Article

Photometric monitoring of the doubly imaged quasar UM673: possible evidence for chromatic microlensing

08/2005; DOI: 10.1051/0004-6361:20052877
Source: arXiv

ABSTRACT We present the results of two-band CCD photometric monitoring of the gravitationally lensed quasar Q 0142-100 (UM 673).The data, obtained at ESO-La Silla with the 1.54 m Danish telescope in the Gunn i-band (October 1998 - September 1999) and in the Johnson V-band (October 1998 to December 2001), were analyzed using three different photometric methods. The light-curves obtained with all methods show variations, with a peak-to-peak amplitude of 0.14 magnitude in $V$. Although it was not possible to measure the time delay between the two lensed QSO images, the brighter component displays possible evidence for microlensing: it becomes bluer as it gets brighter, as expected under the assumption of differential magnification of a quasar accretion disk

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    ABSTRACT: We present optical $V$ and $i$-band light curves of the gravitationally lensed BAL quasar HE 2149-2745. The data, obtained with the 1.5 m Danish Telescope (ESO-La Silla) between October 1998 and December 2000, are the first from a long-term project aimed at monitoring selected lensed quasars in the Southern Hemisphere. A time delay of $103\pm12$ days is determined from the light curves. In addition, VLT/FORS1 spectra of HE 2149-2745 are deconvolved in order to obtain the spectrum of the faint lensing galaxy, free of any contamination by the bright nearby two quasar images. By cross-correlating the spectrum with galaxy-templates we obtain a tentative redshift estimate of $z=0.495\pm0.01$. Adopting this redshift, a $\Omega=0.3$, $\Lambda=0.7$ cosmology, and a chosen analytical lens model, our time-delay measurement yields a Hubble constant of $H_{0}=66 \pm 8\, {\rm km}~{\rm s^{-1}}~{\rm Mpc^{-1}}$ (1$\sigma$ error) with an estimated systematic error of $\pm3\, {\rm km}~{\rm s^{-1}}~{\rm Mpc^{-1}}$. Using non-parametric models yields $H_{0}=65 \pm 8\, {\rm km}~{\rm s^{-1}}~{\rm Mpc^{-1}}$ (1$\sigma$ error) and confirms that the lens exhibits a very dense/concentrated mass profile. Finally, we note, as in other cases, that the flux ratio between the two quasar components is wavelength dependent. While the flux ratio in the broad emission lines – equal to 3.7 – remains constant with wavelength, the continuum of the brighter component is bluer. Although the data do not rule out extinction of one quasar image relative to the other as a possible explanation, the effect could also be produced by differential microlensing by stars in the lensing galaxy.
    Astronomy and Astrophysics 01/2002; · 5.08 Impact Factor

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