Time-Domain Photometry and Activity Evolution of Interstellar Comet 3I/ATLAS with BHTOM

Abstract

Time-domain photometric monitoring is essential for characterizing cometary evolution, particularly for rare interstellar objects with limited observing opportunities. 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. We obtained 70 days of time-series photometry of 3I/ATLAS from 2025 July 4 - September 11 using 16 telescopes and 1554 images. The data were processed and calibrated with the BHTOM pipeline. High-cadence, multi-band imaging was used to measure the rotation period and color evolution, while the dust activity was quantified via Afp measurements. We present a pre-perihelion light curve of 3I/ATLAS from Rh = 3.18 - 2.19 au, which exhibited a steady increase of ~3 magnitudes with no evidence of anomalous behavior. We measured a rotation period of P_rot = 15.98 +/- 0.08 h. The relative dust production increased from A(0)fp ~600 - 1100 cm, and the upper limit on the dust mass-loss rate increased from \leq 217 kg/s to \leq 328 kg/s. We measured an activity index of n = -1.24 +/- 0.02, consistent with a well-developed dust coma. 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.

Figures (15)

• Figure 1: Geographic locations of the observatories from which photometric data were obtained. Colors correspond to observatory names and telescope apertures given in the legend. Insets indicate regions with clustered sites.
• Figure 2: Apparent magnitude vs MJD of interstellar comet 3I/ATLAS. The calibrated magnitudes are shown as a function of Modified Julian Date, combining observations from multiple telescopes and filters. Marker shapes distinguish individual telescope datasets. $g', r'$ and $i'$ refer to the Sloan filter system and $R$ and $I$ refer to the J-C filter system, and $L$ corresponds to the broad GOTO $L$-band. The vertical dashed line marks the discovery epoch, and the grey shaded regions indicate intervals when the Moon–comet elongation fell below 30°.
• Figure 3: The top panel shows the phase-folded rotational light curve of 3I/ATLAS. Calibrated magnitudes were detrended for the long-term brightening and folded on the adopted rotation period of $P_{\mathrm{rot}} = 15.98 \pm 0.08$ h. LCOGT $r'$, UZO $R$ and D154T $R$ are shown in the foreground with the best-fitting three-harmonic sinusoidal model for the LCOGT $r'$-band over-plotted. The background shows the entire remaining dataset from LCOGT $g'$, $i'$, UZO $B$, $V$ and D154T $B$, $V$ and $I$. The middle panel shows the residuals of the LCOGT data relative to the LCOGT $r'$ harmonic model. The bottom panel shows the LS periodogram of the detrended LCOGT residuals; the shaded region marks the strongest periodicity and the dashed horizontal line indicates the 1% FAP level.
• Figure 4: The aperture photometry light curve of 3I/ATLAS is displayed in the top panel and the dust production parameter $A(0^\circ)f\rho$ vs time in MJD is displayed in the middle panel. The lower panel shows $A(0^\circ)f\rho$ vs heliocentric distance, with the solid line representing the $A(0^\circ)f\rho$ activity index, $n$. The red markers on the plots represent the 1 m LCOGT $r$-band measurements.
• Figure 5: Representative high-cadence intra-night photometry of 3I/ATLAS. (a) shows UZO observations on MJD 60880 over 4.8 hours and (b) shows the D154T observations on MJD 60885 over 1.2 hours. The blue, green, red and maroon points represent the J-C $B, V, R$ and $I$ bands respectively. The remaining high-cadence light curves are shown in the Appendices \ref{['uzo_append']} and \ref{['danish_append']}.