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

This deposit contains a 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 86 highly reliable 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 documentation details the upstream FITS anomaly discovery (explicitly flagging 8 events compromised by truncated archives on Zenodo) and provides a worked demonstration. 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 of the sky distributions and density percentile rankings are also included.