0 Assigned ID: 4885 ----- Attempting Submission 1 (Tue Jul 18 18:40:02 GMT 2023) ----- ORBITAL VISIBILITY OVERRUN ORBITAL VISIBILITY OVERRUN We have changed the constraints according to one-gyro mode requirements. We apply various Orient constraints to avoid diffraction spikes landing near our target. -906542435 ----- Attempting Submission 1 (Fri Jul 28 14:52:41 GMT 2023) ----- ----- Attempting Submission 2 (Tue Oct 17 20:52:34 GMT 2023) ----- ----- Attempting Submission 3 (Thu Jul 04 03:27:14 GMT 2024) ----- APT has reported errors regarding Visit 01, which has already been executed. We propose to confirm an isolated dark stellar remnant candidate identified based on the recently established method of VLTI-GRAVITY interferometric microlensing. The present mass measurement of the lens (2.20^{+0.31}_{-0.23} Msun) is consistent with either a neutron star or a low-mass black hole in the so-called "mass gap", and it will improve to a much better precision of ~5% when the complete light curve becomes available in 2024. However, the lens can also be a normal star blended with the microlensed red giant source, and the possibility of a luminous lens can only be definitively distinguished from a dark lens with the proposed HST observations. If the lens is confirmed to be dark by HST, it will be the first isolated low-mass dark stellar remnant having a mass measurement, with exciting potential implications on the physics of compact objects and their formation pathways. We will use the UVIS channel of WFC3 to image the target in both F555W (V) and F814W (I) at two epochs. (An adjustment is made from F606W to F555W proposed in Phase 1 to yield a more precise result.) The first epoch is very time-critical, and the sooner it is executed, the higher the magnification for leveraging the blended flux constraint. The upcoming available window is in August 2023, and if possible, it should be scheduled by mid-August when the event is magnified with A~1.2. The scheduling of first epoch should absolutely be no later than October 2023 when the event is still magnified with A=1.1. The second epoch needs to be scheduled in 2024 when the event returns to the baseline. The UVIS2-C1K1C-SUB subarray mode will be used to avoid buffer dumps, optimize observing efficiency, and minimize the CTE effects. For the first epoch (~Aug 15), we expect the target to be at (V, I) ~ (18.7, 15.2), after making careful magnitude calibrations. We plan to take multiple exposures for each filter to mitigate the possible effects of CRs, artifacts, or imperfectly subtracted hot pixels. For F555W and F814W, seven and five exposures will be taken, respectively. For each exposure, we use ETC to ensure obtaining sufficient SNR and also the peak pixel being well below saturation. We will follow the 7-point and 5-point dithering patterns in Instrument Science Report WFC3 2020-07 (Anderson 2020). The seven/five points spread the target around in the pixel phase to obtain a well-sampled image for optimizing high-precision photometry. The second epoch has the same observing strategy as the first epoch to optimize differential photometry. Subo Dong, Andrew Gould and Przemek Mroz are experts on gravitational microlensing with decades of extensive research experience. Dong was the lead author of the paper reporting first successful interferometric resolution of microlensed images. Gould is the PI of the KMTNet project, which discovered the target (KMT-2023-BLG-0025) in this proposal. Gould and Dong also led two previously successful HST programs constraining blended light of microlensing events. Antoine Merand and Jinyi Shangguan are experts on VLTI-GRAVITY interferometric observations and data analysis. Merand is the PI of an ongoing VLTI-GRAVITY microlensing program, from which the interferometric observations of KMT-2023-BLG-0025 were conducted. Stellar Populations and the Interstellar Medium Stellar Physics and Stellar Types 6 false SUPPORTED STAR K III-I J2000 J2000 I ~ 15.2+/-0.1 WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W 1720063530742 /private/var/folders/57/yq8tfsbs55v4c_qwl0r022240000gn/T/OPVisit3977111193505590204.xml The scheduling data was generated by CASM 32.1.1. CASM was run in Phase II mode. This visit must be made as soon as possible, by mid-August 2023 if at all possible, but absolutely no later than October 2023. 1692004916000 88.02470047756441 (Tic-Visit :proposal-id 17397 :visit-id "01" :proposal-version -1 :trans-version "20240604" :date "04-Jul-2024 11:25:30.0" :trans-errors '(:WARNINGS 2 :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 213904 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17397 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17397 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2796 :al-overhead-at-end 87 :internal-dur-at-end 270 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3034 :pcs-gap 0 ) true 1 1 1720063434541 /Users/dong/Nutstore Files/proposals/HST/KB230025_HST/phase2/OP-cache/newphase2-v02.xml The scheduling data was generated by CASM 32.1.1. CASM was run in Phase II mode. We apply various Orient constraints to avoid diffraction spikes landing near our target. (Tic-Visit :proposal-id 17397 :visit-id "02" :proposal-version -1 :trans-version "20240604" :date "04-Jul-2024 11:23:54.0" :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 213904 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 17397 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17397 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2654 :al-overhead-at-end 87 :internal-dur-at-end 270 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3034 :pcs-gap 0 ) true 1 0