Apache Point rapid response characterization of primitive imminent impactor 2024 RW$_1$

Imminent impactors may be detected only a few hours before their impact with Earth, providing a brief opportunity to characterize them before impact. We describe the characterization of imminent impactor 2024 RW$_1$, which was discovered by the Catalina Sky Survey on 2024 September 4 at 05:43 UTC, before it entered the atmosphere near the northern Philippines at 16:39 UTC. We observed 2024 RW$_1$ with the Astrophysical Research Consortium Telescope Imaging Camera on the Apache Point Astrophysical Research Consortium's 3.5-m telescope on 2024 September 4 10:16 UTC. We obtained g, r, i, and z photometry of 2024 RW$_1$, yielding color indices of g-r = 0.47$\pm$0.04, r-i = 0.13$\pm$0.04, i-z = -0.11$\pm$0.07, and g-i = 0.60$\pm$0.04, corresponding to a spectral slope of 0.67$\pm$0.40~$\%$/100 nm. The closest match to an asteroid spectral type is with B-type asteroids from the C-complex. We detect variations in the time series photometry of the asteroid with an amplitude of $\sim$0.75, and a double-peaked rotation period of $\sim$1900 s. Assuming a visible albedo of 0.07$\pm$0.03, a density of $\sim$1500 kg/m$^3$, and a calculated absolute magnitude of 30.92$\pm$0.05, we estimate that the asteroid has a diameter of 3.3$\pm$0.7 m and a total mass of $\sim$28,000 kg. Comparing our astrometric orbital solutions to NEOMOD3, the most likely source of 2024 RW$_1$ is the 3:1 main belt mean motion resonance (77\% probability) followed by the $\nu_6$ resonance (13\% probability), consistent with its organic B-type nature.