18190( 4) - 14-Sep-2026 19:19:25 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18190 Version: 4 Check-in Time: 14-Sep-2026 19:19:25 STScI Edit Number: 0 Title Transient LMXBs in globular clusters ------------------------------------------------------------------------------------ Type Cycle GO 33 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Jeroen Homan Eureka Scientific Inc. Y CoI: Dr. Maureen van den Berg Harvard University Y CoI: Dr. David A. Pooley Eureka Scientific Inc. N CoI: Dr. Craig Heinke University of Alberta N ------------------------------------------------------------------------------------ Abstract Deep Chandra observations of globular clusters have revealed that globular clusters contain numerous quiescent LMXBs, any of which could go into outburst. Recent Chandra studies have confirmed that globular clusters can indeed harbor multiple transients and that more quiescent LMXBs are hiding under the sensitivity limits of even relatively deep Chandra exposures. Both findings have significant implications for studies of accreting neutron stars and have provided better insight into globular cluster compact binary populations. Here we propose a Chandra program to precisely localize new transients in globular clusters. As part of this effort we further request HST observations to identify the optical counterparts of these transients. ------------------------------------------------------------------------------------ Observations Description ------------------------ * General background for the program The goal of this joint Chandra/HST Target-of-Opportunity (ToO) proposal is to measure accurate coordinates of the next two X-ray transients in a globular cluster with Chandra, and to use HST subsequently to look for their optical counterparts inside the Chandra positional error circles. The Chandra error circle is large enough (about 0.6" radius) that it likely contains multiple globular-cluster stars. Comparison of the photometry that is extracted from the observations requested here with photometry from archival HST data reveals highly variable objects in the error circle, and hence the identity of the true counterpart. Our choice for the WFC3/UVIS is driven by the existence of a large body of archival globular-cluster images taken with this camera. We have been allocated time to do Chandra/HST follow-up for two X-ray transients. For the HST component we were granted 4 HST orbits in total (2 orbits per transient). * The first trigger - September 2026 Late August 2026, Einstein Probe detected an outburst of an X-ray transient in the globular cluster NGC6440. Subsequent Swift observations in September 2026 confirmed the association of the transient with NGC6440. This X-ray transient has not been confidently identified with any of the known X-ray transients in the cluster. We have triggered our Chandra ToO program to obtain an accurate X-ray position of the source. The Chandra observations will be performed on **late September / early October**. We have triggered the HST component of our 18190( 4) - 14-Sep-2026 19:19:25 - [ 2] program to search for the optical counterpart. * Visit 1 Our program for Visit 1 to observe NGC6440 consists of observations in the F606W and F814W filters as there are already archival UVIS data in these filters that we can use for comparison. The observations are organized in a 4-point dither-box pattern to improve the resolution. We aim for a S/N>10 for a star of mag [TBD] (F606W, F814W). We also include short observations to get photometry of bright stars, and of stars near bright stars. ****** Note added 14-sep-2026: this is a preliminary setup to check the visibility of our target for HST ***** ------------------------------------------------------------------------------------ 18190( 4) - 14-Sep-2026 19:19:25 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 NGC6440-TRAN STELLAR CLUSTER, RA=17H48M52.2240S +/- 2.6", J2000 V = 16 SIENT GLOBULAR CLUSTER DEC=-20D21'36.86" +/- 2.6" Reference Frame: ICRS Extended: NO Comments: Our target is a to-be-identified star in the globular cluster NGC6440. The counterpart that we aim to identify is expected to have a magnitude in F606W m(F435W) >~ 16. Epoch of Position RA proper motion (seconds of time/yr) DEC Proper Motion (arcsec/yr) Annual Parallax (arcsec) 2000 0.0 0.0 0.0 ------------------------------------------------------------------------------------------------------------------------------------ 18190( 4) - 14-Sep-2026 19:19:25 - [ 4] TARGET LIST Generic Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Selection Criteria | Flux data No | Name | Description | | ------------------------------------------------------------------------------------------------------------------------------------ 2 GCTRANSIENT2 STAR, X-RAY TRANSIENT FIRST TRIGGER OF GLOBULAR-CLUSTER V = 19 X-RAY TRANSIENT PROGRAM Comments: next X-ray transient discovered in a Galactic globular cluster, whose position can only be accurately determined with Chandra ------------------------------------------------------------------------------------------------------------------------------------ 18190( 4) - 14-Sep-2026 19:19:25 - [ 5] Patterns ------------------------------------------------------------------------------------------------------------------------------------ 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 ------------------------------------------------------------------------------------------------------------------------------------ 18190( 4) - 14-Sep-2026 19:19:25 - [ 6] Visit: 01 (NGG6440) Visit Requirements: ON HOLD ; TOO MIN RESPONSE TIME 25.0D; TOO MAX RESPONSE TIME 30.0D On Hold Comments: Target of Opportunity 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 NGC6440-TRA WFC3/UVI ACCUM UVIS2 F606W 1 531 S PATTERN 2 1 NSIENT S (2124 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 NGC6440-TRA WFC3/UVI ACCUM UVIS2 F606W FLASH=15 1 30 S (30 NSIENT S S) ------------------------------------------------------------------------------------------------------------------------------------ 3 