arXiv subjects

Luke Weisenbach

Publications and source records attributed to Luke Weisenbach.

13 recordsLinked to original sources

arXiv2026

A Salpeter IMF and an NFW halo: Disentangling the dark and stellar mass of an elliptical galaxy through precise lens modelling of a double-source-plane system

We present a strong lensing analysis of the double source plane lens J0946+1006 (colloquially "Jackpot" lens) to measure the inner dark matter density profile, the stellar-to-halo mass ratio, and the stellar initial mass function normalisation using a two component stellar plus dark matter mass model. The stellar mass follows a multi-Gaussian expansion light model with a free global mass-to-light ratio and an allowed radial $M/L$ gradient, while the dark matter is described by an elliptical generalised NFW halo. The double-source-plane geometry provides additional leverage against the mass-sheet transformation and helps constrain the radial mass profile. Despite allowing both a radial stellar $M/L$ gradient and a generalised NFW halo, the data prefer the canonical picture: an approximately constant stellar mass-to-light ratio with a Salpeter-like IMF normalisation, and a dark matter halo consistent with NFW. We infer $M_{\star} = 4.4^{+0.25}_{-0.39}\times 10^{11}\,M_{\odot}$ and an inner halo slope $γ_{\rm in}^{\rm halo} = 1.04^{+0.10}_{-0.14}$. The halo mass is $M_{200}^{\rm halo} = 1.11^{+0.37}_{-0.32}\times 10^{13}\,M_{\odot}$, implying $\log_{10}(M_{200}/M_{\star})=1.41^{+0.13}_{-0.14}$. At fixed halo mass, the inferred stellar mass lies $\sim0.1$ dex above typical literature stellar halo mass relations at similar redshift, which is comparable to the intrinsic scatter of these relations. We expect this approach to provide a practical template for future dark matter studies with the large double-source-plane lens samples from Euclid.

astro-ph.GA↗

arXiv2026

Discovery of Quasar Variability and Early Accretion Disk Signatures at Cosmic Dawn

In the nearby universe, quasars are well known to exhibit variability in their brightness over time, offering a powerful tool to probe the physics of accretion onto the SMBH and directly measure the mass of the SMBH. However, detecting variability in early quasars remains challenging. Here, we report the detection of multi-wavelength infrared and X-ray variability in a quasar observed just 850 million years after the Big Bang. The infrared variability spans five filters, tracing rest-frame ultraviolet and optical emission from the accretion disk, while the X-ray variability probes the corona. The variable spectrum reveals that the accretion disk has a geometrically thin, optically thick structure. This provides observational constraints on the accretion disk structure at early times, when quasars are accreting at high Eddington ratios and reside in extreme environments. Our findings demonstrate the feasibility of characterizing accretion physics using variability in the early universe, laying the groundwork for studies exploiting upcoming facilities such as the Rubin Observatory and Roman Space Telescope. These facilities will discover large samples of variable high-redshift quasars, enabling population-level variability studies of accretion physics and black hole masses, filling key missing ingredients in understanding early SMBH growth.

astro-ph.GA↗

arXiv2026

Lensed stars in galaxy-galaxy strong lensing -- a JWST prediction for the Cosmic Horseshoe

We explore for the first time the possibility of detecting lensed star transients in galaxy-galaxy strong lensing systems upon repeated, deep imaging using the {\it James-Webb Space Telescope} ({\it JWST}). Our calculation predicts that the extremely high recent star formation rate of $\sim 140\,M_{\odot}\textrm{yr}^{-1}$ over the last 50 Myr (not accounting for image multiplicity) in the ``Cosmic Horseshoe'' lensed system ($z = 2.381$) generates many young, bright stars, of which their large abundance is expected to lead to a detection rate of $\sim 60$ transients per pointing in {\it JWST} observations with a $5σ$ limiting magnitude of $\sim 29\,m_{AB}$. With the high expected detection rate and little room for uncertainty for the lens model compared with cluster lenses, our result suggests that the Cosmic Horseshoe could be an excellent tool to test the nature of dark matter based on the spatial distribution of transients, and can be used to constrain axion mass if dark matter is constituted of ultra-light axions. We also argue that the large distance modulus of $\sim46.5\,$mag at $z \approx 2.4$ can act as a filter to screen out less massive stars as transients and allow one to better constrain the high-mass end of the stellar initial mass function based on the transient detection rate. Follow-up {\it JWST} observations of the Cosmic Horseshoe would allow one to better probe the nature of dark matter and the star formation properties, such as the initial mass function at the cosmic noon, via lensed star transients.

