Variable Radio Echoes from a Jet in QSO 0957 + 561. II. Statistical Significance of the Result

At the previous publication (Oknyanskij, 1997) we considered and used a new algorithm for time-delay investigations in case when the time delay was a linear function of time and the echo response intensity was a power-low function of the delay. We applied this method to investigate optical-to-radio delay in the double quasar 0957 + 561 (generally accepted to be a case of gravitational lensing). It was found in this way that the radio variations (5 MHz) followed the optical ones, but the time delay was a linear function of time with the mean value being about 2370 day and with the rate of increase V≈ 110 days/year. Here we use Monte-Carlo simulations to estimate the significance of the results. We estimate (with the 95% confidence level) that the probability of getting the same result by chance (if real V value was equal to 0 days/year) is less then 5%. We also show that the method can determinate the actual rate of increase Va of the time delay in the artificial light curves, which have the same data spacing, power spectrum and noise level as real ones. We briefly consider some other possible fields for using the method. This revised version was published online in July 2006 with corrections to the Cover Date.     

CCD Photometry of QSO 0957+561 A & B for the period 1987–1992 using La Palma archive

We present the results of CCD photometry of the first gravitational lenssystem QSO 0957+561, obtained on the base of La Palma archive. Usingcross-correlation analysis we have found the time delay between A and Bquasar images to be 430 ± 15 days. Our photometry of QSO 0957+561 A & Bgives additional independent confirmation of the recent results on the time delay about 425 days.     

Five nights of intensive R- and V-band photometry of QSO 0957+561A,B

We present R - and V -band photometry of the gravitational lens system QSO 0957+561 from five nights (one in 2000 January and four in 2001 March, corresponding to the approximate time delay for the system) of uninterrupted monitoring at the Nordic Optical Telescope. In the photometry scheme we have stressed careful magnitude calibration as well as corrections for the lens galaxy contamination and the crosstalk between the twin (A and B) quasar images. The resulting, very densely sampled, light curves are quite stable, in conflict with earlier claims derived from the same data material. We estimate high-precision timelag-corrected B/A flux ratios in both colour bands, as well as V – R colour indices for A and B, and discuss the short time-scale variability of the system.     

Evidence for accretion disc reprocessing in QSO 0957+561

We use archival g - and r -band photometry of the gravitational lens system QSO 0957+561A, B to estimate the intrinsic variability of the quasar during 1996 February–June. The light curves span 234 d with temporal resolutions of about 2.5 d. Both light curves display a single large-amplitude event, of ∼0.1 mag (max-to-min) in about 100 d, followed by small-amplitude variations of ∼0.02 mag on time-scales of tens of days. We find the r -band variations lag those at g by 3.4_−1.4^+1.5 d for the large-amplitude event. This lag is greater than zero at no less than 98 per cent confidence. The delayed coupling of the rest-frame UV intrinsic variations strongly suggests the existence of a stratified reprocessing region extending ∼light-days from the putative central black hole source. The observed lag is consistent with that expected from a reverberation within an irradiated accretion disc structure. However, any definitive statement requires further detailed theoretical modelling and high-quality, signal-to-noise ratio of about 100, optical/IR simultaneous monitoring with about 3-d resolution for approximately 6 months.     

GLITP optical monitoring of QSO 0957+561: VR light curves and variability

The Gravitational Lenses International Time Project (GLITP) collaboration observed the first gravitational lens system (QSO 0957+561) from 2000 February 3 to March 31. The daily VR observations were made with the 2.56-m Nordic Optical Telescope at Roque de los Muchachos Observatory, La Palma, Spain. We have derived detailed and robust VR light curves of the two components Q0957+561A and Q0957+561B. In spite of the excellent sampling rate, we have not found evidence in favour of true daily variability. With respect to variability on time-scales of several weeks, we measure VR gradients of about −0.8 mmag d⁻¹ in Q0957+561A and +0.3 mmag d⁻¹ in Q0957+561B. The gradients are very probably originated in the far source. Thus, adopting this reasonable hypothesis (intrinsic variability), we compare them to the expected gradients during the evolution of a compact supernova remnant at the redshift of the source quasar. The starburst scenario is roughly consistent with some former events, but the new gradients do not seem to be caused by supernova remnant activity.     

