The influence of gravitational lensing on the spectra of lensed QSOs

We consider the influence of (milli/micro)lensing on the spectra of lensed QSOs. We propose a method for the observational detection of microlensing in the spectra of lensed QSOs and apply it to the spectra of the three lensed QSOs (PG 1115+080, QSO 1413+117 and QSO 0957+561) observed with Hubble Space Telescope (HST). We find that the flux ratio between images A1 and A2 of PG 1115+080 is wavelength-dependent and shows differential magnification between the emission lines and the continuum. We interpret this magnification as arising from millilensing. We also find that the temporal variations in the continuum of image C of QSO 1413+117 may be caused by microlensing, while the temporal variation observed in QSO 0957+561 was probably an intrinsic one.Comment: 11 pages, accepted for publication in MNRA

New VR magnification ratios of QSO 0957+561

We present VR magnification ratios of QSO 0957+561, which are inferred from the GLITP light curves of Q0957+561A and new frames taken with the 2.56m Nordic Optical Telescope about 14 months after the GLITP monitoring. From two photometric approaches and a reasonable range for the time delay in the system (415-430 days), we do not obtain achromatic optical continuum ratios, but ratios depending on the wavelength. These new measurements are consistent with differential extinction in the lens galaxy, the Lyman limit system, the damped Ly-alpha system, or the host galaxy of the QSO. The possible values for the differential extinction and the ratio of total to selective extinction in the V band are reasonable. Moreover, crude probability arguments suggest that the ray paths of the two components cross a similar dusty environment, including a network of compact dust clouds and compact dust voids. As an alternative (in fact, the usual interpretation of the old ratios), we also try to explain the new ratios as caused by gravitational microlensing in the deflector. From magnification maps for each of the gravitationally lensed images, using different fractions of the surface mass density represented by the microlenses, as well as different sizes and profiles of the V-band and R-band sources, several synthetic distributions of V-band and R-band ratios are derived. In some gravitational scenarios, there is an apparent disagreement between the observed pair of ratios and the simulated distributions. However, several microlensing pictures work well. To decide between either extinction, or microlensing, or a mixed scenario (extinction + microlensing), new observational and interpretation efforts are required.Comment: PS and PDF versions are created from the LaTeX file and 5 EPS figures, two additional figues (Figs. 6 and 7) in JPEG format, scheduled for the ApJ 20 January 2005 issu

A Large Brightness Enhancement of the QSO 0957+561 A Component

We report an increase of more than 0.2 mag in the optical brightness of the leading image (A) of the gravitational lens Q0957+561, detected during the 09/2000 -- 06/2001 monitoring campaign (2001 observing season). The brightening is similar to or even greater than the largest change ever detected during the 20 years of monitoring of this system. We discuss two different provisional explanations to this event: intrinsic source variability or microlensing (either short timescale microlensing or cessation of the historical microlensing). An exhaustive photometric monitoring of Q0957+561 is needed until summer of 2002 and during 2003 to discriminate between these possibilities.Comment: 13 pages including 3 figures and 1 table. Accepted for publication in ApJ Let

New aperture photometry of QSO 0957+561; application to time delay and microlensing

We present a re-reduction of archival CCD frames of the doubly imaged quasar 0957+561 using a new photometry code. Aperture photometry with corrections for both cross contamination between the quasar images and galaxy contamination is performed on about 2650 R-band images from a five year period (1992-1997). From the brightness data a time delay of 424.9 +/- 1.2 days is derived using two different statistical techniques. The amount of gravitational microlensing in the quasar light curves is briefly investigated, and we find unambiguous evidence of both long term and short term microlensing. We also note the unusual circumstance regarding time delay estimates for this gravitational lens. Estimates by different observers from different data sets or even with the same data sets give lag estimates differing by typically 8 days, and error bars of only a day or two. This probably indicates several complexities where the result of each estimate depends upon the details of the calculation.Comment: 14 pages, 16 figures (several in color

Deep Imaging of the Double Quasar 0957+561: New Constraints on H_0

We present new results from extremely deep, high-resolution images of the field around the double quasar QSO 0957+561. A possible gravitational arc system near the double quasar has recently been reported, which, if real, would set strong constraints on determinations of the Hubble constant from the time delay in the double quasar. We find that both the morphology and the colors of the claimed arc systems suggest that they are chance alignments of three and two different objects, and not gravitationally lensed arcs. Hence, the constraints on $H_0$-determinations from the arcs are not valid. Also, a small group of galaxies at $z=0.5$ near the line-of-sight which was required to have a very large mass in the physically interesting arc models, is most likely insignificant. From our deep images we are able to use weak lensing of faint background galaxies in the field to map the gravitational potential in the main cluster. This sets new constraints on determinations of $H_0$. We find that the Hubble constant is constrained to be less than 70km/(s Mpc), if the time delay between the two images of the QSO is equal to or larger than 1.1 years.Comment: (uuencoded and compressed postscipt including 3 figures); 14 page

Short-timescale Fluctuations in the Difference Light Curves of QSO 0957+561A,B: Microlensing or Noise?

From optical R band data of the double quasar QSO 0957+561A,B, we made two new difference light curves (about 330 days of overlap between the time-shifted light curve for the A image and the magnitude-shifted light curve for the B image). We observed noisy behaviours around the zero line and no short-timescale events (with a duration of months), where the term event refers to a prominent feature that may be due to microlensing or another source of variability. Only one event lasting two weeks and rising - 33 mmag was found. Measured constraints on the possible microlensing variability can be used to obtain information on the granularity of the dark matter in the main lensing galaxy and the size of the source. In addition, one can also test the ability of the observational noise to cause the rms averages and the local features of the difference signals. We focused on this last issue. The combined photometries were related to a process consisting of an intrinsic signal plus a Gaussian observational noise. The intrinsic signal has been assumed to be either a smooth function (polynomial) or a smooth function plus a stationary noise process or a correlated stationary process. Using these three pictures without microlensing, we derived some models totally consistent with the observations. We finally discussed the sensitivity of our telescope (at Teide Observatory) to several classes of microlensing variability.Comment: MNRAS, in press (LaTeX, 14 pages, 22 eps figures