0 Assigned ID: 570 ----- Attempting Submission 1 (Fri Mar 06 17:08:51 GMT 2020) ----- HST Phase I Proposal 570 successfully submitted. Receipt: # 570-1 ----- Attempting Submission 2 (Fri Mar 06 21:10:27 GMT 2020) ----- HST Phase I Proposal 570 successfully submitted. Receipt: # 570-2 ----- Attempting Submission 3 (Fri Mar 06 21:33:34 GMT 2020) ----- HST Phase I Proposal 570 successfully submitted. Receipt: # 570-3 ----- Attempting Submission 4 (Fri Mar 06 22:17:32 GMT 2020) ----- Visit 01 is ready to execute. This is our last orbit for this program. Hope this works! 1700217941 ----- Attempting Submission 1 (Tue Jun 30 14:39:00 GMT 2020) ----- HST Phase II Proposal 16200 successfully submitted. Receipt: # 16200-1 ----- Attempting Submission 2 (Tue Jun 30 14:51:59 GMT 2020) ----- HST Phase II Proposal 16200 successfully submitted. Receipt: # 16200-2 ----- Attempting Submission 3 (Tue Jun 30 15:44:34 GMT 2020) ----- HST Phase II Proposal 16200 successfully submitted. Receipt: # 16200-3 ----- Attempting Submission 4 (Mon Jul 06 18:49:19 GMT 2020) ----- ----- Attempting Submission 5 (Thu May 06 21:46:46 GMT 2021) ----- ----- Attempting Submission 6 (Wed Jun 16 20:12:36 GMT 2021) ----- ----- Attempting Submission 7 (Wed Jul 28 13:20:15 GMT 2021) ----- ----- Attempting Submission 8 (Mon Jan 30 21:01:25 GMT 2023) ----- Visit 1 ready to execute. There should be ~100 million black holes (BHs) in our Galaxy, of which the majority should be single. Yet there has been no unambiguous detection of a solitary BH--not surprisingly, since they emit essentially no radiation. Astrometric microlensing--the relativistic deflection of light from a background star--is the only technique that can detect isolated BHs. HST has demonstrated its capability to measure such tiny deflections. A multi-year HST program of precision astrometry of long-duration microlensing events in the Galactic bulge is underway, aimed at the first detection of isolated BHs. This program has achieved clear detections of deflections in two events, consistent with non-luminous massive lenses. However, the timing of the observations was sub-optimal due to some guide-star failures and the HST gyro failure in 2018. As a result, there remains a degeneracy between deflection, parallax, and proper motion; an unusually slow-moving low-mass lens remains a possibility. One more observation of these two targets will resolve this degeneracy. Observations of another event had to be abandoned when HST revealed a blended source. Because of the 2018 HST gyro failure, initiation of two new events had to be delayed until 2019. These events are still ongoing and need a total of five orbits for completion. One of the BH candidate sources is extremely red, which can be best explained by a shell around the putative BH. A 1-orbit STIS spectrum, and a final WFC3 image in Cycle 29, will test this hypothesis. Thus a total of 9 orbits (7 in Cycle 28 and 2 in Cycle 29) are needed to achieve the original objectives of the program, and to confirm the potentially exciting findings. WFC3 imaging to measure astrometric deflections. We will employ an observational strategy similar to that used in the GO-15318 program. We will image each field using WFC3/UVIS, primarily with the F814W filter, but also with F606W. In a rich stellar field such as the Galactic bulge, there is always a large number of reference stars available in the WFC3 field, even with the subarrays that we will use to avoid buffer dumps. This allows the highest possible astrometric accuracy. For the astrometric analysis, we will basically use procedures described in previous HST studies, including Anderson & King (2004) and Bellini (2014). Very similar techniques were used by Sahu et al. (2017) to detect the relativistic deflection of a background star by the nearby white dwarf Stein2051B, leading to a mass determination for this isolated object. STIS spectroscopy of OGLE-2017-BLG-328. We will use G750L with a 0.2" slit to obtain a classification spectrum of the source. This setting covers a wavelength range of 5240-10265 A. The STIS ETC indicates that a one-orbit exposure will reach a S/N per resolution element of ~22 at 7500 A, which is more than adequate for spectral classification and inference of the intrinsic color. As indicated in phase-I, we may have to re-adjust the orbit distribution for the targets based on the results from the first few observations. So the targets in vists 5 and 6 are specified as "DUMMY", which will be replaced with specific targets later on. Dr. Sahu has led several projects on microlensing, including a recent project where the mass of the nearby white dwarf, Stein 2051B, was measured through astrometric microlensing using HST/WFC3 observations. Dr. Anderson is the world-expert on characterizing HST/PSFs, and astrometry using HST observations. Dr. Bellini is an expert on PSF modeling, geometric distortion, high-precision photometry and astrometry of resolved stellar populations with HST data. Dr. Bond specializes in stellar astronomy, and has extensive experience with HST observing, having received observing time as PI or CoI in all 27 Cycles to date. Dr. Brown has extensive experience observing with Hubble, particularly in the planning and interpretation of observations in crowded stellar populations. Dr. Calamida has vast experience on accurate astrometry and photometry of stars in crowded HST fields, and is also an expert on stellar-evolution models. Dr. Casertano has large experience on the analysis and processing of HST images, especially for astrometry, and has significantly contributed to the interpretation of astrometric microlensing signals for several events. Dr. Udalski is the PI of the OGLE team and is an expert on crowded-field photometry. He is the architect of the OGLE Early Warning System which is essential for choosing our targets for HST observations. Stellar Physics and Stellar Types Stellar Populations and the Interstellar Medium 6 false false false false false false false SUPPORTED EXT-STAR A0-A3 III-I J2000 J2000 EXT-STAR A0-A3 III-I J2000 J2000 WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F555W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F606W STIS/CCD Spectroscopic No No G750L 7751 The scheduling data was generated by CASM 30.3. CASM was run in Phase II mode. 1675111573748 55.4 /private/var/folders/xx/kgq1b0qs5jj32f6tg4vg0m5r00067j/T/OPVisit9643720915428437400.xml (Tic-Visit :proposal-id 16782 :visit-id "03" :proposal-version -1 :trans-version "20220630" :date "30-Jan-2023 15:46:13.0" :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 124784 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 16782 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 16782 :visit-id "03" :orbit-number 1 :pcs-time 383 :si-vis-utilization 2792 :al-overhead-at-end 81 :internal-dur-at-end 173 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3180 :pcs-gap 0 ) false 1 0 The scheduling data was generated by CASM 29.2. CASM was run in Phase II mode. 1627477775096 1645055501000 265.9710476514019 (Tic-Visit :proposal-id 16782 :visit-id "02" :proposal-version -1 :trans-version "20210514" :date "28-Jul-2021 09:09:34.0" :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 160432 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 16782 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 16782 :visit-id "02" :orbit-number 1 :pcs-time 383 :si-vis-utilization 2765 :al-overhead-at-end 81 :internal-dur-at-end 212 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3165 :pcs-gap 0 ) false 1 0