Paper - summarizes the key findings
• Rubin images clearly show the interstellar object 3I/ATLAS with a dust coma, indicative of activity. These observations provide the earliest high resolution evidence of detected cometary activity.
• Rubin multi-filter photometry is consistent with previous bservations in the literature, with significantly smaller error bars. There are not sufficient same-night multi-filter observations to examine surface/coma color or color evolution, but sufficient to exclude photometric variability on short timescales.
• The coma’s radius appeared to increase slightly over the 11 days from ∼ 6,520 km (UT 2025 June 21) to ∼9,380 km (UT 2025 July 02) as measured from azimuthally averaged radial profiles. We estimate an increase in coma level of ∆η ∼0.5 between 2025 June 21 June and 2025 July 2, which we assume to be a lower limit for several reasons, including that 3I/ATLAS was observed nearly head-on (very low phase angle) and the tail could have extended far along the z-axis, as projected on the sky.
• We obtain a V-band absolute magnitude of HV = (13.7 ± 0.2) mag and equivalent effective radius of ∼ (5.6 ± 0.7) km for the nucleus, assuming a spherically symmetric steady-state coma. Due to this simplifying assumption, we consider the latter result to be an upper limit to the true nucleus size. We estimate a mass loss rate ranging from 10 to 100 kg/s, depending on the grain sizes assumed to dominate, and we compute Afρ = (315 ± 15) cm for data obtained on UT 2025 July 2.
• We detect no short-term photometric variability. A sequence of measurements taken on UT 2025 July 2 constrains the apparent brightness variations of 3I/ATLAS to less than 0.1 mag on timescales of less than an hour.
• If the Rubin SSP pipelines had been processing the commissioning data in real time, our modeling shows that there were sufficient SV observations to identify 3I/ATLAS as a moving object.
• If the nominal SV survey strategy continues as planned, 3I/ATLAS should be observed in ten or more observations in each filter through mid August 2025. Future SV observations will likely be able to monitor the coma color as 3I/ATLAS moves towards perihelion.
• Analysis of the derived astrometry suggest that for bright high-SNR and extended active small bodies, the combination of Rubin’s large aperture and LSSTCam’s pixel scale are less impacted by asymmetrical coma provided precise positions for deriving accurate orbital parameters.
Cool :)