17873( 2) - 23-Oct-2024 21:31:43 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 17873 Version: 2 Check-in Time: 23-Oct-2024 21:31:43 STScI Edit Number: 0 Title Transient LMXBs in Globular Clusters ------------------------------------------------------------------------------------ Type Cycle GO 32 ------------------------------------------------------------------------------------ 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 ACS/WFC 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 program consists of two completely independent but identical 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. The structure of these two orbits is the same. We start an orbit with a short exposure (30s) to obtain photometry of relatively bright stars that are saturated in the longer exposures. The short exposure is followed by 4 longer (about 430s) exposures. The exact number of 17873( 2) - 23-Oct-2024 21:31:43 - [ 2] exposures and the exposure times depend of the duration of the orbit and will be determined once the target coordinates are known. We aim for a minimum of 4 long exposures so that they can be arranged in an ACS-DITHER-BOX pattern. These 4 long exposures are put in a Pattern container that is coupled to the default ACS-DITHER-BOX pattern with center=yes. To elevate the background levels, we have added a post-flash of 25e- to each short (=30s) 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 ACS/WFC 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. ------------------------------------------------------------------------------------ 17873( 2) - 23-Oct-2024 21:31:43 - [ 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 ------------------------------------------------------------------------------------------------------------------------------------ 17873( 2) - 23-Oct-2024 21:31:43 - [ 4] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.2637 Line_Spacing 0.1856 Coordinate_Frame POS-TARG Pattern_Orient 20.7 Angle_Between_Sides 69.02 Center_Pattern YES ------------------------------------------------------------------------------------------------------------------------------------ 17873( 2) - 23-Oct-2024 21:31:43 - [ 5] 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 ACS/WFC ACCUM WFC1 F606W FLASH=25 1 30 S (30 1 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 GCTRANSIENT ACS/WFC ACCUM WFC1 F606W 1 430 S PATTERN 1 2 1 (1720 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 GCTRANSIENT ACS/WFC ACCUM WFC1 F814W FLASH=25 1 30 S (30 1 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 GCTRANSIENT ACS/WFC ACCUM WFC1 F814W 1 430 S PATTERN 1 4 1 (1720 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 | ------------------------------------------------------------------------------------------------------------------------------------ GCTRANSIENT1 1 ACS/WFC ACCUM WFC1 F606W none none none none 1 N/A GCTRANSIENT1 2 ACS/WFC ACCUM WFC1 F606W 1 none none none 1 N/A GCTRANSIENT1 2 ACS/WFC ACCUM WFC1 F606W 2 none none none 1 N/A GCTRANSIENT1 2 ACS/WFC ACCUM WFC1 F606W 3 none none none 1 N/A GCTRANSIENT1 2 ACS/WFC ACCUM WFC1 F606W 4 none none none 1 N/A GCTRANSIENT1 3 ACS/WFC ACCUM WFC1 F814W none none none none 2 N/A GCTRANSIENT1 4 ACS/WFC ACCUM WFC1 F814W 1 none none none 2 N/A GCTRANSIENT1 4 ACS/WFC ACCUM WFC1 F814W 2 none none none 2 N/A GCTRANSIENT1 4 ACS/WFC ACCUM WFC1 F814W 3 none none none 2 N/A GCTRANSIENT1 4 ACS/WFC ACCUM WFC1 F814W 4 none none none 2 N/A 17873( 2) - 23-Oct-2024 21:31:43 - [ 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 ACS/WFC ACCUM WFC1 F606W FLASH=25 1 30 S (30 2 S) ------------------------------------------------------------------------------------------------------------------------------------ 2 GCTRANSIENT ACS/WFC ACCUM WFC1 F606W 1 430 S PATTERN 1 2 2 (1720 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 GCTRANSIENT ACS/WFC ACCUM WFC1 F814W FLASH=25 1 30 S (30 2 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 GCTRANSIENT ACS/WFC ACCUM WFC1 F814W 1 430 S PATTERN 1 4 2 (1720 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 | ------------------------------------------------------------------------------------------------------------------------------------ GCTRANSIENT2 1 ACS/WFC ACCUM WFC1 F606W none none none none 1 N/A GCTRANSIENT2 2 ACS/WFC ACCUM WFC1 F606W 1 none none none 1 N/A GCTRANSIENT2 2 ACS/WFC ACCUM WFC1 F606W 2 none none none 1 N/A GCTRANSIENT2 2 ACS/WFC ACCUM WFC1 F606W 3 none none none 1 N/A GCTRANSIENT2 2 ACS/WFC ACCUM WFC1 F606W 4 none none none 1 N/A GCTRANSIENT2 3 ACS/WFC ACCUM WFC1 F814W none none none none 2 N/A GCTRANSIENT2 4 ACS/WFC ACCUM WFC1 F814W 1 none none none 2 N/A GCTRANSIENT2 4 ACS/WFC ACCUM WFC1 F814W 2 none none none 2 N/A GCTRANSIENT2 4 ACS/WFC ACCUM WFC1 F814W 3 none none none 2 N/A GCTRANSIENT2 4 ACS/WFC ACCUM WFC1 F814W 4 none none none 2 N/A 17873( 2) - 23-Oct-2024 21:31:43 - [ 7] Summary Form for Proposal 17873 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=25 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ON HOLD , PATTERN 1 2, PATTERN 1 4, TOO RESPONSE TIME 25.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names GCTRANSIENT1, GCTRANSIENT2 ------------------------------------------------------------------------------------------------------------------------------------