Short time-scale 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 (with an overlap of about 330 d 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 time-scale events (with a duration of months), where the term 'event' refers to a prominent feature that may be a result of 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), 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.     

The JVAS/CLASS search for 6-arcsec to 15-arcsec image separation lensing

The Jodrell Bank–VLA Astrometric Survey (JVAS) and the Cosmic Lens All Sky Survey (CLASS) have been systematically searched for multiple gravitational imaging of sources with image separations between 6 arcsec and 15 arcsec, associated with galaxy group and cluster lensing masses. The radio and optical follow-up observations of all candidates are presented. From a total of ∼15 000 sources only one weak candidate remains and this is not contained in the statistically complete sample of flat-spectrum JVAS/CLASS sources of 11 670 sources. A simple Press–Schechter analysis is performed. For singular isothermal sphere lenses the lack of multiple image systems is inconsistent with the currently favoured cosmologies with     at the 4.2 σ level. Cored isothermal lenses reduce the expected number of lens systems and we suggest that the most probable interpretation of our results is that the surface mass density of groups and clusters of galaxies is not high enough to cause multiple imaging and the presence of the mass concentrations associated with individual galaxies is required to produce image separations such as those in B0957+561.     

The new gravitational lens system B1030 + 074

We report the discovery of a new double-image gravitational lens system, B1030 + 074, which was found during the Jodrell Bank–VLA Astrometric Survey (JVAS). We have collected extensive radio data on the system using the VLA, MERLIN, the EVN and the VLBA, and optical observations using WFPC2 on the HST . The lensed images are separated by 1.56 arcsec and their flux density ratio at centimetric wavelengths is approximately 14:1, although the ratio is slightly frequency-dependent and the images appear to be time-variable. The HST pictures show both the lensed images and the lensing galaxy close to the weaker image. The lensing galaxy has substructure which could be a spiral arm or an interacting galaxy. We have modelled B1030 + 074 using a singular isothermal ellipsoid which yields a time delay of 156/ h ₅₀ d. This lens is likely to be suitable for the measurement of the Hubble constant.     

Gravitational lensing in quasar samples

The first cosmic mirage was discovered approximately 20 years ago as the double optical counterpart of a radio source. This phenomenon had been predicted some 70 years earlier as a consequence of General Relativity. We present here a summary of what we have learnt since. The applications are so numerous that we had to concentrate on a few selected aspects of this new field of research. This review is focused on strong gravitational lensing, i.e. the formation of multiple images, in QSO samples. It is intended to give the reader an up-to-date status of the observations and to present an overview of its most interesting potential applications in cosmology and astrophysics, as well as numerous important results achieved so far. The first section follows an intuitive approach to the basics of gravitational lensing and is developed in view of our interest in multiply imaged quasars. The astrophysical and cosmological applications of gravitational lensing are outlined in Sect. 2 and the most important results are presented in Sect. 5. Sections 3 and 4 are devoted to the observations. Finally, conclusions are summarized in the last section. We have tried to avoid duplication with existing (and excellent) introductions to the field of gravitational lensing. For this reason, we did not concentrate on the individual properties of specific lens models, as these are already well presented in Narayan and Bartelmann (1996) and on a more intuitive ground in Refsdal and Surdej (1994). Wambsganss (1998) proposes a broad view on gravitational lensing in astronomy; the reviews by Fort and Mellier (1994) and Hattori et al. (1999) deal with lensing by galaxy clusters; microlensing in the Galaxy and the local group is reviewed by Paczyński (1996) and a general panorama on weak lensing is given by Bartelmann and Schneider (1999) and Mellier (1999). The monograph on the theory of gravitational lensing by Schneider, Ehlers and Falco (1992) also remains a reference in the field. Received 4 April 2000 / Published online 9 August 2000     

