The Discovery and Evolution of a Possible New Epoch of Cometary Activity by the Centaur (2060) Chiron

The Centaurs are small icy objects residing on dynamically unstable orbits in the giant planet region of the solar system. Centaurs are thought to originate as primordial Kuiper Belt objects (KBOs) on Neptune-crossing orbits that are then scattered by Neptune into the giant planet region (Duncan & Levison 1997; Levison & Duncan 1997; Volk & Malhotra 2008), where they reside on chaotic orbits with an average orbital lifetime of the order of ∼10⁶–10⁸ yr (Hahn & Bailey 1990; Tiscareno & Malhotra 2003; Di Sisto & Brunini 2007; Volk & Malhotra 2008; Bailey & Malhotra 2009). During this time, Centaurs undergo encounters with the giant planets, causing the Centaurs to either impact the giant planets, be ejected out of the solar system altogether, or become short-period comets that include the Jupiter-family comets (JFCs; Tiscareno & Malhotra 2003; Duncan et al. 2004; Bauer et al. 2013; Fernández et al. 2013; Sarid et al. 2019). Centaurs thus form an evolutionary link between the Kuiper Belt and the more physically evolved JFCs, and this intermediate transitionary nature makes them valuable in studying the evolution of JFCs.

Approximately 8%–10% of Centaurs are known to exhibit dust comae, indicating cometary activity (Peixinho et al. 2020). However, for most active Centaurs, especially those with large perihelia, their activity differs from that of JFCs in that it occurs throughout their orbits (Peixinho et al. 2020). Centaur activity appears to be dependent on a Centaur's perihelion distance from the Sun, with a possible boundary existing at ∼14 au beyond which no activity has been detected (Jewitt 2009, 2015; Guilbert-Lepoutre 2012; Li et al. 2020; Lilly et al. 2021, 2024). The mechanism behind this phenomenon currently remains unknown, with newly exposed pockets of volatile surface ices (Prialnik 1992) as well as volatiles released during the amorphous-to-crystalline transition of subsurface ice (Jewitt 2009) being proposed to explain Centaur activity. Known active Centaurs tend to reside in orbits with smaller semimajor axes than inactive ones (Jewitt 2009) with active Centaurs tending to have experienced recent (∼10²–10³ yr) dynamical encounters with the giant planets, causing a significant decrease in their semimajor axes (Fernández et al. 2018; Lilly et al. 2021, 2024). Combined with the comparative lack of planetary encounters experienced by inactive Centaurs, this hints that a sudden decrease in perihelia could trigger the onset of Centaur activity (Fernández et al. 2018; Lilly et al. 2021, 2024).

(2060) 95P/Chiron (henceforth Chiron) is one of the largest members of the Centaur population and has a well-established history of cometary activity (Tholen et al. 1988; Bus et al. 1989; Meech & Belton 1989; Dahlgren et al. 1990; Hartmann et al. 1990; Luu & Jewitt 1990; Meech & Belton 1990; Meech 1990; West 1990; Buratti & Dunbar 1991; Dahlgren et al. 1991; Sykes & Walker 1991; West 1991; Buie et al. 1993; Luu 1993; Marcialis & Buratti 1993; Elliot et al. 1995; Bus et al. 1996, 2001; Lazzaro et al. 1996, 1997; Silva & Cellone 2001; Belskaya et al. 2010). Cometary activity was first discovered in 1989 when a coma was detected around Chiron as it approached perihelion (Meech & Belton 1989; Hartmann et al. 1990; Meech 1990) and remained detectable even when the absolute magnitude of Chiron was at its dimmest recorded value near perihelion (Silva & Cellone 2001). Subsequent analysis of archival photographic plates obtained from 1969 to 1977 has also revealed that Chiron was active near aphelion (Bus et al. 2001). This cometary activity is thought to contribute to the considerable variation in the brightness of Chiron observed over time (Bus et al. 2001; Duffard et al. 2002; Belskaya et al. 2010). Models of Chiron's nucleus and reported ring system require a contribution from cometary outbursts to reproduce Chiron's measured lightcurve (Ortiz et al. 2015).

Between 2021 February and 2021 June, Chiron underwent a sudden brightening in apparent magnitude by ∼1 mag, remaining brighter than previous years for all of its 2021–2022 observing season (Dobson et al. 2021; Betzler 2023; Dobson et al. 2023; Ortiz et al. 2023), potentially indicative of an epoch of either new or increased cometary activity (Dobson et al. 2021, 2023; Ortiz et al. 2023). We refer to this observed brightening as the 2021 Brightening Event hereafter. In this paper, we analyze the 2021 Brightening Event exhibited by Chiron. We utilize multiwavelength photometry from the Asteroid Terrestrial-impact Last Alert System (ATLAS; Tonry et al. 2018a, 2018b), the Zwicky Transient Facility (ZTF; Bellm et al. 2019; Graham et al. 2019), and the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS; Kaiser et al. 2002; Hodapp et al. 2004; Kaiser 2004; Denneau et al. 2013; Chambers et al. 2016; Magnier et al. 2020a, 2020b, 2020c; Flewelling et al. 2020; Waters et al. 2020) spanning multiple observing seasons, in addition to follow-up photometric observations taken by the 8.1 m telescope of the Gemini Observatory (Hook et al. 2004; Gimeno et al. 2016) and the Las Cumbres Observatory (LCO) telescope network (Brown et al. 2013) via the LCO Outbursting Objects Key (LOOK) project (Lister et al. 2022). We further supplement our data set with observations from the Gaia Data Release 3 (DR3; Gaia Collaboration et al. 2023; Tanga et al. 2023), the robotic TRAnsiting Planets and PlanetesImals Small Telescope South (TRAPPIST-South; Jehin et al. 2011), the Dark Energy Survey (DES; The Dark Energy Survey Collaboration 2005), and observations of Chiron reported in the literature. Our paper is structured as follows. In Section 2, we present details of the photometry used in this study and the data reduction techniques used for each data set. We present our methods of analyzing our observations in Section 3. In Section 4, we present our results of analyzing the 2021 Brightening Event for photometric behavior, color change, the search for a coma, and determining the contribution from Chiron's reported ring system. We discuss the results of our analysis and their implications for the cause of the 2021 Brightening Event in Section 5. We list the conclusions of our study in Section 6.

This section briefly summarizes each telescope and/or survey and describes the observations we use in our analysis. Our data set is comprised of targeted observations (TRAPPIST-South, Gemini, and LOOK) and survey observations (Pan-STARRS, DES, Gaia, ATLAS, and ZTF) where Chiron was serendipitously imaged. Our observations range in time from 2012 to 2023, sampling Chiron's lightcurve pre-, during, and post-2021 Brightening Event. Tables of all observations from each telescope/survey are listed in the Appendix, with the number of observations in each filter from each telescope/survey listed in Table 1. Most of our observations are from the ATLAS data set, followed by ZTF, and the combined ATLAS and ZTF cover the largest time span in our sample. These large data sets therefore constitute the majority of our analysis. Pan-STARRS provides our earliest consistent observations of Chiron, extending our baseline to 2009. Our observations from Gemini occurred just after the 2021 Brightening Event and allow us to search for any coma exhibited by Chiron during this time. Our observations span 12 observing seasons when Chiron's on-sky distance from the Sun was sufficient to allow observations. The date ranges of each observing season are listed in Table 2. For clarity, we designate each observing season with a letter as described in Table 2.

