Abstract for Proposal 092207
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. We have an approved
AO-21 TOO program to trigger on one neutron star merger discovered during
O4, and in this proposal we request one additional trigger (two triggers
are warranted by the updated merger rates).
Details on Observing Strategy and Trigger Criteria
We request one TOO trigger, to observe a 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. If no X-ray counterpart
is detected in the first three epochs then we will forgo the fourth epoch.