Mining the Alerts: A Preliminary Catalog of Compact Binaries from the Fourth Observing Run

We present a preliminary catalog of compact binary merger candidates from the ongoing fourth observing run (O4) of Advanced LIGO, Virgo, and KAGRA, based on an analysis of public alerts distributed through GraceDB as of May 2025. We developed and applied methods to estimate the source-frame chirp mass for each candidate by utilizing information from public data products, including source classification probabilities, sky localizations, and observatory status. Combining our O4 analysis with previous catalogs, we provide updated estimates for the local merger rate density. For sources with chirp mass characteristic of binary neutron stars ($[1, 1.5]\,M_\odot$), we find a rate of $56^{+99}_{-40}$ $\textrm{Gpc}^{-3}\,\textrm{yr}^{-1}$. For systems in the expected neutron star--black hole chirp mass range ($[1.5, 3.5]\,M_\odot$), the rate is $36^{+32}_{-20}$ $\textrm{Gpc}^{-3}\,\textrm{yr}^{-1}$, and for heavier binary black holes ($[3.5, 100]\,M_\odot$), we estimate a rate of $19^{+4}_{-2}$ $\textrm{Gpc}^{-3}\,\textrm{yr}^{-1}$. This work provides an early glimpse into the compact binary population being observed in O4; we identify a number of high-value candidates up to signal-to-noise $\sim 80$, which we expect to enable precision measurements in the future.