2.1. ATLAS

2.2. ZTF

2.3. Pan-STARRS

2.4. DES

2.5. Gaia

2.6. LOOK

2.7. TRAPPIST-South

2.8. Gemini

For all our observations, we correct the time of observation for light travel time and calculate reduced magnitudes (the apparent magnitude scaled to a geocentric distance and heliocentric distance of 1 au) and their associated uncertainties, assuming negligible contribution from heliocentric and geocentric distances (obtained from JPL Horizons ²² ) to the errors. To help remove any stellar contamination or spurious data points from our analysis, we split our observations into pre- and post-2021 Brightening Event. We treat each filter of each data set individually. For data sets with <50 observations—namely, DES, Gemini, Pan-STARRS, and LOOK in all filters and TRAPPIST-South in the B and V filters—we visually inspect each image, removing any observations that exhibit evidence of contamination from background sources. To remove possibly spurious data points for data sets with ≥50 observations, we followed the prescription (Dobson et al. 2023) of removing data points with large magnitude uncertainties and σ-clipping our data set.

All observations of Chiron pre-2021 Brightening Event, corresponding to observing seasons A–L (2009–2021), are consistent with a single phase curve slope. Analyzing ATLAS and ZTF observations, which constitute the majority of our data and span observing seasons G–L (2015–2021), utilizing the same method as Dobson et al. (2023), we find that their reduced magnitude values in a given filter all reside on a single phase curve profile (see Figure 6 in Section 4.1 for more details). Therefore, we treat all observing seasons pre-2021 Brightening Event as one. From this time range, we remove all observations with magnitude uncertainties exceeding the 85th percentile of the magnitude uncertainty distribution for ATLAS and TRAPPIST-South in the R filter and 95th percentile for ZTF and Gaia. For each filter in our ATLAS, ZTF, TRAPPIST-South R, and Gaia data sets, we 3σ-clip the remaining observations to eliminate outliers in reduced magnitude.

For all observations post-2021 Brightening Event, we treat each observing season separately due to Chiron's evolving phase curve (see Section 4 for more details). We utilize the 95th percentile of the magnitude uncertainty distribution as the cutoff for our ATLAS and ZTF data sets. For a given observing season, we bin all remaining observations into two bins of phase angle, each of width equal to half the maximum phase angle span of the data set. This ensures that the algorithm does not remove features of the phase curve (e.g., opposition effect) that may not be fully sampled by the data set of one observing season. For each observing season, we 3σ-clip the observations in each bin. The comparatively smaller sizes of the Gemini and LOOK data sets means we instead manually remove any outliers caused by contamination from background sources in the images.

In this section, we analyze the 2021 Brightening Event and its subsequent evolution to explore the potential causes of Chiron's enhanced brightness in all filters. There are several possible explanations for the 2021 Brightening Event that we test with our data set. Cometary activity could cause Chiron to increase in brightness. Chiron is a known active object with previous instances of the Centaur brightening coinciding with detections of a visible coma and/or significant PSF extension (Tholen et al. 1988; Bus et al. 1989; Meech & Belton 1989; Dahlgren et al. 1990; Hartmann et al. 1990; Luu & Jewitt 1990; Meech & Belton 1990; Meech 1990; West 1990; Bus et al. 2001; Silva & Cellone 2001). Alternatively, Chiron's ring system may instead offer an explanation for the 2021 Brightening Event. Occultation studies have revealed Chiron to be surrounded by either symmetric shell-like structures (Ruprecht et al. 2015) or a system of debris rings (Ortiz et al. 2015, 2023). Ortiz et al. (2015) found that by modeling the contribution of rings to the Centaur's absolute magnitude across its orbit, their changing orientation as viewed from Earth could explain previous instances of brightening by Chiron. Betzler (2023) hypothesized that the 2021 Brightening Event could instead be caused by variation in Chiron's surface albedo, with the seasonal appearance of a high-albedo surface feature causing the Centaur to brighten. We explore the brightness evolution and phase curve of Chiron pre-, during, and post-2021 Brightening Event (Section 4.1); search for significant changes in color index across time (Section 4.2); search for a visible coma or PSF extension in our observations (Section 4.3); and compute the effect of Chiron's ring system on its brightness evolution across our baseline of observations (Section 4.4).

4.1. Brightness Evolution

We first look at ATLAS and ZTF observations because they comprise the largest two data sets in our sample and cover the widest time span. Figure 1 plots our ATLAS and ZTF reduced magnitudes versus time, both as measured in each filter and corrected for wavelength to the ATLAS c and o filters (due to them having the largest numbers of observations) using Chiron's pre-Brightening Event c − g and o − r color indices calculated from the absolute magnitude values of Chiron in each filter. The reduced magnitudes separated by both survey and filter are shown in Figures 2 (panels (a) and (c)) and 3 (panels (a) and (c)). The "seesaw" motion of Chiron's brightness over time in each observing season is due to phase curve effects, with the brightness maxima corresponding to the smallest phase angles, and the spread of the data is partly due to the periodic brightness variation caused by the Centaur's rotational lightcurve of amplitude Δm ≲ 0.1 mag (Bus et al. 1989; Groussin et al. 2004; Ortiz et al. 2015; Cikota et al. 2018). As seen in Figure 1, Chiron's brightness varies due to phase angle by approximately the same amount across observing seasons G–L (2015–2021), with peak reduced magnitude values in each filter consistent between observing seasons. These reduced magnitude values are plotted against phase angle in Figure 4. Fitting the phase curve of Chiron in each of these observing seasons separately, we find that Chiron's phase coefficient β is consistent between observing seasons, implying that Chiron's phase curve profile had been constant before the 2021 Brightening Event (see Section 4.1.1 for more details). However, in observing season M (2021–2022), corresponding to the 2021 Brightening Event, Chiron suddenly increases in brightness by ∼1 mag compared to all previous observing seasons. The ATLAS and ZTF observations allow us to constrain the start date of the 2021 Brightening Event to between 2021 February 6 and 2021 June 17. Chiron passed aphelion between these dates, though observations were precluded due to the Centaur being behind the Sun as viewed from Earth. This observed brightness increase far exceeds Chiron's brightness variation due to rotational modulation (≲0.1 mag) and solar phase angle (∼0.5 mag), meaning that neither Chiron's lightcurve nor its phase curve can account for it. Furthermore, Chiron's brightness has continued to evolve. We see from Figure 1 that Chiron's peak magnitude dimmed significantly by ∼0.5 mag between observing seasons M (2021–2022) and N (2022–2023), but it continues to remain at brighter magnitudes than compared to G–L (2015–2021) by ∼0.5 mag.