astro-ph.CO↗

arXiv2026

CROCS Data Release I: Constraints on the Hubble Constant

Recent cosmological surveys and datasets have highlighted a variety of tensions to the concordance model of our universe, $Λ$CDM. Of particular interest is the Hubble tension, the $5.5σ$ discrepancy between measurements of the Hubble constant $H_0$ using high redshift CMB data from Planck ($67.27\pm0.60$km$\text{s}^{-1}\text{Mpc}^{-1}$) and low redshift supernovae from SH0ES ($73.2\pm1.3$km$\text{s}^{-1}\text{Mpc}^{-1}$). To avoid stepping on any toes, we have initiated the CROCS collaboration to resolve this tension, gathering experts from across many fields of cosmology, astrophysics, astronomy, machine learning, data science, philosophy, and astrology. In this paper, we present findings from CROCS Data Release 1, corresponding to the first $\sim3$ days and 27 minutes (rest frame) of observation. We perform a robust statistical analysis, showing that Planck and SH0ES both suffer from imperial biasing systematics (IBS) at $5σ$ significance. Accounting for these errors by converting to metric units reconciles the high and low redshift data, with $H_0 = 69.00\pm0.420$km$\text{s}^{-1}\text{Mpc}^{-1}$. We thus report that our results are sufficient to end the Hubble tension for good.

astro-ph.CO↗

arXiv2025

Microlensing of lensed supernovae Zwicky & iPTF16geu: constraints on the lens galaxy mass slope and dark compact object fraction

To date, only two strongly lensed type Ia supernovae (SNIa) have been discovered with an isolated galaxy acting as the lens: iPTF16geu and SN Zwicky. The observed image fluxes for both lens systems were inconsistent with predictions from a smooth macro lens model. A potential explanation for the anomalous flux ratios is microlensing: additional (de)magnification caused by stars and other compact objects in the lens galaxy. In this work, we combine observations of iPTF16geu and SN Zwicky with simulated microlensing magnification maps, leveraging their standardizable candle properties to constrain the lens galaxy mass slope, $η$, and the fraction of dark compact objects, $f_{\rm dc}$. The resulting mass slopes are $η= 1.70 \pm 0.07$ for iPTF16geu and $η= 1.81 \pm 0.10$ for SN Zwicky. Our results indicate no evidence for a population of dark compact objects, placing upper limits at the $95\%$ confidence level of $f_{\rm dc} < 0.25$ for iPTF16geu and $f_{\rm dc} < 0.47$ for SN Zwicky (for compact objects with masses above $ 0.02 M_{\odot}$). Assuming a constant fraction of dark compact objects for both lensed SNe, we obtain $f_{\rm dc} < 0.19$. These results highlight the potential of strongly lensed SNIa to probe the innermost parts of lens galaxies and learn about compact matter.

astro-ph.GA↗

arXiv2025

Rootin' Tootin' Efficient Ray Shootin': Creating Microlensing Magnification Maps with GPUs

The impending discovery and monitoring of hundreds of new gravitationally lensed quasars and supernovae from upcoming ground and space based large area surveys such as LSST, \textit{Euclid}, and \textit{Roman} necessitates the development of improved numerical methods for studying gravitational microlensing. We present in this work the fastest microlensing map generation code currently publicly available. We utilize graphics processing units to take advantage of the inherent parallelizable nature of creating magnification maps, in addition to using 1) the fast multipole method to reduce the runtime dependence on the number of microlenses and 2) inverse polygon mapping to reduce the number of rays required. The code is available at https://github.com/weisluke/microlensing/.

