16420( 4) - 20-Aug-2021 14:56:01 - [ 1] HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 16420 Version: 4 Check-in Time: 20-Aug-2021 14:56:01 STScI Edit Number: 0 Title Transient LMXBs in Globular Clusters ------------------------------------------------------------------------------------ Type Cycle GO 28 ------------------------------------------------------------------------------------ Investigators Contact? PI: Dr. Jeroen Homan Eureka Scientific Inc. Y CoI: Dr. David A. Pooley Eureka Scientific Inc. N CoI: Dr. Craig Heinke University of Alberta N CoI: Dr. Maureen van den Berg Harvard University Y ------------------------------------------------------------------------------------ Abstract Since the discovery of globular cluster LMXBs in the 1970s, it was long assumed there was only one luminous LMXB per cluster. Deep Chandra observations of globular clusters have revealed that they contain numerous quiescent LMXB systems, any of which could go into outburst. Our past Chandra programs have shown 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. Here we propose to continue our Chandra program to precisely localize new transients in globular clusters, with the aim of gaining better insight into globular cluster LMXB populations. As part of this effort we further request HST observations to identify the optical counterparts of these transients. ------------------------------------------------------------------------------------ Observations Description ------------------------ NOTE added after the trigger: we have triggered this HST observation on an X-ray transient that has been detected in the globular cluster Terzan 6 by Swift. We have adjusted the target coordinates accordingly. In each orbit (one for F606W exposures, one for F814W exposures) we have fitted one short (30s) and four longer (about 475 s) exposures. ***** 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 were granted 4 HST orbits in total. Our 16420( 4) - 20-Aug-2021 14:56:01 - [ 2] program is made up of two completely independent but identical visits of two orbits each. The first orbit in a visit is spent on observations in the F606W band, and the second 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 to 5 longer (about 340s) exposures; the exact number of 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. In this Phase II file we have assumed that 5 long exposures can be accommodated. The first long exposure is to be taken at the target coordinates. The remaining 4 long exposures are put in a Pattern container that is coupled to the default ACS-DITHER-BOX pattern with center=yes. As a result these 4 exposures will surround the first long exposure. 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. ------------------------------------------------------------------------------------ 16420( 4) - 20-Aug-2021 14:56:01 - [ 3] TARGET LIST Fixed Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Target |Coord | Radial | Flux data No | Name | Description | Position |Eqnx | Vel. | ------------------------------------------------------------------------------------------------------------------------------------ 1 TERZAN6-TRAN STELLAR CLUSTER, RA=17H50M46.7000S +/- 0.4", J2000 V = 21.0 SIENT GLOBULAR CLUSTER DEC=-31D16'35.00" +/- 0.4" Reference Frame: ICRS Extended: NO Comments: Our target is a to-be-identified star in the globular cluster Terzan 6. The counterpart that we aim to identify is expected to have a magnitude in F606W m(F606W) > 21. ------------------------------------------------------------------------------------------------------------------------------------ 16420( 4) - 20-Aug-2021 14:56:01 - [ 4] TARGET LIST Generic Targets ------------------------------------------------------------------------------------------------------------------------------------ Tar| Target | Target | Selection Criteria | Flux data No | Name | Description | | ------------------------------------------------------------------------------------------------------------------------------------ 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 ------------------------------------------------------------------------------------------------------------------------------------ 16420( 4) - 20-Aug-2021 14:56:01 - [ 5] Patterns ------------------------------------------------------------------------------------------------------------------------------------ Pattern_Number: 1 Primary_Pattern Secondary_Pattern Pattern_Type ACS-WFC-DITHER-BOX Pattern_Purpose DITHER Number_Of_Points 4 Point_Spacing 0.262 Line_Spacing 0.192 Coordinate_Frame POS-TARG Pattern_Orient 18.39 Angle_Between_Sides 68.14 Center_Pattern YES ------------------------------------------------------------------------------------------------------------------------------------ 