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The phenomenon that caused the 2021 Brightening Event changed Chiron's phase curve profile as well as its absolute magnitude. The color-corrected phase curve of Chiron before, during, and after the 2021 Brightening Event is plotted in Figure 5. These magnitudes were color-corrected to the ATLAS c and o filters using Chiron's pre-Brightening Event c − g and o − r color indices calculated from the absolute magnitude values of linear fits to Chiron's phase curve in each filter. Figures 2 and 3 show the brightness evolution (panels (a) and (c)) and phase curve (panels (b) and (d)) of Chiron in the ATLAS and ZTF data sets, respectively, color-coded by pre-, during, and post-2021 Brightening Event, with the latter two time ranges divided into pre- and postopposition. The ATLAS phase curves of individual post-2021 Brightening Event observing seasons are shown in Figure 6 for clarity. Chiron's phase curves in observing seasons M (2021–2022) and N (2022–2023) are distinct both from each other and compared to the previous five observing seasons pre-2021 Brightening Event, which all lie on the same profile as shown in Figures 2, 3, 5, and 6 (see Section 4.1.1 for more details). Figure 5 also shows that Chiron's phase curve profile is consistent between different filters across the same observing seasons. Figures 2, 3, and 6 also reveal that Chiron's brightness has evolved significantly within the time span of a single observing season. Postopposition magnitudes in observing season M (2021–2022) are brighter than those preopposition at the same phase angles. Measuring this difference for the ZTF r filter, we find the postopposition magnitudes to be brighter than preopposition by ∼2σ on average. The ZTF g-filter data postopposition are too sparse to allow an accurate measurement but are consistent with preopposition within the uncertainties. Pre- and postopposition magnitude values in observing season N (2022–2023) are also consistent in brightness. Chiron's evolving brightness and phase curve prevents us from fitting its phase curve during the 2021 Brightening Event or after, precluding removal of phase angle effects.

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4.1.1. Phase Coefficients

The dense sampling coverage (see Figure 4) of observing seasons I–L before the 2021 Brightening Event by ATLAS allows us to measure the linear phase coefficient of Chiron in each observing season utilizing the same methods and criteria of Dobson et al. (2023). We find that Chiron's phase curve has remained relatively unchanged from 2012 up to the 2021 Brightening Event (observing seasons D–L). The o-filter linear phase coefficient values of each observing season I–M are listed in Table 3 and are consistent to 2σ within uncertainties, in accordance with the findings of Betzler (2023). The comparatively sparser sampling of the ATLAS c filter compared to o means we can only accurately measure the linear phase coefficient for observing season L. Comparing the c-filter linear phase coefficient value for observing season L, β_c = 0.110 ± 0.018 mag deg⁻¹, to that of the entire ATLAS baseline as measured by Dobson et al. (2023; β_c =0.093 ± 0.011 mag deg⁻¹), we find them to be consistent within uncertainties. Figures 2, 3, and 6 highlight that Chiron's phase curve changed in shape during and post-2021 Brightening Event. This would cause different linear phase coefficient values when measured across preoppositon magnitude values compared to postopposition ones and would lead to an inaccurate value when measured across the entire observing season. We therefore select only preopposition data points of observing season M (2021–2022) to reduce the effect of Chiron's evolving phase curve and fit a linear phase curve function to the ATLAS o-filter reduced magnitudes to quantify by how much the phase curve has changed. The preopposition ATLAS c-filter data set for observing season M (2021–2022) is too sparse for an accurate phase curve fit. We find a linear phase coefficient value β_o = 0.226 ± 0.008 mag deg⁻¹, differing significantly from the pre-2021 Brightening Event value measured by Dobson et al. (2023; β_o = 0.097 ± 0.006). The presence of a dust coma may have caused this change in the phase coefficient of Chiron, but the uncertainties and scatter of our ATLAS and ZTF observations prevent us from constraining the presence of a dust coma before Chiron's 2021 Brightening Event. Furthermore, although a standard phase curve model for cometary comae exists (Schleicher 2010), ²³ it may not be applicable to Centaur comae if the mechanism that produces them or the type of dust emitted is different for comets. A detailed analysis of the effect of dust on Chiron's phase curve is therefore beyond the scope of this paper.

Table 3. ATLAS o-filter Phase Coefficients for Observing Seasons Pre- and Post-2021 Brightening Event

Observing Season	βo	No
	(mag deg−1)
I (2017–2018)	0.105 ± 0.012	235
J (2018–2019)	0.115 ± 0.013	176
K (2019–2020)	0.092 ± 0.012	178
L (2020–2021)	0.093 ± 0.011	178
M (preopposition 2021)	0.226 ± 0.008	108

Note. Uncertainties are 2σ errors.

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4.2. Color Evolution

We use the dual filter coverage of the ATLAS and ZTF data sets to look for any potential changes in Chiron's c − o and g − r colors across time. We cannot correct our reduced magnitudes for phase angle as Chiron's phase curve evolved significantly during observing seasons M (2021–2022, the Brightening Event) and N (2022–2023, post-Brightening Event). Therefore, we utilize two methods to minimize the phase angle effects on our observations. First, for a given data point in one filter, we select the data point in the other filter closest in time, under the criterion that it is separated in time by ≤2 days (the approximate observation cadence of ATLAS) in order to identify any sudden changes in Chiron's color index. We use the reduced magnitude value of each observation to calculate Chiron's c − o and g − r color values and associated uncertainties. The second method involves fitting polynomial splines to its reduced magnitude values across time. By interpolating reduced magnitude measurements across time, we may better account for Chiron's evolving brightness and phase curve, including any sudden increase/decrease in magnitude. We generate 10³ synthetic lightcurves for each filter in a given observing season by offsetting each magnitude value by a random number generated from a Gaussian distribution with standard deviation equal to the magnitude uncertainty. We then fit third-order polynomial splines to each synthetic lightcurve using the interpolate package of the scipy Python library (Virtanen et al. 2020). For a given data point of a given synthetic lightcurve in one filter, we extrapolate Chiron's brightness at that time in the other filter based on the fitted spline. We then use the resulting reduced magnitude values to calculate a c − o and g − r color value. We only perform this second method for observing seasons with ≥25 observations in a given filter, ensuring that the polynomial can fit Chiron's magnitude values accurately.

Figure 7 shows the resulting ATLAS c − o and ZTF g − r color indices of Chiron across time, calculated from temporally adjacent points. The ATLAS c − o and ZTF g − r values as calculated from the spline-fitting method are shown in Figures 8 and 9, respectively. We find that Chiron has not undergone any significant color evolution across the baselines of ATLAS and ZTF, as Figures 7, 8, and 9 show that Chiron's color has remained constant within uncertainties across this time.

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Simultaneous g- and r-band measurements of Chiron were also obtained from our Gemini observations at three epochs (see Table A8). The sparse number of data points from Gemini combined with the evolution of Chiron's phase curve means we can only compare the Gemini g − r values within a season. The two data points from season K are separated in phase angle by ≲02, allowing comparison, as the brightness variation due to phase angle will be minimal. The distance-corrected g − r values of the two data points of observing season N (2022–2023), g − r = 0.501 ± 0.057 and g − r = 0.557 ±0.052, and are consistent to 1.0σ.

