18190( 3) - 13-Oct-2025 21:06:39 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 18190 Version: 3 Check-in Time: 13-Oct-2025 21:06:39 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 ------------------------ 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 (to be taken about 25 days after triggering the proposal) 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 have been granted 4 HST orbits in total (two orbits per transient). Our current program consists of two completely independent but identifical visits of two orbits each. One orbit in a visit is spent on observations in the F606W band, and the other orbit on observations in the F814W band. We note that the final filter choice, the observation sequence and the exposure times depend on the target that will be observed. We will try to match the filters to those that were used in archival HST observations of the selected source. This Phase-2 program should therefore be considered as a placeholder. 18190( 3) - 13-Oct-2025 21:06:39 - [ 2] In each filter we take one short exposure (30s) to extract photometry of relatively bright stars that would otherwise saturate in the longer exposures. In addition, we obtain a sequence of longer exposures with a 4-point or 5-point dither pattern depending on the duration of the orbit, which is set once the target cluster is known. The dither pattern is defined by POS-TARG offsets entered manually and taken from ISR_2020-07. To elevate the background levels, we have added a post-flash to each exposure. Given the ToO nature of our program, our targets have been defined as two generic targets with unknown coordinates. Once the coordinates are known, we can fine-tune the observational setup, such as the exposure times and number of exposures, as well as the detailed placement of the target (i.e. the Chandra error circle) on the WFC3/UVIS detectors. ** Impact of reduced-gyro operations The primary impact of reduced-gyro operations is a reduction of HST's field of regard. Therefore, we expect that the main effect on our program is a possible reduced likelihood of responding to our Target-of-Opportunity trigger within the requested time frame of 25 days. ------------------------------------------------------------------------------------ 18190( 3) - 13-Oct-2025 21:06:39 - [ 3] TARGET LIST Generic Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Selection Criteria | Flux data No | Name | Description | | ------------------------------------------------------------------------------------------------------------------------------------ 1 GCTRANSIENT1 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 ------------------------------------------------------------------------------------------------------------------------------------ 2 GCTRANSIENT2 STAR, X-RAY TRANSIENT SECOND 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( 3) - 13-Oct-2025 21:06:39 - [ 4] Visit: 01 (Visit-Transient1) Visit Requirements: ON HOLD ; TOO RESPONSE TIME 25.0D 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 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 2 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F606W FLASH=5 1 376 S (376 POS TARG 0.1719, 0.1837 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 3 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F606W FLASH=5 1 376 S (376 POS TARG 0.3437, 0.3674 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 4 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F606W FLASH=5 1 376 S (376 POS TARG 0.4760, 0.5286 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 5 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F606W FLASH=5 1 376 S (376 POS TARG 0.6544, 0.6796 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 6 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F606W FLASH=15 1 30 S (30 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 7 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F814W FLASH=10 1 376 S (376 POS TARG 0, 0 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 8 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F814W FLASH=10 1 376 S (376 POS TARG 0.1719, 0.1837 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 9 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F814W FLASH=10 1 376 S (376 POS TARG 0.3437, 0.3674 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 10 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F814W FLASH=10 1 376 S (376 POS TARG 0.4760, 0.5286 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 11 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F814W FLASH=10 1 376 S (376 POS TARG 0.6544, 0.6796 1 S S) ------------------------------------------------------------------------------------------------------------------------------------ 12 GCTRANSIENT WFC3/UVI ACCUM UVIS2 F814W FLASH=20 1 30 S (30 1 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( 3) - 13-Oct-2025 21:06:39 - [ 5] 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 | ------------------------------------------------------------------------------------------------------------------------------------ GCTRANSIENT1 1 WFC3/UVI ACCUM UVIS2 F606W none none none none 1 N/A S GCTRANSIENT1 2 WFC3/UVI ACCUM UVIS2 F606W none none none none 1 N/A S GCTRANSIENT1 3 WFC3/UVI ACCUM UVIS2 F606W none none none none 1 N/A S GCTRANSIENT1 4 WFC3/UVI ACCUM UVIS2 F606W none none none none 1 N/A S GCTRANSIENT1 5 WFC3/UVI ACCUM UVIS2 F606W none none none none 1 N/A S GCTRANSIENT1 6 WFC3/UVI ACCUM UVIS2 F606W none none none none 1 N/A S GCTRANSIENT1 7 WFC3/UVI ACCUM UVIS2 F814W none none none none 2 N/A S GCTRANSIENT1 8 WFC3/UVI ACCUM UVIS2 F814W none none none none 2 N/A S GCTRANSIENT1 9 WFC3/UVI ACCUM UVIS2 F814W none none none none 2 N/A S GCTRANSIENT1 10 WFC3/UVI ACCUM UVIS2 F814W none none none none 2 N/A S GCTRANSIENT1 11 WFC3/UVI ACCUM UVIS2 F814W none none none none 2 N/A S GCTRANSIENT1 12 WFC3/UVI ACCUM UVIS2 F814W none none none none 2 N/A S 18190( 3) - 13-Oct-2025 21:06:39 - [ 6] Visit: 02 (Visit-Transient2) Visit Requirements: ON HOLD ; TOO RESPONSE TIME 25.0D 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( 3) - 13-Oct-2025 21:06:39 - [ 7] 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( 3) - 13-Oct-2025 21:06:39 - [ 8] 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 , 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 RESPONSE TIME 25.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names GCTRANSIENT1, GCTRANSIENT2 ------------------------------------------------------------------------------------------------------------------------------------