arXiv subjects

Hum Chand

Publications and source records attributed to Hum Chand.

63 records · Page 4Linked to original sources

arXiv2011

Probing spectral properties of radio-quiet quasars searched for optical microvariability-II

In the context of AGN unification scheme rapid variability properties play an important role in understanding any intrinsic differences between sources in different classes. In this respect any clue based on spectral properties will be very useful toward understanding the mechanisms responsible for the origin of rapid small scale optical variations, or microvariability. Here we have added spectra of 46 radio-quiet quasars (RQQSOs) and Seyfert 1 galaxies to those of our previous sample of 37 such objects, all of which had been previously searched for microvariability. We took new optical spectra of 33 objects and obtained 13 others from the literature. Their \hbeta and \mgii emission lines were carefully fit to determine line widths (FWHM) as well as equivalent widths (EW) due to the broad emission line components. The line widths were used to estimate black hole masses and Eddington ratios, $\ell$. Both EW and FWHM are anticorrelated with $\ell$. Nearly all trends were in agreement with our previous work, although the tendency for sources exhibiting microvariability to be of lower luminosity was not confirmed. Most importantly, this whole sample of EW distributions provides no evidence for the hypothesis that a weak jet component in the radio quiet AGNs is responsible for their microvariability.

astro-ph.CO↗

arXiv2010

Optical microvariability properties of BALQSOs

We present optical light curves of 19 radio quiet (RQ) broad absorption line (BAL) QSOs and study their rapid variability characteristics. Systematic CCD observations, aided by a careful data analysis procedure, have allowed us to clearly detect any such microvariability exceeding 0.01--0.02 mag. Our observations cover a total of 13 nights (~72 hours) with each quasar monitored for about 4 hours on a given night. Our sample size is a factor of three larger than the number of radio-quiet BALQSOs previously searched for microvariability. We introduce a scaled F-test statistic for evaluating the presence of optical microvariability and demonstrate why it is generally preferable to the statistics usually employed for this purpose. Considering only unambiguous detections of microvariability we find that ~11 per cent of radio-quiet BALQSOs (two out of 19 sources) show microvariability for an individual observation length of about 4 hr. This new duty cycle of 11 per cent is similar to the usual low microvariability fraction of normal RQQSOs with observation lengths similar to those of ours. This result provides support for models where radio-quiet BALQSO do not appear to be a special case of the RQQSOs in terms of their microvariability properties.

astro-ph.CO↗

arXiv2010

Probing spectral properties of radio-quiet quasars searched for optical microvariability

We obtained SDSS spectra for a set of 37 radio-quiet quasars (RQQSOs) that had been previously examined for rapid small scale optical variations, or microvariability. Their H-beta and Mg II emission lines were carefully fit to determine line widths (FWHM) as well as equivalent widths (EW) due to the broad emission line components. The line widths were used to estimate black hole masses and Eddington ratios, ell. Both EW and FWHM are anticorrelated with ell. The EW distributions provide no evidence for the hypothesis that a weak jet component in the RQQSOs is responsible for their microvariability.

astro-ph.CO↗

arXiv2009

Constraining fundamental constants of physics with quasar absorption line systems

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 0.5-1.5x10-5 at the 3sigma level over a redshift range of approximately 0.5-2.5 for the fine-structure constant, alpha, the proton-to-electron mass ratio, mu, and a combination of the proton gyromagnetic factor and the two previous constants, gp(alpha^2/mu)^nu, for only one claimed variation of alpha. It is therefore very important to perform new measurements to improve the sensitivity of the numerous methods to at least <0.1x10-5 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 galaxies.

astro-ph.CO↗

arXiv2007

In response to the comments by Murphy et al. (arxiv:0708.3677)

In their comment, Murphy et al. (arxiv:0708.3677) criticize the fitting procedure we used in two previous papers [Srianand et al. 2004(Paper I) and Chand et al. 2004 (Paper II)] and conclude that the above papers offers no stringent test to previous evidence for varying fine structure constant. We think this is a hasty conclusion as (a) our procedure is robust as shown in Paper II; (b) the data used by Murphy et al., in particular the error array, are different from ours and there are differences in the fitting procedure; (c) despite these differences, 70 per cent of their individual measurements are consistent with that quoted in Paper II. Our reanalysis using VPFIT with identical initial guess parameters leads to Δα/α= (0.01+/-0.15)x 10^{-5}$ for 21 systems (excluding two systems that deviate at more than 3$σ$ level) with very little scatter (χ^2_ν~1).

