The Quadruply Lensed Supernova SN 2025wny: Implications for LSST

Technical Summary: The Quadruply Lensed Supernova SN 2025wny: Implications for LSST

Problem Statement
The Legacy Survey of Space and Time (LSST) is expected to detect numerous lensed supernovae (SNae), with quadruply lensed systems offering the most highly constrained models compared to doubly lensed systems. However, the discovery and confirmation of such rare transients face significant delays. The case of SN 2025wny illustrates these challenges: a five-week gap occurred between the initial identification of the transient by the Zwicky Transient Facility (ZTF) and the report of four distinct images. Further delays were incurred before the detection of additional images and the acceleration of follow-up efforts. The paper identifies a critical inefficiency in current alert protocols: the lack of immediate, on-the-fly modeling to predict the positions and magnitudes of trailing images or to "pre-cover" leading images that may have been too faint to trigger an initial alert. Without rapid intervention, the full scientific potential of these events—specifically for time-delay cosmography and lightcurve extraction—is compromised.

Methodology
The authors propose a generic, staged protocol for responding to alerts near known quadruply lensed host galaxies. The core of this methodology relies on "on-the-fly" propagation of alert coordinates through a pre-computed lens model derived from the host galaxy.

The proposed workflow consists of seven stages:

1. Modeling: Pre-computation of the gravitational potential for known incipient hosts (galaxies already identified as quadruply lensed).
2. Alert: Generation of a Difference Image Analysis (DIA) alert.
3. Flagging: Association of the alert with a pre-selected list of incipient hosts based on angular proximity.
4. Propagation: Tracing the alert position back to the host redshift and projecting it forward through the lens model to predict the number, positions, magnifications, and time delays of additional images.
5. Pre-/Re-covery: Searching for additional images at predicted locations using the alert packet and ancillary observations (e.g., forced photometry on prior exposures).
6. Corroboration: Confirming the system by detecting at least one additional predicted image.
7. Phasing, Verification, and Reconstruction: Establishing lightcurve phases, verifying the transient as a supernova via spectroscopy, and iteratively reconstructing the lens model as new data becomes available.

The technical implementation employs the Witt-Wynne geometric lens model (based on a singular isothermal elliptical potential) combined with Falor's exact, forward, differentiable solution. This approach allows for rapid computation of image positions without the computational overhead of full ray-tracing, balancing accuracy with the speed required for real-time alerts.

Application to SN 2025wny
The authors apply this protocol retrospectively to SN 2025wny to illustrate its efficacy and identify potential pitfalls.

• Modeling: Using cursor measurements from discovery figures (due to the absence of tabulated coordinates in initial papers), the authors constructed a Witt-Wynne model for the host galaxy PS1J0716+3821.
• Propagation: The model successfully predicted the positions of the three non-triggering images based on the position of the triggering image. The scatter between predicted and observed positions was approximately 0.1 arcseconds, deemed sufficient for "re-covery" and "pre-covery."
• Limitations Identified: The study notes that the Witt-Wynne model, while useful for rapid prediction, has shortcomings. The host system involves two lensing galaxies (G1 and G2), causing deviations from the single-potential model. Furthermore, the model predicted a large ellipticity (~0.36) and significant time delays (up to 175 days for the trailing image), which complicates "pre-covery" of the leading image (Image D) due to observational constraints (e.g., solar elongation).
• Broker Role: The paper envisions a "broker" system that would ingest the alert, perform the propagation, and broadcast predicted coordinates and time delays (or "pre-lays") to the community. This would enable immediate follow-up, potentially bypassing the standard 80-hour embargo on LSST data.

Key Results

• Feasibility of Rapid Response: The study demonstrates that a pre-computed geometric model can successfully predict the positions of multiple lensed images with sufficient accuracy (sub-arcsecond) to guide immediate follow-up observations.
• Protocol Viability: The seven-stage protocol provides a structured framework for moving from an initial alert to a confirmed, multi-image system.
• Time Delay Constraints: The analysis of SN 2025wny suggests that while the system is a quadruple lens, the large time delays and specific geometry may render it less suitable for time-delay cosmography than initially hoped, highlighting the need for precise modeling to assess scientific utility early in the discovery process.
• Micro-lensing and Thresholds: The paper notes that micro-lensing may affect image brightness and that lowering detection thresholds for known incipient hosts could facilitate the detection of fainter, leading images.

Significance and Claims
The paper claims that the immediate exploitation of lensed supernova alerts requires a shift from passive observation to active, model-driven prediction. The primary significance lies in the proposal of a specific, actionable protocol for LSST brokers to:

1. Accelerate Confirmation: Reduce the time between initial detection and the confirmation of a quadruply lensed system from weeks to hours.
2. Enable Pre-covery: Allow for the detection of leading images that might otherwise be missed or detected too late for useful lightcurve analysis.
3. Optimize Resources: Direct telescope time (e.g., HST, JWST, ground-based spectroscopy) efficiently by providing precise coordinates and expected magnitudes.

The authors emphasize that while the Witt-Wynne model has theoretical limitations (particularly in complex, multi-galaxy lens systems), its computational speed makes it the appropriate tool for the initial "on-the-fly" phase of the alert response. The paper concludes that refining this protocol is essential for maximizing the scientific return of LSST's transient surveys, particularly for the rare and valuable class of quadruply lensed supernovae.