Deprojection-Response Diagnostics for ACT DR6 × NILC Cross-Spectra: Beam-Amplification Systematics and Scale-Cut Recommendations

Abstract: We quantify how switching the ACT+Planck needlet internal linear combination (NILC) temperature map from a standard to a thermal Sunyaev–Zel'dovich (tSZ) deprojected configuration affects cross-power spectra with the six ACT Data Release 6 (DR6) frequency channels. For each channel we construct the deprojection-response ratio using Monte Carlo–calibrated pseudo-Cℓ transfer functions, orthogonal split-difference null tests, and beam-envelope uncertainty propagation. Over the multipole range analyzed, five of six channels yield inverse-variance–weighted mean ratios consistent with unity at the sub-percent level. The remaining channel, pa4_f220, exhibits a mild excess traced to beam-deconvolution amplification rather than a physical deprojection effect. Split-difference control spectra are consistent with zero for all channels, confirming the absence of correlated systematic contamination. These results validate the ACT–NILC cross-spectrum framework for cosmological analyses and motivate a conservative scale cut that excludes the 220 GHz channel above this threshold.

Subjects:	astro-ph.CO; physics.data-an; astro-ph.IM
Cite as:	PX:2604.00013