astro-ph↗

arXiv2007

Line shift, line asymmetry, and the 6Li/7Li isotopic ratio determination

Context: Line asymmetries are generated by convective Doppler shifts in stellar atmospheres, especially in metal-poor stars, where convective motions penetrate to higher atmospheric levels. Such asymmetries are usually neglected in abundance analyses. The determination of the 6Li/7Li isotopic ratio is prone to suffering from such asymmetries, as the contribution of 6Li is a slight blending reinforcement of the red wing of each component of the corresponding 7Li line, with respect to its blue wing. Aims: The present paper studies the halo star HD 74000 and estimates the impact of convection-related asymmetries on the Li isotopic ratio determination. Method: Two methods are used to meet this aim. The first, which is purely empirical, consists in deriving a template profile from another element that can be assumed to originate in the same stellar atmospheric layers as Li I, producing absorption lines of approximately the same equivalent width as individual components of the 7Li I resonance line. The second method consists in conducting the abundance analysis based on NLTE line formation in a 3D hydrodynamical model atmosphere, taking into account the effects of photospheric convection. Results: The results of the first method show that the convective asymmetry generates an excess absorption in the red wing of the 7Li absorption feature that mimics the presence of 6Li at a level comparable to the hitherto published values. This opens the possibility that only an upper limit on 6Li/7Li has thus far been derived. The second method confirms these findings. Conclusions: From this work, it appears that a systematic reappraisal of former determinations of 6Li abundances in halo stars is warranted.

astro-ph↗

arXiv2006

On the variation of the fine-structure constant: Very high resolution spectrum of QSO HE 0515-4414

We present a detailed analysis of a very high resolution (R\approx 112,000) spectrum of the quasar HE 0515-4414 obtained using the High Accuracy Radial velocity Planet Searcher (HARPS) mounted on the ESO 3.6 m telescope at the La Silla observatory. The HARPS spectrum, of very high wavelength calibration accuracy (better than 1 mÅ), is used to search for possible systematic inaccuracies in the wavelength calibration of the UV Echelle Spectrograph (UVES) mounted on the ESO Very Large Telescope (VLT). We have carried out cross-correlation analysis between the Th-Ar lamp spectra obtained with HARPS and UVES. The shift between the two spectra has a dispersion around zero of σ\simeq 1 mÅ. This is well within the wavelength calibration accuracy of UVES (i.e σ\simeq 4 mÅ). We show that the uncertainties in the wavelength calibration induce an error of about, Δα/α\le 10^{-6}, in the determination of the variation of the fine-structure constant. Thus, the results of non-evolving Δα/αreported in the literature based on UVES/VLT data should not be heavily influenced by problems related to wavelength calibration uncertainties. Our higher resolution spectrum of the z_{abs}=1.1508 damped Lyman-αsystem toward HE 0515-4414 reveals more components compared to the UVES spectrum. Using the Voigt profile decomposition that simultaneously fits the high resolution HARPS data and the higher signal-to-noise ratio UVES data, we obtain, Δα/α=(0.05\pm0.24)x10^{-5} at z_{abs}=1.1508. This result is consistent with the earlier measurement for this system using the UVES spectrum alone.

astro-ph↗

arXiv2004

Probing the time-variation of the fine-structure constant: Results based on Si IV doublets from a UVES sample

We report a new constraint on the variation of the fine-structure constant based on the analysis of 15 Si IV doublets selected from a ESO-UVES sample. We find Δα/α= (+0.15+/-0.43) x 10^-5 over a redshift range of 1.59< z < 2.92 which is consistent with no variation in α. This result represents a factor of three improvement on the constraint on Δα/αbased on Si IV doublets compared to the published results in the literature. The alkali doublet method used here avoids the implicit assumptions used in the many-multiplet method that chemical and ionization inhomogeneities are negligible and isotopic abundances are close to the terrestrial value.

astro-ph↗

arXiv2004

Limits on the time variation of the electromagnetic fine-structure constant in the low energy limit from absorption lines in the spectra of distant quasars

Most of the successful physical theories rely on the constancy of few fundamental quantities (such as the speed of light, $c$, the fine-structure constant, α, the proton to electron mass ratio, μ, etc), and constraining the possible time variations of these fundamental quantities is an important step toward a complete physical theory. Time variation of αcan be accurately probed using absorption lines seen in the spectra of distant quasars. Here, we present the results of a detailed many-multiplet analysis performed on a new sample of Mg II systems observed in high quality quasar spectra obtained using the Very Large Telescope. The weighted mean value of the variation in αderived from our analysis over the redshift range 0.4<z<2.3 is Δα/α= (-0.06+/-0.06) x 10^{-5}. The median redshift of our sample (z=1.55) corresponds to a look-back time of 9.7 Gyr in the most favored cosmological model today. This gives a 3σlimit, -2.5 x 10^{-16} yr^-1 <(Δα/αΔt) <+1.2x10^{-16} yr^-1, for the time variation of α, that forms the strongest constraint obtained based on high redshift quasar absorption line systems.

astro-ph↗