Impact of Calibration Systematics on Dark Energy Constraints from LSST Type Ia Supernovae

Technical Summary: Impact of Calibration Systematics on Dark Energy Constraints from LSST Type Ia Supernovae

Problem Statement
The Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) is poised to deliver an unprecedented sample of Type Ia supernovae (SNe Ia), significantly expanding the dataset available for cosmological analysis. As the sample size ($N$) increases, statistical uncertainties are expected to shrink proportionally to $1/\sqrt{N}$. Consequently, systematic effects—specifically those related to flux calibration—are projected to dominate the error budget. Even millimagnitude-level calibration errors can significantly impact the cosmological signal derived from SNe Ia. Recent analyses have indicated that miscalibration can introduce dataset tensions and potentially generate spurious evidence for evolving dark energy. Therefore, accurately quantifying how uncertainties in LSST passbands propagate into biases in SN distance moduli and, subsequently, dark energy equation of state parameters ($w_0$ and $w_a$) is a central challenge for the Rubin-era supernova cosmology program.

Methodology
To assess the impact of LSST calibration uncertainties, the authors employ the supernova analysis software SNANA. Unlike previous systematic analyses that focused on zeropoint or wavelength shifts, this study directly distorts the LSST passbands using linear and quadratic tilts. This approach is chosen to capture chromatic calibration errors that cannot be adequately represented by simple offsets.

The analysis procedure involves the following steps:

1. Simulation: An idealized, noise-free sample of LSST Deep Drilling Field supernovae is simulated.
2. Systematic Injection: Calibration systematics are introduced by modifying the passbands within the K-Correction file used during light-curve fitting. The authors intentionally introduce exaggerated passband tilts to clearly illustrate their impact on cosmological parameters.
3. Propagation: The distorted passbands alter the inferred distance moduli, resulting in a shifted Hubble diagram and shifted cosmological parameters.
4. Quantification: The study evaluates the resulting changes in the $w_0 - w_a$ contours, specifically measuring the shift in the contour centroid (best-fit values) and the fractional increase in the contour area (uncertainty) as a function of the systematic amplitude.

Key Results
The analysis quantifies the sensitivity of dark energy constraints to passband systematics:

• Linear Tilts: For linear passband tilts, the study finds a linear relationship between the amplitude of the tilt and the bias in cosmological parameters. Specifically, for every 1%/100nm increase in tilt:
  • The best-fit $(w_0, w_a)$ values shift by approximately 0.025$\sigma$.
  • The area of the $w_0 - w_a$ contour increases by approximately 5%.
• Quadratic Tilts: The results regarding quadratic passband tilts are described as less conclusive and warrant further exploration.
• 2D Analysis: The authors also evaluated contour changes over a 2D grid of linear and quadratic coefficients, visualizing the shifts in the centroid and the fractional area increase as a function of both tilt types simultaneously.

Significance and Claims
The primary significance of this work lies in its direct quantification of how passband uncertainties translate into biases in dark energy constraints. By establishing a quantitative link between the amplitude of passband systematics and the resulting shifts in $w_0$ and $w_a$, this analysis aims to inform the calibration accuracy required for LSST to achieve its dark energy science objectives. The paper does not propose new experimental setups or future applications beyond this calibration requirement; rather, it serves as a diagnostic tool to define the necessary precision for the LSST calibration system to ensure that systematic errors do not compromise the survey's cosmological goals.