0 Assigned ID: 6919 ----- Attempting Submission 1 (Fri Aug 29 14:02:49 GMT 2025) ----- HST Phase I Proposal 6919 successfully submitted. Receipt: # 6919-1 Sensitive exposures should have an ETC run number provided. FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser FLASH level may be too low for this exposure or a short subexposure. See extended explanation in the diagnostic browser a 4-orbits visit split into two 2-orbits visits; flashes added after imaging exposures. 962148351 ----- Attempting Submission 1 (Tue Oct 07 02:05:00 GMT 2025) ----- ----- Attempting Submission 2 (Wed Feb 11 20:33:08 GMT 2026) ----- the ETC calculations were sent via email to the reviewing scientist X-ray emissions of a peculiar hyperluminous X-ray source We propose a joint Chandra-HST study of the most extreme hyperluminous X-ray source selected from a cross-correlation study of Chandra and XMM-Newton catalogues. It reached a peak X-ray luminosity of 5e42 erg/s in 2012; it was 100 times fainter in 2009 and 2023. It is a strong off-nuclear IMBH candidate in a thermal dominant state. We will study its current luminosity and spectral state with Chandra, and the brightness and colours of its optical counterpart with HST, constraining its BH mass and testing between alternative scenarios (repeated outbursts or one-off TDE). This is an imaging observation to determine the multi-band photometry of our target for a SED study. Prof. Albert Kong has been working on observational high-energy astrophysics focusing on compact binary systems for over 20 years. He has been awarded observing time as a PI in all modern X-ray and gamma-ray observing facilities. In this project, he will coordinate the observing campaign and perform some of the data analysis. Prof. Roberto Soria is an experienced high-energy astrophysicist. His current research focuses on multi-wavelength observations of ultraluminous X-ray sources. In the last 20 years, he has been awarded observing time as a PI in all major X-ray and optical observing facilities. In this project, he will help some of the data analysis and arrange follow-up ground-based observations. Yi-Chi Chang is a PhD student of Prof. Albert Kong. She is currently working on multiwavelength observations of ultraluminous X-ray sources. She previously worked with the COSI team and has good experience in gamma-ray data analysis. In this project, she will perform most of the data analysis. Dr. Igor Chilingarian is a research scientist at Center for Astrophysics - Harvard and Smithsonian. He is a specialist in dwarf galaxies, compact galaxies, stellar populations, and AGN with the emphasis on active intermediate-mass black holes, in 2018 he published a bona-fide sample of Intermediate-Mass Black Holes obtained using data mining in the spectroscopic part of SDSS and X-ray surveys (Chilingarian et al. 2018, ApJ, 863, 1). We will be responsible for the joint interpretation of X-ray and optical datasets. Dr. Kirill Grishin is a postdoc at Universite de Paris (France); IC and KG published the first bona fide sample of intermediate-mass black holes (Chilingarian et al. 2018, ApJ, 863, 1) which made it to the top 3% of the most cited papers in astrophysics published in 2018. His current research is mostly focused on the analysis of optical X-ray data for AGN. 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