astro-ph.GA↗

arXiv2025

A GPU Code for Finding Microlensing Critical Curves and Caustics

Advancements in analyses of caustic crossing events in gravitationally microlensed quasars and supernovae can benefit from numerical simulations which locate the caustics in conjunction with the creation of magnification maps. We present a GPU code which efficiently solves this problem; the code is available at https://github.com/weisluke/microlensing/. We discuss how the locations of the microcaustics can be used to determine the number of caustic crossings and the distances to caustics, both of which can be used to constrain the space of nuisance parameters such as source position and velocity within magnification maps.

astro-ph.GA↗

arXiv2025

Stars as cosmic scales: measuring stellar mass with microlensed supernovae

Gravitational microlensing is a unique probe of the stellar content in strong lens galaxies. Flux ratio anomalies from gravitationally lensed supernovae (glSNe), just like lensed quasars, can be used to constrain the stellar mass fractions at the image positions. Type Ia supernovae are of particular interest as knowledge of the intrinsic source brightness helps constrain the amount of (de)magnification from the macromodel predictions that might be due to microlensing. In addition, the presence or absence of caustic crossings in the light curves of glSNe can be used to constrain the mass of the microlenses. We find that a sample of 50 well-modeled glSNe Ia systems with single epoch observations at peak intrinsic supernova luminosity should be able to constrain an average stellar mass-to-light ratio to within $\sim 15\%$. A set of systems with light curve level information providing the location (or absence) of caustic crossing events can also constrain the mass of the microlenses to within $\sim 50\%$. Much work is needed to make such a measurement in practice, but our results demonstrate the feasibility of microlensing to place constraints on astrophysical parameters related to the initial mass function of lensing galaxies without any prior assumptions on the stellar mass.

astro-ph.GA↗

arXiv2024

Microlensing near macro-caustics

Microlensing near macro-caustics is a complex phenomenon in which swarms of micro-images produced by micro-caustics form on both sides of a macro-critical curve. Recent discoveries of highly magnified images of individual stars in massive galaxy cluster lenses, predicted to be formed by these micro-image swarms, have stimulated studies on this topic. In this Chapter, we explore microlensing near macro-caustics using both simulations and analytic calculations. We show that the mean total magnification of the micro-image swarms follows that of an extended source in the absence of microlensing. Micro-caustics join into a connected network in a region around the macro-critical line of a width proportional to the surface density of microlenses; within this region, the increase of the mean magnification toward the macro-caustic is driven by the increase of the number of micro-images rather than individual magnifications of micro-images. The maximum achievable magnification in micro-caustic crossings decreases with the mass fraction in microlenses. We conclude with a review of applications of this microlensing phenomenon, including limits to the fraction of dark matter in compact objects, and searches of Population III stars and dark matter subhalos. We argue that the discovered highly magnified stars at cosmological distances already imply that less than $\sim$ 10\% of the dark matter may be in the form of compact objects with mass above $\sim 10^{-6}\, M_{\odot}$.

astro-ph.GA↗

arXiv2024

How to Break the Mass Sheet Degeneracy with the Lightcurves of Microlensed Type Ia Supernovae

The standardizable nature of gravitationally lensed Type Ia supernovae (glSNe Ia) makes them an attractive target for time delay cosmography, since a source with known luminosity breaks the mass sheet degeneracy. It is known that microlensing by stars in the lensing galaxy can add significant stochastic uncertainty to the unlensed luminosity which is often much larger than the intrinsic scatter of the Ia population. In this work, we show how the temporal microlensing variations as the supernova disc expands can be used to improve the standardization of glSNe Ia. We find that SNe are standardizable if they do not cross caustics as they expand. We estimate that this will be the case for $\approx$6 doubly imaged systems and $\approx$0.3 quadruply imaged systems per year in LSST. At the end of the ten year LSST survey, these systems should enable us to test for systematics in $H_0$ due to the mass sheet degeneracy at the $1.00^{+0.07}_{-0.06}$\% level, or $1.8\pm0.2$\% if we can only extract time delays from the third of systems with counter images brighter than $i=24$ mag.