NGC6440-TRA WFC3/UVI ACCUM UVIS2 F814W FLASH=10 1 524 S PATTERN 2 3 NSIENT S (2096 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 NGC6440-TRA WFC3/UVI ACCUM UVIS2 F814W FLASH=20 1 30 S (30 NSIENT S 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 | ------------------------------------------------------------------------------------------------------------------------------------ NGC6440-TRANSIENT 1 WFC3/UVI ACCUM UVIS2 F606W 1 none none none 1 N/A S NGC6440-TRANSIENT 1 WFC3/UVI ACCUM UVIS2 F606W 2 none none none 1 N/A S NGC6440-TRANSIENT 1 WFC3/UVI ACCUM UVIS2 F606W 3 none none none 1 N/A S NGC6440-TRANSIENT 1 WFC3/UVI ACCUM UVIS2 F606W 4 none none none 1 N/A S NGC6440-TRANSIENT 2 WFC3/UVI ACCUM UVIS2 F606W none none none none 1 N/A S NGC6440-TRANSIENT 3 WFC3/UVI ACCUM UVIS2 F814W 1 none none none 2 N/A S NGC6440-TRANSIENT 3 WFC3/UVI ACCUM UVIS2 F814W 2 none none none 2 N/A S NGC6440-TRANSIENT 3 WFC3/UVI ACCUM UVIS2 F814W 3 none none none 2 N/A S NGC6440-TRANSIENT 3 WFC3/UVI ACCUM UVIS2 F814W 4 none none none 2 N/A S NGC6440-TRANSIENT 4 WFC3/UVI ACCUM UVIS2 F814W none none none none 2 N/A S 18190( 4) - 14-Sep-2026 19:19:25 - [ 7] Visit: 02 (Visit-Transient2) Visit Requirements: ON HOLD ; TOO MIN RESPONSE TIME 25.0D; TOO MAX RESPONSE TIME 30.0D On Hold Comments: Target of Opportunity 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 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F606W FLASH=5 1 376 S (376 POS TARG 0, 0 2 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F606W FLASH=5 1 376 S (376 POS TARG 0.1719, 0.1837 2 S S) ------------------------------------------------------------------------------------------------------------------------------------ 3 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F606W FLASH=5 1 376 S (376 POS TARG 0.3437, 0.3674 2 S S) ------------------------------------------------------------------------------------------------------------------------------------ 4 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F606W FLASH=5 1 376 S (376 POS TARG 0.4760, 0.5286 2 S S) ------------------------------------------------------------------------------------------------------------------------------------ 5 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F606W FLASH=5 1 376 S (376 POS TARG 0.6544, 0.6796 2 S S) ------------------------------------------------------------------------------------------------------------------------------------ 6 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F606W FLASH=15 1 30 S (30 2 S S) ------------------------------------------------------------------------------------------------------------------------------------ 7 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F814W FLASH=10 1 376 S (376 POS TARG 0, 0 2 S S) ------------------------------------------------------------------------------------------------------------------------------------ 8 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F814W FLASH=10 1 376 S (376 POS TARG 0.1719, 0.1837 2 S S) ------------------------------------------------------------------------------------------------------------------------------------ 9 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F814W FLASH=10 1 376 S (376 POS TARG 0.3437, 0.3674 2 S S) ------------------------------------------------------------------------------------------------------------------------------------ 10 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F814W FLASH=10 1 376 S (376 POS TARG 0.4760, 0.5286 2 S S) ------------------------------------------------------------------------------------------------------------------------------------ 11 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F814W FLASH=10 1 376 S (376 POS TARG 0.6544, 0.6796 2 S S) ------------------------------------------------------------------------------------------------------------------------------------ 12 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F814W FLASH=20 1 30 S (30 2 S 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 | ------------------------------------------------------------------------------------------------------------------------------------ 18190( 4) - 14-Sep-2026 19:19:25 - [ 8] 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 | ------------------------------------------------------------------------------------------------------------------------------------ GCTRANSIENT2 1 WFC3/UVI ACCUM UVIS2 F606W none none none none 1 N/A S GCTRANSIENT2 2 WFC3/UVI ACCUM UVIS2 F606W none none none none 1 N/A S GCTRANSIENT2 3 WFC3/UVI ACCUM UVIS2 F606W none none none none 1 N/A S GCTRANSIENT2 4 WFC3/UVI ACCUM UVIS2 F606W none none none none 1 N/A S GCTRANSIENT2 5 WFC3/UVI ACCUM UVIS2 F606W none none none none 1 N/A S GCTRANSIENT2 6 WFC3/UVI ACCUM UVIS2 F606W none none none none 1 N/A S GCTRANSIENT2 7 WFC3/UVI ACCUM UVIS2 F814W none none none none 2 N/A S GCTRANSIENT2 8 WFC3/UVI ACCUM UVIS2 F814W none none none none 2 N/A S GCTRANSIENT2 9 WFC3/UVI ACCUM UVIS2 F814W none none none none 2 N/A S GCTRANSIENT2 10 WFC3/UVI ACCUM UVIS2 F814W none none none none 2 N/A S GCTRANSIENT2 11 WFC3/UVI ACCUM UVIS2 F814W none none none none 2 N/A S GCTRANSIENT2 12 WFC3/UVI ACCUM UVIS2 F814W none none none none 2 N/A S 18190( 4) - 14-Sep-2026 19:19:25 - [ 9] Summary Form for Proposal 18190 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures UVIS2 ------------------------------------------------------------------------------------------------------------------------------------ Configurations WFC3/UVIS ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=10, FLASH=15, FLASH=20, FLASH=5 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ON HOLD , PATTERN 2 1, PATTERN 2 3, POS TARG 0, 0, POS TARG 0.1719, 0.1837, POS TARG 0.3437, 0.3674, POS TARG 0.4760, 0.5286, POS TARG 0.6544, 0.6796, TOO MAX RESPONSE TIME 30.0D, TOO MIN RESPONSE TIME 25.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names GCTRANSIENT2, NGC6440-TRANSIENT ------------------------------------------------------------------------------------------------------------------------------------