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1. Time delay lens modelling challenge

   https://doi.org/10.1093/mnras/stab484  

   Ding, X ; Treu, T ; Birrer, S ; Chen, G C-F ; Coles, J ; Denzel, P ; Frigo, M ; Galan, A ; Marshall, P J ; Millon, M ; et al ( March 2021 , Monthly Notices of the Royal Astronomical Society)

   null (Ed.)

   ABSTRACT In recent years, breakthroughs in methods and data have enabled gravitational time delays to emerge as a very powerful tool to measure the Hubble constant H0. However, published state-of-the-art analyses require of order 1 yr of expert investigator time and up to a million hours of computing time per system. Furthermore, as precision improves, it is crucial to identify and mitigate systematic uncertainties. With this time delay lens modelling challenge, we aim to assess the level of precision and accuracy of the modelling techniques that are currently fast enough to handle of order 50 lenses, via the blind analysis of simulated data sets. The results in Rungs 1 and 2 show that methods that use only the point source positions tend to have lower precision ($10\!-\!20{{\ \rm per\ cent}}$) while remaining accurate. In Rung 2, the methods that exploit the full information of the imaging and kinematic data sets can recover H0 within the target accuracy (|A| < 2 per cent) and precision (<6 per cent per system), even in the presence of a poorly known point spread function and complex source morphology. A post-unblinding analysis of Rung 3 showed the numerical precision of the ray-traced cosmological simulations to be insufficient to test lens modelling methodology at the percent level, making the results difficult to interpret. A new challenge with improved simulations is needed to make further progress in the investigation of systematic uncertainties. For completeness, we present the Rung 3 results in an appendix and use them to discuss various approaches to mitigating against similar subtle data generation effects in future blind challenges. 

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2. TDCOSMO: X. Automated modeling of nine strongly lensed quasars and comparison between lens-modeling software

   https://doi.org/10.1051/0004-6361/202244909  

   Ertl, S. ; Schuldt, S. ; Suyu, S. H. ; Schmidt, T. ; Treu, T. ; Birrer, S. ; Shajib, A. J. ; Sluse, D. ( April 2023 , Astronomy & Astrophysics)

   When strong gravitational lenses are to be used as an astrophysical or cosmological probe, models of their mass distributions are often needed. We present a new, time-efficient automation code for the uniform modeling of strongly lensed quasars withGLEE, a lens-modeling software for multiband data. By using the observed positions of the lensed quasars and the spatially extended surface brightness distribution of the host galaxy of the lensed quasar, we obtain a model of the mass distribution of the lens galaxy. We applied this uniform modeling pipeline to a sample of nine strongly lensed quasars for which images were obtained with the Wide Field Camera 3 of theHubbleSpace Telescope. The models show well-reconstructed light components and a good alignment between mass and light centroids in most cases. We find that the automated modeling code significantly reduces the input time during the modeling process for the user. The time for preparing the required input files is reduced by a factor of 3 from ~3 h to about one hour. The active input time during the modeling process for the user is reduced by a factor of 10 from ~ 10 h to about one hour per lens system. This automated uniform modeling pipeline can efficiently produce uniform models of extensive lens-system samples that can be used for further cosmological analysis. A blind test that compared our results with those of an independent automated modeling pipeline based on the modeling softwareLenstronomyrevealed important lessons. Quantities such as Einstein radius, astrometry, mass flattening, and position angle are generally robustly determined. Other quantities, such as the radial slope of the mass density profile and predicted time delays, depend crucially on the quality of the data and on the accuracy with which the point spread function is reconstructed. Better data and/or a more detailed analysis are necessary to elevate our automated models to cosmography grade. Nevertheless, our pipeline enables the quick selection of lenses for follow-up and further modeling, which significantly speeds up the construction of cosmography-grade models. This important step forward will help us to take advantage of the increase in the number of lenses that is expected in the coming decade, which is an increase of several orders of magnitude.

