Browsing by Author "Stalin, C. S."

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    Long-term optical photometric monitoring of the quasar SDSS J153259.96 - 003944.1
    (2005-02-01) Stalin, C. S.; Srianand, R.
    We report optical Cousins R and I band monitoring observations of the high redshift (z = 4.67) QSO SDSS J153259.96−003944.1 that does not show detectable emission lines in its optical spectrum. We show this object varies with a maximum amplitude of ∼0.4 mag during a year and three months of monitoring. Combined with two other epochs of photometric data available in the literature, we show the object has gradually faded by ∼0.9 mag during the period June 1998 − April 2001. A linear least squares fit to all available observations gives a slope of ∼0.35 mag/yr which translates to ∼1.9 mag/yr in the rest frame of the quasar. Such a variability is higher than that typically seen in QSOs but consistent with that of BL Lacs, suggesting that the optical continuum is Doppler boosted. Alternatively, within photometric errors, the observed lightcurve is also consistent with the object going through a microlensing event. Photoionization model calculations show the mass of the Broad Line Region to be few tens of M⊙ similar to that of low luminosity Seyfert galaxies, but ∼2 orders of magnitude less than that of luminous quasars. Further frequent photometric/spectroscopic monitoring is needed to support or refute the different alternatives discussed here on the nature of SDSS J153259.96−003944.1
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    Optical identification of XMM sources in the CFHTLS
    (2009-05-01) Stalin, C. S.; Petitjean, Patrick; Srianand, R.; et al.
    We summarize the attempts by our group and others to derive constraints on variations of fundamental constants over cosmic time using quasar absorption lines. Most upper limits reside in the range 0.5 1.5 10 5 at the 3 level over a redshift range of approximately 0:5 2:5 for the fine-structure constant, , the proton-to-electron mass ratio, and a combination of the proton gyromagnetic factor and the two previous constants, gp( 2= ) , for only one claimed variation of . It is therefore very important to perform new measurements to improve the sensitivity of the numerous methods to at least <0.1 10 5 which should be possible in the next few years. Future instrumentations on ELTs in the optical and/or ALMA, EVLA and SKA pathfinders in the radio will undoutedly boost this field by allowing to reach much better signal-to-noise ratios at higher spectral resolution and to perform measurements on molecules in the ISM of high redshift galaxi