Discovering periodic and repeating nuclear transients in the XMM-Newton archives

Abstract: The regions around massive black holes can show X-ray variability on timescales from seconds to decades. Observing many black holes over different timescales can enhance our chances of detecting variability coming from (partial) tidal disruption events, massive black hole binaries, changing state AGN, blazar activity and much more. X-ray catalogues with hundreds of thousands of detections are treasure troves of such sources, which require innovative methods to identify these black holes. We present the current XMM-Newton catalogues available and describe several examples of tidal disruption events (TDEs) and quasi-periodic eruption sources that have been found whilst mining this data. We describe preliminary work on a search for periodic variables in the XMM-Newton EPIC archival data, with the example of finding new massive black hole binaries. We also describe the STONKS pipeline that is now in the XMM-Newton automatic reduction pipeline and the near real-time alert system that allows the follow-up of new and fading transients. We provide examples of fading sources that are newly identified candidate TDEs.

• The paper discovers and catalogs periodic nuclear X-ray transients using advanced Fourier analysis and time-domain folding.
• It employs multi-epoch XMM-Newton data to distinguish between classical TDEs, partial repeating TDEs, QPEs, and AGN variability.
• Results outline candidate MBH binaries with orbital separations of 0.002–0.01 mpc, promising future multi-messenger detections.

Discovery and Characterization of Periodic and Repeating Nuclear Transients in the XMM-Newton Archives

XMM-Newton Archival Resources and Nucleus-Centric X-Ray Catalogues

The study leverages extensive archival resources provided by the XMM-Newton observatory, emphasizing the utility of large-scale, multi-epoch X-ray data for the identification and characterization of nuclear transients. Principal catalogues discussed include the 4XMM-DR14 detection catalogue, which aggregates over 1,000,000 X-ray detections, the stacked-sensitivity 4XMM-DR14s catalogue, and the all-sky XMMSL3 slew catalogue. These resources collectively offer temporal coverage up to four decades and multi-wavelength associations, facilitating systematic searches for variability and transient activity among galactic nuclei.

The cataloguing methodology incorporates advanced routines for source photometry, lightcurve extraction, and multi-wavelength cross-correlation. The 4XMM-DR14 provides detailed photometric and temporal data for over 692,000 sources, with median soft-band fluxes of approximately $5.2 \times 10^{-15}$ erg cm$^{-2}$ s$^{-1}$ (0.2–2.0 keV), complemented by long-term variability metrics and galaxy catalog cross-identifications. The XMMSL3, covering 93.7% of the sky, enables transient searches across most of the celestial sphere, albeit at shallower sensitivity.

Typology and Physical Mechanisms of Nuclear Transients

The investigation details the principal mechanisms responsible for nuclear X-ray transients, focusing on both classical and repeating TDEs, QPEs, and AGN state transitions. Stellar tidal disruption events (TDEs) are described in terms of Roche radius crossing, with subsequent accretion-driven X-ray outbursts reaching luminosities up to $10^{45}$ erg s$^{-1}$ [stru09]. Examples of TDEs identified via XMM-Newton data include sources with black hole masses ranging from $10^4$ to $10^7$ M$_\odot$, some residing in the nuclei of dwarf galaxies, thereby implying the existence of IMBHs [lin20].

Partial, repeating TDEs such as HLX-1 manifest as multi-epoch, quasi-periodic outbursts with recurrence times spanning hundreds of days; their evolution may be consistent with eccentric intermediate mass ratio inspirals and subsequent outspirals. The paper catalogs several similar repeating events discovered by XMM-Newton and corroborated by eROSITA and Swift, elucidating the impact parameter statistics and encounter frequency advantage for partial over full tidal disruptions [guil13].

Quasi-periodic eruptions (QPEs) are also documented, characterized by soft X-ray bursts recurring on timescales of hours to months [mini19, arco21]. The variability in burst intervals, energies, and non-sinusoidal profiles is highlighted, reflecting diversity in disk-star interaction mechanisms and possible eccentric accretion orbits [arco22].

