Transient LMXBs in Globular Clusters

STScI submission for the PI of the accepted joint Chandra-HST proposal. -1858514195 Assigned ID: 5934 ----- Attempting Submission 1 (Mon Sep 30 20:51:23 GMT 2024) ----- HST Phase I Proposal 5934 successfully submitted. Receipt: # 5934-1 ----- Attempting Submission 2 (Mon Sep 30 20:58:32 GMT 2024) ----- This is the Phase 2 setup for our ToO Chandra/HST program on transient LMXBs in globular clusters. -2074606670 ----- Attempting Submission 1 (Wed Oct 23 21:25:58 GMT 2024) ----- Transient LMXBs in Globular Clusters 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. 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 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. First co-I responsible for observations (Maureen van den Berg) Stellar Physics and Stellar Types Stellar Populations and the Interstellar Medium 6 false false false false false false false true SECONDARY CHANDRA 26400419 false SUPPORTED STAR X-RAY TRANSIENT next X-ray transient discovered in a Galactic globular cluster, whose position can only be accurately determined with Chandra first trigger of globular-cluster X-ray transient program 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 0.2637 LINE 0.1856 false true 69.02 4 20.7 ACS-WFC-DITHER-BOX 1

false

1729632165491 /Users/maureen/Research/Observe/hst_chandra24/final/OP-cache/apt17873-v01.xml The scheduling data was generated by CASM 32.1.3. CASM was run in Phase II mode. (Tic-Visit :proposal-id 17873 :visit-id "01" :proposal-version -1 :trans-version "20240604" :date "22-Oct-2024 23:22:45.0" :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 17873 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17873 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2624 :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 3005 :pcs-gap 0 ) (Tic-Orbit :proposal-id 17873 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2605 :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 3005 :pcs-gap 0 ) true 2 0 Target of Opportunity 1729632167352 /Users/maureen/Research/Observe/hst_chandra24/final/OP-cache/apt17873-v02.xml The scheduling data was generated by CASM 32.1.3. CASM was run in Phase II mode. (Tic-Visit :proposal-id 17873 :visit-id "02" :proposal-version -1 :trans-version "20240604" :date "22-Oct-2024 23:22:47.0" :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 17873 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 17873 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2624 :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 3005 :pcs-gap 0 ) (Tic-Orbit :proposal-id 17873 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2605 :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 3005 :pcs-gap 0 ) true 2 0 Target of Opportunity