18189( 4) - 12-Feb-2026 11:28:44 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18189 Version: 4 Check-in Time: 12-Feb-2026 11:28:44 STScI Edit Number: 1 Title X-ray emissions of a peculiar hyperluminous X-ray source ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Prof. Albert Kong National Tsing Hua University Y CoI: Dr. Roberto Soria INAF - Osservatorio Astronomico di Trieste N CoI: Dr. Igor Chilingarian Smithsonian Institution Astrophysical N Observatory CoI: Dr. Kirill Grishin Universite Paris Diderot (Paris 7) N CoI: Dr. Alister W. Graham Swinburne University of Technology N CoI: Ms. Yi Chi Chang National Tsing Hua University N ------------------------------------------------------------------------------------ Abstract We propose a joint Chandra-HST study of the most extreme hyperluminous X-ray source selected from a cross-correlation study of Chandra and XMM-Newton catalogues. It reached a peak X-ray luminosity of 5e42 erg/s in 2012; it was 100 times fainter in 2009 and 2023. It is a strong off-nuclear IMBH candidate in a thermal dominant state. We will study its current luminosity and spectral state with Chandra, and the brightness and colours of its optical counterpart with HST, constraining its BH mass and testing between alternative scenarios (repeated outbursts or one-off TDE). ------------------------------------------------------------------------------------ Observations Description ------------------------ This is an imaging observation to determine the multi-band photometry of our target for a SED study. ------------------------------------------------------------------------------------ 18189( 4) - 12-Feb-2026 11:28:44 - [ 2] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6099 GALAXY, ACCRETION DISK, RA=16H15M34.2000S, J2000 V = 25 +/- 0.1 DWARF COMPACT DEC=+19D27'7.70" Reference Frame: ICRS ------------------------------------------------------------------------------------------------------------------------------------ 18189( 4) - 12-Feb-2026 11:28:44 - [ 3] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-SBC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.179 Line_Spacing 0.116 Coordinate_Frame POS-TARG Pattern_Orient 20.02 Angle_Between_Sides 63.65 Center_Pattern NO ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 2 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: 3 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 ------------------------------------------------------------------------------------------------------------------------------------ 18189( 4) - 12-Feb-2026 11:28:44 - [ 4] Visit: 01 Visit Requirements: BETWEEN 11-APR-2026 AND 25-APR-2026 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 NGC6099 ACS/SBC ACCUM SBC F140LP 1 500 S PATTERN 1 1 see below (2332 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 | ------------------------------------------------------------------------------------------------------------------------------------ NGC6099 1 ACS/SBC ACCUM SBC F140LP 1 none none none 1 583.0 S NGC6099 1 ACS/SBC ACCUM SBC F140LP 2 none none none 1 583.0 S NGC6099 1 ACS/SBC ACCUM SBC F140LP 3 none none none 1 583.0 S NGC6099 1 ACS/SBC ACCUM SBC F140LP 4 none none none 1 583.0 S 18189( 4) - 12-Feb-2026 11:28:44 - [ 5] Visit: 02 Visit Requirements: BETWEEN 11-APR-2026 AND 25-APR-2026 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 NGC6099 WFC3/UVI ACCUM UVIS F555W FLASH=2 1 444 S PATTERN 3 1 S (1332 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC6099 WFC3/UVI ACCUM UVIS F300X FLASH=18 1 352 S PATTERN 2 2 S (1408 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC6099 WFC3/UVI ACCUM UVIS F621M FLASH=12 1 432 S PATTERN 3 3 S (1296 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 | ------------------------------------------------------------------------------------------------------------------------------------ NGC6099 1 WFC3/UVI ACCUM UVIS F555W 1 none none none 1 N/A S NGC6099 1 WFC3/UVI ACCUM UVIS F555W 2 none none none 1 N/A S NGC6099 1 WFC3/UVI ACCUM UVIS F555W 3 none none none 1 N/A S NGC6099 2 WFC3/UVI ACCUM UVIS F300X 1 none none none 1 N/A S NGC6099 2 WFC3/UVI ACCUM UVIS F300X 2 none none none 1 N/A S NGC6099 2 WFC3/UVI ACCUM UVIS F300X 3 none none none 2 N/A S NGC6099 2 WFC3/UVI ACCUM UVIS F300X 4 none none none 2 N/A S NGC6099 3 WFC3/UVI ACCUM UVIS F621M 1 none none none 2 N/A S NGC6099 3 WFC3/UVI ACCUM UVIS F621M 2 none none none 2 N/A S NGC6099 3 WFC3/UVI ACCUM UVIS F621M 3 none none none 2 N/A S 18189( 4) - 12-Feb-2026 11:28:44 - [ 6] Visit: 03 Visit Requirements: BETWEEN 11-APR-2026 AND 25-APR-2026 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 NGC6099 WFC3/UVI ACCUM UVIS F390W FLASH=15 1 437 S PATTERN 3 1 S (1311 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC6099 WFC3/UVI ACCUM UVIS F336W FLASH=19 1 352 S PATTERN 2 2 S (1408 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC6099 WFC3/UVI ACCUM UVIS F814W FLASH=5 1 434 S PATTERN 3 3 S (1302 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 | ------------------------------------------------------------------------------------------------------------------------------------ NGC6099 1 WFC3/UVI ACCUM UVIS F390W 1 none none none 1 N/A S NGC6099 1 WFC3/UVI ACCUM UVIS F390W 2 none none none 1 N/A S NGC6099 1 WFC3/UVI ACCUM UVIS F390W 3 none none none 1 N/A S NGC6099 2 WFC3/UVI ACCUM UVIS F336W 1 none none none 1 N/A S NGC6099 2 WFC3/UVI ACCUM UVIS F336W 2 none none none 1 N/A S NGC6099 2 WFC3/UVI ACCUM UVIS F336W 3 none none none 2 N/A S NGC6099 2 WFC3/UVI ACCUM UVIS F336W 4 none none none 2 N/A S NGC6099 3 WFC3/UVI ACCUM UVIS F814W 1 none none none 2 N/A S NGC6099 3 WFC3/UVI ACCUM UVIS F814W 2 none none none 2 N/A S NGC6099 3 WFC3/UVI ACCUM UVIS F814W 3 none none none 2 N/A S 18189( 4) - 12-Feb-2026 11:28:44 - [ 7] Summary Form for Proposal 18189 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures SBC, UVIS ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/SBC, WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=12, FLASH=15, FLASH=18, FLASH=19, FLASH=2, FLASH=5 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements BETWEEN 11-APR-2026 AND 25-APR-2026, PATTERN 1 1, PATTERN 2 2, PATTERN 3 1, PATTERN 3 3 ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F140LP, F300X, F336W, F390W, F555W, F621M, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names NGC6099 ------------------------------------------------------------------------------------------------------------------------------------