0 Assigned ID: 4268 ----- Attempting Submission 1 (Wed May 24 06:39:12 GMT 2023) ----- HST Phase I Proposal 4268 successfully submitted. Receipt: # 4268-1 0 ----- Attempting Submission 1 (Wed Sep 06 14:22:09 GMT 2023) ----- Quasi-periodic eruptions (QPEs) are recurrent, high-amplitude bursts of X-ray emission in the nuclei of galaxies hosting low mass (10^5-6 M_sun) black holes. Their nature is still hotly debated, with the main two model classes including various flavours of accretion disk instabilities, and two (or multi-)body interactions between the central SMBH and a stellar-mass perturber. The latter can involve a main sequence star, white dwarf, or stellar mass black hole, and the eruptions are variously related to tidal stripping of orbiting companions, Roche lobe overflow, or interactions of the perturber with an accretion disk. The main QPE properties are short recurrence times (2-20 hrs), short durations (27 min - few hrs) and high amplitudes (factor 10-100). Discovered and well studied in X-rays, the UV, IR and radio properties remain poorly constrained. We propose to obtain the first deep observations in the UV (currently constrained only with the XMM-Newton Optical monitor), with high cadence (3 min) HST FUV imaging of a QPE with eruptions lasting only 27 min and recurring every 114 min. These timescales are perfectly suited to perform the first time-resolved UV variability search during the X-ray eruptions. The HST spatial resolution will resolve out the host galaxy emission; with HST sensitivity in the FUV this will lead to a >2 orders of magnitude improvement compared to current constraints. A detection of UV variability and how it correlates with the X-rays will have field-defining implications that will redraw the theoretical landscape. Non-detections will provide very strong additional constraints on the geometry, extent and properties of the putative accretion disks. HST/STIS/FUV-MAMA F25QTZ time-tag imaging observations of 1 target in 2 identical visits taken strictly simultaneous with XMM-Newton. This will be done through a 5-orbit contiguous visit, repeated twice. No time restrictions exist on the relative timing of the 2 visits in the cycle. We wish to obtain the highest possible time resolution, so use time-tag mode for imaging to fill the entire orbital visibility period. F25QTZ filter was chosen to obtain the lowest background with good sensitivity to FUV photons. Buffer times estimated from the ETC are > the orbital visibility time, so this has been set to 20000. All investigators are actively contributing to the rapid development of the QPE field. Wevers was the PI of the first systematic study of their host galaxies, and has experience analysing X-ray, UV and optical data of transient events through photometry and spectroscopy, for QPEs and possibly related TDEs. Pasham and Arcodia are experts in the field, having discovered half of the QPE population. They have (co-)authored nearly all observational papers on QPEs since the first publication in 2019. Wevers and Pasham are experts in the adjacent field of tidal disruption events, including in the analysis of their (time-resolved) UV/optical and X-ray properties. They both have hands-on experience with optical and UV photometry, including with HST. All authors have in-depth experience with detailed analysis of XMM-Newton data. Together the team has all the knowledge and expertise necessary to analyse the requested data. We will also collaborate with experts in the theoretical part of the community to maximise the impact of these observations in the field. 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