What is the Most Promising Electromagnetic Counterpart of a Neutron Star Binary Merger?
Metzger, B. D.
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CitationMetzger, B. D., and E. Berger. 2012. What is the Most Promising Electromagnetic Counterpart of a Neutron Star Binary Merger?. The Astrophysical Journal 746, no. 1: 48. doi:10.1088/0004-637x/746/1/48.
AbstractThe inspiral and coalescence of double neutron star (NS-NS) and neutron star-black hole (NS-BH) binaries are likely to be detected by advanced networks of ground-based gravitational wave (GW) interferometers. Maximizing the science returns from such a discovery will require the identification and localization of an electromagnetic (EM) counterpart. Here we critically evaluate and compare several possible counterparts, including short-duration gamma-ray bursts (SGRBs), “orphan” optical and radio afterglows, and ∼ day-long optical transients powered by the radioactive decay of heavy nuclei synthesized in the merger ejecta (“kilonovae”). We assess the promise of each potential counterpart in terms of four “Cardinal Virtues”: detectability, high fraction, identifiability, and positional accuracy. For viewing angles within the half-opening angle of the jet (θobs . θj ) the SGRB and associated afterglow are easily detectable within the range of Advanced LIGO/Virgo if the jet energy (Ej ) and the circumburst density (n) are similar to those inferred from existing SGRB observations. For modest off-axis angles (θobs . 2θj), the orphan optical afterglow is detectable with LSST if Ej,50 n 7/8 0 & 0.002; the fraction of such events is ∼ 7θ 2 j ∼ 0.1. At even larger viewing angles (i.e., the majority of observers) the isotropic kilonova emission dominates, with a peak optical brightness of ∼ 19 − 22 mag within the Advanced LIGO/Virgo volume, detectable with LSST using a specialized 1-day cadence. Radio afterglow emission from an initially off-axis jet or from sub-relativistic ejecta is also isotropic, but peaks on a timescale of months-years; this signal is detectable provided that Ej,50 n 7/8 0 (v/c) 11/4 & 0.2 (for off-axis afterglows, v/c ∼ 1). However, existing SGRB afterglows do not satisfy this criterion, indicating a low probability of radio detections. Taking into account the search strategy for typical error regions of tens of square degrees, our primary conclusion is that SGRBs are the most useful EM counterparts to confirm the cosmic origin of a few GW events, and to test the association with NS-NS/NS-BH mergers. However, for the more ambitious goal of localizing and obtaining redshifts for a large sample of GW events, kilonovae are instead preferred. Off-axis optical afterglows will be detectable for at most ∼ 10% of all events, while radio afterglows are promising only for the unique combination of energetic relativistic ejecta in a high density medium, and even then will require hundreds of hours of EVLA time per event spread over months-years. Our main recommendations from this analysis are: (i) an all-sky γ-ray satellite is essential for temporal coincidence detections, and for GW searches of γ-ray triggered events; (ii) LSST should adopt a 1-day cadence follow-up strategy, ideally with ∼ 0.5 hr per pointing to cover GW error regions (the standard 4-day cadence and depth will severely limit the probability of a unique identification); and (iii) radio searches should only focus on the relativistic case, which requires observations for a few months.
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