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

TL;DR

The study investigates whether X-ray follow-up of gravitational-wave detected binary neutron star mergers can reveal newborn millisecond magnetars. It combines GW population synthesis and detector-calibrated detectability with a magnetar spin-down X-ray lightcurve model that accounts for ejecta geometry and opacity, anchored by NR-informed ejecta results. The results indicate that 2%–16% of BNS mergers may form millisecond magnetars, with up to ~1 magnetar detection per year for LVKI O5 and potentially orders of magnitude more with next-generation detectors, though detectability hinges on the EoS and magnetic field. The work outlines optimized observing strategies that pair narrow-FoV and wide-FoV X-ray instruments to maximize joint GW–EM detections, highlighting the significance for constraining NS physics, merger dynamics, and GRB central engines.

Abstract

The nature of the remnant of a binary neutron star (BNS) merger is uncertain. Though certainly a black hole (BH) in the cases of the most massive BNSs, X-ray lightcurves from gamma-ray burst (GRB) afterglows suggest a neutron star (NS) as a viable candidate for both the merger remnant as well as the central engine of these transients. When jointly observed with gravitational waves (GWs), X-ray lightcurves from BNS merger events could provide critical constraints on the remnant's nature. We aim to assess the current and future capabilities to detect a NS remnant through X-ray observations following GW detections. To this end, we simulate GW signals from BNS mergers and the subsequent X-ray emission from newborn millisecond magnetars. The GW detectability is modeled for both current and next-generation interferometers, while the X-ray emission is reproduced using a dedicated numerical code that models magnetar spin-down and ejecta dynamics informed by numerical-relativity simulations. In our simulations, 2% - 16% of BNS mergers form millisecond magnetars. Among these, up to 70% could be detectable, amounting to up to 1 millisecond magnetar detection per year with SVOM/MXT-like instruments during the LIGO Virgo KAGRA LIGO India (LVKI) O5 run, with optimal detectability occurring about 2 hours post-merger. For next-generation GW interferometers, this rate could increase by up to three orders of magnitude, with peak detectability 3 to 4 hours post-merger. We also explore how the magnetar's magnetic field strength and observer viewing angle affect detectability and discuss optimized observational strategies. Although more likely with upcoming GW interferometers, detecting the spin-down emission of a millisecond magnetar may already be within reach, warranting sustained theoretical and observational efforts given the profound implications for mergers, GRBs, and NS physics of a single detection.

Figures (15)

• Figure 1: PSD curves (i.e. strain noise amplitude as a function of frequency) chosen for the GW simulation depending on the configuration. O4 PSD curves are shown in filled lines, while O5 are shown in dashed lines, each for the LIGO (blue) and Virgo (green) instruments.
• Figure 2: Distribution of erroboxes obtained on the O4 (panel (a)) and O5 (panel (b)) GW runs simulations. All the BNS of the synthetic population that went through the GW detection pipeline are shown in blue, all detected systems are shown in orange, and the localized systems are shown with colors dependent on their localization accuracy (noted here $\Delta \Omega$).
• Figure 3: Distribution of GW localization accuracy from et_data data on 2 ET configurations: a triangle of 10 km arms by itself (a) or with the one CE interferometer (b). The localized systems are shown with colors dependent on their localization accuracy (noted $\Delta \Omega$).
• Figure 4: Schematized ejecta geometry, seconds after the merger. A fraction of the disk goes into post-merger ejecta. The dynamical ejecta is extracted at the time of the merger, and expands rapidly. The different emission zones are delimited by dashed lines, with corresponding opening angles reported.
• Figure 5: Simulated lightcurves for a magnetar of dipolar magnetic field 10$^{14}$ G (panel (a), left) 10$^{15}$ G (panel (b), middle) and 10$^{16}$ G (panel (c), right) assuming the DD2 EoS with a 2.37 M$_\odot$ NS mass. The top panels show the emission from the two considered viewing angles, and the bottom panels detail the physical origins of the emission. Times are measured relative from merger.