4.3. Search for a Coma and PSF Extension

Beyond photometry, we also analyze our Gemini and TRAPPIST-South observations for a visible coma and PSF extension. Chiron has previously exhibited a visible coma during instances of brightening (Hartmann et al. 1990; Silva & Cellone 2001; Duffard et al. 2002). However, Dobson et al. (2021) found no visible coma or PSF extension in ATLAS and LOOK observations during Chiron's 2021 Brightening Event. We stack our Gemini observations to search for any faint coma or PSF extension exhibited by Chiron during or post-2021 Brightening Event. The stacked images at the coordinates of Chiron from each of the five epochs listed in Table A8 are shown in Figure 10 in the r and i filters. Our filter choice is motivated by the physical reality that the r and i filters are free from the brightest gas emission features in comets (Figure 1 in Feldman et al. 2004); the only detection of optical gas emission at Chiron previously reported was the CN (0–0) band at 3883 Å by Bus et al. (1991) during its active phase in 1990. No comae or tails are visible in any of the stacked Gemini images. We search for PSF extension by measuring the radial profiles of Chiron's PSF in each stacked image and comparing the result with field stars. We utilize SExtractor Python (Bertin & Arnouts 1996; Barbary 2016) to detect background sources in our observations. We select field stars for comparison to Chiron on the basis that (1) the peak flux count of their PSF does not exceed 50,000 electrons, to ensure they are not saturated; (2) they are distant from neighboring stars by at least the outer radius of any applied circular annulus for photometry; (3) they are brighter than 22nd magnitude, to ensure they are clearly visible in our images; (4) the uncertainties of both their catalog magnitude and the difference between their instrumental magnitudes and their Pan-STARRS1 catalog magnitudes are both less than 0.075 mag; (5) their catalog g − r colors are within the range −0.5 ≤ g − r ≤ 2.0; (6) the difference between their catalog PSF magnitudes and Kron magnitudes in the i filter is less than 0.05 mag so that we can separate stars from galaxies; and (7) their residuals to an unweighted linear fit to their difference in instrumental and catalog magnitudes as a function of catalog color index g − i are less than 2.5σ. We utilize the RadialProfile function from the photutils Python library (Bradley 2023) to measure the radial profiles of Chiron and the background field stars. We apply a circular aperture of radius 13 pixels to both Chiron and the background field stars. To remove the flux contribution from the background, we utilize a background annulus aperture of inner and outer radii of 21.4 and 35.7 pixels, respectively, calculating the total background value from the product of the annulus area and the 3σ-clipped median background value from the annulus. We subtract this total background flux value from the corresponding radial profile. An example of the resulting radial profiles is shown in Figure 11, where no visible coma is detected. Chiron's PSF is consistent with those of the background stars in all filters for all five epochs of observation.

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We also searched for signs of activity in our TRAPPIST-South observations (pre-Brightening Event, spanning 2012–2015), whose relatively large data sets and time span allow a probe into past cometary behavior. Neither visible comae nor tails are visible in any of our TRAPPIST-South observations. We searched for PSF extension by comparing the PSF profiles of Chiron to those of the field stars using the same method as Devogèle et al. (2021). We created two stacks for each night, a first one on Chiron, compensating for its proper motion, and a second one on the background field stars. We then computed the distances of each pixel from the centroids of the PSFs of Chiron and the background stars, respectively. The radial profiles of Chiron were found to be identical to the field stars for every epoch.

4.4. Rings

To ascertain if Chiron's reported ring system together with a changing viewing angle as seen from Earth could explain the Centaur's observed brightness evolution, we reconstruct the ring model as constructed by Ortiz et al. (2015). We include flux contributions from Chiron's nucleus, its two rings as reported by Ortiz et al. (2015) based on observations of Chiron's 2011 occultation, and the 1970 cometary outburst, utilizing a ring albedo ²⁴ value of p_V = 0.18 and ring widths of 6.6 and 4.7 km (private communication with [anonymized for dual-anonymous review]). We calculate the aspect angle δ of Chiron at a given time and corresponding ecliptic coordinates according to the equation

$$\begin{eqnarray}&&\begin{array}{l}\delta =\displaystyle \frac{\pi }{2}-\arcsin [\sin ({\beta }_{e})\sin ({\beta }_{p})\\ \,+\cos ({\beta }_{e})\cos ({\beta }_{p})\cos ({\lambda }_{e}-{\lambda }_{p})],\end{array}\end{eqnarray} \tag{ 1 }$$

where β_e and λ_e are the ecliptic latitude and longitude, respectively, of Chiron as observed from Earth and β_p and λ_p are the ecliptic latitude and longitude of the pole orientation of Chiron and its ring system. We then calculate the projected area A_p (δ) of Chiron at a given epoch and corresponding aspect angle δ according to the equation

$$\begin{eqnarray}&&{A}_{p}(\delta )=\displaystyle \frac{{\mathrm{Area}}_{\max }(\delta )+{\mathrm{Area}}_{\min }(\delta )}{2},\end{eqnarray} \tag{ 2 }$$

where ${{\rm{Area}}}_{\max }(\delta )$ and ${{\rm{Area}}}_{\min }(\delta )$ are the maximum and minimum projected area defined by the following equations:

$$\begin{eqnarray}&&{{\rm{Area}}}_{\max }(\delta )=\pi a{[{b}^{2}{\cos }^{2}(\delta )+{c}^{2}{\sin }^{2}(\delta )]}^{1/2}\end{eqnarray} \tag{ 3 }$$

and

$$\begin{eqnarray}&&{\mathrm{Area}}_{\min }(\delta )=\pi b{[{a}^{2}{\cos }^{2}(\delta )+{c}^{2}{\sin }^{2}(\delta )]}^{1/2},\end{eqnarray} \tag{ 4 }$$

where a, b, and c are the semimajor axes of Chiron's nucleus assuming a triaxial ellipsoid shape in hydrostatic equilibrium. We use the same equation as Ortiz et al. (2015) to calculate the ratio of Chiron's total computed flux F_Chiron to that of the Sun F_Sun at a given time t and corresponding aspect angle α, given by

$$\begin{eqnarray}&&\begin{array}{c}\displaystyle \frac{{F}_{\mathrm{Chiron}}}{{F}_{\mathrm{Sun}}}={A}_{p}{p}_{V}f{(\alpha )+{p}_{V}^{\mathrm{Ring}1}f^{\prime} (\alpha )\pi (({a}_{1}+{W}_{1})}^{2}-{a}_{1}^{2})\mu \\ \,+{p}_{V}^{\mathrm{Ring}2}f^{\prime} (\alpha )\pi \left({\left({a}_{2}+{W}_{2}\right)}^{2}-{a}_{2}^{2}\right)\mu +{A}_{c}{p}_{d}\exp (-(t-{t}_{0})/{\tau }_{d}),\end{array}\end{eqnarray} \tag{ 5 }$$

where A_p is the projected area of Chiron's nucleus; p_V is the geometric albedo of the nucleus; f(α) is the solar phase function of the nucleus (taken to equal 1); ${p}_{V}^{{\rm{Ring}}1}$ and ${p}_{V}^{{\rm{Ring}}2}$ are the geometric albedos of the first and second rings, respectively; $f^{\prime} (\alpha )$ is the solar phase function of the rings (taken to equal 1); μ is the absolute value of the cosine of the rings' aspect angle; $\pi ({({a}_{i}+{W}_{i})}^{2}-{a}_{i}^{2})$ is the projected area of the i ring, where W_i is the width of the i ring and a_i is the radial distance of each ring to Chiron's nucleus; A_c is the initial scattering cross section of the coma caused by Chiron's 1970 cometary outburst; t₀ is the initial time at which the coma dust cloud starts to decay; and τ_d is the decay time of the coma. We utilize the same parameter values of Ortiz et al. (2015) for the cometary outburst contribution. We then compute Chiron's resulting absolute magnitude H_V across time using the following equation:

$$\begin{eqnarray}&&{H}_{V}={H}_{V,\mathrm{Solar}}+2.5{\mathrm{log}}_{10}\left(\pi \displaystyle \frac{{\left(1.496\times {10}^{8}\right)}^{2}}{\left({F}_{\mathrm{Chiron}}/{F}_{\mathrm{Sun}}\right)}\right),\end{eqnarray} \tag{ 6 }$$

where H_V,Solar is the absolute magnitude of the Sun, which we take to equal −26.76 (Willmer 2018).

