Science Keywords: Select from 2 to 5 items. Science Keywords is a required field. The total number of Primary Orbits used in all Cycles must equal the total number of Primary Orbits specified on all the Observations. (Compare totals reported on the first and second pages of the PDF Preview.) Only letters, numerals, '.', '+' and '-' are allowed in target names. Blank spaces are not allowed. Target names are limited to 2 to 31 characters. Only letters, numerals, '.', '+' and '-' are allowed in target names. Blank spaces are not allowed. Target names are limited to 2 to 31 characters. Please specify your scheduling constraints or 'No Constraints' on all Observation forms. Please specify your scheduling constraints or 'No Constraints' on all Observation forms. 0 Assigned ID: 1125 ----- Attempting Submission 1 (Wed Aug 19 13:43:43 GMT 2020) ----- Joint Chandra/HST A small change has been made to the target coordinates of visit 1. Also, the orientation angle for visit 1 has been set to 112deg. Visit 2 is still 'on hold' for now. 97719985 ----- Attempting Submission 1 (Thu Sep 17 18:39:38 GMT 2020) ----- ----- Attempting Submission 2 (Wed Aug 18 15:29:16 GMT 2021) ----- HST Phase II Proposal 16420 successfully submitted. Receipt: # 16420-2 ----- Attempting Submission 3 (Fri Aug 20 14:52:12 GMT 2021) ----- 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. 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 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. n/a Stellar Physics and Stellar Types Stellar Populations and the Interstellar Medium 6 false false false false false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER NO 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. J2000 STAR X-RAY TRANSIENT next X-ray transient discovered in a Galactic globular cluster, whose position can only be accurately determined with Chandra second trigger of globular-cluster X-ray transient program ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W ACS/WFC Imaging No No F606W ACS/WFC Imaging No No F814W POS-TARG DITHER 0.262 LINE 0.192 false true 68.14 4 18.39 ACS-WFC-DITHER-BOX 1 false 1629470670993 /Users/maureen/Research/Observe/hst_chandra20/trigger_terzan6/OP-cache/apt16420_trigger_Terzan6_v2-v01.xml The scheduling data was generated by CASM 29.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 16420 :visit-id "01" :proposal-version -1 :trans-version "20210514" :date "20-Aug-2021 16:44:30.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 2842240 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 16420 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 16420 :visit-id "01" :orbit-number 1 :pcs-time 383 :si-vis-utilization 2796 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3180 :pcs-gap 0 ) (Tic-Orbit :proposal-id 16420 :visit-id "01" :orbit-number 2 :pcs-time 392 :si-vis-utilization 2785 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3180 :pcs-gap 0 ) true 2 0 Target of Opportunity 1629471088407 /private/var/folders/_g/8j8s9lr56w9_4gvlrc8llvd00000gn/T/OPVisit5859266208837664046.xml The scheduling data was generated by CASM 29.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 16420 :visit-id "02" :proposal-version -1 :trans-version "20210514" :date "20-Aug-2021 16:51:28.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "27" :si-bright-earth-limb-angle 20.0 :data-volume 3410688 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 16420 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 16420 :visit-id "02" :orbit-number 1 :pcs-time 383 :si-vis-utilization 2742 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3142 :pcs-gap 0 ) (Tic-Orbit :proposal-id 16420 :visit-id "02" :orbit-number 2 :pcs-time 392 :si-vis-utilization 2703 :al-overhead-at-end 0 :internal-dur-at-end 347 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 3142 :pcs-gap 0 ) true 2 0 Target of Opportunity