0 Assigned ID: 6915 ----- Attempting Submission 1 (Wed Aug 27 19:54:57 GMT 2025) ----- HST Phase I Proposal 6915 successfully submitted. Receipt: # 6915-1 0 ----- Attempting Submission 1 (Wed Oct 01 17:22:23 GMT 2025) ----- Community Discovery Program: panchromatic CXO/VLA/HST observations of a NS merger Chandra was the first facility to detect X-ray light from a neutron star (NS) merger. This ToO program continues Chandra's legacy in the revolutionary field of gravitational wave astronomy. Our goals are three-fold: (i) to characterize the diversity of emission from NS-NS mergers; (ii) to enable breakthroughs, such as the discovery of emission from a mass-gap or neutron star-black hole (NS-BH) merger; and (iii) to model panchromatic jet emission to constrain system parameters, particularly inclination and jet structure. This program will maximize the scientific output of LVK O4 and the first year of operations of new novel surveys, including Rubin/LSST and the La Silla Schmidt Southern Survey. We will support these Chandra observations via coordinated follow-up with the VLA and HST. This is a joint TOO program with Chandra as the main facility (and the VLA as an additional secondary facility) focused on gravitational wave (GW) events. There are two targets in this proposal: a GW trigger that occurs prior to Cycle 27, and a GW trigger that occurs during Cycle 27. All observations are imaging with WFC3/UVIS F606W and are non-disruptive TOO. Below we outline the nominal plan although it is subject to change based on the specific event details. For the pre-cycle 27 target, we will trigger 2 epochs of 6 orbits each to roughly align in time with our Chandra observations (target 3-sigma limit of m = 28.5 AB). If a source is detected in the second epoch, we may trigger one final epoch to serve as a host galaxy template. For the cycle 27 target, we will trigger one 3-orbit epoch to occur approximately 20-30 days after the GW trigger, to achieve a 3-sigma limit of m = 28 AB. We will trigger 2 epochs of 6 orbits each to roughly align in time with our Chandra observations, beyond 100 days after the GW trigger (target 3-sigma limit of m = 28.5 AB). If a source is detected in the second epoch, we may trigger one final epoch to serve as a host galaxy template. The total time allocated to this program is 33 orbits. Our collaboration is broad and includes scientists based in Canada, Europe and the US. A shared PI-ship testifies to the collaboration among the teams. Specifically, this proposal combines the efforts from Co-PIs: S. Campana, W. Fong, D. Haggard, and R. Margutti. A complete, alphabetized list of co-investigators is appended at the end of this document. The four Co-PI's are committed to the management of this large team to make timely decisions, informed by a wide range of expertise. Each of the Co-PI's also have experience as PI's of their own Chandra programs and collectively have two decades of experience in this arena. Our collective expertise includes X-ray (Chandra, Swift, etc.), optical/UV (including HST), and radio (including VLA) data analysis and interpretation, transient discovery and follow-up searches, and theoretical modelling of explosions and outflows associated with compact object mergers (e.g., GRBs and kilonovae). Resources and Survey Access: Our team has a strong track record of triggering, analyzing, and rapidly publishing transient X-ray, radio, and multi-wavelength studies. Our prompt publications featuring Chandra, HST, and VLA observations of GW170817 provide a proof-of-concept. To identify optical and near-IR counterparts, the Co-PI's are collectively involved in or leading efforts using the Dark Energy Camera and BlackGEM in Chile, the Canada-France-Hawaii Telescope, a network of telescopes in Southern Arizona (SAGUARO), the 2.6m VLT Survey Telescope (VST) in Paranal, the 60cm robotic REM telescope in La Silla (one of the telescopes which promptly observed GW170817), along with the JWST, Swift, NuSTAR, and XMM-Newton observatories. We have full access to optical telescopes to spectroscopically follow up any counterpart, including the TNG (3.6m in Canary Islands), and the LBT (binocular 8m) telescope in the US, in addition to Gemini, Keck, MMT, Lick and Magellan. GRAvitational Wave Inaf TeAm (GRAWITA) is also part of the ENGRAVE collaboration (Co-I Levan is chair of the ENGRAVE Executive Committee, Co-I Tanvir is chair of the ENGRAVE Governing Council), as well as European Co-Is of this proposal, and have access to ESO-VLT telescopes through a Large Program. O4b will overlap with the start of scientific operations of the LSST carried out by the Rubin Observatory. Co-PI Margutti chairs the GW follow-up group within the LSST Science Collaborations and many Co-Is of this proposal are members of this group. Our teams also belong to the rapid-response networks that report transients from other observatories (e.g., via OpenLVEM, GCNs, and ATels). Our involvement in these efforts ensures that Chandra observations will be used efficiently. Additionally, we have gathered a premier team of theorists to help interpret our program's incoming observations, which will likely offer new challenges to existing paradigms. Multi-messenger astrophysics is entering a new era and the observations proposed here will ensure that Chandra continues its important legacy in this exciting domain. Stellar Physics and Stellar Types 6 false false false false false false true SECONDARY CHANDRA 27510170 false SUPPORTED GW trigger for which we have also triggered or plan to trigger Chandra observations GW event STAR GAMMA RAY BURSTER NEUTRON STAR GW trigger for which we have also triggered or plan to trigger Chandra observations GW event WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F606W /private/var/folders/6l/jvctrz396nz_jsh8d02xh8400000gn/T/OPVisit9207865328025129577.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. pre-cycle 27 target, 100-150 day epoch (Tic-Visit :proposal-id 18194 :visit-id "01" :proposal-version -1 :trans-version "20250624" :date "01-Oct-2025 12:21:46.