18486( 2) - 16-Oct-2026 14:37:53 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18486 Version: 2 Check-in Time: 16-Oct-2026 14:37:53 STScI Edit Number: 0 Title Luminous X-ray Sources Below 0.3 keV in Galaxies: A New Population ------------------------------------------------------------------------------------ Type Cycle GO 34 ------------------------------------------------------------------------------------ Investigators Contact? PI: Prof. Jimmy A. Irwin University of Alabama Y CoI: Mr. Mustafa Muhibullah University of Alabama N CoI: Dr. Rosanne Di Stefano Smithsonian Institution Astrophysical N Observatory CoI: Dr. Yuanyuan Su University of Kentucky N CoI: Dr. Ralph Kraft Smithsonian Institution Astrophysical N Observatory CoI: Dr. Gerrit Schellenberger Smithsonian Institution Astrophysical N Observatory CoI: Dr. Renato A. Dupke Eureka Scientific Inc. N ------------------------------------------------------------------------------------ Abstract An exciting new class of luminous X-ray emitters with little to no detectable emission above 0.3 keV has recently been discovered within galaxies observed with early ACIS-S data, dubbed `hypersoft' X-ray sources. Within the narrow 0.15-0.3 keV bandpass these sources emit up to 1e38 erg/s with bolometric luminosities potentially exceeding 1e39 erg/s. We propose to utilize the very soft response of HRC-I coupled with a far UV HST/UVIS/F225W exposure of the nearby galaxy M94 which has previously been observed to harbor luminous hypersoft sources to determine the sources' temperatures and therefore the crucial bolometric correction. We also request ACIS-S observations of a very hot Milky Way dwarf with a known blackbody temperature to translate X-ray hardness ratios into temperatures. ------------------------------------------------------------------------------------ Observations Description ------------------------ Our observations are designed to mimic the WFC3/UVIS/F225W observations of previous HST programs (11583; PI: Bregman, 18091; PI: Irwin) of the galaxies NGC 4697 and NGC 3379 as closely as possible, except that we break the 5-orbit F225W observation into two visits of 3 and 2 orbits, separated by 25-40 days if possible. We use the UVIS-CENTER aperture and CR-SPLIT=NO to match Programs 11583/18091, and remove cosmic rays by combining the dithered frames. Visit 01 (3 orbits) uses the WFC3-UVIS-DITHER-LINE-3PT pattern with two exposures of 1242 s at each of its three dither points, for 6 exposures and 7452 s in total. Visit 02 (2 orbits) uses the WFC3-UVIS-DITHER-BOX pattern with a single exposure of 1232 s at each of its four points, for 4 exposures and 4928 s in total. In both visits two exposures are obtained per orbit. Exposure durations were packed to fill each orbit using the Orbit Planner's Auto-Adjust. Because the exposures are long (more than 348 s), buffer dumps are hidden during the following exposure, maximizing on-target time. 18486( 2) - 16-Oct-2026 14:37:53 - [ 2] Since Program 11583 did not increase the flash level, we did not add post-flash either, so the APT diagnostic that the FLASH level may be too low is expected. ------------------------------------------------------------------------------------ 18486( 2) - 16-Oct-2026 14:37:53 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 M-94 GALAXY, SPIRAL RA=12H50M53.0738S, J2000 V = 8.24 DEC=+41D07'12.90" Reference Frame: ICRS Extended: YES Comments: This object was generated by the targetselector and retrieved from the SIMBAD database. SIMBAD listed proper motion for this target. When retrieving targets with PM from SIMBAD, APT requests the coordinates be calculated with an epoch of the year 2000. Do not modify this epoch. Always review coordinates using the Target Confirmation tool, which graphically displays the PM. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 6.318603560735761E-5 8.0E-4 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 18486( 2) - 16-Oct-2026 14:37:53 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.173 Line_Spacing 0.112 Coordinate_Frame POS-TARG Pattern_Orient 23.884 Angle_Between_Sides 81.785 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 Primary_Pattern Secondary_Pattern Pattern_Type WFC3-UVIS-DITHER-LINE-3PT Pattern_Purpose DITHER Number_Of_Points 3 Point_Spacing 0.135 Line_Spacing Coordinate_Frame POS-TARG Pattern_Orient 46.84 Angle_Between_Sides Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ 18486( 2) - 16-Oct-2026 14:37:53 - [ 5] Visit: 01 Visit Requirements: 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 M-94 WFC3/UVI ACCUM UVIS-CENT F225W 2 1200 S PATTERN 2 1 S ER see below (7412 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 | ------------------------------------------------------------------------------------------------------------------------------------ M-94 1 WFC3/UVI ACCUM UVIS-CEN F225W 1 none 1 none 1 1242.0 S S TER M-94 1 WFC3/UVI ACCUM UVIS-CEN F225W 1 none 2 none 1 1242.0 S S TER M-94 1 WFC3/UVI ACCUM UVIS-CEN F225W 2 none 1 none 2 1232.0 S S TER M-94 1 WFC3/UVI ACCUM UVIS-CEN F225W 2 none 2 none 2 1232.0 S S TER M-94 1 WFC3/UVI ACCUM UVIS-CEN F225W 3 none 1 none 3 1232.0 S S TER M-94 1 WFC3/UVI ACCUM UVIS-CEN F225W 3 none 2 none 3 1232.0 S S TER 18486( 2) - 16-Oct-2026 14:37:53 - [ 6] Visit: 02 Visit Requirements: AFTER 01 BY 25 D TO 40 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 M-94 WFC3/UVI ACCUM UVIS-CENT F225W 1 1200 S PATTERN 1 1 S ER see below (4948 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 | ------------------------------------------------------------------------------------------------------------------------------------ M-94 1 WFC3/UVI ACCUM UVIS-CEN F225W 1 none none none 1 1242.0 S S TER M-94 1 WFC3/UVI ACCUM UVIS-CEN F225W 2 none none none 1 1242.0 S S TER M-94 1 WFC3/UVI ACCUM UVIS-CEN F225W 3 none none none 2 1232.0 S S TER M-94 1 WFC3/UVI ACCUM UVIS-CEN F225W 4 none none none 2 1232.0 S S TER 18486( 2) - 16-Oct-2026 14:37:53 - [ 7] Summary Form for Proposal 18486 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS-CENTER ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements AFTER 01 BY 25 D TO 40 D, PATTERN 1 1, PATTERN 2 1 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F225W ------------------------------------------------------------------------------------------------------------------------------------ Target Names M-94 ------------------------------------------------------------------------------------------------------------------------------------