NICMOS images of JVAS/CLASS gravitational lens systems

We present Hubble Space Telescope ( HST ) infrared images of four gravitational lens systems from the JVAS/CLASS gravitational lens survey and compare the new infrared HST pictures with previously published WFPC2 HST optical images and radio maps. Apart from the wealth of information that we get from the flux ratios and accurate positions and separations of the components of the lens systems, which we can use as inputs for better constraints on the lens models, we are able to discriminate between reddening and optical/radio microlensing as the possible cause of differences observed in the flux ratios of the components across the three wavelength bands. Substantial reddening has been known to be present in the lens system B1600+434 and has been further confirmed by the present infrared data. In the two systems B0712+472 and B1030+074 microlensing has been pinpointed as the main cause of the flux ratio discrepancy both in the optical/infrared and in the radio, the radio possibly caused by the substructure revealed in the lensing galaxies. In B0218+357, however, the results are still not conclusive. If we are actually seeing the two 'true' components of the lens system then the flux ratio differences are attributed to a combination of microlensing and reddening or are alternatively the result of some variability in at least one of the images. Otherwise the second 'true' component of B0218+357 may be completely absorbed by a molecular cloud and the anomalous flux density ratios and large difference in separation between the optical/infrared and radio that we see can be explained by emission either from a foreground object or from part of the lensing galaxy.     

Analysis of the Difference Light Curve of the Gravitational Mirage QSO 0957+561

From optical data of Q0957+561 for 1995/1996 seasons (in the g band) and 1996/1997 seasons (in the R band) they are derived upper limits on the amplitude of the rapid microlensing fluctuations of 0.07 mag and 0.05 mag, respectively. The photometry in the R band (1996/1997 seasons) was taken at Teide Observatory (IAC-80 Telescope), Instituto de Astrofisica de Canarias, Spain. In this contribution, we also study the microlensing history (from 1981 to 1996) of the Twin QSO, which is a curious system with two optical images of similar brightness. This revised version was published online in July 2006 with corrections to the Cover Date.     

On the mass and distance of the Quasi-Stellar Object 3C 273

For the QSO 3C 273 we derive, on the basis of two different theoretical models, expressions for a lower limit to the mass of the QSO, as a function of its distance. We conclude that an appreciable gravitational redshift component is consistent with the observational data only if the QSO mass is at least Galactic in magnitude. The setting of an independent upper limit to the QSO massM10¹⁰ M could indicate that the QSO redshift is predominantly cosmological in nature.     

IRAS F02044+0957: An interacting system

A list of 750 objects has been compiled using the Astrophysical CATalogs Support System (CATS) database, by cross-identifying sources in the IRAS catalogues and the catalogue of the Texas survey at 365 MHz. We have carried out a search for optical counterparts of those objects, where the difference in positions between the two catalogues and the APM is less than 3. One of these sources, IRAS F02044+0957, was observed with the RATAN-600 radio telescope at four frequencies in April 1999. Optical spectroscopy of the components of the system was made with the 2.1-m telescope of the Guillermo Haro Observatory. The radio and optical spectra, the NVSS radio map and the optical and infrared images allow us to conclude that the steep spectrum (=–0-94 ± 0-02) radio source IRAS F02044+0957 is a pair of interacting galaxies, a LINER and a HII galaxy, at z=0.093.Published in Astrofizika, Vol. 48, No. 1, pp. 113–124 (February 2005).     

Contribution of Vacuum Field to Angular Deviation of Light Path and Radar Echo Delay

The discovery of ‘twin quasistellar objects’ arose interests among astronomers and astrophysicists to study gravitational lensing problems. The deviation of light from its straight line path is caused by two sources according to the general theory of relativity: (i) the presence of massive objects, i.e. the presence of gravitational field and (ii) the presence of a ‘vacuum field’ which arises because there is a non-zero cosmological vacuum energy. Recently, the research on the relationship between cosmological constant and gravitational lensing process is rather active (see reference [1, 2, 3]. According to the Kottler space time metric, we have deduced an explicit representation of the angular deviation of light path. The deviation term is found to be simply , where M is the mass of the ‘astronomical lens’, r_min is the distance between the point of nearest approach and the centre of M, other symbols have their usual meaning. The presence of this term may be meaningful to the study of cosmological constant using the concept of gravitational lensing; however more sophisticated analysis awaits. Consider a signal radar to be sent from one planet to another. We have found that the radar echo delay contributed by the existence of the cosmological constant Λ is expressible as This revised version was published online in July 2006 with corrections to the Cover Date.     

