17400( 4) - 20-Feb-2025 19:01:01 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17400 Version: 4 Check-in Time: 20-Feb-2025 19:01:01 STScI Edit Number: 1 Title Hunting for Black Holes with Astrometric Microlensing ------------------------------------------------------------------------------------ Type Cycle GO 31 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Jessica Ryan Lu University of California - Berkeley Y CoI: Dr. William Anthony Dawson Lawrence Livermore National Laboratory N CoI: Casey Lam Carnegie Institution of Washington N CoI: Dr. Peter E. Nugent Lawrence Berkeley National Laboratory N CoI: Michael Medford University of California - Berkeley N CoI: Dr. Nathan Golovich Lawrence Livermore National Laboratory N ------------------------------------------------------------------------------------ Abstract Although there are likely 10^7 - 10^9 stellar mass black holes (BHs) in the Milky Way, only a handful have been detected in the Universe, all in X-ray binaries or BH-BH mergers; no isolated BHs have been definitively detected. A census of isolated BHs will provide important constraints on stellar evolution, the Milky Way BH mass function, supernovae physics, and BH/neutron star formation. Gravitational microlensing is ideal for finding isolated BHs, as properties of the lens can be inferred from changes in the brightness and position of a background star. By combining both the photometric and astrometric microlensing signal, we can directly determine the lens mass. We propose to use HST to measure the astrometric shift of 4 likely BH microlensing candidates. Combined with other ongoing measurements, this will allow us to constrain the number of BHs in the Milky Way to better than 50%, a major improvement over the current orders of magnitude uncertainty. This will also enable the first constraints on the Milky Way BH mass function, binarity, and kick velocities. ------------------------------------------------------------------------------------ Observations Description ------------------------ Four black hole candidates will be selected from photometric microlensing surveys for follow-up astrometric observations with HST WFC3-UVIS. The 4 targets will be non-disruptive ToOs and, once selected, 5 HST measurements will be obtained spread as evenly as possible over Cycle 29. The targets will be changing brightness, thus exposure times and number of exposures will be adjusted prior to each new observation. Further astrometric follow-up will be obtained in Cycles 30 and 31. Each target observation consists of >5 dither positions (to maximize pixel phase coverage and thus astrometric precision) in a single orbit. The exposure times will be chosen in order to not saturate the source star or any bright neighbor stars within a few arcseconds. We will observing using 2k x 2k sub-array mode as the astrometric reference stars are those closest to the target and we would like to minimize the astrometric impacts of CTE. In order to maximize observability and time sampling throughout the year, we 17400( 4) - 20-Feb-2025 19:01:01 - [ 2] have placed no restrictions on roll angle. ------------------------------------------------------------------------------------ 17400( 4) - 20-Feb-2025 19:01:01 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 2 OB230215 STELLAR CLUSTER, BULGE, RA=18H20M20.3700S, J2000 V = 21.09 +/- 0.1 I-Mag 18.37 GRAVITATIONAL LENS DEC=-21D35'14.90" Reference Frame: ICRS Extended: NO ------------------------------------------------------------------------------------------------------------------------------------ 17400( 4) - 20-Feb-2025 19:01:01 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type SPIRAL Pattern_Purpose DITHER Number_Of_Points 6 Point_Spacing 0.13 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 45 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 17400( 4) - 20-Feb-2025 19:01:01 - [ 5] Visit: 01 (OB230215_vis1) Visit Requirements: ORIENT 268.2585D TO 268.2585 D; BETWEEN 01-FEB-2027 AND 04-APR-2027; TOO RESPONSE TIME 14.0D On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 OB230215 WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=15 1 285 S (285 POS TARG 0.0000, 0.0000 S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 2 OB230215 WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=15 1 285 S (285 POS TARG 0.0927, 0.3376 S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 3 OB230215 WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=15 1 285 S (285 POS TARG 0.1851, 0.1978 S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 4 OB230215 WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=15 1 285 S (285 POS TARG 0.2382, 0.5130 S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 5 OB230215 WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=15 1 285 S (285 POS TARG 0.3305, 0.3732 S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 6 OB230215 WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=15 1 285 S (285 POS TARG 0.5024, 0.5171 S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ OB230215 1 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB OB230215 2 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB OB230215 3 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB OB230215 4 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB OB230215 5 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB OB230215 6 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB 17400( 4) - 20-Feb-2025 19:01:01 - [ 6] Visit: 02 (OB230215_vis2) Visit Requirements: ORIENT 0D TO 0D FROM 01; AFTER 01 BY 30 D TO 180 D On Hold Comments: Additional Comments: Exposures ------------------------------------------------------------------------------------------------------------------------------------ Exp | Target | Instr | Oper. | Aper |Spectral|Central| Optional |Num| Time | Special Num | Name | Config | Mode | or FOV |Element |Waveln.| Parameters |Exp| (Total) | Requirements ------------------------------------------------------------------------------------------------------------------------------------ 1 OB230215 WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=15 1 285 S (285 POS TARG 0.0000, 0.0000 S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 2 OB230215 WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=15 1 285 S (285 POS TARG 0.0927, 0.3376 S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 3 OB230215 WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=15 1 285 S (285 POS TARG 0.1851, 0.1978 S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 4 OB230215 WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=15 1 285 S (285 POS TARG 0.2382, 0.5130 S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 5 OB230215 WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=15 1 285 S (285 POS TARG 0.3305, 0.3732 S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ 6 OB230215 WFC3/UVI ACCUM UVIS2-2K2 F814W FLASH=15 1 285 S (285 POS TARG 0.5024, 0.5171 S C-SUB S) ------------------------------------------------------------------------------------------------------------------------------------ Sub Exposures ------------------------------------------------------------------------------------------------------------------------------------ Target | Exp |Instr | Oper. | Aper |Spectral|Cent.|Primary |Secondary |Iteration |CR-SPLIT |Orbit |Duration Name | Num |Config | Mode | or FOV |Element |Wave.|Pattern Pos |Pattern Pos |Num |Num |Number | ------------------------------------------------------------------------------------------------------------------------------------ OB230215 1 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB OB230215 2 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB OB230215 3 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB OB230215 4 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB OB230215 5 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB OB230215 6 WFC3/UVI ACCUM UVIS2-2K F814W none none none none 1 N/A S 2C-SUB 17400( 4) - 20-Feb-2025 19:01:01 - [ 7] Summary Form for Proposal 17400 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2-2K2C-SUB ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=15 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 01 BY 30 D TO 180 D, BETWEEN 01-FEB-2027 AND 04-APR-2027, ORIENT 0D TO 0D FROM 01, ORIENT 268.2585D TO 268.2585 D, POS TARG 0.0000, 0.0000, POS TARG 0.0927, 0.3376, POS TARG 0.1851, 0.1978, POS TARG 0.2382, 0.5130, POS TARG 0.3305, 0.3732, POS TARG 0.5024, 0.5171, TOO RESPONSE TIME 14.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names OB230215 ------------------------------------------------------------------------------------------------------------------------------------