Luis J. Goicoechea

The Nature and Size of the Optical Continuum Source in QSO 2237+0305

From the peak of a gravitational microlensing high-magnification event in the A component of QSO 2237+0305, which was accurately monitored by the Gravitational Lenses International Time Project collaboration, we derived new information on the nature and size of the optical V-band and R-band sources in the distant quasar. If the microlensing peak is caused by a microcaustic crossing, we first obtain that the standard accretion disk is a scenario more reliable/feasible than other typical axially symmetric models. Moreover, the standard scenario fits both the V-band and R-band observations with reduced χ2 values very close to 1. Taking into account all these results, a standard accretion disk around a supermassive black hole is a good candidate for the optical continuum main source in QSO 2237+0305. Second, using the standard source model and a robust upper limit on the transverse galactic velocity, we infer that 90% of the V-band and R-band luminosities are emitted from a region with a radial size less than 1.2 × 10-2 pc (=3.7 × 1016 cm, at a 2 σ confidence level).

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.

Time Delay of QSO 0957+561 and Cosmological Implications

We obtain the time delay between the arrival time of the A and B images of the QSO 0957+561. The results of applying two different methods (the discrete cross-correlation function and the dispersion estimation technique) to the observed light curves of the A and B images are presented. The adopted value (time delay) is of ΔτBA = 424 ± 3 days (1 σ). We have used this time delay as well as a recent measurement of the one-dimensional velocity dispersion of the main lensing galaxy (σgal) to estimate H0. Two H0 = H0(ΔτBA, σgal) relations based on different pictures of the lens galaxy, lead to H0 = 64+ 14−15 km s-1 Mpc-1 (2 σ) (softened power-law sphere) and H0 = 66+ 15−14 km s-1 Mpc-1 (2 σ) (King profile plus a point-mass at the center).

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.

New two-colour light curves of Q0957+561: time delays and the origin of intrinsic variations

Aims. We extend the gr-band time coverage of the gravitationally lensed double quasar Q0957+561. New gr light curves permit us to detect significant intrinsic fluctuations, to determine new time delays, and thus to gain perspective on the mechanism of intrinsic variability in Q0957+561. Methods. We use new optical frames of Q0957+561 in the g and r passbands from January 2005 to July 2007. These frames are part of an ongoing long-term monitoring with the Liverpool robotic telescope. We also introduce two photometric pipelines that are applied to the new gr frames of Q0957+561. The transformation pipeline incorporates zero-point, colour, and inhomogeneity corrections to the instrumental magnitudes, so final photometry to the 1–2% level is achieved for both quasar components. The two-colour final records are then used to measure time delays. Results. The gr light curves of Q0957+561 show several prominent events and gradients, and some of them (in the g band) lead to a time delay between components ΔtBA = 417 ± 2d( 1σ). We do not find evidence of extrinsic variability in the light curves of Q0957+561. We also explore the possibility of a delay between a large event in the g band and the corresponding event in the r band. The gr cross-correlation reveals a time lag Δtrg = 4.0 ± 2. 0d( 1σ ;t heg-band event is leading) that confirms a previous claim of the existence of a delay between the g and r band in this lensed quasar. Conclusions. The time delays (between quasar components and between optical bands) from the new records and previous ones in similar bands indicate that most observed variations in Q0957+561 (amplitudes of ∼100 mmag and timescales of ∼100 d) are very probably due to reverberation within the gas disc around the supermassive black hole.

Around-the-Clock Observations of the Q0957+561A,B Gravitationally Lensed Quasar. II. Results for the Second Observing Season

We report on an observing campaign in 2001 March to monitor the brightness of the later arriving Q0957+561B image in order to compare with the previously published brightness observations of the (first-arriving) A image. The 12 participating observatories provided 3543 image frames, which we have analyzed for brightness fluctuations. From our classical methods for time-delay determination, we find a 417.09 ± 0.07 day time delay, which should be free of effects due to incomplete sampling. During the campaign period, the quasar brightness was relatively constant and only small fluctuations were found; we compare the structure function for the new data with structure function estimates for the 1995-1996 epoch and show that the structure function during our observing interval is unusually depressed. We also examine the data for any evidence of correlated fluctuations at zero lag. We discuss the limits of our ability to measure the cosmological time delay if the quasars emitting surface is time resolved, as seems likely.

