ORAC-NT v9: A 2PN Matched Filter Pipeline for Gravitational Wave Detection and 3-Detector Sky Localization

We present ORAC-NT v9, an open-source gravitational wave analysis pipeline implemented as an interactive Streamlit application. The pipeline processes real strain data from the LIGO-Virgo Open Science Center (GWOSC) and performs matched filter detection using a 2nd post-Newtonian (2PN) Stationary Phase Approximation (SPA) chirp template.

Key technical contributions: (1) a zero-padded linear FFT matched filter that eliminates circular wrap-around artifacts; (2) template-peak aligned SNR trimming ensuring correct inter-detector delay sign; (3) MAD (Median Absolute Deviation) normalization robust to signal self-suppression; (4) causal peak search constrained to the physical light-travel window between detector pairs; (5) 3-detector sky localization via Rodrigues rotation of probability rings in ECEF coordinates, with a numerical ring intersection finder returning a 2D (RA, Dec) candidate.

Validated against four LIGO events: GW150914, GW170104, GW170814, and GW170817. For GW150914, the measured inter-detector delay Δt(H1-L1) = −4.15 ms reproduces the correct sign and order of magnitude of the published value (−6.9 ms). For GW170814 (first 3-detector event), the ring intersection falls in the correct sky quadrant. For GW170817, the neutron star template correctly dominates over the black hole template.

The pipeline is intended as an educational and prototyping tool. Single-template SNR values below the 8σ detection threshold are expected; the full LIGO detection pipeline employs approximately 250,000 matched templates. Sky geometry and timing are physically validated.