Abstract for Proposal 090270

Probing the Astrophysics of Neutron Star Mergers with X-ray Afterglows

The afterglow of GW170817 probed the merger geometry as well as the morphology and energy of the fast ejecta launched by the binary neutron star merger. Here we propose X-ray afterglow follow up of two neutron star mergers, that will be discovered in the upcoming LIGO-Virgo O4 gravitational-wave run, having well-localized electromagnetic counterparts. Importantly, the X-rays will provide key information on ultra-relativistic jets and wide-angle outflows (cocoons), merger environments, magnetar remnants and may also help disambiguate binary neutron star and neutron-star black-hole mergers.



Details on Observing Strategy and Trigger Criteria
We request 2 TOO triggers, to observe two NS mergers discovered in LIGO-Virgo run O4, satisfying any one of the following criterion: 1) a BNS merger that is nearby (<~150 Mpc), 2) a BNS merger residing in a relatively dense environment or 3) the first NS-BH with an electromagnetic (EM) counterpart. The XMM-Newton observations would be triggered only if an EM counterpart with ~arcminute or better localization exists. Based on our simulations, we propose that the optimal strategy for each merger is to carry out the first observation as rapidly as possible (<5 days post-merger), and subsequent epochs at ~10--20 days, 1--3 months and 3--6 months. Accordingly, we request 4 epochs per trigger. For each trigger, if no X-ray counterpart is detected in the first three epochs then we will forgo the fourth epoch.