We also collate all absolute magnitude measurements of Chiron from the literature (Bus et al. 2001; Silva & Cellone 2001; Duffard et al. 2002; Bauer et al. 2003; Romon-Martin et al. 2003; Groussin et al. 2004; Bagnulo et al. 2006; Belskaya et al. 2010; Fornasier et al. 2013; Galiazzo et al. 2016; Cikota et al. 2018), applying a consistent phase angle correction to all values utilizing the HG phase curve model with slope parameter G = 0.70 as determined by Bus et al. (2001). We transform the magnitude measurements from our own observations to the Johnson–Cousins V filter. For a given filter, we calculate the mean reduced magnitude of any observations taken before the 2021 Brightening Event. We then calculate the mean reduced magnitude of any V-filter observations (our own or from the literature) that coincide in time. The difference between these means is subtracted from every observation in the given filter. No V-filter measurements exist post-2021 Brightening Event, so we use the filter-corrected ATLAS o observations, divided by pre- and postopposition in each observing season, to transform the Gemini and LOOK magnitudes. We transform our filter-corrected reduced magnitudes to absolute magnitudes H_V by applying a phase angle correction using G = 0.70 as per Bus et al. (2001). All the collated absolute magnitude values of Chiron, both from this work and from previous literature studies, are listed in Table 4.

Table 4. Estimated Absolute Magnitudes of Chiron

UT Year	Geocentric	Heliocentric	Phase	Apparent	Apparent	Reduced	Filter	Estimated	Estimated	Reference
	Distance	Distance	Angle	Magnitude	Magnitude	Magnitude		Absolute	Absolute
					Uncertainty			Magnitude	Magnitude
									Uncertainty
	(au)	(au)	(deg)
2023.098537	19.343	18.807	2.485	18.111	0.055	5.3068	$r^{\prime}$	5.2661	0.055	This work
2023.098531	19.343	18.807	2.485	18.475	0.079	5.6708	$g^{\prime}$	5.2734	0.079	This work
2023.098524	19.343	18.807	2.486	18.127	0.056	5.3228	$r^{\prime}$	5.2821	0.056	This work
2023.098518	19.343	18.807	2.486	18.485	0.078	5.6808	$g^{\prime}$	5.2834	0.078	This work
2023.093077	19.314	18.807	2.540	18.125	0.063	5.3240	$r^{\prime}$	5.2812	0.063	This work
2023.093071	19.314	18.807	2.540	18.453	0.074	5.6520	$g^{\prime}$	5.2526	0.074	This work
2023.093064	19.314	18.807	2.540	18.164	0.060	5.3630	$r^{\prime}$	5.3202	0.060	This work
2023.093058	19.314	18.807	2.541	18.556	0.078	5.7550	$g^{\prime}$	5.3556	0.078	This work
2023.076629	19.225	18.809	2.689	18.071	0.082	5.2799	$r^{\prime}$	5.2319	0.082	This work
2023.076623	19.225	18.809	2.689	18.611	0.093	5.8199	$g^{\prime}$	5.4152	0.093	This work

Note. The paper Bus et al. (2001) does not provide values for apparent magnitude, its associated uncertainty, or reduced magnitude.

Only a portion of this table is shown here to demonstrate its form and content. A machine-readable version of the full table is available.

Download table as:  Machine-readable (MRT)Typeset image

Figure 12 shows the estimated absolute magnitude of Chiron over time combining our data sets and measurements from the literature as compared to Ortiz et al. (2015). Chiron's absolute magnitude across our baseline of observations is shown in Figure 13 for clarity. As seen in Figures 12 and 13, the latter showing the post-Brightening Event era, Chiron's brightness evolution across our baseline of observations is inconsistent with that predicted by Ortiz et al. (2015). The high-precision Pan-STARRS, DES, ZTF, and Gaia measurements show that even before 2021, Chiron was already significantly brighter than predicted by Ortiz et al. (2015). The ring model predicted that Chiron would slowly dim across observing seasons M (2021–2022, the 2021 Brightening Event) and N (2022–2023, post-Brightening Event). However, Chiron's measured absolute magnitude from ATLAS, ZTF, LOOK, and Gemini was significantly brighter than the Ortiz et al. (2015) ring model during this time, differing by ∼0.5–1 mag and reaching a peak magnitude comparable to its historical maximum brightness in the 1970s (Bus et al. 2001). The Ortiz et al. (2015) ring model fails to match Chiron's brightness evolution across our baseline of observations and therefore cannot explain the 2021 Brightening Event.

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Chiron's observed increase in brightness from 2021 onward far exceeds its amplitude of rotational modulation, predicted to be ≲0.05 mag during 2021 (Ortiz et al. 2015) when phase-folded to its measured rotation period of 5.917813 hr (Bus et al. 1989; Marcialis & Buratti 1993), and was too small to be measured in ATLAS data from Dobson et al. (2021). Furthermore, Chiron's observed increase in brightness exceeds its brightness variation due to its phase function. We therefore rule out both Chiron's rotation and phase curve as being responsible for the 2021 Brightening Event. Betzler (2023) hypothesized that Chiron's observed evolution in brightness could instead be due to variation in surface albedo, with areas of differing albedo becoming visible to Earth as the Centaur's aspect angle changes throughout its orbit. However, comparing pre- and postopposition ATLAS and ZTF observations from observing season M (2021–2022), we have seen that Chiron's brightness evolved significantly, at an appreciable rate across the time span of a single observing season. The short timescale of Chiron's observing season M (0.6 yr) compared to its orbit around the Sun (50.8 yr) means that Chiron has only swept through 1.2% of its orbit during this time. Combined with the measured ecliptic coordinates of Chiron's axial pole of λ = 151° ± 8° and β = 18° ± 11° (Ortiz et al. 2023) implying a near face-on obliquity to Earth, as well as the Centaur's small amplitude of rotational modulation, we consider it implausible for a surface feature on Chiron as suggested by Betzler (2023) to enter and exit the view from Earth across such a small percentage of the object's orbit. We thus reject surface albedo variations as being responsible for Chiron's brightening and dimming across 2021–2023.

We also consider the Chiron ring system model proposed by Ortiz et al. (2015) to be an unlikely cause of the object's observed brightening during and since 2021. Reconstructing this model, we find Chiron's measured absolute magnitudes to be significantly brighter than predicted by ∼0.5–1 mag. Furthermore, the timescale on which Chiron's 2021 Brightening Event occurred is too short to be explained by the ring model, which predicts variation in Chiron's absolute magnitude across decadal time spans. The Ortiz et al. (2015) lightcurve also fails to replicate the long-term evolution of Chiron's absolute magnitude both pre- and post-2021 Brightening Event. Chiron was predicted to slowly dim over time when it instead was observed to brighten in 2021. Additionally, Chiron was also significantly brighter pre-2021 Brightening Event than predicted by the ring model. We note, however, that although the Ortiz et al. (2015) model cannot explain either the 2021 Brightening Event or Chiron's brightness evolution before this across our observation baseline, this does not completely rule out the possibility that Chiron's rings are not well described by current models and are at least partially responsible for this brightening in reduced magnitude. Analyses of stellar occultations by Chiron in 2018 and 2022 indicate evolution of the material surrounding Chiron on relatively short timescales (Braga-Ribas et al. 2023; Ortiz et al. 2023; Sickafoose et al. 2023). Sickafoose et al. (2023) found Chiron's rings, as measured during the 2018 occultation, to be of reduced optical depth compared to 2011, with a reported feature, potentially indicative of a third ring, detected during immersion but not during emersion. Furthermore, Ortiz et al. (2023) detected signals indicative of additional, previously undiscovered ringlike features during Chiron's 2022 occultation with the signals of the already-known rings being more pronounced compared to 2011, indicating a significant change in the ring system's configuration. Occultation studies also reveal that Chiron's rings are not azimuthally homogeneous in width (Ortiz et al. 2023), exhibiting agglomerations of ring particles possibly caused by small, unseen shepherd satellites. Collisions of satellites have been proposed as one mechanism for creating Centaur rings (Braga-Ribas et al. 2014; Ortiz et al. 2015; Melita et al. 2017), and such an event could act to augment Chiron's ring system and thus its apparent brightness as seen from Earth. Future occultation studies are necessary to reveal the true nature of the reported ring system surrounding Chiron in order to better estimate its contribution to the Centaur's changing brightness across time.