Feeding Monsters – A Study of Active Galaxies

We present new interferometric observations of the molecular gas distribution and kinematics in the nuclei of different active galaxies at high angular resolution and high sensitivity carried out with the IRAM Plateau de Bure interferometer (PdBI). The observations cover galaxies in a redshift range of 0.03–1.4. We have so far observed five different active galaxies: NGC3718, NGC1068, HE1029–1831, 3C48 and Q0957+561. The first two objects belong to the NUGA (NUclei of GAlaxies) project – an international collaboration mainly between Spain, France and Germany – containing about 30 Seyfert and LINER galaxies. Both are at the same distance of 14Mpc and show a warped gas and dust disk. The new mosaic map of NGC3718 corrected for short-spacing effects with IRAM 30 m observations well demonstrates the existence of the warped gas disk with several secondary maxima in the projected gas distribution caused by orbit crowding effects. Based on these new data we have improved recent simulations of the warped disk in NGC3718. HE1029–1831 and 3C48 are nearby QSOs. HE1029–1831 is taken out of the Cologne nearby QSO sample. The maps of the integrated CO(1–0) and CO(2–1) emission clearly show that the molecular gas is mostly related to the central bulge with a non-negligible fraction distributed along the bar known from optical observations. Our new CO data of 3C48 unveil new information about the kinematics of its molecular gas complementing and improving further studies based on previous infrared observations and detailed multi-particle simulations. Finally, new measurements of the CO(1–0) line in Q0957+561 – a highly-red-shifted, gravitationally lensed quasar – will be presented as a link to earlier evolutionary stages of active galaxies and their hosts.     

ARCS, the Arcminute Radio Cluster-lens Search – I. Selection criteria and initial results

We present the results of an unbiased radio search for gravitational lensing events with image separations between 15 and 60 arcsec, which would be associated with clusters of galaxies with masses >10^13–14 M_⊙. A parent population of 1023 extended radio sources stronger than 35 mJy with stellar optical identifications was selected using the FIRST radio catalogue at 1.4 GHz and the APM optical catalogue. The FIRST catalogue was then searched for companions to the parent sources stronger than 7 mJy and with separation in the range 15 to 60 arcsec. Higher-resolution observations of the resulting 38 lens candidates were made with the VLA at 1.4 and 5 GHz, and with MERLIN at 5 GHz in order to test the lens hypothesis in each case. None of our targets was found to be a gravitational lens system. These results provide the best current constraint on the lensing rate for this angular scale, but improved calculations of lensing rates from realistic simulations of the clustering of matter on the relevant scales are required before cosmologically significant constraints can be derived from this null result. We now have an efficient, tested observational strategy with which it will be possible to make an order-of-magnitude larger unbiased search in the near future.     

The rise and fall of OJ 287 in 1983–1984: Periodicities and wavelength-dependent delays

The flare behaviour of OJ 287 during the years 1983 and 1984 is studied on the basis of all the available data in the different wavebands from these years. A strong correlation between the flux and the spectral index is found in the optical wavebands. At the same time there is weak evidence for a 9.3 days periodicity in the same waveband. Time lags of the flares and sharp drops are studied between different wavebands. The sharp events seem to be simultaneous in the region ultraviolet (0.25 m)-infrared (25 m) but between these and millimeter wavelengths there seems to be a timelag of 40–60 days so that the short wavelength variations precede the radio ones. In longer wavelengths (>3 cm) the variations are much slower and it is very difficult to find the exact delay times.     

