We report the discovery of a transient source in the central regions of galaxy cluster A267. The object, which we call "PALS-1," was found in a survey aimed at identifying highly magnified Lyman break galaxies in the fields of intervening rich clusters. At discovery, the source had Un > 24.7 (2 σ; AB), g = 21.96 ± 0.12, and very blue g - r and r - i colors; i.e., PALS-1 was a "U-band dropout," characteristic of star-forming galaxies and quasars at z ∼3. However, 3 months later the source had faded by more than 3 mag. Further observations showed a continued decline in luminosity, to R > 26 A at 7 months after discovery. Although the apparent brightness suggests a supernova at roughly the cluster redshift, we show that the photometry and light curve argue against any known type of supernova at any redshift. The spectral energy distribution and location near the center of a galaxy cluster are consistent with the hypothesis that PALS-1 is a gravitationally lensed transient at z ≈ 3.3. If this interpretation is correct, the source is magnified by a factor of 4-7, and two counterimages are predicted. Our lens model predicts that there are time delays between the three images of 1-10 yr and that we have witnessed the final occurrence of the transient. The intense luminosity (MAB ∼-23.5 after correcting for lensing) and blue UV continuum (implying T ≳ 50,000 K) argue that the source may have been a flare resulting from the tidal disruption of a star by a 106-108 M· black hole. Regardless of its physical nature, PALS-1 highlights the importance of monitoring regions of high magnification in galaxy clusters for distant time-varying phenomena.

Galaxies and galaxy groups located along the line of sight towards gravitationally lensed quasars produce high-order perturbations of the gravitational potential at the lens position. When these perturbation are too large, they can induce a systematic error on H0 of a few per cent if the lens system is used for cosmological inference and the perturbers are not explicitly accounted for in the lens model. In this work, we present a detailed characterization of the environment of the lens system WFI 2033−4723 (zsrc = 1.662, zlens = 0.6575), one of the core targets of the H0LiCOW project for which we present cosmological inferences in a companion paper. We use the Gemini and ESO-Very Large telescopes to measure the spectroscopic redshifts of the brightest galaxies towards the lens, and use the ESO-MUSE integral field spectrograph to measure the velocity-dispersion of the lens (σlos = 250+−1521 km s−1) and of several nearby galaxies. In addition, we measure photometric redshifts and stellar masses of all galaxies down to i < 23 mag, mainly based on Dark Energy Survey imaging (DR1). Our new catalogue, complemented with literature data, more than doubles the number of known galaxy spectroscopic redshifts in the direct vicinity of the lens, expanding to 116 (64) the number of spectroscopic redshifts for galaxies separated by less than 3 arcmin (2 arcmin) from the lens. Using the flexion-shift as a measure of the amplitude of the gravitational perturbation, we identify two galaxy groups and three galaxies that require specific attention in the lens models. The ESO MUSE data enable us to measure the velocity-dispersions of three of these galaxies. These results are essential for the cosmological inference analysis presented in Rusu et al.