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Item Molecules in the Zabc = 2.8112 damped system toward PKS 0528-250(2015-03-11) Srianand, R.; Petitjean, PatrickItem Do the central engines of quasars evolve by accretion(2015-03-11) Srianand, R.; Krishna-GopalItem High resolution study of associated C IV absorption systems in NGC 5548(2015-03-01) Srianand, R.Item The Z abs~Z em absorption line systems toward QSO J2233-606 in the hubble deep field south: Ne VIIIλλ770,780 absorption and partial coverage(2015-03-01) Petitjean, Patrick; Srianand, R.Item Probing the BLR in AGNs using time variability of associated absorption lines(2015-03-01) Srianand, R.; Shankaranarayanan, S.It is know that most of the clouds producing associated absorption in the spectra of AGNs and quasars do not completely cover the background source (continuum + broad emission line region, BLR). We note that the covering factor derived for the absorption is the fraction of photons occulted by. The absorbing clouds, and is not necessarily the same as the fractional area covered. We show that the variability in absorption lines can be produced by the changes in the covering factor caused by the variation in the continuum and the finite light travel time across the BLR. We discuss how such avariability can be distinguished from the variability caused by other effects and how one can use the variability in the covering factor to probe the BLR.Item Dynamical history of Ly α clouds(2015-02-07) Srianand, R.Item Analysis of low z absorbers in the QSO spectra(2015-02-07) Srianand, R.Item Analysis of Ly α absorption lines in the vicinity of QSOS(2015-02-07) Srianand, R.; Khare, PushpaItem Near-solar metallicity damped lyman-alpha system toward the BAL quasar Tol 1037 - 2703(2001-02-01) Srianand, R.; Petitjean, PatrickWe 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.Item Probing the dark ages with redshift distribution of GRBs(2002-01-01) Roy Choudhury, T.; Srianand, R.In this article, we explore the possibility of using the properties of gamma ray bursts (GRBs) to probe the physical conditions in the epochs prior to reionization. The redshift distribution of GRBs is modelled using the Press-Schechter formalism with an assumption that they follow the cosmic star formation history. We reproduce the observed star formation rate obtained from galaxies in the redshift range 0 < z < 5, as well as the redshift distribution of the GRBs inferred from the luminosity-variability correlation of the burst light curve.We show that the fraction of GRBs at high redshifts, whose afterglows cannot be observed in R and I band due to HI Gunn Peterson optical depth can, at the most, account for one third of the dark GRBs. The observed redshift distribution of GRBs, with much less scatter than the one available today, can put stringent constraints on the epoch of reionization and the nature of gas cooling in the epochs prior to reionization