We present narrowband images of the gravitational lens system Q 2237 + 0305 made with the Nordic Optical Telescope in eight different filters covering the...

Context. PKS 1510-089 is a flat spectrum radio quasar strongly variable in the optical and GeV range. To date, very high-energy (VHE, > 100 GeV) emission has...

We determine the extinction curve in the zl=0.83 lens galaxy of the gravitational lens SBS 0909+532 from the wavelength dependence of the flux ratio between the...

Context. QSO B0218+357 is a gravitationally lensed blazar located at a redshift of 0.944. The gravitational lensing splits the emitted radiation into two...

We study a recently observed gravitational microlensing peak in the V-band light curve of Q2237+0305A using a relatively simple physical model, but one that is...

The sub-population of quasars reddened by intrinsic or intervening clouds of dust are known to be underrepresented in optical quasar surveys. By defining a...

We present the serendipitous discovery of a physical pair of quasars with estimated cosmological redshift z = 1.76. Long-slit spectroscopic observations of the...

We report on the discovery of very high energy (VHE) γ-ray emission from the BL Lacertae object 1ES 1011+496. The observation was triggered by an optical...

The most frequently proposed model for the origin of quasars holds that the high accretion rates seen in luminous active galactic nuclei (AGN) are primarily...