Rodrigo Gil-Merino

QSO 2237+0305 VR Light Curves from Gravitational LensES International Time Project Optical Monitoring

We present VR observations of QSO 2237+0305 conducted by the Gravitational Lensing International Time Project collaboration from 1999 October 1 to 2000 February 3. The observations were made with the 2.56 m Nordic Optical Telescope at Roque de los Muchachos Observatory, La Palma (Spain). The point-spread function (PSF) fitting method and an adapted version of the ISIS subtraction method have been used to derive the VR light curves of the four components (A-D) of the quasar. The mean errors range in the intervals 0.01-0.04 mag (PSF fitting) and 0.01-0.02 mag (ISIS subtraction), with the faintest component (D) having the largest uncertainties. We address the relatively good agreement between the A and D light curves derived using different filters, photometric techniques, and telescopes. The new VR light curves of component A extend the time coverage of a high-magnification microlensing peak, which was discovered by the OGLE team.

The AAT/WFI survey of the Monoceros Ring and Canis Major dwarf galaxy – I. From l= (193–276)°

We present the results of an Anglo-Australian Telescope (AAT) wide field camera survey of the stars in the Monoceros Ring (MRi) and purported Canis Major (CMa) overdensity in the Galactic longitudes of l = (193‐276) ◦ . Current numerical simulations suggest that both of these structures are the result of a single on-going accretion event, although an alternative solution is that the warped and flared disc of the Galaxy can explain the origin of both of these structures. Our results show that, with regards the MRi, the warped and flared disc is unable to reproduce the locations and strengths of the detections observed around the Galaxy. This supports a non-Galactic origin for this structure. We report eight new detections and two tentative detections of the MRi in this survey. The exact nature of the CMa overdensity is still unresolved, although this survey provides evidence that invoking the Galactic warp is not a sufficient solution when compared with observation. Several fields in this survey are highly inconsistent with the current Galactic disc models that include a warp and flare, to such an extent that explaining their origins with these structures is problematic. We also report that the blue plume stars previously invoked to support the dwarf galaxy hypothesis are unfounded, and associating them with an outer spiral arm is equally problematic. Standard Galactic models are unable to accommodate all the observations of these new structures, leading away from a warped/flared disc explanation for their origins and more toward a non-Galactic source. Additionally, evidence is presented in favour of a detection of the CMa dwarf stream away from the CMa region. As the outer reaches of the Galactic disc continue to be probed, the fascinating structures that are the MRi and CMa overdensity will no doubt continue to inform us of the unique structure and formation of the Milky Way.

A Fast and Very Accurate Approach to the Computation of Microlensing Magnification Patterns Based on Inverse Polygon Mapping

A new method of calculating microlensing magnification patterns is proposed that is based on the properties of the backward gravitational lens mapping of a lattice of polygonal cells defined at the image plane. To a first-order approximation, the local linearity of the transformation allows us to compute the contribution of each image-plane cell to the magnification by apportioning the area of the inverse image of the cell (transformed cell) among the source-plane pixels covered by it. Numerical studies in the κ = 0.1-0.8 range of mass surface densities demonstrate that this method (provided with an exact algorithm for distributing the area of the transformed cells among the source-plane pixels) is more efficient than the inverse ray-shooting technique (IRS). Magnification patterns with relative errors of ~5 × 10-4 are obtained with an image-plane lattice of only 1 ray per unlensed pixel. This accuracy is, in practice, beyond the reach of IRS performance (more than 10,000 rays should be collected per pixel to achieve this result with the IRS) and is obtained in a small fraction (less than 4%) of the computing time that is used by the IRS technique to achieve an error more than an order of magnitude larger. The computing time for the new method is reduced to below 1% of the IRS computing time when the same accuracy is required of both methods. We have also studied the second-order approximation to control departures from linearity that could induce variations in the magnification within the boundaries of a transformed cell. This approximation is used to identify and control the cells enclosing a critical curve.

BVRI Photometry of QSO 0957+561A, B: Observations, New Reduction Method, and Time Delay

CCD observations of the gravitational lens system Q0957+561A, B in the BVRI bands are presented in this paper. The observations, taken with the 82 cm IAC-80 telescope at Teide Observatory, Spain, were made from the beginning of 1996 February to 1998 July, as part of an ongoing lens-monitoring program. Accurate photometry was obtained by simultaneously fitting a stellar two-dimensional profile on each component by means of DAOPHOT software. This alternative method is equal to and even improves on the results obtained with previous techniques. The final data set is characterized by its high degree of homogeneity, since it was obtained using the same telescope and instrumentation during a period of almost 3 yr. The resulting delay, obtained with a new method, the δ2 test, is of 425 ± 4 days, slightly higher than the value previously accepted (417 days), but concordant with the results obtained by other researchers.

