A Star's Death by a Thousand Cuts: The Runaway Periodic Eruptions of AT2023uqm

TL;DR

AT2023uqm presents a compelling rpTDE, showing at least five periodic optical flares with an unprecedented exponential growth in flare energy, signaling progressive stellar destruction on a bound orbit around a SMBH. Multi-wavelength follow-up and spectroscopy reveal features consistent with rpTDEs, including intermediate-width Balmer lines, Fe II, coronal lines, and Bowen fluorescence, while the double-peaked light curves and a stable ~526.75-day period constrain the orbital and disk geometry. The analysis favors a low-mass star (potentially giant) undergoing repeated partial disruptions, with a plausible future disruption after one or two more passages; a giant-star scenario can explain the large peak-separation-to-period ratio, whereas a disk-collision model offers an alternative path to the observed emission. Overall, AT2023uqm provides a crucial framework for modeling rpTDEs, constraining stellar properties and disruption physics, and guiding future time-domain surveys to uncover similar systems.

Abstract

Stars on bound orbits around a supermassive black hole may undergo repeated partial tidal disruption events (rpTDEs), producing periodic flares. While several candidates have been suggested, definitive confirmation of these events remains elusive. We report the discovery of AT2023uqm, a nuclear transient that has exhibited at least five periodic optical flares, making it only the second confirmed case of periodicity after ASASSN-14ko. Uniquely, the flares from AT2023uqm show a nearly exponential increase in energy--a "runaway" phenomenon signaling the star's progressive destruction. This behavior is consistent with rpTDEs of low-mass, main-sequence stars or evolved giant stars. Multiwavelength observations and spectroscopic analysis of the two most recent flares reinforce its interpretation as an rpTDE. Intriguingly, each flare displays a similar double-peaked structure, potentially originating from a double-peaked mass fallback rate or two discrete collisions per orbit. The extreme ratio of peak separation to orbital period draws attention to the possibility of a giant star being disrupted, which could be distinguished from a low-mass main-sequence star by its future mass-loss evolution. Our analysis demonstrates the power of rpTDEs to probe the properties of disrupted stars and the physical processes of tidal disruption, though it is currently limited by our knowledge of these events. AT2023uqm emerges as the most compelling rpTDE thus far, serving as a crucial framework for modeling and understanding these phenomena.

Figures (6)

• Figure 1: Multiband light curves of AT2023uqm. Facilities and filters are indicated by the legends. For the purpose of presentation, we shifted the light curves vertically and scaled the WISE photometry by a factor of 10. Vertical gray lines indicate the time of the dip of the double-peaked flare, calculated using the period derived in Section \ref{['sec:period']}. Gray shaded regions spanning $\pm 30$ from each vertical line, highlight the flare events.
• Figure 2: Optical spectra of AT2023uqm. Vertical lines mark the prominent emission features, including the Balmer series, Bowen fluorescence $\rm O\,\textsc{iii}$, $\rm Fe\, \textsc{ii}$ and high ionization coronal lines. The pink shaded regions highlight the prominent telluric bands. The brown arrow points to a spurious feature in the NOT spectrum, which may likely due to the incomplete subtraction of a night-sky emission line.
• Figure 3: The zoomed-in ZTF light curve for each flare. The green and red points depict the ZTF photometric data, while curves of the same color indicate the best double-Gaussian fits (see Section \ref{['sec:lcshape']}). Vertical dashed lines mark the spectrum acquisition times. The timing of each flare is relative to the corresponding dip in the Gaussian fit. For the latest flare, wfst-r band (dark red square) and scaled photometry from other bands (magenta square, mainly Swift) are also included.
• Figure 4: Multi-band light curves for the quiescent phase preceding the recent outburst (lower two panels) and during the outburst (upper panels). In the top panel, multi-band optical/UV photometry—identified in the legend—has been vertically offset for clarity. In the X-ray panels, gray downward triangles represent marginal detections or upper limits from Swift/XRT observations. Black squares indicate binned results, with the horizontal error bars reflecting the bin width in MJD. For upper limits obtained from EP observations (orange downward triangles), those with horizontal error bars correspond to binned data.
• Figure 5: The evolution of integrated energy (left y-axis) and mass loss (right y-axis) as a function of flare number. The red points and green squares represent the integrated energy for each flare converted from ZTF-r and ZTF-g monochromatic fluxes, respectively (see details in the text). Solid lines of the same colors show exponential function fits to the corresponding data. The magenta dashed lines indicate the mass loss evolution for a Sun-like star, based on hydrodynamic simulations by LiuChang2025, with different values of $\beta$ labeled on each line. The black dotted lines represent the mass loss evolution for a $1\,\rm M_\odot$ ZAMS star with $\beta=0.6$, given by the simulations of Bandopadhyay2025.