GWTC-4 Cross-Referenced Catalogue with Line-of-Sight Environmental Properties

This deposit contains an updated cross-referenced dataset joining the LIGO-Virgo-KAGRA (LVK) GWTC-4.0 gravitational-wave catalogue with independent line-of-sight electromagnetic (EM) density proxies. Designed to assist researchers conducting standard siren cosmology and propagation tests, the primary dataset provides precise sky coordinates and 90% credible sky areas for a total of 85 successfully extracted events from the O4 run (along with the canonical multi-messenger event GW170817). Each event is cross-referenced with two environmental density metrics: Milky Way foreground extinction (E(B-V) derived from Schlafly & Finkbeiner dust maps) and a proxy for cosmological integrated density (extended source galaxy counts from the 2MASS XSC within a 1° radius). In addition to the primary dataset, this deposit includes the end-to-end Python extraction pipeline and formal methodological documentation. The Version 2 pipeline corrects a previous matching bug to successfully identify and explicitly flag all 8 events compromised by truncated FITS archives on Zenodo. The documentation provides a worked demonstration where GW170817 is ranked strictly against the remaining 77-event reliable O4 comparison sample. By applying a source-physics jet-launch subtraction to GW170817, the pipeline establishes a strict phenomenological upper-bound constraint on hypothetical density-dependent propagation delays (κ) for both proxy scales. Standalone publication-ready visualizations—now expanded to a 3-panel figure featuring sky distributions, E(B-V) extinction, and Cosmological 2MASS distributions—are also natively generated and included.