A cycle-offset fit gives a -42.8 MHz per day peak-frequency slope in 19 selected FRB 20180916B cycles

We test peak-frequency changes within activity cycles of FRB 20180916B in CHIME/FRB Catalog 2 release 20251231. We fit a separate offset for each cycle. The 103 selected events give 81 daily means. Only 19 cycles, with 46 daily means, carry slope information. The pooled slope is -42.8 MHz per day. A nominal 95 percent cycle-bootstrap interval is -67.3 to -15.8 MHz per day. A conditional within-cycle permutation test gives p = 0.00245 under exchangeable daily frequencies. Thirteen of 19 slopes are negative; their sign test gives p = 0.167. Quality cuts and removal of any one cycle keep the pooled slope negative. Period changes that leave the groups intact do not change the estimate. Only one informative cycle is after 2021, so later persistence is not confirmed. The fit tests a local spectral association. Detection selection, spectral calibration, band limits, and intrinsic burst structure prevent a physical cause from being established. No independent telescope confirms these contrasts. Code, selected inputs, source hashes, random draws, checks and figures are included.

astronomy · CHIME · FRB 20180916B · chromatic activity · cycle offsets · spectral selection