This is for our joint program with Chandra with Chandra as the primary facility. 1042998817 Assigned ID: 7881 ----- Attempting Submission 1 (Fri Oct 02 22:28:18 GMT 2026) ----- Chandra made history with the first X-ray detection of a neutron star (NS) merger. This ToO program builds directly on that legacy to drive the next wave of discoveries in multi-messenger astrophysics. Our goals are three-fold: (i) to characterize the diverse X-ray emission profiles of NS-NS mergers; (ii) to pioneer the discovery of X-ray counterparts to mass-gap or neutron star-black hole (NS-BH) mergers; and (iii) to leverage panchromatic jet modeling to tightly constrain system parameters, particularly viewing angle and jet structure. Supported by coordinated follow-up with the VLA and HST, this program will maximize the scientific yield of the IGWN network and the inaugural operations of major time-domain surveys like Rubin/LSST and the La Silla Schmidt Southern Survey. 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 (in alphabetical order): S. Campana, W. Fong, D. Haggard, and R. Margutti. 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, NuSTAR etc.), optical/UV (including HST and Rubin), 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) Stellar Physics and Stellar Types 6 false false false false false false false true SECONDARY CHANDRA 28510223 false SUPPORTED J2000 J2000 J2000 J2000 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