HATPI Pre-Perihelion Time-series Photometry of the Interstellar Comet 3I/ATLAS

Abstract

HATPI is a recently commissioned time-domain facility at Las Campanas Observatory, Chile, that uses 64 wide-angle, 9.6 cm diameter lenses and back-illuminated CCDs, yielding a mosaic field-of-view of 7,100 square arcdegrees, observing the night sky at a cadence of 45 s and a spatial scale of 19.7 arcsec pixel$^{-1}$. In this paper, we present moving object time-series photometry with this facility, focusing on the interstellar comet 3I/ATLAS. 3I/ATLAS was first robustly recovered by HATPI on the night of 2025 July 2 (one night after its discovery) at a Gaia $G$-band magnitude of $G = 17.796 \pm 0.082$ mag ($\pm 0.030$ mag systematic uncertainty). The comet then increased in brightness to $G = 14.071 \pm 0.073$ mag $\pm 0.030$ mag by 2025 Sep 13, after which it became unobservable by HATPI as it approached perihelion. Before 3I/ATLAS achieved a brightness of $G = 16.396 \pm 0.029$ mag $\pm 0.030$ mag on 2025 Aug 6, it could be detected when stacking all HATPI observations from a single night, while after this date it is sufficiently bright to detect in individual 45 s exposures. We do not detect evidence for significant short-time-scale variations in the brightness of 3I/ATLAS after Aug 6. Compared to other light curves in the literature, the HATPI photometry exhibits a somewhat steeper rise in brightness with decreasing heliocentric distance, $r_{H}$. The HATPI magnitudes are well-fit as a power law function of $r_{H}$, with an exponential index of $n = 5.167 \pm 0.095$, over the range $2.14$ AU $ < r_{H} < 4.44$ AU, compared to $n = 3.94 \pm 0.10$ when fitting together with other literature observations. We find that the phase function is constrained to $β= 0.0552 \pm 0.0032$ mag deg$^{-1}$.

Figures (7)

• Figure 1: One hour stacked HATPI subtracted images of 3I/ATLAS from each of the nights on which 3I/ATLAS is formally detected with $3\sigma$ confidence in the full night binned light curve (nights corresponding to blue points in Fig. \ref{['fig:maglcnightly']}). Individual 45 s exposures that are flagged as problematic (Sec \ref{['sec:cleaning']}) are excluded from the stacking. Each image stamp is $21\arcmin\times21\arcmin$, displayed with North up and East to the left. The red lines indicate the position of the nucleus of 3I/ATLAS in the center of each stamp. Grey rectangular regions are masked from the image subtraction process due to the presence of saturated stars. Other sources are variable objects, residual Poisson noise from bright stars, or other systematic residuals from poor subtractions (typically most notable for bright stars). Because 3I/ATLAS was at low Galactic latitude, the density of such objects is high in these stacked subtracted images. Individual 45 s exposures are aligned on the position of 3I/ATLAS before stacking, causing background variable star sources (which can be either positive or negative) to show trails. For each night, we show a single IHU/Field as indicated in the labels after the date, while additional IHU/Fields may be available and included in the full night binned light curves. Note that the stacked images are for display purposes only. The photometry is performed on the individual 45 s exposures, and binning is then performed on the photometric measurements rather than the images. The detections on May 20, May 23, June 20, June 23, July 6, July 11, and July 21 are likely affected by contamination from neighboring variable sources, and are excluded from the analysis. The first night on which we consider the recovery of 3I/ATLAS to be reliable is 2025 Jul 2. Additional stamps are shown in Fig. \ref{['fig:imgstamps2']}.
• Figure 2: A continuation of Fig. \ref{['fig:imgstamps']}. Here we show observations from Aug 7 through Sep 13.
• Figure 3: Full, un-binned, HATPI light curve of 3I/ATLAS shown as fluxes $F$. These are calibrated to the Gaia$G$-band system such that the Gaia magnitude is equal to $-2.5\log_{10}(F)$. Top: all points are shown, including those that are rejected by the cleaning methods described in Section \ref{['sec:cleaning']}. The color coding shows points with different quality flags. Bottom: same as above, but now including only the "clean" points from the top panel.
• Figure 4: Top: clean HATPI light curve of 3I/ATLAS shown as fluxes $F$ on the Gaia$G$-band system such that the Gaia magnitude is equal to $-2.5\log_{10}(F)$. We bin all observations obtained on a given night by a given IHU/Field using the weighted mean. Each point thus represents up to one hour of observations. Different colors indicate the different IHU/Fields. Middle: same as above, but now binning together all observations obtained on a given night. Each point in this case represents up to $\sim 6$ hours of observations. Bottom: same as middle, but showing the nightly binned light curve converted to magnitudes. Circles show points for which a non-zero flux is measured with at least $3\sigma$ confidence, while arrows indicate $3\sigma$ upper-limits on the brightness. The solid line shows the expected $T$ magnitude from JPL/Horizons.
• Figure 5: The nightly binned HATPI light curve of 3I/ATLAS for three different concentric, circular apertures. Here we omit upper-limit measurements. As a result, on some nights fewer than 3 apertures are displayed. All other figures show the 1.45 pixel aperture measurements. Large inconsistencies between the different apertures for some of the early-time observations indicate measurements that may be contaminated by blending with residual variable sources. At late times, as the cometary tail becomes resolved by HATPI, the largest aperture measurements are systematically brighter than the smaller aperture measurements.