0 Assigned ID: 6924 ----- Attempting Submission 1 (Wed Sep 17 15:43:59 GMT 2025) ----- HST Phase I Proposal 6924 successfully submitted. Receipt: # 6924-1 ----- Attempting Submission 2 (Thu Sep 18 01:53:15 GMT 2025) ----- HST Phase I Proposal 6924 successfully submitted. Receipt: # 6924-2 Could not connect to APT Status Server 0 ----- Attempting Submission 1 (Thu Oct 16 00:10:10 GMT 2025) ----- HST Phase II Proposal 18213 successfully submitted. Receipt: # 18213-1 ----- Attempting Submission 2 (Thu Oct 16 16:53:54 GMT 2025) ----- ----- Attempting Submission 3 (Sun Nov 23 06:16:12 GMT 2025) ----- HST Phase II Proposal 18213 successfully submitted. Receipt: # 18213-3 ----- Attempting Submission 4 (Wed Nov 26 04:10:06 GMT 2025) ----- A warning "Warning (Server): Could not connect to APT Status Server" occurred but I found no problem in my side. Since the launch of EP, peculiar fast X-ray transients (FXT) have been discovered, blurring the boundaries between the properties of Gamma-ray bursts (GRB) and jetted tidal disruption events (TDE). One example is the FXT EP241021a at z=0.748, which shows an X-ray plateau with luminosities of several 10^44 erg/s lasting for ~10 days. The time duration of the plateau is far longer than all known GRBs, while it is similar to jetted TDEs. The plateau luminosity is two orders of magnitude lower than jetted TDEs, suggesting a possible origin of TDE by an intermediate-mass black hole. In addition, during the X-ray plateau, there is a clear optical rebrightening by two magnitudes, peaking at around -21 mag, which is also rarely seen. Recent ground-based observations show that the optical light curve no longer declines in accordance with the early power-law trend but flattens. The flattening may originate in the thermal radiation from the transient source, or the continuum of the host galaxy, neither of which was detected previously. We propose a new and timely imaging observation of EP241021a at a phase of ~200 days with HST WFC3/UVIS and WFC3/IR. Our intermediate goal is to distinguish between the possible origins of the optical flattening, based on whether the target is extended and whether the central position coincides with the transient position measured by VLBA. In either case, we can further tell an exotic GRB or a peculiar jetted TDE as the favored origin of EP241021a with the constraints on the positional offset and broad-band SED. We will observe EP241021a with WFC3/UVIS with F606W and F814W filters, and with WFC3/IR with F110W and F160W filters. The proposing team consists of the original discovery group of EP241021a, including the PI of the Einstein Probe mission (Weimin Yuan) and other core members of the EP Science Topical Panels 1 (STP1, TDEs and AGNs). Since the discovery of EP241021a, the team has initiated multi-wavelength follow-up observations and carried out a comprehensive analysis of the transient. The results were presented in Shu et al. (2025, ApJL, 990, 29). The PI, Luming Sun, has experience in multiband observations and data analyses of supernovae and tidal disruption events. The co-I, Ning Jiang, has extensive experience in HST imaging analysis (e.g., Jiang et al. 2013, ApJ, 770, 3; Jiang et al. 2018, ApJ, 869, 49). He will contribute to analyzing the HST observing data. Furthermore, as the PI of four previous DDT programs (ID: 16493, 17609, 17849 and 17933), he will assist with reviewing the details of the Phase 2 preparation. The co-I, Xinwen Shu, is the first author of the discovery paper for EP241021a. He is an expert in X-ray and radio astronomy. He has also participated in the HST CLASH, HFF, and ASTRODEEP programs, leading to the discovery of magnified distant galaxies at z>9 and the HST-dark galaxies at z~4 in the cluster fields. The co-I, Chichuan Jin, is the chair of EP STP1. He has led the discovery of EP240222a, the first TDE candidate discovered by EP, which is likely to involve an intermediate black hole (Jin et al. 2025, arXiv:2501.09580). The co-I, Jane Dai and Rongfeng shen are experts on TDE theoretical models, and Bing Zhang is an expert in theoretical studies of GRBs, SNe and TDEs. They will contribute to interpretating the HST observing results in the context of TDEs or stellar explosions. Stellar Physics and Stellar Types Supermassive Black Holes and Active Galaxies 6 false SUPPORTED GALAXY HIGH REDSHIFT GALAXY The position of the target, used in WFC3/IR observations J2000 J2000 GALAXY HIGH REDSHIFT GALAXY The center of the field of view that can observe four Gaia stars together with the target, used in WFC3/UVIS observations J2000 WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/IR Imaging No No F110W WFC3/IR Imaging No No F160W POS-TARG DITHER 1.908 LINE false false 2 41.788 WFC3-IR-DITHER-LINE 1 false POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 2 false POS-TARG DITHER 0.145 LINE false false 2 46.84 WFC3-UVIS-DITHER-LINE 3 false 1764129933197 The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. C:\Users\傅狸\AppData\Local\Temp\OPVisit9324655871401901152.xml Observations of EP241021a in two UVIS fitlers and two IR filters 1762164024000 16.0 (Tic-Visit :proposal-id 18213 :visit-id "01" :proposal-version -1 :trans-version "20250929" :date "26-Nov-2025 12:05:32.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 2219952 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18213 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18213 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2492 :al-overhead-at-end 96 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2977 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18213 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2550 :al-overhead-at-end 0 :internal-dur-at-end 348 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2977 :pcs-gap 0 ) true 2 0 1763796261198 The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. D:\workforshu\202503_EP241021a\HSTprop\OP-cache\EP241021a_HST_reobservation-v51.xml Observations of EP241021a in two UVIS fitlers and two IR filters - repeat of last four exposures (Tic-Visit :proposal-id 18213 :visit-id "51" :proposal-version -1 :trans-version "20250929" :date "22-Nov-2025 15:24:21.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "30" :si-bright-earth-limb-angle 20.0 :data-volume 487440 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18213 :visit-id "51" :obset-id "51" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18213 :visit-id "51" :orbit-number 1 :pcs-time 380 :si-vis-utilization 746 :al-overhead-at-end 0 :internal-dur-at-end 614 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2977 :pcs-gap 0 ) true 1 0