0 Assigned ID: 6907 ----- Attempting Submission 1 (Wed Aug 06 07:11:33 GMT 2025) ----- HST Phase I Proposal 6907 successfully submitted. Receipt: # 6907-1 Two alloted orbits by the TTRB after succesful observation of a target in the first orbit. 290811852 ----- Attempting Submission 1 (Thu Aug 14 17:23:38 GMT 2025) ----- HST Phase II Proposal 18175 successfully submitted. Receipt: # 18175-1 ----- Attempting Submission 2 (Tue Sep 16 01:44:01 GMT 2025) ----- Chasing Binary Black Hole Merger candidate AT2025smm We propose three single-orbit HST WFC3/UVIS observations to test whether AT2025smm is an EM counterpart to the BBH event S250727dc. Swift/UVOT found a blue transient 4 hours after the trigger that faded in less than 6 hours, with later non-detections, motivating rapid UV imaging. Only HST can establish whether the transient is nuclear by resolving and registering it relative to the host nucleus. Each visit uses F336W and F606W to ~27 mag (5 sigma) to measure UV color and evolution, separate host and transient light, and discriminate UV-breakout vs shock-cooling scenarios. The first visit should occur by Sep 21, followed by Oct-Nov and mid-Dec windows. We will obtain multi-filter observations with WFC3/UVIS of AT2025smm in the F336W and F606W. A first observation should occur in August-September, ideally by the end of August. Upon confirmation of a transient, the second observation will occur between October and November, the final observation can occur in December. Our team is very well placed to carry out the proposed observations with HST. PI Hall is a second year graduate student at Carnegie Mellon University and has 8 year experience in transient science. Co-I Palmese and Andreoni are experts in transient and multi-messenger astrophysics at CMU, she has led many projects using space telescopes. Prof. Graham, Dr. McKernan, and Dr. Perna are all seasoned expert in AGN mechanics and BBH merger modeling. Dr. Hu is an expert in data processing for transients with from various telecopes, including space telescopes such as JWST. Supermassive Black Holes and Active Galaxies 0 false SUPPORTED UNIDENTIFIED JET SHOCK FRONT NO J2000 J2000 WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F336W WFC3/UVIS Imaging No No F606W POS-TARG DITHER 0.135 LINE false false 3 46.84 WFC3-UVIS-DITHER-LINE-3PT 1 false POS-TARG DITHER 0.173 WFC3-IR-DITHER-BOX-MIN 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 2 false C:\Users\20xha\AppData\Local\Temp\OPVisit1881833533729128097.xml Timing link suitability includes constraints ([02 After 01]) The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. Visit that ideally occurs before the end of August, at the latest by September 23rd. 1757217239000 251.52797535819946 (Tic-Visit :proposal-id 18175 :visit-id "01" :proposal-version -1 :trans-version "20250624" :date "15-Sep-2025 21:41:29.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 106960 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18175 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18175 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2612 :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 2992 :pcs-gap 0 ) true 1 0 C:\Users\20xha\AppData\Local\Temp\OPVisit8036232129555222464.xml Timing link suitability includes constraints ([02 After 01] [03 After 02]) The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. October 5th to November 11th Visit (Tic-Visit :proposal-id 18175 :visit-id "02" :proposal-version -1 :trans-version "20250624" :date "15-Sep-2025 21:41:33.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 71312 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18175 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18175 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2610 :al-overhead-at-end 74 :internal-dur-at-end 114 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2992 :pcs-gap 0 ) true 1 0 C:\Users\20xha\AppData\Local\Temp\OPVisit7792136467883855511.xml Timing link suitability includes constraints ([03 After 02]) The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. December 7th to December 20th (Tic-Visit :proposal-id 18175 :visit-id "03" :proposal-version -1 :trans-version "20250624" :date "15-Sep-2025 21:41:16.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 1 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 71312 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18175 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18175 :visit-id "03" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2610 :al-overhead-at-end 74 :internal-dur-at-end 114 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2992 :pcs-gap 0 ) true 1 0