    

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3. Find the haystacks, then look for needles: the rate of strongly lensed supernovae in galaxy–galaxy strong gravitational lenses

   https://doi.org/10.1093/mnras/stae2486  

   Sainz de Murieta, Ana ; Collett, Thomas_E ; Magee, Mark_R ; Pierel, Justin_D_R ; Enzi, Wolfgang_J_R ; Lokken, Martine ; Gagliano, Alex ; Ryczanowski, Dan ( November 2024 , Monthly Notices of the Royal Astronomical Society)

   The time-delay between appearances of multiple images of a gravitationally lensed supernova (glSN) is sensitive to the Hubble constant, $H_0$. As well as time-delays, a lensed host galaxy is needed to enable precise inference of $H_0$. In this work, we investigate the connection between discoverable lensed transients and their host galaxies. We find that the Legacy Survey of Space and Time (LSST) will discover at least 90 glSNe per year, of which 54 per cent will also have a strongly lensed host. The rates are uncertain by approximately 30 per cent depending primarily on the choice of the unlensed SN population and uncertainties in the redshift evolution of the deflector population, but the fraction of glSNe with a lensed host is consistently around a half. LSST will discover around 20 glSNe per year in systems that could plausibly have been identified by Euclid as galaxy–galaxy lenses before the discovery of the glSN. Such systems have preferentially longer time-delays and therefore are well suited for cosmography. We define a golden sample of glSNe Ia with time-delays over 10 d, image separations greater than 0.8 arcsec, and a multiply imaged host. For this golden sample, we find 91 per cent occur in systems that should already be discoverable as galaxy–galaxy lenses in Euclid. For cosmology with glSNe, monitoring Euclid lenses is a plausible alternative to searching the entire LSST alert stream.

    

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4. TDCOSMO: III. Dark matter substructure meets dark energy. The effects of (sub)halos on strong-lensing measurements of H 0

   https://doi.org/10.1051/0004-6361/202038829  

   Gilman, D. ; Birrer, S. ; Treu, T. ( October 2020 , Astronomy & Astrophysics)

   Time delay cosmography uses the arrival time delays between images in strong gravitational lenses to measure cosmological parameters, in particular the Hubble constant H 0 . The lens models used in time delay cosmography omit dark matter subhalos and line-of-sight halos because their effects are assumed to be negligible. We explicitly quantify this assumption by analyzing mock lens systems that include full populations of dark matter subhalos and line-of-sight halos, applying the same modeling assumptions used in the literature to infer H 0 . We base the mock lenses on six quadruply imaged quasars that have delivered measurements of the Hubble constant, and quantify the additional uncertainties and/or bias on a lens-by-lens basis. We show that omitting dark substructure does not bias inferences of H 0 . However, perturbations from substructure contribute an additional source of random uncertainty in the inferred value of H 0 that scales as the square root of the lensing volume divided by the longest time delay. This additional source of uncertainty, for which we provide a fitting function, ranges from 0.7 − 2.4%. It may need to be incorporated in the error budget as the precision of cosmographic inferences from single lenses improves, and it sets a precision limit on inferences from single lenses. 

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5. TDCOSMO III: Dark matter substructure meets dark energy -- the effects of (sub)halos on strong-lensing measurements of H0

   Gilman, Daniel ; Birrer, Simon ; Treu, Tommaso ( July 2020 , Astronomy astrophysics)

   Time delay cosmography uses the arrival time delays between images in strong gravitational lenses to measure cosmological parameters, in particular the Hubble constant H0. The lens models used in time delay cosmography omit dark matter subhalos and line-of-sight halos because their effects are assumed to be negligible. We explicitly quantify this assumption by analyzing realistic mock lens systems that include full populations of dark matter subhalos and line-of-sight halos, applying the same modeling assumptions used in the literature to infer H0. We base the mock lenses on six quadruply-imaged quasars that have delivered measurements of the Hubble constant, and quantify the additional uncertainties and/or bias on a lens-by-lens basis. We show that omitting dark substructure does not bias inferences of H0. However, perturbations from substructure contribute an additional source of random uncertainty in the inferred value of H0 that scales as the square root of the lensing volume divided by the longest time delay. This additional source of uncertainty, for which we provide a fitting function, ranges from 0.6−2.4%. It may need to be incorporated in the error budget as the precision of cosmographic inferences from single lenses improves, and sets a precision limit on inferences from single lenses. 

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