astro-ph.GA↗

arXiv2023

Lensed type Ia supernovae in light of SN Zwicky and iPTF16geu

Strong gravitationally lensed supernovae (glSNe) are a powerful probe to obtain a measure of the expansion rate of the Universe, but they are also extremely rare. To date, only two glSNe with multiple images strongly lensed by galaxies have been found, but their short time delays make them unsuitable for cosmography. Here, we simulate a realistic catalogue of lensed supernovae and study the characteristics of the population of detectable systems for different surveys. Compared to previous studies, our simulations also account for the effect of microlensing and its impact on the glSNe yields. We show that the properties of glSNe in shallow surveys (such as the Zwicky Transient Facility; ZTF) are determined by the need for large magnifications, which favours systems of four images with short time delays and low image separations. This picture is consistent with the properties of iPTF16geu and SN Zwicky, but is not representative of the population found in deeper surveys, which are limited by the volume of the Universe that is strongly lensed. For deeper surveys, such as the Legacy Survey of Space and Time (LSST), glSNe show longer time delays and greater angular separations, and the inclusion of microlensing results in 8$\%$ of glSNe becoming demagnified under the detection threshold. In the 10 years of the survey LSST should be able to find $\approx$ 180 systems, of which 70 will be suited for cosmography enabling a $\approx$ 1.2$\%$ precision $H_0$ measurement with LSST glSNe.

astro-ph.CO↗

arXiv2021

"Worst-Case" Micro-Lensing in the Identification and Modeling of Lensed Quasars

Although micro-lensing of macro-lensed quasars and supernovae provides unique opportunities for several kinds of investigations, it can add unwanted and sometimes substantial noise. While micro-lensing flux anomalies may be safely ignored for some observations, they severely limit others. "Worst-case" estimates can inform the decision whether or not to undertake an extensive examination of micro-lensing scenarios. Here, we report "worst-case" micro-lensing uncertainties for point sources lensed by singular isothermal potentials, parameterized by a convergence equal to the shear and by the stellar fraction. The results can be straightforwardly applied to non-isothermal potentials utilizing the mass sheet degeneracy. We use micro-lensing maps to compute fluctuations in image micro-magnifications and estimate the stellar fraction at which the fluctuations are greatest for a given convergence. We find that the worst-case fluctuations happen at a stellar fraction $κ_\star=\frac{1}{|μ_{macro}|}$. For macro-minima, fluctuations in both magnification and demagnification appear to be bounded ($1.5>Δm>-1.3$, where $Δm$ is magnitude relative to the average macro-magnification). Magnifications for macro-saddles are bounded as well ($Δm > -1.7$). In contrast, demagnifications for macro-saddles appear to have unbounded fluctuations as $1/μ_{macro}\rightarrow0$ and $κ_\star\rightarrow0$.

astro-ph.GA↗

arXiv2019

Magnifications of paired micro-images emerging from a micro-lensing critical curve

Studies of the inner regions of micro-lensed AGN during caustic crossing events have often relied upon the approximation that the magnification near a fold caustic is inversely proportional to the square root of the source-caustic distance. We examine here the behavior of the individual micro-images (one a micro-minimum, the other a micro-saddle) that emerge as a point source crosses a micro-fold caustic. We provide a variety of statistics on both the behavior of the two newly created micro-images, and some parameters which appear in higher order approximations for the magnification. We compare the predictions of these higher order approximations to the actual image magnifications of our simulations.

astro-ph.GA↗