Despite the need for an improved understanding of Chiron's rings, we argue that Chiron's observed evolution in brightness from 2021 onward is better described by an epoch of cometary activity. The increase in brightness in 2021 followed by a dimming across time is consistent with Chiron's observed brightness evolution during previous cometary episodes (e.g., Bus et al. 1989; Meech & Belton 1989). This years-spanning change in brightness, though far longer than the weeks- (Trigo-Rodríguez et al. 2008; Miles 2016; Schambeau et al. 2017, 2019; Peixinho et al. 2020) or months-long (Choi et al. 2006a, 2006b; Jaeger et al. 2011; Rousselot et al. 2016; James 2018; Kareta et al. 2019; Seccull et al. 2019) cometary outbursts of the active Centaurs 29P/Schwassman–Wachmann 1 and 174P/Echeclus, respectively, is nevertheless in keeping with the duration of previous epochs of Chiron's cometary activity, which have also lasted for years (Tholen et al. 1988; Bus et al. 1989; Meech & Belton 1989; Dahlgren et al. 1990; Hartmann et al. 1990; Luu & Jewitt 1990; Meech & Belton 1990; Meech 1990; West 1990; Bus et al. 2001; Silva & Cellone 2001). Additionally, Ortiz et al. (2023) theorized that material ejected due to an increase in cometary activity in 2021 could augment Chiron's rings, thereby explaining the apparent change in configuration in Chiron's ring system between occultations. Whether such a cometary outburst would represent an increase from quiescence or an entirely new epoch of cometary activity from a previously quiescent state is unclear. We did not detect any visible indicators of activity in the TRAPPIST-South observations spanning the years 2012–2015, yet Cikota et al. (2018) detected a tentative ∼5'' length tail-like feature of surface brightness 25.3 mag arcsec⁻² in their 2015 observations of Chiron. Cikota et al. (2018) also reported that Chiron exhibited fluctuations in brightness exceeding that of background field stars, implying low-level "microactivity." Therefore, we consider the most likely cause of Chiron's 2021 Brightening Event to be an epoch of either new or increased cometary activity.

If Chiron is presently undergoing cometary activity, an explanation for why we do not detect any visible coma could be that it is too faint to be directly observed beyond the central point spread at Chiron's present heliocentric distance. Previous measurements of Chiron's coma report V-band surface brightness values of 24.6 mag arcsec⁻² (West 1990) in 1990, when Chiron was 11.2 au from the Sun, and 26.0 mag arcsec⁻² (Silva & Cellone 2001) in 1998 at a heliocentric distance of 8.938 au. We calculate the limiting magnitudes of our Gemini deep-stacked observations. We utilize the same method of measuring the radial profiles of Chiron and the field stars as Section 4.3 and calculate the limiting surface brightness of each observation at a radial distance from the centroid of Chiron's PSF of 3 times the FWHM of a Gaussian fit to the PSF. We recalculate Chiron's literature coma surface brightness values to the corresponding heliocentric and geocentric distances for each Gemini observation. We transform the limiting surface brightnesses and Chiron's literature coma surface brightnesses M_surface into total magnitudes M according to the equation

$$\begin{eqnarray}&&M={M}_{\mathrm{surface}}-2.5{\mathrm{log}}_{10}(2\pi {r}^{2}),\end{eqnarray} \tag{ 7 }$$

where r is the radial distance of the surface brightness measurement from the PSF centroid (Lowry & Fitzsimmons 2005). We then transform Chiron's coma values to the corresponding Pan-STARRS filter of each Gemini observation, transforming first from Johnson–Cousins to SDSS photometric systems and then from SDSS to Pan-STARRS. To transform from Johnson–Cousins to SDSS, we utilize the transformation equations of Jester et al. (2005) for stars with Johnson–Cousins color indices R_c − I_c < 1.15 and U − B ≥ 0. These require B − V and R_c − I_c color index measurements of Chiron's coma; we use the B − V = 0.3 measurement from West (1991) for this transformation, and in the absence of a historical R_c − I_c measurement of Chiron's coma, we utilize the color index value R − I = 0.36 (Galiazzo et al. 2016) of Chiron itself as a proxy. We then use the transformation equations of Finkbeiner et al. (2016) to transform these SDSS values into the Pan-STARRS photometric filter system. We utilize Chiron's g − i color as measured in each epoch of Gemini observations; if an epoch lacks such a measurement, we instead use the closest g − i measurement in time to that epoch. Table 5 shows our resulting 3σ limiting magnitudes for each observation with Chiron's corresponding predicted coma brightness. For all our Gemini observations, for both surface brightness and total magnitude, we find Chiron's coma brightness to be fainter than the 3σ limiting magnitude of the observation, assuming a similar coma to that observed in 1990.

Alternatively, the lack of a detected coma could be due to the mass of the Centaur's nucleus. The large size of Chiron (volume-equivalent radius ∼98 km; Braga-Ribas et al. 2023) compared to most cometary nuclei means its gravitational force is nonnegligible. Some material emitted during an active epoch may be bound to the nucleus if its escape velocity exceeds the grain velocities of the emitted particles, preventing the formation of a spatially extended coma detectable from Earth-based observations. A bound coma around Chiron was previously discovered in 1993 by Meech et al. (1997) using the Hubble Space Telescope. These revealed that Chiron's brightness profile could only be explained by a coma whose structure comprised two components, one spatially extended and the other bound to the nucleus within its exopause (Meech et al. 1997). To ascertain the possibility that an entirely bound coma could exist around Chiron, we estimate its nuclear escape velocity via the equation

$$\begin{eqnarray}&&{v}_{\mathrm{esc}}=\sqrt{\displaystyle \frac{4\pi G\rho {abc}}{3r}},\end{eqnarray} \tag{ 8 }$$

where G is the gravitational constant; ρ is the density of Chiron's nucleus; a, b, and c are the semimajor axes of the nucleus assuming the triaxial ellipsoidal shape of Braga-Ribas et al. (2023); and r is the equivalent volume radius of the nucleus. Braga-Ribas et al. (2023) estimated Chiron's density to be 1.119 g cm⁻³ based on a Jacobi ellipsoidal model of its shape derived from stellar occultations under the assumption of hydrostatic equilibrium. As this value is model-dependent, we also estimate a range of escape velocity values for Chiron by utilizing the density estimates of the two known Neptune-crossing Centaur binaries: Ceto/Phorcys (ρ = 1.4 g cm⁻³; Grundy et al. 2007) and Typhon/Echidna (ρ = 0.44 g cm⁻³; Grundy et al. 2008). This range of values is also consistent with the upper limit of Chiron's density of ρ < 1.0 g cm⁻³ as inferred by Meech et al. (1997). Figure 14 shows the escape velocity range of Chiron along with the value estimated by Braga-Ribas et al. (2023) with respect to measured dust grain velocities from outbursts by the JFC 67P/Churyumov–Gerasimenko (Grün et al. 2016; Agarwal et al. 2017; Rinaldi et al. 2018) and the Centaur 29P/Schwassman–Wachmann 1 (Miles 2016; Wierzchos & Womack 2020), which we use as a proxy for grain velocities on Chiron and list in Table 6.

