present, the stochasticity of α can help alleviate catastrophic dynamo quenching, even in the absence of helicity ﬂuxes. One can obtain ﬁnal ﬁeld strengths up to a fraction ∼ 0.01 of the equipartition ﬁeld Beq for dynamo numbers |D| ∼ 40, while ﬁelds comparable to Beq require much larger degree of α ﬂuctuations or shear. This type of dynamomay be particularly useful for amplifying ﬁelds in the central regions of disk galaxies.
General relativistic treatment of LISA optical links
(2008-05) Dhurandhar, Sanjeev; Vinet, J-Y.; Nayak, K. R.
LISA is a joint spae mission of the NASA and the ESA for deteting low frequeny gravitational waves in the band 10−5 − 1 Hz. In order to attain the requisite sensitivity for LISA, the laser frequeny noise must be suppressed below the other seondary noises suh as the optial path noise, a eleration noise et. This is ahieved by ombining time-delayed data for whih preise knowledge of time-delays is required. The gravitational eld, mainly that of the Sun and the motion of LISA a e t the time-delays and the opti al links. Further, the e e t of the gravitational eld of the Earth on the orbits of spaeraft is inluded. This leads to additional exing over and above that of the Sun. We have written anumerial ode whih omputes the optial links, that is, the time-delays with great auray∼ 10−2 metres - more than what is required for time delay interferometry (TDI) - for most of the orbit and with su ient auray within ∼ 10 metres for an integrated time window of about six days, when one of the arms tends to be tangent to the orbit. Our analysis of the optial links is fully general relativisti and the numerial ode takes into aount e ets suh as the Sagna, Shapiro delay, et. We show that with the deemed parameters in the design of LISA, there are symmetries inherent in the on guration of LISA and in the physi s, whih may be used e etively to suppress the residual laser noise in the modi ed rst generation TDI. We demonstrate our results for some important TDI variables.
Giant Radio Jet Ejected by An Ultramassive Black Hole in a Single-Lobed Radio Galaxy
(2008-01) Bagchi, Joydeep
We report the discovery of a very unusual, highly asymmetric radio galaxy whose radio jet, the largest yet detected, emits strongly polarized synchrotron radiation and can be traced all the way from the galactic nucleus to the hot spot located ∼ 440 kpc away. This jet emanates from an extremely massive black-hole (> 109 M⊙) and forms a strikingly compact radio lobe. No radio lobe is detected on the side of the counter-jet, even though it is similar to the main jet in brightness up to a scale of tens of kilo parsecs. Thus, contrary to the nearly universal trend, the brightness asymmetry in this radio galaxy increases with distance from the nucleus. With several unusual properties, including a predominantly toroidal magnetic ﬁeld, this Fanaroﬀ-Riley type II mega-jet is an exceptionally useful laboratory for testing the role of magnetic ﬁeld in jet stabilization and radio lobe formation.
Hard Electron Energy Distribution in the Relativistic Shocks of GRB Afterglows
(2008-04) Resmi, L.; Bhattacharya, Dipankar
Particle acceleration in relativistic shocks is not a very well understood subject. Owing to that diﬃculty, radiation spectra from relativistic shocks, such as those in GRB afterglows, have been often modelled by making assumptions about the underlying electron distribution. One such assumption is a relatively soft distribution of the particle energy, which need not be true always, as is obvious from observations of several GRB afterglows. In this paper, we describe modiﬁcations to the afterglow standard model to accommodate energy spectra which are ‘hard’. We calculate the overall evolution of the synchrotron and compton ﬂux arising from such a distribution. We also model two afterglows, GRB010222 and GRB020813, under this assumption and estimate the physical parameters.
HD molecules at high redshift: A low astration factor of deuterium in a solar-metallicity DLA system at z=2.418
(2008-08) Noterdaeme, P.; Srianand, R.
We present the detection of deuterated molecular hydrogen (HD) in the remote Universe in a damped Lyman-α cloud at zabs = 2.418 toward the quasar SDSS J143912.04+111740.5. This is a unique system in which H2 and CO molecules are also detected. The chemical enrichment of this gas derived from Zn ii and S ii is as high as in the Sun. We measure N(HD)/2N(H2) = 1.5+0.6 −0.4 × 10−5, which is signiﬁcantly higher than the same ratio measured in the Galaxy and close to the primordial D/H ratio estimated from the WMAP constraint on the baryonic matter density (Ωb). This indicates a low astration factor of deuterium that contrasts with the unusually high chemical enrichment of the gas. This can be interpreted as the consequence of an intense infall of primordial gas onto the associated galaxy. Detection of HD molecules at high-z also opens the possibility to obtain an independent constraint on the cosmological-time variability of , the proton-to-electron mass ratio.
Kinematic alpha effect in isotropic turbulence simulations
(2008-01) Sur, Sharanya; Brandenburg, Axel; Subramanian, Kandaswamy
Using numerical simulations at moderate magnetic Reynolds numbers up to 220 it is shown that in the kinematic regime, isotropic helical turbulence leads to an alpha effect and a turbulent diffusivitywhose values are independent of the magnetic Reynolds number,Rm, provided Rm exceeds unity. These turbulent coefﬁcients are also consistent with expectations from the ﬁrst order smoothing approximation. For small values of Rm, alpha and turbulent diffusivity are proportional to Rm. Over ﬁnite time intervals meaningful values of alpha and turbulent diffusivity can be obtained even when there is small-scale dynamo action that produces strong magnetic ﬂuctuations. This suggests that small-scale dynamo-generated ﬁelds do not make a correlated contribution to the mean electromotive force.
