Opening
When a pair of neutron stars collide, they unleash a cascade of electromagnetic radiation across the spectrum—but catching that light in the critical first hours and days requires the right tools pointed at the right place at the right time. A new study by Di Xiao examines whether the upcoming Xue-shan-mu-chang 15-meter SubMillimeter Telescope (XSMT) can reliably detect the submillimeter afterglows from neutron-star mergers, and finds that the answer depends critically on what happens in the merger's aftermath: whether a long-lived magnetar forms or a black hole promptly collapses.
What they found
Xiao presents a unified numerical framework that models how merger ejecta evolve dynamically and produce synchrotron emission, accounting for self-absorption and relativistic effects. The key question is whether XSMT's sensitivity—1.5, 2.9, and 10.2 mJy at 230, 345, and 460 GHz respectively for a 5σ detection in 1 hour—is sufficient to catch these events.
The results hinge on the merger's central engine. For a magnetar-powered scenario (where a neutron star survives the merger), the ejecta afterglow peaks on timescales of weeks to months, remaining detectable long enough to permit delayed follow-up observations. Under current binary neutron star merger-rate estimates, Xiao predicts an all-sky detection rate of approximately 0.05–1.7 events per year at 230 GHz, though this assumes all mergers produce long-lived magnetars and represents an upper limit. The actual rate scales linearly with the fraction of mergers that genuinely form magnetars rather than black holes.
!Fiducial light curves showing submillimeter flux evolution for ejecta-only scenario
Relativistic jets, by contrast, present a different challenge. While they produce intense early-time signals, their narrow beaming geometry and rapid evolution severely constrain detectability. The window for observation is fleeting, and only a small fraction of mergers will have jets pointed toward Earth.
Why it matters
Submillimeter observations fill a critical gap in multi-messenger astronomy. At early times, centimeter-band emission is often optically thick or has not yet peaked, making submillimeter wavelengths uniquely sensitive to the energetics and composition of the outflow. Distinguishing between a long-lived magnetar remnant and a promptly formed black hole has profound implications for understanding the neutron-star equation of state and the maximum mass a neutron star can support. XSMT's facility-specific forecasts provide a quantitative basis for prioritizing gravitational-wave triggers in real time.
What's next
The framework Xiao develops enables observers to construct parameter-space detectability maps tailored to XSMT's capabilities. Future work will likely refine magnetar-fraction estimates from gravitational-wave and electromagnetic observations, test the predictions against actual detections, and optimize observing cadences to maximize scientific yield during the multi-messenger era.
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