Transient LMXBs in globular clusters

The UVIS aperture has been changed to UVIS2, which is a better choice for out program. note added 14-sep-2026: this is an intermediate version of our observation setup, to check the visibility of our ToO target 647941735 ----- 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) ----- ----- Attempting Submission 3 (Mon Sep 14 18:55:56 GMT 2026) ----- 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. * General background for the program 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 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 were granted 4 HST orbits in total (2 orbits per transient). * The first trigger - September 2026 Late August 2026, Einstein Probe detected an outburst of an X-ray transient in the globular cluster NGC6440. Subsequent Swift observations in September 2026 confirmed the association of the transient with NGC6440. This X-ray transient has not been confidently identified with any of the known X-ray transients in the cluster. We have triggered our Chandra ToO program to obtain an accurate X-ray position of the source. The Chandra observations will be performed on **late September / early October**. We have triggered the HST component of our program to search for the optical counterpart. * Visit 1 Our program for Visit 1 to observe NGC6440 consists of observations in the F606W and F814W filters as there are already archival UVIS data in these filters that we can use for comparison. The observations are organized in a 4-point dither-box pattern to improve the resolution. We aim for a S/N>10 for a star of mag [TBD] (F606W, F814W). We also include short observations to get photometry of bright stars, and of stars near bright stars. ****** Note added 14-sep-2026: this is a preliminary setup to check the visibility of our target for HST ***** 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 false true SECONDARY CHANDRA 27400223 false SUPPORTED STELLAR CLUSTER GLOBULAR CLUSTER NO Our target is a to-be-identified star in the globular cluster NGC6440. The counterpart that we aim to identify is expected to have a magnitude in F606W m(F435W) >~ 16. J2000 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 WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F814W POS-TARG DITHER 0.173 LINE 0.112 false false 81.785 4 23.884 WFC3-UVIS-DITHER-BOX 2

false

1789411850951 /Users/maureen/Research/Observe/hst_chandra25/trigger1-ngc6440/OP-cache/apt18190_trigger1_checkvis-v01.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18190 :visit-id "01" :proposal-version -1 :trans-version "20260828" :date "14-Sep-2026 20:50:50.0" :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 2887488 :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 301 :si-vis-utilization 2676 :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 301 :si-vis-utilization 2676 :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 1789411583400 /Users/maureen/Research/Observe/hst_chandra25/trigger1-ngc6440/OP-cache/apt18190_trigger1_checkvis-v02.xml The scheduling data was generated by CASM 34.2. CASM was run in Phase II mode. (Tic-Visit :proposal-id 18190 :visit-id "02" :proposal-version -1 :trans-version "20260828" :date "14-Sep-2026 20:46:23.0" :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 301 :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 301 :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