The joint detection of GWs and light from the neutron star (NS) merger GW 170817 ushered us into a new era. Deep X-ray observations of GW 170817 constrained the structural properties of a relativistic outflow launched by an NS merger for the first time. After these landmark discoveries, the frontier is now to map the X-ray properties of a population of sources. We propose to exploit the unique capabilities of XMM-Newton to constrain the broad-band properties of very nearby NS-bearing mergers, selected from LVKC-O4 GW triggers or electromagnetically identified kilonovae. Our immediate goals are: (i) map the diversity of emission from NS mergers, (ii) test the nature of the remnant object, (iii) determine if all mergers launch a jet, and (iv) enable the first X-ray detections of NS-BH mergers. Our detailed follow-up strategy is as follows: [I] Prompt-bright observation (< 2d), trigger if: (i) detection of a bright (> 5x10-13 erg s-1 cm-2, 0.5-10 keV) X-ray counterpart, OR (ii) bright blue/UV-optical counterpart (V < 18); then trigger one 60 ks XMM-Newton observation within 2d to exploit EPIC (and if bright) RGS science, to search for soft components and absorption studies, and OM with u(the most efficient) filter in timing mode (> 150,000 counts). No particular feasibility is needed. The event should be bright given the triggering criteria and the selected observing modes should avoid pile-up. [II] Follow-up observations at t > 10d if: (i) discovery and localisation of an EM (gamma-rays, X-rays, optical, radio) of a GW-detected NS-bearing event, OR (ii) EM-identified kilonova then (indicatively) we will execute three observations. (1) before peak: 60ks XMM-Newton/EPIC/FF/Thin + HST F606W (1orb) + VLA (2hr@6GHz+2hr@10GHz); (2) at the peak: 60ks XMM-Newton/EPIC/FF/Thin + HST F606W (1orb) + VLA (2hr@6GHz+2hr@10GHz) + 60ks NuSTAR; (3) after peak: 100ks XMM-Newton/EPIC/FF/Thin + HST F606W (1orb) + VLA (2hr@6GHz+2hr@10GHz). For the VLA, we require reaching 3sigma limits of 10 microJy (1.5 hr on-source, 2 hr total) in C-band. If detected, we will trigger another 2-hr observation at 10 GHz to constrain the spectral slope. In one orbit observation HST can reach F606W ~27 mag. HST and VLA will be mandatory to firmly constrain the spectral slope and search for cooling frequency breaks in the spectrum. NuSTAR will detect the afterglow if it is brighter than >10-13 erg s-1 cm-2 (0.5-10 keV). This will allow us to extend the wavelength range at peak on a broader wavelength scale. Our total request is 280 ks XMM-Newton time, 3 orbits of HST, 12 hr of VLA and 60 ks of NuSTAR. A shared PI-ship testifies for the collaboration among our US-based and Europe-based teams. Our team has a strong track record of triggering, analyzing, and rapidly publishing transient X-ray through radio multi-wavelength studies, as we demonstrated with GW170817. To identify optical and near-IR counterparts, our team is collectively involved in or leading efforts using the Dark Energy Camera, BlackGEM, 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), the Gravitational-wave Optical Transient Observer (GOTO) on La Palma, and the La Silla Schmidt Southern Survey (LS4). We have full access to optical telescopes to spectroscopically follow up any counterpart, including the TNG (3.6m in Canary Islands), the LBT (binocular 8m in the US), 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 XMM-Newton observations will be used efficiently. Additionally, we have gathered a premier team of theorists to help interpret our program's 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 XMM-Newton continues its important legacy in this exciting domain. Stellar Physics and Stellar Types 6 false false false false false false true SECONDARY XMM_NEWTON 096133 200 13 true SUPPORTED STAR GAMMA RAY BURSTER NEUTRON STAR Electromagnetic counterpart of a GW trigger GW trigger WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F606W WFC3/UVIS Imaging No No F606W 1738862487130 The scheduling data was generated by CASM 33.0. 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