Probing millisecond magnetar formation in binary neutron star mergers through X-ray follow-up of gravitational wave alerts

The nature of the remnant of a binary neutron star (BNS) merger is uncertain. Although it certainly is a black hole in the cases of the most massive BNSs, X-ray light-curves from short gamma-ray burst afterglows suggest a neutron star (NS) as a viable candidate for the merger remnant and central engine of these transients. When jointly observed with gravitational waves (GWs), X-ray light-curves from BNS merger events could provide critical constraints on the remnant nature. We assess the current and future capabilities for detecting an NS remnant through X-ray observations following GW detections. To this end, we simulated GW signals from BNS mergers and the subsequent X-ray emission from newborn millisecond magnetars. We modeled the GW detectability for the current and next-generation GW interferometers, and we reproduced the X-ray emission using a dedicated numerical code that models magnetar spin-down and ejecta dynamics informed by numerical relativity simulations. In our simulations, 2% - 16% of the BNS mergers form millisecond magnetars. Up to ∼ 70% of these might be detectable, which means up to 1.0 +0.3 _ -0.3 millisecond magnetar detections per year with instruments such as SVOM/MXT during the LIGO Virgo KAGRA LIGO India (LVKI) O5 run. The best detectability occurs about two hours post merger. For next-generation GW interferometers, this rate might increase by up to three orders of magnitude, with the peak detectability three to four hours post merger. We also explored how the magnetar magnetic field strength and observer viewing angle affect detectability, and we discuss optimized observational strategies. Although more likely with upcoming GW interferometers, the detection of the spin-down emission of a millisecond magnetar may already be within reach. This warrants sustained theoretical and observational efforts given the profound implications for mergers, gamma-ray bursts, and NS physics of a single detection.