Pre-discovery TESS Observations of Interstellar Object 3I/ATLAS

TL;DR

This study leverages serendipitous TESS sector 92 data to extract pre-discovery observations of the interstellar object $3I/ATLAS$, providing a 26-day light curve from two camera/CCD setups despite the object’s faintness. By combining background modeling, a Taylor-expanded PRF-based position correction, and track-enabled image stacking, the authors obtain 15 measurements with $\mathrm{SNR}>3$ and reveal a gradual brightening corresponding to $H_V \approx 13.8 \pm 0.4$ and a possible faint activity before perihelion. The analysis highlights that TESS can contribute valuable pre-discovery information for interstellar objects, even with large pixel scales and astrometric uncertainties dominated by the WCS, and emphasizes the need for advanced moving-object detection methods in upcoming surveys. The work reinforces the role of small, high-cadence surveys in constraining the dynamical and physical evolution of interstellar visitors and informs future observational strategies with Rubin Observatory and similar facilities.

Abstract

3I/ATLAS, also known as C2025 N1 (ATLAS), is the third known macroscopic interstellar object to pass through our Solar System. We report serendipitous Transiting Exoplanet Survey Satellite (TESS) observations of 3I/ATLAS taken between 2025-05-07 and 2025-06-02, 55 days prior to the discovery date (2025-07-01). We retrieve the TESS pixel data, perform a robust background correction and use a data-driven approach to compute the objects position on the TESS detectors. We find a consistent offset between the targets observed and predicted positions which is dominated by uncertainty in the TESS World Coordinate System (WCS) rather than ephemeris errors. 3I/ATLAS is too faint to be detected in the individual 200 second TESS integrations, so we stack images to improve detectability. We perform aperture and Pixel Response Function (PRF) photometry on the stacked images to create two light curves. Each light curve consists of 15 measurements with $\text{SNR}>3$, collected across two different TESS cameras during the 26 days that the object was observed. The PRF light curve, which is more robust against image noise, in the TESS bandpass shows a gradual increase in brightness from $T_{\text{mag}}=20.9\pm0.29$ to $19.57\pm0.15$. This is expected as 3I/ATLAS approaches the inner Solar System. Its absolute magnitude decreases from $H_{V}=14.3\pm0.4$ to $13.7\pm0.3$ and shows signs of faint activity consistent with other observations. This paper highlights the power of using TESS for Solar System science by increasing the number of pre-discovery observations, in an otherwise sparsely populated region of the light curve, the long-term behavior of 3I/ATLAS can be investigated.

Figures (7)

• Figure 1: Background modeling, as described in Section \ref{['subsec:bkg_corr']}. The left/right panel shows an example of a cadence with/without strong scattered light (SL). We show the raw flux (top), our background model that accounts for scattered light and stars (middle) and the residuals (bottom). The "holes" in the residuals are pixels that were identified as outliers and rejected.
• Figure 2: Full image stack of 3I/ATLAS, using all frames available from TESS sector 92 after quality filtering. Data from camera 2 CCD 3 (left) and camera 1 CCD 2 (right) are stacked separately to preserve instrument characteristics. The pixels with $\text{SNR}>3$ are highlighted in red; these are used for aperture photometry (see Section \ref{['subsec:phot']}).
• Figure 3: Stacked images of 3I/ATLAS from TESS sector 92. We stacked 901 ($\sim 2.1$ days) and 520 ($\sim 1.2$ days) frames from camera 2 CCD 3 (first row) and camera 1 CCD 2 (second and third row), respectively. The aperture masks used for photometry are plotted in red; the masks are different for each camera to account for instrument characteristics. Due to the stacking procedure, the row and column pixel values do not represent the location in the original TESS FFI and are only shown for reference. The images are aligned so that the target is always centered. See Section \ref{['subsec:stacking']} for details on the stacking procedure and Section \ref{['subsec:phot']} for details on the photometry.
• Figure 4: Normalized flux for every pixel in the image cutout with respect to the expected position of 3I/ATLAS. Data correspond to observations from TESS sector 92 camera 2 CCD 3 (top row) and camera 1 CCD 2 (bottom row). The left panels show the original ephemeris from JPL Horizons (accessed on 2025-07-28), while the right panels show the corrected ephemeris as described in Section \ref{['subsec:ephem_corr']}. A stronger signal is represented by redder points.
• Figure 5: 3I/ATLAS light curves using aperture (top) and PRF (bottom) photometry in flux (left) and magnitude (right), measured from the stacked images. Data from camera 2 CCD 3 is shown in blue squares, while orange circles represent data from camera 1 CCD 2. Error bars in the time axis represent the window used for image stacking. The solid red line and shaded region represent the mean background flux and $1\sigma$ intervals. The horizontal dashed red line represents the $3\sigma$ average detection limit across all stacked images. The x-axis is defined with respect to the time of discovery 2025-07-01.