Tormund’s return: Hints of quasi-periodic eruption features from a recent optical tidal disruption event

Abstract

Context. Quasi-periodic eruptions (QPEs) are repeating thermal X-ray bursts associated with accreting massive black holes, the precise underlying physical mechanisms of which are still unclear. Aims. We present a new candidate QPE source, AT 2019vcb (nicknamed Tormund by the Zwicky Transient Facility Collaboration), which was found during an archival search for QPEs in the XMM-Newton archive. It was first discovered in 2019 as an optical tidal disruption event (TDE) at z = 0.088, and its X-ray follow-up exhibited QPE-like properties. Our goals are to verify its robustness as QPE candidate and to investigate its properties to improve our understanding of QPEs. Methods. We performed a detailed study of the X-ray spectral behaviour of this source over the course of the XMM-Newton archival observation. We also report on recent Swift and NICER follow-up observations to constrain the source’s current activity and overall lifetime, as well as an optical spectral follow-up. Results. The first two Swift detections and the first half of the 30 ks XMM-Newton exposure of Tormund displayed a decaying thermal emission typical of an X-ray TDE. However, the second half of the exposure showed a dramatic rise in temperature (from 53.5−7.7+9.2 eV to 113.8−2.7+2.9 eV) and 0.2–2 keV luminosity (from 3.2−1.0+1.6 × 1042 erg s−1 to 1.19−0.05+0.05 × 1044 erg s−1) over ∼15 ks. The late-time NICER follow-up indicates that the source is still X-ray bright more than three years after the initial optical TDE. Conclusions. Although only a rise phase was observed, Tormund’s strong similarities with a known QPE source (eRO-QPE1) and the impossibility to simultaneously account for all observational features with alternative interpretations allow us to classify Tormund as a candidate QPE. If confirmed as a QPE, it would further strengthen the observational link between TDEs and QPEs. It is also the first QPE candidate for which an associated optical TDE was directly observed, constraining the formation time of QPEs.