0 Assigned ID: 6899 ----- Attempting Submission 1 (Sun Apr 13 15:01:57 GMT 2025) ----- HST Phase I Proposal 6899 successfully submitted. Receipt: # 6899-1 ----- Attempting Submission 2 (Mon Apr 14 13:12:31 GMT 2025) ----- HST Phase I Proposal 6899 successfully submitted. Receipt: # 6899-2 0 ----- Attempting Submission 1 (Sun Apr 27 06:50:20 GMT 2025) ----- HST Phase II Proposal 17933 successfully submitted. Receipt: # 17933-1 ----- Attempting Submission 2 (Sun Apr 27 06:56:31 GMT 2025) ----- HST Phase II Proposal 17933 successfully submitted. Receipt: # 17933-2 ----- Attempting Submission 3 (Tue Apr 29 14:21:21 GMT 2025) ----- Turn-on AGN or IMBH-TDE? UV Spectroscopic Observations May Tell The quasi-periodic eruptions (QPEs) are an intriguing transient phenomenon associated with supermassive balck holes (SMBHs) discovered in the past few years. There is growing observational evidence that some (if not all) QPEs are connected with tidal disruption events (TDEs), which has promoted the "EMRI+TDE=QPE" model. Very recently, another remarkable QPE source has been reported after an optical outburst in the nearby galaxy SDSSJ1335+0728. It is suggested that the QPEs in this galaxy are linked to a turn-on active galactic nuclei (AGN) rather than a TDE as its slow decay and AGN-like variability differ greatly from noraml TDEs. Therefore, this discovery may broaden the possible formation channels for QPEs. However, its blue optical color, soft X-ray emission and lack of broad emissions are unusual for AGNs, but could be consistent with a TDE scenario. In fact, such a long, blue and faint optical outburst can be a strong evidence for TDEs by intermediate-mass black holes (IMBHs). To further explore the nature of the intriguing outburst (turn-on AGN or IMBH-TDE), we propose to conduct STIS UV spectroscopic observations on SDSSJ1335+0728 with 4 orbits time. The immediate objectives are mainly in two folds: 1) to detect possible characteristic UV emission lines; 2) to better characterize the outburst SED down to 1000 A, both of which may help to distinguish between the AGN and TDE scenarios. If it is indeed an IMBH-TDE, it provides another representative example of QPE-TDE associations in a rare IMBH-TDE system while in the case of a turn-on AGN, it also provides valuable, yet rarely studied, SED information for a newly formed AGN disk. We propose to observe the UV spectrum of SDSSJ1335+0728 with STIS. We require to use G140L and G230L gratings to obtain a continuous wavelength coverage from 1120 to 3100 A in the rest frame. A slit width of 0.2" is preferred to cover the galactic nucleus and minimize the starlight contamination. The magnitudes in UVW1, UVM2, UVW2 bands measured from the latest Swift/UVOT observations on March 12 are 17.91+-0.05, 18.17+-0.05 and 18.05+-0.10, respectively. After subtracting the host contribution from the host galaxy, the UVM2 (with a central wavelength of 2246A) magnitude is around 18.9. We expect the magnitude to be comparable after a few months, since its evolution is rather slow. We try to estimate the needed exposure time using the Exposure Time Calculator (ETC) of HST/STIS. We assume a blackbody spectrum with a temperature of 30,000K, and an AB magnitude of 18.9 in the UVM2 band. To reliably detect a possible N III]1750 line, a continuum signal-to-noise ratio (SNR) of 10 at 1792A is required. This requires an exposure time of about 5000 seconds (~2 orbits) with the G230L grating. Based on our previous experience in analyzing the STIS data, a similar time is required in G140L. Therefore, a total of 4 orbits is sufficient for both gratings, including overheads. Our team has been focusing on various transients associated with supermassive black holes (SMBHs) in the past five years, particularly on tidal disruption events (TDEs). The PI, Dr. Ning Jiang, is also the PI of three previous HST DDT proposals, which have resulted in one published paper (Sun,Jiang+2024,A&A,692,A262) and the other one submitted. The PI is also the co-author of another two TDE publications basing on HST/STIS UV spectra (Yang et al.2017,ApJ,846,150; Sheng et al.2021,ApJL,920,25). Other CoIs are also experts on multi-wavelength observations of TDEs and numerical simulation of TDEs. Supermassive Black Holes and Active Galaxies 0 false SUPPORTED GALAXY SEYFERT J2000 J2000 STIS/FUV-MAMA Spectroscopic No No G140L 1425 STIS/NUV-MAMA Spectroscopic No No G230L 2376 /home/jnac/proposal/HST/DDT/J1335/OP-cache/J1335-phase2-v01.xml The scheduling data was generated by CASM 33.0. CASM was run in Phase II mode. 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