Related papers: Optical Continuum Sources in Gravitationally Lense…

We present multi-frequency radio continuum VLBI observations of the gravitational lens system B0218+357 carried out using a global VLBI network and the VLBA. The source has been observed with resolutions from 0.2 milliarcsec to 5…

We demonstrate that gravitationally lensed quasars are easily recognized using image subtraction methods as time variable sources that are spatially extended. For Galactic latitudes |b|>20 deg, lensed quasars dominate the population of…

We report on a program of spectroscopic observations of gravitationally-lensed QSOs with multiple images. We seek to establish whether microlensing is occurring in each QSO image using only single-epoch observations. We calculate flux…

This paper presents results of a search for short-term variability in the optical band of selected high-luminosity, high-redshift radio-quiet quasars. Each quasar has been monitored typically for 2 - 4 hours with a time resolution of 2 - 5…

As the Universe expands, the redshift of distant sources changes with time. Here we discuss gravitational lensing phenomena that are consequence of the redshift drift between lensed source, gravitational lens, and observer. When the source…

We present 21 new long-term variable radio sources found commensally in two years of weekly MeerKAT monitoring of the low-mass X-ray binary GX 339-4. The new sources vary on time scales of weeks to months and have a variety of light curve…

We report on the second installment of an X-ray monitoring project of seven luminous radio-quiet quasars (RQQs). New {\sl Chandra} observations of four of these, at $4.10\leq z\leq4.35$, yield a total of six X-ray epochs, per source, with…

In previous studies we have shown that the optical variability of quasars increases, on average, with redshift. We explained this dependance in terms of a hardening of the spectrum during bright phases, coupled with the increase of the…

The aim of this contribution is to present the two first phases of the optical monitoring programme of the Gravitational Lenses group at the Universidad de Cantabria (GLUC, http://grupos.unican.es/glendama/). In an initial stage (2003…

Gravitational lensing offers unique opportunities to learn about the astrophysical origin of distant sources, the abundance of intervening objects acting as lenses, and gravity and cosmology in general. However, all this information can…

In 2021 BL Lacertae underwent an extraordinary activity phase, which was intensively followed by the Whole Earth Blazar Telescope (WEBT) Collaboration. We present the WEBT optical data in the BVRI bands acquired at 36 observatories around…

We collected from literature the information about large-separated pairs of QSOs, which however once were suspected as gravitationally lensed system. We discuss some new results on time delay determinations including optical-radio…

We present a new method for determining time delays among the light curves of various images in a gravitational lens. The method is based on constructing a simple model for the source variation and forming a chi squared measure of the…

In the last few years, a mysterious new class of astrophysical objects has been uncovered. These are spatially coincident with the nuclei of external galaxies and show X-ray variations that repeat on timescales of minutes to a month. They…

We report multi-epoch GMRT HI observations of the z = 0.3127 damped absorber towards the quasar PKS 1127-145, which reveal variability in both the absorption profile and the flux of the background source, over a time-scale of a few days.…

The V=16.9 quasar FBQ 0951+2635 at redshift z=1.24 appears double on CCD exposures taken in subarcsecond seeing. The two objects are separated by 1.1" and differ in brightness by 0.9 mag. VLA observations show the radio source to be double…

We present new spectroscopic and polarimetric observations of the first discovered gravitational lens Q0957+561 obtained with the 6m telescope of the Special Astrophysical Observatory (SAO, Russia). We explore spectropolarimetric parameters…

We present ROSAT All-Sky Survey and ROSAT pointed observations (PSPC and HRI) of a complete sample of 113 bright soft X-ray AGN selected from the ROSAT Bright Source Catalog. We compare these observations in order to search for extreme…

We report on an observing campaign in March 2001 to monitor the brightness of the later arriving Q0957+561 B image in order to compare with the previously published brightness observations of the (first arriving) A image. The 12…

Recent spectropolarimetry of QSOs has revealed a surprising rise in polarization in the Lyman continuum of several objects. We discuss several recent attempts to interpret this feature, including the role of Lyman limit absorption in PG…