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As seen in Figure 14, most of the outbursts from comet 67P/Churyumov–Gerasimenko have dust grain velocities that do not exceed our estimated range of values for Chiron's escape velocity. While most of the grain velocities from the Centaur 29P/Schwassman–Wachmann 1 are much greater than Chiron's estimated escape velocity, the range of values from Wierzchos & Womack (2020) nevertheless overlaps with our range of estimated escape velocity values, including that of the shape model and density as reported by Braga-Ribas et al. (2023). Assuming these dust grain velocities to be representative of cometary activity on Chiron, this first-order comparison shows that an entirely bound coma is a plausible outcome of a cometary outburst. In this situation, any ejecta that remains bound to Chiron's nucleus may act to augment the ring system around the Centaur, and evidence of this activity may be detectable from differences in detected structures around Chiron in (future) occultation studies (Ortiz et al. 2023). However, without accurate values of its nuclear escape velocity and dust grain velocities from its cometary outbursts, we cannot rule out the possibility that Chiron is exhibiting a spatially extended coma that is too faint to be observed at aphelion.

Chiron's heliocentric distance of 18.9 au at the time of the first observation of the 2021–2022 observing season places it far beyond the boundary of direct sublimation of surface water ice at ∼3 au (Meech & Svoren 2004; Womack et al. 2017), therefore ruling out this mechanism as being responsible for Chiron's 2021 epoch of cometary activity. Beyond this boundary, cometary activity in the solar system is thought to be dominated by the sublimation of the volatile species CO and CO₂ (A'Hearn et al. 2012; Ootsubo et al. 2012; Reach et al. 2013; Bauer et al. 2015; Cochran et al. 2015; Womack et al. 2017) and the transition of water ice from an amorphous to a crystalline configuration (Jewitt 2009; Guilbert-Lepoutre 2012; Li et al. 2020). Although Chiron resides at distances too close to the Sun for these species to be maintained on its surface (Jewitt 2009), subsurface pockets of these species could persist across the Centaur's lifetime before being exposed to insolation via surface disruption (Prialnik 1992). This is further corroborated by the findings of Birch & Umurhan (2024) that imply that large quantities of CO could be preserved in the interiors of small KBOs. An alternative mechanism proposed to explain activity throughout the Centaur population is the release of volatiles from the transition of subsurface water ice from an amorphous to a crystalline configuration caused by the propagation of the thermal wave through the object's interior (Jewitt 2009). Guilbert-Lepoutre (2012) found that this transition can occur for Centaurs at heliocentric distances up to 16 au. Though Chiron resided beyond this outer boundary during its 2021 Brightening Event, we cannot rule out Chiron being active in the years preceding the outburst, as evidenced from a possible detection of a tail-like feature by Cikota et al. (2018). Combined with the considerably different size of Chiron (volume-equivalent radius 98 km; Braga-Ribas et al. 2023) compared to the Centaur model of Guilbert-Lepoutre (2012; 50 km), this means we cannot rule out the amorphous-to-crystalline transition of water ice as being responsible for Chiron's 2021 Brightening Event. We therefore consider newly exposed pockets of volatile ices and the amorphous-to-crystalline water-ice transition to both be viable mechanisms for Chiron's 2021 Brightening Event.

We have analyzed a ∼1 mag brightening in reduced/apparent magnitude by the large Centaur Chiron that started in 2021, referred to in this study as the 2021 Brightening Event. Combining multifilter observations from multiple instruments, we find that from 2021 June to 2023 February, Chiron remained brighter than its pre-2021 Brightening Event magnitude and did not exhibit any significant change in color index across time. The ∼1 mag increase of Chiron's brightness and its continued brightening across the 2021–2022 observing season disfavors rotational modulation, solar phase angle effects, and seasonal albedo features as plausible causes for this behavior. We find that the Ortiz et al. (2015) model of Chiron's reported ring system also does not account for the observed brightening. Chiron's enhanced brightening followed by its slow dimming across its 2022–2023 observing season, combined with its duration according with that of previous cometary outbursts, means we propose that the best explanation for the 2021 Brightening Event is an epoch of either new or increased cometary activity, caused either by the amorphous-to-crystalline water-ice transition or by newly exposed pockets of subsurface volatiles. We cannot completely rule out Chiron's ring system playing a role in its observed brightening, potentially by a collision of two small, unseen shepherd moons within the ring system, highlighting the need for further occultation observations over the coming years. We find no visible coma in any of our deep-stack Gemini images, yet brightness values of Chiron's historical comae imply that the Centaur could be exhibiting a coma that is too faint to be observed near aphelion or, alternatively, is bound to the nucleus.

We highlight that the multiyear baseline and high cadence of the ATLAS observations allowed Chiron's brightening to be detected and discerned via analysis of the Centaur's phase curve. The Rubin Observatory Legacy Survey of Space and Time (LSST) by the Vera C. Rubin Observatory (LSST Science Collaboration et al. 2009; Ivezić et al. 2019; Jurić et al. 2019; Schwamb et al. 2023) will allow a 10 yr data set of multifilter, high-cadence observations of approximately 5 million solar system objects, including the Centaur population (Guy et al. 2022). This will enable the discovery and analysis of potentially unseen Centaur activity via phase curve analysis for many more Centaurs. Our analysis thus serves as an example of how such a data set can be used to detect cometary activity exhibited by Centaurs, including Chiron, itself a possible flyby target for future space missions (Singer et al. 2019).

M.E.S. was supported by the UK Science Technology Facilities Council (STFC) grants ST/V000691/1 and ST/X001253/1. M.M.D. was supported by the UK Science Technology Facilities Council (STFC) grant ST/V506990/1. We acknowledge travel support provided by STFC for UK participation in LSST through grant ST/N002512/1 in addition to travel support provided by the Royal Astronomical Society. L.J.S. acknowledges support by the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (ERC Advanced Grant KILONOVA No. 885281). P.H.B. acknowledges support from the DIRAC Institute in the Department of Astronomy at the University of Washington, United States. The DIRAC Institute is supported through generous gifts from the Charles and Lisa Simonyi Fund for Arts and Sciences and the Washington Research Foundation. J.M. acknowledges support from the Department for the Economy (DfE) Northern Ireland postgraduate studentship scheme. M.S.K. was supported by the NASA Solar System Observations program (80NSSC20K0673).

This research has made use of data and/or services provided by the International Astronomical Union's Minor Planet Center.

