Scientific Category is a required field. Science Keywords: Select from 2 to 5 items. Small GO Proposals are not allowed in Cycle 26 unless they are Joint Programs. Please specify your scheduling constraints or 'No Constraints' on all Observation forms. 0 Assigned ID: 39 ----- Attempting Submission 1 (Thu Aug 02 13:15:39 GMT 2018) ----- Joint Chandra/HST This target is not used in this proposal. We are resubmitting this Phase II setup after we triggered this ToO observation on an X-ray transient that has been detected in the globular cluster Terzan 6. -1928733264 ----- Attempting Submission 1 (Thu Sep 20 19:47:28 GMT 2018) ----- HST Phase II Proposal 15616 (apt15616.final.apt) successfully submitted. Receipt: # 15616-1 ----- Attempting Submission 2 (Thu Jun 06 16:06:00 GMT 2019) ----- This is a ToO proposal to do optical follow-up of an X-ray transient in a Galactic globular cluster. The coordinates of our target are not known yet, therefore we cannot run the Visit Planner. 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 have revealed that globular clusters can contain numerous quiescent LMXBs, 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. 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. We further request HST observations to identify the optical counterparts of these transients, to allow for more detailed follow-up studies. 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 five longer (about 370s) exposures. The Chandra ToO observation has been triggered as well, and will be executed in the week of June 10. *** Observing description: The goal of this joint Chandra/HST Target-of-Opportunity (ToO) proposal is to measure accurate coordinates of the next X-ray transient in a globular cluster with Chandra, and to use HST subsequently to look for the optical counterpart inside the Chandra positional error circle. The Chandra error circle is large enough (about 0.6" radius or less) 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 one X-ray transient, and for this we were granted 2 HST orbits. Our program is made up of one visit that comprises two orbits. The first orbit 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) that is meant to get photometry of relatively bright stars that are saturated in the longer exposures. The short exposure is followed by 4 to 5 longer (about 360s) exposures; the exact number of exposures and the exposure times depend of the duration of the orbit and will be known once the target coordinates are known. We aim for a minimum number of long exposures of 4 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 the short exposure. Given the ToO nature of our program, our target has been defined as a generic target 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. 6 false false false false false false false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER Our target is a to-be-identified star in the globular cluster Terzan 6. The listed V-magnitude is the integrated V magnitude of the entire cluster. The counterpart that we aim to identify is expected to have a magnitude in F606W around 21 or fainter. J2000 STAR X-RAY TRANSIENT next X-ray transient discovered in a Galactic globular cluster, whose position can only be accurately determined with Chandra trigger of globular-cluster X-ray transient program 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 1560198800289 /private/var/folders/46/4jpf0wpx40jbz16h87362w240001d2/T/OPVisit6706946793725568349.xml The scheduling data was generated by CASM 27.0. CASM was run in Phase II mode. (Tic-Visit :proposal-id 15616 :visit-id "01" :proposal-version -1 :trans-version "20181130" :date "10-Jun-2019 16:33:19.0" :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 15616 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 15616 :visit-id "01" :orbit-number 1 :pcs-time 333 :si-vis-utilization 2348 :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 15616 :visit-id "01" :orbit-number 2 :pcs-time 302 :si-vis-utilization 2365 :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