The UVIS aperture has been changed to UVIS2, which is a better choice for out program. 2127433838 ----- Attempting Submission 1 (Thu Oct 02 22:21:32 GMT 2025) ----- HST Phase II Proposal 18190 successfully submitted. Receipt: # 18190-1 ----- Attempting Submission 2 (Mon Oct 13 18:59:49 GMT 2025) ----- 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 WFC3/UVIS 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 current program consists of two completely independent but identifical 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. We note that the final filter choice, the observation sequence and the exposure times depend on the target that will be observed. We will try to match the filters to those that were used in archival HST observations of the selected source. This Phase-2 program should therefore be considered as a placeholder. In each filter we take one short exposure (30s) to extract photometry of relatively bright stars that would otherwise saturate in the longer exposures. In addition, we obtain a sequence of longer exposures with a 4-point or 5-point dither pattern depending on the duration of the orbit, which is set once the target cluster is known. The dither pattern is defined by POS-TARG offsets entered manually and taken from ISR_2020-07. To elevate the background levels, we have added a post-flash to each 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 WFC3/UVIS 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. All team members have extensive expertise in Chandra data analysis and are experts on (transient) X-ray sources in globular clusters. They all have led past Chandra observations of globular clusters. Dr. Heinke and Dr. Homan have studied and modeled crustal cooling or accretion heated neutron stars. Dr. Homan and Dr. Pooley will lead the analysis of the Chandra data. Dr. van den Berg has extensive expertise with the analysis of HST observations and will lead the analysis of the HST data. All team members will contribute to the interpretation of the data and publishing of the results. Stellar Populations and the Interstellar Medium Stellar Physics and Stellar Types 6 false false false false false false true SECONDARY CHANDRA 27400223 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 WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W /private/var/folders/w3/782d_wzj0fxdx9skhhtlhjt80000gn/T/OPVisit1848171922811986270.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18190 :visit-id "01" :proposal-version -1 :trans-version "20250624" :date "13-Oct-2025 20:57:49.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 3464992 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18190 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18190 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2585 :al-overhead-at-end 127 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18190 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2595 :al-overhead-at-end 129 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0 Target of Opportunity /private/var/folders/w3/782d_wzj0fxdx9skhhtlhjt80000gn/T/OPVisit17217704440301619988.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18190 :visit-id "02" :proposal-version -1 :trans-version "20250624" :date "13-Oct-2025 20:57:51.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 2 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 3464992 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18190 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18190 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2585 :al-overhead-at-end 127 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18190 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2595 :al-overhead-at-end 129 :internal-dur-at-end 668 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 2 0 Target of Opportunity