Monthly quasi-periodic eruptions from repeated stellar disruption by a massive black hole

The study investigates the nature of a newly discovered X-ray nuclear transient, Swift J023017.0+283603 (Swift J0230), that exhibits quasi-periodic outbursts on week-to-month timescales. Prior work has identified hour-scale X-ray quasi-periodic eruptions (QPEs) in galactic nuclei, often interpreted as repeated partial tidal disruptions of compact stars by modest-mass black holes, and longer-timescale optical periodic nuclear transients (PNTs), potentially due to partial stripping of main-sequence stars by more massive black holes. The central question is whether Swift J0230 represents a system that bridges these two classes and whether its behavior can be explained by repeated, partial tidal disruption events (rpTDEs) feeding a supermassive black hole. Establishing this connection would unify disparate transient classes and clarify accretion physics in galactic nuclei.

Previous discoveries of QPEs in galactic nuclei showed recurrent soft X-ray flares on hour-long periods and were proposed to arise from rpTDEs of white dwarfs by ~10^5 M⊙ black holes. Separately, PNTs with much longer recurrence times (tens to hundreds of days) observed in the optical/UV have been suggested to result from rpTDEs of main-sequence stars by more massive (~10^7–10^8 M⊙) black holes (for example ASASSN-14ko and AT2018fyk). Alternative origins such as AGN variability, ULXs, and delayed pulsar-driven supernova emission do not match the observed spectra, luminosities, or timescales of Swift J0230. Formation channels for bound, highly eccentric donor-star orbits include the Hills mechanism, wherein a binary is tidally separated by the central black hole, plausibly placing one star on an eccentric orbit with the observed period for a modest-mass black hole. Prior theoretical work also emphasizes the role of relativistic precession and tidal debris stream interactions in producing variability in rpTDE accretion flows.

Discovery and monitoring: Swift J0230 was discovered serendipitously on 2022-06-22 by the Living Swift-XRT Point Source (LSXPS) catalogue’s real-time transient detector in Swift-XRT data. The field had 11 prior Swift observations (Dec 2021–Jan 2022) with no detection. Follow-up Swift monitoring was initiated the day of discovery, with daily 1 ks exposures for about 12 days, then weekly, and later near-daily monitoring until 2023-03-19, subject to Sun/Moon constraints. A 3 ks Chandra DDT observation was obtained during a later outburst but coincided with a quiescent phase. Optical/UV photometry was obtained from Swift-UVOT and the Liverpool Telescope (LT). A host-galaxy optical spectrum was obtained with the Nordic Optical Telescope (NOT) and analyzed to measure redshift and emission-line ratios. X-ray analysis: Swift-XRT 0.3–10 keV light curves were constructed with one bin per observation; overlapping observations were handled by per-observation light curves and replacement of affected bins. Consecutive non-detections were merged to improve quiescent limits. For detected epochs, spectra were fit with an absorbed blackbody (tbabs with Galactic NH fixed at 1.12×10^21 cm^-2; ztbabs with host NH free at the host redshift). Fluxes (0.3–2 keV) and luminosities were derived using the host luminosity distance (160.7 Mpc). A hardness ratio using 0.9–2 keV and 0.3–0.9 keV was computed to track spectral changes. Temperature–luminosity trends were assessed with Spearman rank tests. To characterize recurrence, outbursts were defined via a luminosity threshold (3×10^41 erg s^-1), and start/end times were bracketed between consecutive points crossing this level. A Lomb–Scargle periodogram (Astropy) with bootstrap significance (10,000 shuffles) assessed periodicities; a window function was computed. Simulations inserted GSN 069-like short-period modulations into the real observing cadence to test detectability; the analysis showed such signals would have been recovered if present. Optical/UV analysis: Swift-UVOT pre-discovery and discovery-epoch photometry were compared; no significant changes were found. LT imaging after galaxy light subtraction provided upper limits on any variable nuclear component (g, r, i, z bands). NOT spectroscopy measured the host redshift and placed the galaxy on BPT diagrams to assess AGN activity, indicating a quiescent or very weak AGN host. Interpretive modeling: Simple energetics estimated the accreted mass per outburst. A standard accretion disc temperature scaling was used to estimate the black hole mass from the observed soft X-ray blackbody temperature (~100 eV). Scenario testing considered supernova association, ULX, AGN variability, and rpTDEs, incorporating formation via Hills capture and variability from tidal stream interactions and relativistic precession.