AGN variability is contextualized within accretion disk physics, state transitions, and the dynamics of MBH binaries created in galaxy mergers. The significance of dual AGN, disk instability models, and changing-look AGN is emphasized. The paper delineates electromagnetic and gravitational wave signatures, noting that MBH binaries with milliparsec separations and masses $>$10$^7$ M$^{-2}$0 should eventually produce LISA-detectable GW events [bara15, cocc24, fous25].

Automated Pipeline for Periodicity and Transient Detection

A systematic search for periodic X-ray emission in the 4XMM-DR13 catalogue was conducted using accelerated Fourier techniques [rans02], the HENDRICS/Stingray suite [hupp19, bach18b], and robust time-domain folding (epoch-folding, $^{-2}$1) [leah83, bach21]. Quality criteria and barycentric corrections were applied to minimize false positives due to background flaring and red noise. The pipeline demonstrated recoverability of known periodic sources and yielded 1355 detections with pulsations, of which 345 were spatially coincident with galaxy nuclei (GLADE+ catalog). Approximately a dozen sources exhibited $^{-2}$2 periodic variability, including non-sinusoidal modes suggestive of orbital eccentricity or evolving system geometry. Estimated nuclear MBH masses range from $^{-2}$3–$^{-2}$4 M$^{-2}$5, with one dwarf candidate at $^{-2}$61300 M$^{-2}$7. Inferred orbital separations for candidate binaries are 0.002–0.01 mpc, falling within reach of future LISA campaigns. However, confirmation of periodicity origin, discriminating persistent periodicity from statistical red noise, remains ongoing.

STONKS Pipeline and Near Real-Time Transient Alerts

Near real-time detection of X-ray transients has been enabled via the STONKS transient pipeline integrated into the XMM-Newton data reduction workflow [quin24]. Detection criteria require flux variability by at least a factor of five relative to archival measurements. Alerts for significant transients are disseminated post-manual validation, contingent on public access clearance. This system has uncovered several fading candidates for TDEs, despite limited follow-up prospects due to declining source flux.

Specific cases, such as 4XMM J071928.1+591102, J081738.2+012402.9, J093220.2+460634.9, and J093306.2+460750.1, exhibit long-term declines in X-ray output over multi-decade baselines, spectral transitions from soft to hard states, and Luminosities typically in the $^{-2}$8–$^{-2}$9 erg s$^{-1}$0 regime. The inferred nuclear MBH masses are %%%%21$_\odot$22$^{-2}$023%%%% M$^{-1}$4, corresponding to sub-Eddington accretion rates at peak. However, sparse temporal sampling hinders definitive classification as TDEs or AGN variability.

Implications, Theoretical Outlook, and Future Prospects

The results establish the XMM-Newton archives as critical infrastructure for the population synthesis and characterization of both classical and exotic nuclear transients. The demonstrated periodic search pipeline facilitates discovery of candidate MBH binaries at separations and masses relevant for electromagnetic and GW observation synergies. The STONKS pipeline enhances rapid transient identification, catalyzing timely multi-wavelength and multi-messenger follow-up.

From a theoretical perspective, the work underscores the complexity of tidal interactions, disk instability physics, and accretion variability at galactic nuclei. Observation of repeated TDEs and QPEs suggests that partial disruptions and disk-star interactions play a more significant role than previously modeled, advocating for continued hydrodynamical and magnetohydrodynamical simulations incorporating non-linear and eccentric dynamical regimes [dai10, dai18].

The upcoming 5XMM catalogue, featuring expanded multi-wavelength associations, long-term variability tracking, photometric redshifts, and systematic spectral fitting, is anticipated to further refine nuclear transient classification, source modeling, and MBH demographic mapping.

Conclusion

This work showcases the integration of advanced archival mining, time-domain analysis, and real-time transients pipelines in the XMM-Newton archives for the systematic study of nuclear transients. The identification of TDEs, repeated partial disruptions, QPEs, and candidate MBH binaries marks significant progress in empirical constraints on galactic nucleus evolution and transient phenomenology. Continuing development of catalogues and detection algorithms, in concert with GW observatories, promises enhanced temporal resolution and source characterization in the study of black hole accretion, dynamics, and growth.