0 Assigned ID: 7481 ----- Attempting Submission 1 (Thu Apr 16 17:01:36 GMT 2026) ----- HST Phase I Proposal 7481 successfully submitted. Receipt: # 7481-1 ----- Attempting Submission 2 (Thu Apr 16 17:05:59 GMT 2026) ----- HST Phase I Proposal 7481 successfully submitted. Receipt: # 7481-2 ----- Attempting Submission 3 (Thu Apr 16 17:43:35 GMT 2026) ----- Could not connect to APT Status Server duplicated pattern and increased post-flash to increase science exposure times 890410465 ----- Attempting Submission 1 (Tue Aug 11 08:03:06 GMT 2026) ----- ----- Attempting Submission 2 (Wed Aug 12 06:48:45 GMT 2026) ----- A warning says Could not connect to APT Status Server But I think all info is syned. EP260324a/AT2026hir is a luminous X-ray and optical nuclear transient at z=0.087 that defies classification within known transient populations. Its nuclear location and quiescent massive host galaxy are consistent with a luminous tidal disruption event (LTDE), yet its anomalously rapid light-curve evolution is more reminiscent of Luminous Fast Blue Optical Transients (LFBOTs). Panchromatic early-time follow-up leaves the nature of AT2026hir ambiguous: it may be a TDE powered by the supermassive black hole in the galaxy nucleus, or an LFBOT powered by a much lower-mass black hole at a slightly off-nuclear location. To resolve this ambiguity, we propose joint late-time HST (2 orbits) + XMM-Newton (30 ks) observations targeting two key measurements: the positional offset of the transient relative to the host nucleus, and the mass of the central black hole (Mbh). HST UV+optical imaging will improve the current ground-based astrometric uncertainty of 0.3 arcsec by an order of magnitude, enabling detection or exclusion of offsets down to 50 pc. Joint HST+XMM-Newton broadband SED modeling will precisely constrain Mbh via relativistic disk theory, leveraging both the X-ray spectral shape and the normalization of the UV/optical plateau. This proposal consists of two orbits of HST imaging obseravtions of the optical transient AT2026hir. The HST observation is coordinated with a 30ks XMM-Newton observation. First orbit: ACS/SBC Imaging, F150LP filter Second orbit: WFC3/UVIS Imaging, F625W (optical) and F275W (NUV) filters. For the scheduling, we focus on the first half of Cycle 34 to maximize the probability of getting a high-SNR X-ray spectrum before the disk cools significantly. Three suitable HST visibility windows are listed below (based on the Phase II Visit Planner result). 2026 Nov 7-22 2027 Feb 9-21 2027 March 29-30; April 2-9 Dr. Yuhan Yao is a Miller postdoctoral fellow at UC Berkeley, specialized in using time domain surveys such as the Zwicky Transient Facility to select TDEs and multi-wavelength characterization of transients. She is the co-chair of the LS4 MBH science working group and an expert in both tidal disruption events and luminous fast blue optical transients. Prof. Ryan Chornock is an expert on optical/UV observations of TDEs and other optical transients. He is the co-chair of the ZTF MBH Science Working Group. Prof. Ning Jiang is a research associate at the University of Science and Technology of China. He is a member of the Einstein Probe team and currently leads the TDE working group for the Wide Field Survey Telescope (WFST) project. He has extensive experience in HST imaging analysis. Dr. Andy Mummery is a postdoctoral fellow at the Institute for Advanced Study. He developed the relativistic black hole accretion disk theory that will be used in this program to measure black hole mass, and which has been widely applied to high-energy transients. Dr. Nayana A. J. is a postdoctoral researcher at UC Berkeley. She is an expert in radio and millimeter observations of transients and is leading the VLA and NOEMA observations of AT2026hir. Prof. Raffaella Margutti is an Associate Professor at UC Berkeley specializing in high-energy observations of transients. She is coordinating the SMA observations of AT2026hir. Prof. Jane Dai is a Professor at the University of Hong Kong and an expert in TDE theory. She is the co-chair of the Einstein Probe AGN/TDE Working Group. Prof. Charlotte Ward is an Assistant Professor at the Pennsylvania State University. She has extensive experience in forward modeling of multi-epoch transient-host imaging and developed scarlet2 --- the software that will be used to measure the positional offset of AT2026hir from the HST data. Supermassive Black Holes and Active Galaxies Stellar Physics and Stellar Types 6 false true false false false false false true PRIMARY 30 true SUPPORTED UNIDENTIFIED ULTRAVIOLET EMITTER YES ZTF26AAPVIIM EP260324A J2000 J2000 ACS/SBC Imaging No No F150LP WFC3/UVIS Imaging No No F275W WFC3/UVIS Imaging No No F625W POS-TARG DITHER 0.135 LINE false false 3 46.84 WFC3-UVIS-DITHER-LINE-3PT 1 false POS-TARG DITHER 0.472 LINE false false 2 44.4 ACS-SBC-DITHER-LINE 2 false 1786516997167 /private/var/folders/51/8dbdmtw12rn86njf20zwrsc80000gn/T/OPVisit8823153246132852870.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. - 1 orbit: ACS/SBC Imaging in F150LP - 1 orbit: WFC3/UVIS imaging in F225W+F625W (Tic-Visit :proposal-id 18433 :visit-id "01" :proposal-version -1 :trans-version "20260618" :date "12-Aug-2026 14:43:17.0" :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 144000 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18433 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Obset :proposal-id 18433 :visit-id "01" :obset-id "A1" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18433 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2801 :al-overhead-at-end 0 :internal-dur-at-end 77 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3181 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18433 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2799 :al-overhead-at-end 79 :internal-dur-at-end 153 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3181 :pcs-gap 0 ) true 2 0 This F625W image will measure the nucleus of the host galaxy.