Allowing a color offset reverses a fixed delay comparison for Q0957+561 in a shared OU test of 2024–2025 ZTF data

We test whether later public ZTF data support a reported 15.4 day red delay over zero delay for the northern image of Q0957+561. Both bands share a stationary Ornstein–Uhlenbeck (OU) signal. We fit seven parameters before 2024, separately for each fixed delay. We then score the joint conditional density of 51 red nightly observations from 2024–2025, given all blue data and early red data. The 15.4 day model has the higher primary score by 4.245 nats. An exploratory test allows a common later red offset with a zero-mean Gaussian distribution and a standard deviation of 0.02 mag. The score difference becomes −3.706 nats, so zero delay has the higher score. Using only the first exposure each night gives a near tie, at −0.179 nats. The primary models also have large conditional residual statistics. These results show that this fixed delay comparison depends on the treatment of later color and repeated exposures. The test uses later blue observations and is a conditional reconstruction, not a forecast. It keeps early parameter estimates fixed and does not determine a physical disk delay or show a change in that delay. The package includes the raw query, source code, complete results, four figures, provenance, and reproduction instructions.

Q0957+561 · Zwicky Transient Facility · ZTF DR24 · quasar variability · continuum delay · reverberation mapping · Ornstein-Uhlenbeck process · conditional validation · color offset · reproducible analysis