16420( 4) - 20-Aug-2021 14:56:01 - [ 6] Visit: 01 (Visit-Terzan6-transient) Visit Requirements: ORIENT 112D TO 112 D; 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 TERZAN6-TRA ACS/WFC ACCUM WFC1 F814W FLASH=25 1 30 S (30 NSIENT S) ------------------------------------------------------------------------------------------------------------------------------------ 2 TERZAN6-TRA ACS/WFC ACCUM WFC1 F814W 1 473 S PATTERN 1 2 NSIENT (1892 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 TERZAN6-TRA ACS/WFC ACCUM WFC1 F606W FLASH=25 1 30 S (30 NSIENT S) ------------------------------------------------------------------------------------------------------------------------------------ 4 TERZAN6-TRA ACS/WFC ACCUM WFC1 F606W 1 480 S PATTERN 1 4 NSIENT (1920 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 | ------------------------------------------------------------------------------------------------------------------------------------ TERZAN6-TRANSIENT 1 ACS/WFC ACCUM WFC1 F814W none none none none 1 N/A TERZAN6-TRANSIENT 2 ACS/WFC ACCUM WFC1 F814W 1 none none none 1 N/A TERZAN6-TRANSIENT 2 ACS/WFC ACCUM WFC1 F814W 2 none none none 1 N/A TERZAN6-TRANSIENT 2 ACS/WFC ACCUM WFC1 F814W 3 none none none 1 N/A TERZAN6-TRANSIENT 2 ACS/WFC ACCUM WFC1 F814W 4 none none none 1 N/A TERZAN6-TRANSIENT 3 ACS/WFC ACCUM WFC1 F606W none none none none 2 N/A TERZAN6-TRANSIENT 4 ACS/WFC ACCUM WFC1 F606W 1 none none none 2 N/A TERZAN6-TRANSIENT 4 ACS/WFC ACCUM WFC1 F606W 2 none none none 2 N/A TERZAN6-TRANSIENT 4 ACS/WFC ACCUM WFC1 F606W 3 none none none 2 N/A TERZAN6-TRANSIENT 4 ACS/WFC ACCUM WFC1 F606W 4 none none none 2 N/A 16420( 4) - 20-Aug-2021 14:56:01 - [ 7] 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 1 340 S (340 2 S) ------------------------------------------------------------------------------------------------------------------------------------ 3 GCTRANSIENT ACS/WFC ACCUM WFC1 F606W 1 340 S PATTERN 1 3 2 (1360 S) ------------------------------------------------------------------------------------------------------------------------------------ 4 GCTRANSIENT ACS/WFC ACCUM WFC1 F814W FLASH=25 1 30 S (30 2 S) ------------------------------------------------------------------------------------------------------------------------------------ 5 GCTRANSIENT ACS/WFC ACCUM WFC1 F814W 1 340 S (340 2 S) ------------------------------------------------------------------------------------------------------------------------------------ 6 GCTRANSIENT ACS/WFC ACCUM WFC1 F814W 1 340 S PATTERN 1 6 2 (1360 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 none none none none 1 N/A GCTRANSIENT2 3 ACS/WFC ACCUM WFC1 F606W 1 none none none 1 N/A GCTRANSIENT2 3 ACS/WFC ACCUM WFC1 F606W 2 none none none 1 N/A GCTRANSIENT2 3 ACS/WFC ACCUM WFC1 F606W 3 none none none 1 N/A GCTRANSIENT2 3 ACS/WFC ACCUM WFC1 F606W 4 none none none 1 N/A GCTRANSIENT2 4 ACS/WFC ACCUM WFC1 F814W none none none none 2 N/A GCTRANSIENT2 5 ACS/WFC ACCUM WFC1 F814W none none none none 2 N/A GCTRANSIENT2 6 ACS/WFC ACCUM WFC1 F814W 1 none none none 2 N/A GCTRANSIENT2 6 ACS/WFC ACCUM WFC1 F814W 2 none none none 2 N/A GCTRANSIENT2 6 ACS/WFC ACCUM WFC1 F814W 3 none none none 2 N/A GCTRANSIENT2 6 ACS/WFC ACCUM WFC1 F814W 4 none none none 2 N/A 16420( 4) - 20-Aug-2021 14:56:01 - [ 8] Summary Form for Proposal 16420 Item Used in this proposal ------------------------------------------------------------------------------------------------------------------------------------ Apertures WFC1 ------------------------------------------------------------------------------------------------------------------------------------ Configurations ACS/WFC ------------------------------------------------------------------------------------------------------------------------------------ Opmodes ACCUM ------------------------------------------------------------------------------------------------------------------------------------ Optional Parameters FLASH=25 ------------------------------------------------------------------------------------------------------------------------------------ Special Requirements ON HOLD , ORIENT 112D TO 112 D, PATTERN 1 2, PATTERN 1 3, PATTERN 1 4, PATTERN 1 6, TOO RESPONSE TIME 25.0D ------------------------------------------------------------------------------------------------------------------------------------ Spectral Elements F606W, F814W ------------------------------------------------------------------------------------------------------------------------------------ Target Names GCTRANSIENT2, TERZAN6-TRANSIENT ------------------------------------------------------------------------------------------------------------------------------------