Microlensing of a Biconical Broad-Line Region

The influence of microlensing on the profiles of emission lines generated in a biconical geometry is discussed. Microlensing amplification in this anisotropic model is not directly related to the bicones intrinsic size but, rather, depends on the orientation of the bicone axis and on the cone aperture. The orientation of the projected bicone with respect to the shear of the magnification pattern can induce very interesting effects, such as quasi-periodic enhancements of the red or blue part of the emission-line profile or a lack of correlation between the broad-line region (BLR) and continuum light curve for QSOs. The emission-line profiles of a BLR moving in a high caustic concentration exhibit sharp features that are well defined in wavelength. These features (spikes) correspond to a scanning of the kinematics of the BLR by the caustic clusters. The biconical model can qualitatively reproduce, with a transverse (with respect to the shear) movement of the BLR, the recurrent blue-wing enhancement detected in the emission-line profile of image A of the lensed quasar SDSS J1004+4112. The probability of observing such a repetitive event is almost 2% for a fraction of matter in stars of 5%. This result makes plausible the detection of the spectral variability in SDSS J1004+4112 under the hypothesis that it reflects the microlensing of a bicone.

Quasar microlensing: When compact masses mimic smooth matter

The magnification induced by gravitational microlensing is sensitive to the size of a source relative to the Einstein radius, the natural microlensing scale length. This paper investigates the effect of source size in the case in which the microlensing masses are distributed with a bimodal mass function, with solar-mass stars representing the normal stellar masses and smaller masses (down to 8.5 × 10-5 M☉) representing a dark matter component. It is found that there exists a critical regime in which the dark matter is initially seen as individual compact masses, but with increasing source size the compact dark matter acts as a smooth mass component. This study reveals that interpretation of microlensing light curves, especially claims of small-mass dark matter lenses embedded in an overall stellar population, must consider the important influence of the size of the source.

New VR Magnification Ratios of QSO 0957+561

We present VR magnification ratios of QSO 0957+561 that are inferred from the GLITP light curves of Q0957+561A and new frames taken with the 2.56 m Nordic Optical Telescope about 14 months after the GLITP monitoring. To extract the fluxes of the two close components, two different photometric techniques are used, pho2comC and psfphot. From the 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. Our final global measurements ΔmAB(λV) = 0.077 ± 0.023 mag and ΔmAB(λR) = 0.022 ± 0.013 mag (1 σ intervals) are in agreement with the Oslo group results (using the same telescope in the same seasons but another photometric task and only one time delay of about 416 days). These new measurements are consistent with differential extinction in the lens galaxy, the Lyman limit system, the damped Lyα 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 being 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 [ΔmAB(λV), ΔmAB(λR)] pairs 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, microlensing, or a mixed scenario (extinction + microlensing), new observational and interpretation efforts are required.

A 5.5-year robotic optical monitoring of Q0957+561: substructure in a non-local cD galaxy

New light curves of the gravitationally lensed double quasar Q0957+561 in the gr bands during 2008‐2010 include densely sampled, sharp intrinsic fluctuations with unprecedentedly high sig nal-to-noise ratio. These relatively violent flux variatio ns allow us to very accurately measure the g-band and r-band time delays between the two quasar images A and B. Using correlation functions, we obtain that the two time delays are inconsistent with each other at t he 2σ level, with the r-band delay exceeding the 417-day delay in the g band by about 3 days. We also studied the long-term evolution of the delay-corrected flux ratio B/A from our homogeneous two-band monitoring with the Liverpool Robotic Telescope between 2005 and 2010 ⋆ . This ratio B/A slightly increases in periods of violent activity, which seems to be correlated with the flux level in t hese periods. The presence of the previously reported dense cloud within the cD lensing galaxy, along the line of sight to the A image, could account for the observed time delay and flux ratio anomal ies.

The dark halo of the main lens galaxy in QSO 0957 + 561

We present an analysis of infrared/optical/ultraviolet spectra of the two images of the first gravitationally lensed quasar Q0957 + 561A,B. The Hubble Space Telescope observations of Q0957 + 561A and Q0957 + 561B are separated in time by the known time delay in this system, so we can directly deduce the flux ratios. These flux ratios of images lead to important information on the dark halo of the main lens galaxy (a giant elliptical at redshift z = 0.36). Our measurements for the continuum are in good agreement with extinction in the elliptical galaxy and a small fraction of mass in collapsed objects (no need for gravitational microlensing). From the continuum and emission-line ratios, we also show evidence in favour of the existence of a network of compact dusty clouds. Ke yw ords: gravitational lensing ‐ dust, extinction ‐ galaxies: elliptical and lenticular, cD ‐ galaxies: haloes ‐ quasars: individual: QSO 0957 + 561 ‐ dark matter.

Interpreting microlensing signal in QSO 2237+0305: Stars or planets?

The multiply imaged, gravitationally lensed quasar, QSO 2237+0305, has been the subject of recent optical monitoring campaigns, with its light curves displaying uncorrelated variability attributed to gravitational microlensing by masses in the foreground galaxy. Based on these light curves, it has been claimed that the dominant microlensing population must be a population of free-floating Jupiter-like objects; such a conclusion is not new, with several similar claims in the literature. Historically, however, it has been shown that such conclusions are flawed, with an incorrect interpretation of the complex caustic network that arises at significant optical depth. This paper examines this more recent claim, illustrating that it too is flawed.