Results from a Stellar Occultation by (623) Chimaera

Abstract

We present results from a stellar occultation by the asteroid (623) Chimaera on October 21st, 2025, when it passed in front of V magnitude 13.86 star UCAC4 626-004193. Chimaera is the largest asteroid in the Chimaerid asteroid family and will be subject to a spacecraft flyby in the 2030s. Eighteen telescopes were deployed, spanning the United States from Arizona to North Carolina, incrementally spaced by $\sim 4.4$\,km with respect to the cross-track of the shadow. In total, sixteen telescopes collected useful data. This multi-chord observation, when compiled into a single sky-plane plot, suggests a profile which is well-described by an elongated elliptical fit, with a semi-major axis $a=26.2\pm 0.8$ km and a semi-minor axis $b=16.0\pm 1.0$ km. The projected area from ellipse fitting is 1316 km${}^2$, and a limb fit is calculated to enclose 1312 km${}^2$. Photometry collected in the Gaia G band over a seven-month period coinciding with the occultation corresponds to a geometric albedo $p_G=0.059\pm 0.005$; comparing with archival occultation observations and direct observations gives $p_G=0.051\pm 0.004$ and $p_V=0.051\pm 0.004$. We compare occultation sky-plane projections with 3-D modeling from inversion techniques and demonstrate that the models can accurately predict the orientation of Chimaera at the time of each occultation.