Time-domain photometry and activity evolution of interstellar comet 3I/ATLAS with BHTOM

Context. Time-domain photometric monitoring is essential for characterizing cometary evolution, particularly for rare interstellar objects with limited observing opportunities.

Aims. We aimed to characterize the pre-perihelion photometric behavior and dust activity of the interstellar comet 3I/ATLAS and to test the capability of the Black Hole Target and Observation Manager (BHTOM) platform and telescope network for coordinated high-cadence non-sidereal observations.

Methods. We obtained 70 days of time-series photometry of 3I/ATLAS from 2025 July 4 to September 11 using 16 telescopes and 1554 images. The data were processed and calibrated with the BHTOM pipeline. High-cadence multiband imaging was used to measure the rotation period and color evolution, while the dust activity was quantified via Afρ measurements.

Results. We present a pre-perihelion light curve of 3I/ATLAS from Rh = 3.18–2.19 au, which exhibited a steady brightening by ∼3 magnitudes with no evidence of anomalous behavior. We measured a rotation period of Prot = 15.98 ± 0.08 h. The colors were statistically non-changing, with only a weak, non-significant tendency for 3I/ATLAS to become bluer at 3.5 > Rh > 2.2 au. The relative dust production increased from A(0°)fρ ∼600 to 1100 cm, and the upper limit on the dust mass-loss rate increased from ≤217 kg s−1 to ≤328 kg s−1. We measured an activity index of n = −1.24 ± 0.02, which is consistent with a well-developed dust coma.