- Discovery of a nuclear soft X-ray transient, Swift J0230, coincident with the galaxy 2MASX J02301709+2836050 at redshift z = 0.03657 ± 0.00002 (luminosity distance 160.7 Mpc). - X-ray outbursts recur quasi-periodically on week-to-month timescales: eight subsequent outbursts after discovery with typical separations near 22–25 days and variability of several days between events. A long ~70 d gap with only weak activity occurred before resumption. Outburst durations vary from less than a day to about 20 days (one event up to ~32 d or two closely spaced events). - The first outburst lasted ~4 days; its end showed a rapid decay with luminosity dropping by a factor of ~20 in 57 ks, followed by a brief rebrightening by a factor of ~4.5 in 6 ks. - Lomb–Scargle analysis shows significant peaks centered at periods ~22.1 d and ~25.0 d (each ~1 d wide), above 3σ significance via bootstrapping, confirming quasi-periodicity rather than strict periodicity. - Peak soft X-ray luminosity reached ~4 × 10^42 erg s^-1 (0.3–2 keV). Outbursts defined above a threshold luminosity of 3 × 10^41 erg s^-1. - Spectral properties: very soft X-ray emission with no photons detected above 2 keV. Spectra are well fit by a simple blackbody plus Galactic absorption only; no evidence for intrinsic absorption. The source hardens with increasing luminosity; the 0.9–2 keV to 0.3–0.9 keV hardness ratio is strongly correlated with luminosity (Spearman test p ≈ 1.3 × 10^-10). The blackbody temperature increases with luminosity (strong Spearman correlation). - No optical or UV brightening associated with the X-ray outbursts: UVOT magnitudes during outburst are consistent with pre-discovery values; LT imaging yields only upper limits after host subtraction. - Energetics imply an accreted mass per typical outburst of order ~10^-4 M⊙. - The observed soft temperature (~100 eV) implies a modest-mass central black hole (of order a few 10^5 M⊙) when compared with standard accretion disc scalings, consistent with QPE-like black hole masses but with longer recurrence times and outburst durations than hour-scale QPEs.

The properties of Swift J0230—soft X-ray spectrum, peak luminosity ~4 × 10^42 erg s^-1, lack of optical/UV variability, and quasi-periodic outbursts on ~3–4 week timescales—are inconsistent with supernova remnants, ULXs, or typical AGN variability. The data favor a scenario in which a star in a highly eccentric bound orbit around the galaxy’s central black hole undergoes repeated partial tidal stripping at each pericentre passage (rpTDE), supplying gas to an accretion flow that powers the X-ray outbursts. The observed blackbody temperature suggests a modest-mass black hole (few × 10^5 M⊙), and the inferred accreted mass per event (~10^-4 M⊙) is compatible with partial envelope loss from a non-degenerate (main-sequence) donor. This places Swift J0230 in between previously known hour-scale X-ray QPEs (likely white dwarf donors around ~10^5 M⊙ black holes) and longer-timescale optical PNTs (likely main-sequence donors around ~10^7–10^8 M⊙ black holes), thereby bridging the two classes. Formation of the required eccentric orbit is plausibly achieved via the Hills mechanism disrupting an incoming binary, which can produce a bound star with an orbital period comparable to the observed ~25 days for a modest-mass black hole. Variations in outburst duration and structure, including sudden dips and sharp terminations, can be naturally produced by complex interactions between returning inner and outer tidal debris streams, relativistic precession, and perturbations of the accretion flow when the star revisits pericentre. The system thus provides evidence supporting a unified rpTDE framework for nuclear transients across a broad range of timescales.

This work reports the discovery and characterization of Swift J0230, a nuclear X-ray transient exhibiting quasi-periodic outbursts on ~monthly timescales with soft spectra and no optical/UV counterpart variability. The observations support a repeating partial tidal disruption model of a main-sequence star orbiting a modest-mass supermassive black hole, bridging hour-scale X-ray QPEs and longer-period optical PNTs within a unified rpTDE framework. The event was enabled by low-latency detection from a real-time Swift-XRT transient pipeline, demonstrating the importance of sensitive, rapid X-ray transient searches. Future work should include theoretical modeling of tidal stream–disc interactions and relativistic precession effects, improved positional constraints and deeper X-ray coverage to exclude alternative associations, and continued monitoring to map long-term evolution. Upcoming wide-field X-ray missions such as eROSITA and Einstein Probe are expected to discover additional members of this class, enabling population studies and tests of the unified scenario.

- Positional uncertainty remains: a Chandra observation intended to refine the position coincided with a quiescent phase, leaving a small chance of association with SN 2020rht unruled, though a supernova origin is disfavored by spectral and temporal properties. - Observing cadence had gaps due to Sun/Moon constraints and scheduling, leading to incomplete coverage and some ambiguity in identifying and delimiting individual outbursts; outburst definitions relied on a luminosity threshold and visual inspection. - Spectral modeling used simple blackbody fits with Galactic absorption only; black hole mass estimates from disc-temperature scaling are approximate. - No detection above 2 keV and lack of multiwavelength (optical/UV) variability limit constraints on alternative models and detailed accretion physics. - Theoretical interpretation involving complex debris stream interactions and disc dynamics was qualitative; full numerical modeling is deferred to future work.