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

The study of galaxies exhibiting nuclear quasi-periodic eruptions (QPEs) with periods of several hours has gained significant traction in recent years. These events bear resemblance to tidal disruption events (TDEs), where a star is tidally disrupted by a supermassive black hole. A compelling model posits that repeated, partial stripping of a white dwarf in a highly eccentric orbit around a black hole of ~10⁵ solar masses is responsible for these QPEs. Separately, another class of periodic nuclear transients (PNTs), characterized by considerably longer timescales, has been identified optically. These PNTs are hypothesized to result from the partial stripping of a main-sequence star by a more massive black hole (~10⁷ solar masses). However, a clear connection between these two classes remains elusive. This paper presents the discovery of an X-ray nuclear transient, Swift J0230, exhibiting quasi-periodic outbursts with a period of weeks. This discovery is significant as it potentially bridges the gap between the previously identified QPEs and PNTs, offering a unique opportunity to refine our understanding of these phenomena. The rapid identification and follow-up of Swift J0230, facilitated by the new live Swift-XRT transient detector, underscore the crucial role of low-latency, sensitive searches for X-ray transients in unraveling the mysteries of galactic nuclei.

Existing literature details two classes of periodic nuclear transients: QPEs and PNTs. QPEs, with periods of several hours, are thought to originate from the repeated partial stripping of a white dwarf star by a relatively low-mass (~10⁵ solar masses) black hole. This model explains the observed quasi-periodic nature of the X-ray emissions. On the other hand, PNTs, featuring much longer periods (days to years), are attributed to similar partial stripping events but involving main-sequence stars and significantly more massive (~10⁷-⁸ solar masses) black holes. The differences in timescales and the types of stars involved are key distinctions. While both classes suggest repeated interactions between stars and black holes, a unified model encompassing both QPEs and PNTs has not been fully established.

The discovery of Swift J0230 was made possible by the Living Swift-XRT Point Source (LSXPS) catalogue’s real-time transient detector. The source was initially detected on June 22, 2022, with a 0.3–10 keV count rate exceeding the 3σ upper limit established from previous observations. Its location was determined to be consistent with the nucleus of galaxy 2MASX J02301709+2836050 (z=0.03657 ± 0.00002), although also marginally consistent with a Type II supernova, SN 2020rht. Regular monitoring of Swift J0230 was conducted using Swift, initially daily and later weekly, supplemented by a Chandra observation. Data analysis included the construction of light curves (0.3–10 keV), spectral fitting (absorbed black-body model), and Lomb-Scargle periodogram analysis to identify periodicities. Optical and ultraviolet data were also acquired from Swift UVOT and the Liverpool Telescope to assess variability at these wavelengths. The team developed specific methods to merge upper limit bins and to define outbursts, ensuring robust results.

Swift J0230 exhibited a series of quasi-periodic X-ray outbursts with a recurrence timescale of approximately 25 days, although with variability of several days. The outbursts displayed significant variability in duration, ranging from less than a day to approximately 20 days. The X-ray spectrum during outbursts was very soft (no emission above 2 keV), well-modeled by a simple black-body emitter. A strong correlation was observed between luminosity and spectral hardness, ruling out changes in absorption as the primary cause of flux variations. Optical and ultraviolet observations revealed no evidence of outbursting behavior, further constraining potential models. While the Chandra observation did not definitively rule out association with SN 2020rht, this hypothesis is considered unlikely given the observed spectral and temporal properties of Swift J0230. The peak luminosity of ~4 × 10⁴² erg s⁻¹ and lack of optical/UV variability support the hypothesis of near-periodic mass supply into an accretion flow onto a central supermassive black hole. Based on energetics and spectral temperature (~100 eV), the estimated black hole mass is ~2 × 10⁶ solar masses. The authors proposed a repeating, partial tidal disruption event (rpTDE) model, where a star in a highly eccentric orbit loses some of its envelope at each pericentre passage. The quasi-periodic nature of the outbursts is attributed to relativistic precession of the stellar debris orbits and their interaction.

The findings presented strongly suggest that Swift J0230 is a new type of transient source, bridging the gap between previously identified QPEs and PNTs. The observed characteristics, including the weekly periodicity, soft X-ray spectrum, and absence of significant optical/UV variability, are consistent with an rpTDE model. The estimated black hole mass of ~2 × 10⁶ solar masses, intermediate between the masses associated with QPEs and PNTs, supports this conclusion. The rpTDE model, involving a main-sequence star orbiting a moderately massive black hole, successfully explains the observed timescales and luminosities. The variability in outburst duration and recurrence might arise from interactions between the inner and outer tidal streams, affected by relativistic precession and the perturbing influence of the returning star on the accretion flow. This work establishes Swift J0230 as a prototype for a previously unknown class of transient sources, and the discovery methodology highlights the importance of real-time transient detection capabilities for uncovering these elusive phenomena.

This paper reports the discovery of Swift J0230, a novel X-ray transient exhibiting monthly quasi-periodic outbursts. The observed characteristics strongly support a model of repeated, partial tidal disruption events (rpTDEs) of a main-sequence star by a moderately massive black hole (~2 × 10⁶ solar masses), bridging the gap between previously known QPEs and PNTs. The discovery highlights the effectiveness of real-time transient detection and the potential for future discoveries of similar systems using sensitive wide-field X-ray instruments.

The Chandra observation, unfortunately conducted during a relatively quiet phase of the outburst, did not achieve higher positional accuracy to definitively rule out the association with the nearby supernova SN 2020rht. The proposed rpTDE model is a plausible explanation, but more detailed numerical modeling is needed to fully account for the complex interactions between the tidal streams, accretion flow, and the orbiting star, especially concerning the observed outburst variability.