Vyacheslav N. Shalyapin

Recently, the TDCOSMO Collaboration presented a sample of eight doubly imaged quasars (doubles) that will be used to measure the Hubble constant from time-delay cosmography. While multi-band observations of this sample with the Hubble Space Telescope enabled the TDCOSMO Collaboration to make an initial reconstruction of the Fermat potential difference between the two images of each double (assuming an isothermal mass distribution of the lensing galaxy plus external shear), three lens redshifts remain unmeasured or without a direct spectroscopic confirmation, and these are crucial ingredients for time-delay cosmography and measuring the Hubble constant using this sample. From archive spectroscopic observations, we extracted the spectra of the two lensing galaxies with an as-yet-unknown redshift, and obtained z _l = 0.595 ± 0.001 for DES J0407−5006 and z _l = 0.255 ± 0.001 for DES J0602−4335 . Additionally, archive spectroscopic data of SDSS J1515+1511 allowed us to confirm the previously adopted value for the lens redshift and estimate its uncertainty, z _l = 0.742 ± 0.001. We also discussed the role that deviations from isothermality and external convergences can play in a time-delay cosmography from the sample of doubles.

We analyze microlensing variability in 15 seasons of optical monitoring data and 4 epochs of new X-ray observations of the doubly imaged gravitationally lensed quasar SDSS J133907.23+131038.6 to place empirical constraints on the size and structure of that system’s X-ray and optical continuum emission regions. Employing a Bayesian Monte Carlo method, we analyzed ground-based optical light curves to constrain the half-light radius of the far-UV source log ( r 1 / 2 , FUV / cm ) = 15.7 8 − 0.28 + 0.26 at 1930 Å, the rest-frame center of the r band, assuming a 60° inclination angle. This size corresponds to ∼100 r _g for a 4.0 × 10 ⁸ M _⊙ black hole. We measured the half-light radius of the full band (0.2−8.0 keV) X-ray continuum emission region log ( r 1 / 2 , X full / cm ) = 14.3 2 − 0.31 + 0.23 , a size measurement that is consistent with the radius of the innermost stable circular orbit (ISCO) in the Schwarzschild metric. Two shifted Fe K α lines caused by microlensing are detected in the stacked spectrum of image A at 5.9 and 8.9 keV at >99% significance.

We present extended optical monitoring of the quadruply-imaged gravitationally lensed quasar QSO 2237+0305, the Einstein Cross, including observations from different observatories in both hemispheres and using a new photometric technique. This technique uses a region far enough from the lens system to accurately determine the sky background level and minimises contamination from the lensing galaxy by combining analytical and numerical modelling of its structure. The resulting light curves of the four quasar images describe variations across practically the entire optical spectrum and span about 9000 days in the VRI bands. We captured the multi-band microlensing variability with an unprecedented level of detail, and a preliminary microlensing analysis reveals an almost linear scaling of the source radius with wavelength, providing direct evidence for the wavelength-dependent structure of the region contributing to optical passband fluxes. Specifically, assuming a mean microlens mass ⟨ M ⟩ = 0.3 M _⊙ and concentric Gaussian sources that move according to the velocity distribution peaks (speed and direction) reported in a previous microlensing analysis, we find that the half-light radius of the g -band source is 9.6 ± 2.7 lt-day and the size of the sources grows with wavelength, with a power-law index of α = 0.94 ± 0.05. We conclude that these long-term light curves set stringent empirical constraints on models of quasar emission and microlensing physics.

Although dark matter in galaxies may consist of elementary particles different from those that make up ordinary matter and that would be smoothly distributed (still undetected), the so-called primordial black holes (PBHs) formed soon after the initial Big Bang are also candidates to account for a certain fraction of mass in galaxies. In this paper, we focus on the main lensing galaxy (z = 0.260) of the doubly imaged, gravitationally lensed quasar FBQ 0951+2635 (z = 1.246) for probing possible PBH populations. Assuming that the mass of the galaxy is due to smoothly distributed matter (SDM), stars, and PBHs, the 16-yr microlensing variability observations were compared in detail with simulated microlensing signals generated by 90 different physical scenarios. Among other details, the simulated signals were sampled as the observed one, and the observed variability in its entirety and over the long term were used separately for comparison. While none of the scenarios considered can reproduce the overall observed signal, the observed long-term variability favours a small mass fraction in PBHs with a mass of the order of the mean stellar mass. Furthermore, it is possible to obtain strong constraints on the galaxy mass fraction in Jupiter-mass PBHs, provided that a reverberation-based measurement of the source size is available and relatively small. To constrain the mass fraction in ∼10 M_⊙ PBHs, light curves five times longer are probably required.

We conducted long-term monitoring of the doubly imaged gravitationally lensed quasar SDSS J1001+5027 consisting of spectro-photometric observations separated by ∼120 days (time delay between both quasar images), as well as test and auxiliary data. This monitoring approach allowed us to reliably find a strong microlensing-induced chromatic variation of the quasar continuum in the period 2022–2025. The ongoing microlensing event has caused the delay-corrected spectral flux ratio in 2025 to have a dramatic changing look, opening the door to very promising observations of the system in the coming years. These future follow-up observations of such a rare event are expected to provide critical information to discuss, among other things, the structure of the inner accretion flow towards the central supermassive black hole in SDSS J1001+5027.