Time delay of SBS 0909+532

The time delays between the components of a lensed quasar are basic tools to analyze the expansion of the Universe and the structure of the main lens galaxy halo. In this paper, we focus on the variability and time delay of the double system SBS 0909+532A,B as well as the time behaviour of the field stars. We use VR optical observations of SBS 0909+532A,B and the field stars in 2003. The frames were taken at Cal ar Alto, Maidanak and Wise observatories, and the VR light curves of the field stars and quasar components are derived from aperture and point-spread function fitting met hods. We measure the R-band time delay of the system from the � 2 and dispersion techniques and 1000 synthetic light curves based on the observed records. One nearby field star (S BS 0909+532c) is found to be variable, and the other two nearby field stars are non-vari able sources. With respect to the quasar components, the R-band records seem more reliable and are more densely pop- ulated than the V-band ones. The observed R-band fluctuations permit a pre-conditioned measurement of the time delay. From the � 2 minimization, if we assume that the quasar emission is observed first in B and afterwards in A (in agreeme nt with basic observations of the system and the corresponding predictions), we obtain ��BA =− 45 +1 −11 days (95%

Two-dimensional Spectroscopy Reveals an Arc of Extended Emission in the Gravitational Lens System Q2237+0305

We present two-dimensional spectroscopy of the gravitational lens system Q2237+0305 (Einstein Cross) obtained with the INTEGRAL fiber system in subarcsecond seeing conditions. The four components of the system appear clearly separated in the continuum intensity maps. However, the intensity map of the C III] ?1909 line exhibits an arc of extended emission connecting the A, D, and B components. This result can be explained if, as is usually assumed, the continuum arises from a compact source 0.05 pc in extent in the nucleus of the object while the line emission comes from a much larger region. A lens model fitted to the positions of the four compact images also accounts for the arc morphology. In the framework of this model, the region generating the C III] ?1909 emission would have dimensions of about 400 h-1 pc across. We interpret the observed arc as a gravitational lens image of the extended narrow line region of the source.

Time Delay and Accretion Disk Size Measurements in the Lensed Quasar SBS?0909+532 from Multiwavelength Microlensing Analysis

We present three complete seasons and two half-seasons of Sloan Digital Sky Survey (SDSS) r-band photometry of the gravitationally lensed quasar SBS?0909+532 from the U.S. Naval Observatory, as well as two seasons each of SDSS g-band and r-band monitoring from the Liverpool Robotic Telescope. Using Monte Carlo simulations to simultaneously measure the systems time delay and model the r-band microlensing variability, we confirm and significantly refine the precision of the systems time delay to , where the stated uncertainties represent the bounds of the formal 1? confidence interval. There may be a conflict between the time delay measurement and a lens consisting of a single galaxy. While models based on the Hubble Space Telescope astrometry and a relatively compact stellar distribution can reproduce the observed delay, the models have somewhat less dark matter than we would typically expect. We also carry out a joint analysis of the microlensing variability in the r and g bands to constrain the size of the quasars continuum source at these wavelengths, obtaining log {(r s, r /cm)[cos i/0.5]1/2} = 15.3 ? 0.3 and log {(r s, g /cm)[cos i/0.5]1/2} = 14.8 ? 0.9, respectively. Our current results do not formally constrain the temperature profile of the accretion disk but are consistent with the expectations of standard thin disk theory.

Multiple delays in QSO 0957+561: observational evidence and interpretation

Q0957+561A,B is a double-imaged quasar that has been intensively observed during the last 10 yr in different optical bands and with several telescopes, and we concentrated on recent public data obtained at the Apache Point Observatory (APO) and the Teide Observatory (TO). When an intrinsic event appears in the light curve of Q0957+561A, its twin event (a similar feature) is seen in the brightness record of Q0957+561B, and thus, one can measure the corresponding time delay. The TO data set includes two prominent twin events, which were detected with a time separation of 425 ′ 4 d. On the other hand, from the APO data set, we found a clear evidence for two different time delays associated with two pairs of twin events: 417.0 ′ 0. 6 (APO main twin events) and 432.0 ′ 1.9 d (APO secondary twin events), where the APO (main)-APO (secondary) difference delay is of -15 ′ 2 d. In agreement with Yoneharas idea, if the three pairs of twin events are originated inside a standard hybrid source (accretion disc and circumnuclear stellar region), the three measured time delays indicate that they do not come from a common zone in the source. Therefore, we can consider that the prominent features are caused by flares in a standard hybrid source and discuss the size and nature of the region of flares. In this paper it is shown that the more plausible interpretation is that two of the three flares are generated at distances (from the central black hole) larger than 90 pc. Some stellar scenarios can explain the two flares far away from the black hole, while phenomena in the accretion disc cannot cause them.