This work has made use of data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. The ATLAS project is primarily funded to search for near-Earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; by-products of the NEO search include images and catalogs from the survey area. This work was partially funded by Kepler/K2 grant J1944/80NSSC19K0112, HST GO-15889, and STFC grants ST/T000198/1 and ST/S006109/1. We acknowledge travel support provided by STFC for UK participation in LSST through grant ST/S006206/1. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen's University Belfast, the Space Telescope Science Institute, the South African Astronomical Observatory, and the Millennium Institute of Astrophysics (MAS), Chile.

This work is based on observations obtained with the Samuel Oschin Telescope 48 inch telescope at the Palomar Observatory as part of the Zwicky Transient Facility project. Major funding has been provided by the U.S. National Science Foundation under grant No. AST-1440341 and by the ZTF partner institutions: the California Institute of Technology, the Oskar Klein Centre, the Weizmann Institute of Science, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron, the University of Wisconsin–Milwaukee, and the TANGO Program of the University System of Taiwan.

This work makes use of observations from the Las Cumbres Observatory global telescope network. Observations with the LCOGT 1 m were obtained as part of the LCO Outbursting Objects Key (LOOK) project (KEY2020B-009).

This research has made use of services provided by NASA's Astrophysics Data System.

This work has also made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC; https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

This work has made use of data from the TRAnsiting Planets and PlanetesImals Small Telescope South (TRAPPIST-South). TRAPPIST is a project funded by the Belgian Fonds (National) de la Recherche Scientifique (F.R.S.-FNRS) under grant PDR T.0120.21. E.J. is an F.R.S.-FNRS Senior Research Associate.

This work has made use of data and services provided by the Horizons system of the Jet Propulsion Laboratory.

This project used public archival data from the Dark Energy Survey (DES). Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of Chicago, the Center for Cosmology and Astro-Particle Physics at the Ohio State University, the Mitchell Institute for Fundamental Physics and Astronomy at Texas A&M University, Financiadora de Estudos e Projetos, Fundação Carlos Chagas Filho de Amparo à Pesquisa do Estado do Rio de Janeiro, Conselho Nacional de Desenvolvimento Científico e Tecnológico and the Ministério da Ciência, Tecnologia e Inovação, the Deutsche Forschungsgemeinschaft, and the Collaborating Institutions in the Dark Energy Survey. The Collaborating Institutions are Argonne National Laboratory, the University of California at Santa Cruz, the University of Cambridge, Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas-Madrid, the University of Chicago, University College London, the DES-Brazil Consortium, the University of Edinburgh, the Eidgenössische Technische Hochschule (ETH) Zürich, Fermi National Accelerator Laboratory, the University of Illinois at Urbana-Champaign, the Institut de Ciències de l'Espai (IEEC/CSIC), the Institut de Física d'Altes Energies, Lawrence Berkeley National Laboratory, the Ludwig-Maximilians Universität München and the associated Excellence Cluster Universe, the University of Michigan, the National Optical Astronomy Observatory, the University of Nottingham, the Ohio State University, the OzDES Membership Consortium, the University of Pennsylvania, the University of Portsmouth, SLAC National Accelerator Laboratory, Stanford University, the University of Sussex, and Texas A&M University. Based in part on observations at Cerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. Database access and other data services are provided by the Astro Data Lab.

This work has made use of data and services from the Panoramic Survey Telescope and Rapid Response System (Pan-STARRS). The Pan-STARRS1 (PS1) surveys and the PS1 public science archive have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg, and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen's University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Inc., the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation. Data from Pan-STARRS were obtained from the MAST data archive at the Space Telescope Science Institute.

This research utilized data and services provided by the Canadian Astronomy Data Center (CADC) Solar System Object Image Search. This research utilized data and services provided by the Catalog Archive Server Jobs System, developed by the Johns Hopkins University/Sloan Digital Sky Survey (JHU/SDSS) team.

This project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 101032479.

This research made use of Photutils, an Astropy package for detection and photometry of astronomical sources (Bradley 2023).

All photometric data used in this study are provided in full as supplementary information accompanying this paper and are located in the Appendix of this paper. Raw and calibrated observations from Las Cumbres Observatory used in this study are available at the LCO Science Archive (https://archive.lco.global; proposal code KEY2020B-009) after an embargo/proprietary period of 12 months.

We would like to thank Jose-Luiz Ortiz for the useful discussions about the Chiron ring model.

The authors thank the anonymous reviewers for their comments and feedback that improved this paper.

Facilities: ATLAS - , PO:1.2m - Palomar Observatory's 1.2 meter Samuel Oschin Telescope (ZTF), PS1 - Panoramic Survey Telescope and Rapid Response System Telescope #1 (Pan-STARRS), Blanco - Cerro Tololo Inter-American Observatory's 4 meter Blanco Telescope (DES), Gaia - , LCOGT - Las Cumbres Observatory Global Telescope (1 m telescopes), TRAPPIST - , Gemini Gillett - .

Software: Astropy (Astropy Collaboration et al. 2013, 2018), BANZAI (McCully et al. 2018), calviacat (Kelley & Lister 2021), ccdproc (Craig et al. 2017), DRAGONS (Labrie et al. 2019, 2023), Jupyter Notebook (Kluyver et al. 2016), math (Van Rossum 2020), Matplotlib (Hunter 2007), NEOExchange (Lister et al. 2021), Numpy (van der Walt et al. 2011; Harris et al. 2020), os (Van Rossum 2020), Pandas (pandas development team 2020), PHOTOMETRYPIPELINE (Mommert 2017), Photutils (Bradley 2023), python (https://www.python.org), SAOImageDS9 (Joye 2019), SciPy (Virtanen et al. 2020), SExtractor Python (Bertin & Arnouts 1996; Barbary 2016).

All observations used in this study are listed in Tables A1–A8 below, separated by telescope/survey.

Table A6. LOOK Observations of Chiron

MJD a	RH b	Δ c	αd	Seeing	Filter	Exposure	Air Mass e	Magnitude	Magnitude
						Time			Uncertainty
	(au)	(au)	(deg)	(arcsec)		(s)
59463.320833	18.867587	17.971398	1.4368	1.73	w	245.0	1.344	17.802	0.044
59463.324306	18.867587	17.971371	1.4366	1.68	w	245.0	1.355	17.795	0.051
59463.327083	18.867587	17.971350	1.4365	1.62	w	245.0	1.367	17.771	0.042
59463.330556	18.867587	17.971323	1.4363	1.61	w	245.0	1.380	17.769	0.043
59827.018750	18.834124	17.981445	1.6770	1.92	$g^{\prime}$	180.1	1.347	18.178	0.033
59827.020833	18.834124	17.981425	1.6769	2.06	$r^{\prime}$	180.1	1.332	17.936	0.034
59827.023611	18.834124	17.981400	1.6768	2.01	$g^{\prime}$	180.1	1.318	18.241	0.032
59827.025694	18.834123	17.981381	1.6767	1.71	$r^{\prime}$	180.1	1.304	17.885	0.031
59840.381944	18.832074	17.882598	1.0286	2.09	$g^{\prime}$	180.1	1.136	18.156	0.022
59840.386806	18.832073	17.882571	1.0284	2.10	$g^{\prime}$	180.1	1.147	18.159	0.022

Notes.

^a Modified Julian Date of observation. ^b Heliocentric distance. ^c Geocentric distance. ^d Solar phase angle. ^e Mean air mass of Chiron during exposure.

Only a portion of this table is shown here to demonstrate its form and content. A machine-readable version of the full table is available.

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