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 6 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1732464 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18194 :visit-id "01" :obset-id "01" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18194 :visit-id "01" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "01" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "01" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "01" :orbit-number 4 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "01" :orbit-number 5 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "01" :orbit-number 6 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :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 6 0 TOO observation /private/var/folders/6l/jvctrz396nz_jsh8d02xh8400000gn/T/OPVisit16560872230659463967.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. pre-cycle 27 target, 200-300 day epoch (Tic-Visit :proposal-id 18194 :visit-id "02" :proposal-version -1 :trans-version "20250624" :date "01-Oct-2025 12:21:50.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 6 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1732464 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18194 :visit-id "02" :obset-id "02" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18194 :visit-id "02" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "02" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "02" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "02" :orbit-number 4 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "02" :orbit-number 5 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "02" :orbit-number 6 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :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 6 0 TOO observation /private/var/folders/6l/jvctrz396nz_jsh8d02xh8400000gn/T/OPVisit6842537897243093308.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. cycle 27 target, 20-30 day epoch (Tic-Visit :proposal-id 18194 :visit-id "03" :proposal-version -1 :trans-version "20250624" :date "01-Oct-2025 12:21:52.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 3 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 866240 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18194 :visit-id "03" :obset-id "03" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18194 :visit-id "03" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "03" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "03" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 352 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) true 3 0 TOO observation /private/var/folders/6l/jvctrz396nz_jsh8d02xh8400000gn/T/OPVisit7372646707315369160.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. cycle 27 target, 100-150 day epoch (Tic-Visit :proposal-id 18194 :visit-id "04" :proposal-version -1 :trans-version "20250624" :date "01-Oct-2025 12:21:54.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 6 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1732464 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18194 :visit-id "04" :obset-id "04" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18194 :visit-id "04" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "04" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "04" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "04" :orbit-number 4 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "04" :orbit-number 5 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "04" :orbit-number 6 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :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 6 0 TOO observation /private/var/folders/6l/jvctrz396nz_jsh8d02xh8400000gn/T/OPVisit13777131883617986796.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. cycle 27 target, 200-300 day epoch (Tic-Visit :proposal-id 18194 :visit-id "05" :proposal-version -1 :trans-version "20250624" :date "01-Oct-2025 12:21:56.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 6 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1732464 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18194 :visit-id "05" :obset-id "05" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18194 :visit-id "05" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "05" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "05" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "05" :orbit-number 4 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "05" :orbit-number 5 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "05" :orbit-number 6 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :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 6 0 TOO observation /private/var/folders/6l/jvctrz396nz_jsh8d02xh8400000gn/T/OPVisit60280580408170619.xml The scheduling data was generated by CASM 33.1. CASM was run in Phase II mode. template for either target (Tic-Visit :proposal-id 18194 :visit-id "06" :proposal-version -1 :trans-version "20250624" :date "01-Oct-2025 12:21:59.0" :trans-errors '(:WARNINGS 0 :ERRORS 0 :HEALTH-AND-SAFETY 0 :FATAL-ERRORS 0 :CONTINUABLE-FATALS 0) :number-of-orbits 6 :saa-model "29" :si-bright-earth-limb-angle 20.0 :data-volume 1732464 :tdrss-contact-duration 0 :coron NIL ) (Tic-Obset :proposal-id 18194 :visit-id "06" :obset-id "06" :pcs-mode "FGS" ) (Tic-Orbit :proposal-id 18194 :visit-id "06" :orbit-number 1 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "06" :orbit-number 2 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "06" :orbit-number 3 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "06" :orbit-number 4 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "06" :orbit-number 5 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :internal-dur-at-end 0 :dark-offset NIL :dark-utilization 0 :max-dark-utilization 1920 :low-sky "N" :pcs-visibility 2979 :pcs-gap 0 ) (Tic-Orbit :proposal-id 18194 :visit-id "06" :orbit-number 6 :pcs-time 380 :si-vis-utilization 2408 :al-overhead-at-end 116 :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 6 0 TOO observation