Optical and X-ray variability of Seyfert galaxies NGC 5548, NGC 7469, NGC 3227, NGC 4051, NGC 4151, Mrk 509, Mrk 79, and Akn 564 and quasar 1E 0754

The results of a cross-correlation analysis of the optical and X-ray light curves for eight Seyfert galaxies, NGC 5548, NGC 7469, NGC 3227, NGC 4051, NGC 4151, Mrk 509, Mrk 79, and Akn 564 and for the optical spectra of the quasar 1E 0754 are presented. In the case of the galaxies NGC 5548 and NGC 7469, the maximum values of cross-correlation coefficients for optical and X-ray variations proved to be high (0.73 and 0.79, respectively). The lag time, determined from the maximum of cross-correlation function, is 2.800 _?1.58 ^+3.12 days for NGC 5548 and 0.6 _?4.3 ^+0.9 days for NGC 7469. This result favors downscattering of the X-ray emission into the optical range (direct Compton effect) for NGC 5548 and NGC 7469. In addition to the main maximum, which corresponds to the lag of the optical flux variations behind the X-ray flux, six objects (excluding Akn 564 and NGC 4151) show the wings in the intervals of cross-correlation functions that correspond to the time lags of X-ray emission behind the optical emission of approximately 10 days. A method of determining the masses of central black holes in AGNs through spectral line widths is presented; with this method the mass of the central black hole in the quasar 1E 0754 was found (M _BH = 1.01 × 10⁸ M _sun). The position of the quasar 1E 0754 in the mass-luminosity diagram meets the position of other NLS 1 galaxies.     

QSO lensing magnification associated with galaxy groups

We simulated both the matter and light (galaxy) distributions in a wedge of the Universe and calculated the gravitational lensing magnification caused by the mass along the line-of-sight of galaxies and galaxy groups identified in sky surveys. A large volume redshift cone containing cold dark matter particles mimics the expected cosmological matter distribution in a flat universe with low matter density and a cosmological constant. We generate a mock galaxy catalogue from the matter distribution and identify thousands of galaxy groups in the luminous sky projection. We calculate the expected magnification around galaxies and galaxy groups and then the induced quasi-stellar object (QSO)–lens angular correlation due to magnification bias. This correlation is observable and can be used both to estimate the average mass of the lens population and to make cosmological inferences. We also use analytical calculations and various analyses to compare the observational results with theoretical expectations for the cross-correlation between faint QSOs from the 2dF Survey and nearby galaxies and groups from the Automated Plate Measurement and Sloan Digital Sky Survey Early Data Release. The observed QSO–lens anticorrelations are stronger than the predictions for the cosmological model used. This suggests that there could be unknown systematic errors in the observations and data reduction, or that the model used is not adequate. If the observed signal is assumed to be solely due to gravitational lensing, then the lensing is stronger than expected, due to more massive galactic structures or more efficient lensing than simulated.     

Illuminating protogalaxies? The discovery of extended Lyman-α emission around a QSO at z=4.5

We have discovered extended Lyman-α emission around a z=4.5 QSO in a deep long-slit spectrum with Keck/LRIS at moderate spectral resolution (R≈ 1000). The line emission extends 5 arcsec beyond the continuum of the QSO and is spatially asymmetric. This extended line emission has a spectral extent of 1000km/s, much narrower in velocity spread than the broad Lyman-α from the QSO itself and slightly offset in redshift. No evidence of continuum is seen for the extended emission line region, suggesting that this recombination line is powered by reprocessed QSO Lyman continuum flux rather than by local star formation. This phenomenon is rare in QSOs which are not radio loud, and this is the first time it has been observed at z>4. It seems likely that the QSO is illuminating the surrounding cold gas of the host galaxy, with the ionizing photons producing Lyman-α fluorescence. As suggested by Haiman and Rees (2001), this `fuzz' around a distant quasar may place strong constraints on galaxy formation and the extended distribution of cold, neutral gas. This revised version was published online in August 2006 with corrections to the Cover Date.