We conducted long-term monitoring of the doubly imaged gravitationally lensed quasar SDSS J1001+5027 consisting of spectro-photometric observations separated by $\sim$120 days (time delay between both quasar images), as well as test and auxiliary data. This monitoring approach allowed us to reliably find a strong microlensing-induced chromatic variation of the quasar continuum in the period 2022$-$2025. The ongoing microlensing event has caused the delay-corrected spectral flux ratio in 2025 to have a dramatic changing look, opening the door to very promising observations of the system in the coming years. These future follow-up observations of such a rare event are expected to provide critical information to discuss, among other things, the structure of the inner accretion flow towards the central supermassive black hole in SDSS J1001+5027.

We report the spectroscopic confirmation of a bright blue Einstein ring in the Kilo-Degree Survey (KiDS) footprint: the Einstein “blue eye.” Spectroscopic data from X-Shooter at the Very Large Telescope (VLT) show that the lens is a typical early-type galaxy (ETG) at z _l = 0.9906, while the background source is a Lyα emitter at z _s = 2.823. The reference lens modeling was performed on a high-resolution Y-band adaptive-optics image from HAWK-I at VLT. Assuming a singular isothermal ellipsoid total mass density profile, we inferred an Einstein radius R _Ein = 10.47 ± 0.06 kpc. The average slope of the total mass density inside the Einstein radius, as determined by a joint analysis of lensing and isotropic Jeans equations, is γ tot = 2.1 4 − 0.07 + 0.06 , showing no systematic deviation from the slopes of lower-redshift galaxies. This can be the evidence of ETGs developing through dry mergers plus moderate dissipationless accretion. Stellar population analysis with eight-band (gri ZYJHK _s ) photometries from KiDS and VIKING shows that the total stellar mass of the lens is M* = (3.95 ± 0.35) × 10¹¹ M _⊙ (Salpeter initial mass function (IMF)), implying a dark matter fraction inside the effective radius of f _DM = 0.307 ± 0.151. We finally explored the dark matter halo slope and found a strong degeneracy with the dynamic stellar mass. Dark matter adiabatic contraction is needed to explain the posterior distribution of the slope, unless an IMF heavier than Salpeter is assumed.

As there is a long-standing controversy over the time delay between the two images of the gravitationally lensed quasar FBQ 0951+2635, we combined early and new optical light curves to robustly measure a delay of 13.5 ± 1.6 d (1σ interval). The new optical records covering the last 17 yr were also used to trace the long-timescale evolution of the microlensing variability. Additionally, the new time-delay interval and a relatively rich set of further observational constraints allowed us to discuss the mass structure of the main lensing galaxy at a redshift of 0.26. This lens system is of particular interest because the external shear from secondary gravitational deflectors is relatively low, but the external convergence is one of the highest known. When modelling the galaxy as a singular power-law ellipsoid without hypotheses or priors on the power-law index, ellipticity, and position angle, we demonstrated that its mass profile is close to isothermal, and there is a good agreement between the shape of the mass distribution and that of the near-IR light. We also recovered the true mass scale of the galaxy. Finally, a constant mass-to-light ratio model also worked acceptably well.

As there is a long-standing controversy over the time delay between the two images of the gravitationally lensed quasar FBQ 0951+2635, we combined early and new optical light curves to robustly measure a delay of 13.5 +/- 1.6 d (1sigma interval). The new optical records covering the last 17 yr were also used to trace the long-timescale evolution of the microlensing variability. Additionally, the new time delay interval and a relatively rich set of further observational constraints allowed us to discuss the mass structure of the main lensing galaxy at redshift 0.26. This lens system is of particular interest because the external shear from secondary gravitational deflectors is relatively low, but the external convergence is one of the highest known. When modelling the galaxy as a singular power-law ellipsoid without hypotheses/priors on the power-law index, ellipticity and position angle, we demonstrated that its mass profile is close to isothermal, and there is good alignment between mass and near-IR light. We also recovered the true mass scale of the galaxy. Finally, it is worth mentioning that a constant mass-to-light ratio model also worked acceptably well.

To better understand which sources contribute to optical passband fluxes of Q0957+561, we present and analyse light curves of the doubly imaged gravitationally lensed quasar from its discovery to 2023. After an early microlensing event, the difference light curves (describing delay-corrected flux ratios between the two quasar images) only show prominent microlensing gradients over the last 17 yr. In addition to these long time-scale gradients in the gr bands, we detect short time-scale, extrinsically induced differential variations that are highly correlated with the short time-scale intrinsic variability of the quasar in those bands. Both the accretion disc and the broad emission-line region (BELR) contribute to optical passband fluxes, and we also show that realistic contributions of the BELR account for the observed correlations in the gr bands. We would like to highlight that the BELR sources of optical passband fluxes of Q0957+561 should be taken into account when measuring accretion-disc source sizes from microlensing simulations.