MHD modes of solar wind flow tubes
(2008-02) Chandra, Suresh
Nakariakov & Roberts (1995) and Nakariakov et al. (1996) investigated the linear mag- netosonic waves trapped within solar wind ow tubes by accounting for a slab having boundaries at x = d and extended up to in nity in y and z directions. They obtained dispersion relations for sausage and kink surface waves in incompressible plasma. Following the approach of Nakariakov & Roberts (1995), we have obtained dispersion relations for sausage and kink surface waves in a compressible plasma. Values of the parameter a !=kCAo are found to vary in the ranges 0.755 0.849 and 1.080 1.356 for the sausage waves and in the ranges 0.850 0.882 and 1.358 1.538 for the kink waves in the compressible plasma. It shows an interesting feature that the sausage and kink surface waves are exclusive from each other.
Modeling the interstellar aromatic infrared bands with co-added spectra of PAHs
(2008-04) Pathak, Amit; Rastogi, Shantanu
Aims. The observed variations in pro les of the interstellar aromatic infrared bands correlate with the object type and are indicative of PAH populations existing in di erent sources. Spectroscopic studies of PAHs can provide tools for interpreting the variations accompanying the AIBs. As the observed spectra results from a mix of possible species in the region, we try to model this composite spectra by co-adding emissions from PAHs in di erent size groups. Methods. Theoretical IR data of PAHs having 10 to 96 carbon atoms is used to obtain emission spectra. The models are taken in size groups made up of small, medium, and large PAHs. Results. The models show a good pro le match with observations for the 7.7 m complex having sub-features at 7.6 and 7.8 m. The 7.6 m sub-feature dominates in the spectra of medium-sized PAH cations, matching observations from UV-rich interstellar environments. The 7.8 m component is more intense in the spectra of large PAH cations (model III), correlating with observations from benign astrophysical regions. A possible interpretation of the observations of C H out-of-plane bend modes and the weak outliers on the blue side of the intense 11.2 m band is proposed. The models provide pointers to possible PAH populations in di erent regions
Modelling interstellar extinction and polarization with spheroidal grains
(2008-01) Voshchinnikov, N. V.; Das, H.K.
We calculate the wavelength dependence of the ratio of the linear polarization degree to extinction (polarizing eﬃciency) P(λ)/A(λ) from the ultraviolet to ear-infrared. The prolate and oblate particles with aspect ratios from a/b = 1.1 up to 10 are assumed to be rotating and partially aligned with the mechanism of paramagnetic relaxation (Davis-Greenstein). Size/shape/orientation eﬀects are analyzed. It is found that the wavelength dependence of P(λ)/A(λ) is mainly determined by the particle composition and size whereas the values of P(λ)/A(λ) depend on the particle shape, degree and direction of alignment.
Molecular hydrogen in high-redshift damped lyman-alpha systems: the VLT/UVES database
(2008-02) Noterdaeme, P.; Srianand, R.
Aims. We present the current status of ongoing searches for molecular hydrogen in high-redshift (1.8 < zabs ≤ 4.2) Damped Lyman-α systems (DLAs) capitalising on observations performed with the ESO Very Large Telescope (VLT) Ultraviolet and Visual Echelle Spectrograph (UVES). Methods. We identify 77 DLAs/strong sub-DLAs, with log N(Hi) ≥ 20 and zabs > 1.8, which have data that include redshifted H2 Lyman and/or Werner-band absorption lines. This sample of Hi, H2 and metal line measurements, performed in an homogeneous manner, is more than twice as large as our previous sample (Ledoux et al. 2003) considering every system in which searches for H2 could be completed so far, including all non-detections. Results. H2 is detected in thirteen of the systems, which have molecular fractions of values between f ≃ 5 × 10−7 and f ≃ 0.1, where f = N(H2)/(2N(H2) + N(Hi)). Upper limits are measured for the remaining 64 systems with detection limits of typically log N(H2) ∼ 14.3, corresponding to log f < −5. We ﬁnd that about 35% of the DLAs with metallicities relative to solar [X/H] ≥ −1.3 (i.e., 1/20th solar), with X = Zn, S or Si, have molecular fractions log f > −4.5, while H2 is detected – regardless of the molecular fraction – in ∼ 50% of them. In contrast, only about 4% of the [X/H] < −1.3 DLAs have log f > −4.5. We show that the presence of H2 does not strongly depend on the total neutral hydrogen column density, although the probability of ﬁnding log f > −4.5 is higher for log N(Hi) ≥ 20.8 than below this limit (19% and 7% respectively). The overall H2 detection rate in log N(Hi) ≥ 20 DLAs is found to be about 16% (10% considering only log f > −4.5 detections) after correction for a slight bias towards large N(Hi). There is a strong preference for H2-bearing DLAs to have signiﬁcant depletion factors, [X/Fe] > 0.4. In addition, all H2-bearing DLAs have column densities of iron into dust grains larger than log N(Fe)dust ∼ 14.7, and about 40% of the DLAs above this limit have detected H2 lines with log f > −4.5. This demonstrates the importance of dust in governing the detectability of H2 in DLAs. Our extended sample supports neither the redshift evolution of the detection fraction of H2-bearing DLAs nor that of the molecular fraction in systems with H2 detections over the redshift range 1.8 < zabs ≤ 3.