Abstract
The binary neutron star merger gravitational-wave signal GW170817 was followed by three electromagnetic counterparts, including a kilonova arising from the radioactivity of freshly synthesized r-process elements in ejecta from the merger. Finding kilonovae after gravitational-wave triggers is crucial for (i) the search for further counterparts, such as the afterglow, (ii) probing the diversity of kilonovae and their dependence on the system’s inclination angle, and (iii) building a sample for multi-messenger cosmology. During the third observing run of the gravitational-wave interferometer network, no kilonova counterpart was found. We aim to predict the expected population of detectable kilonova signals for the upcoming O4 and O5 observing runs of the LIGO-Virgo-KAGRA instruments. Using a simplified criterion for gravitational-wave detection and a simple GW170817-calibrated model for the kilonova peak magnitude, we determine the rate of kilonovae in reach of follow-up campaigns and their distributions in magnitude for various bands. We briefly consider the case of GW190425, the only binary neutron star merger confirmed since GW170817, and obtain constraints on its inclination angle from the non-detection of its kilonova, assuming the source was below the follow-up thresholds. We also show that non-gravitational-wave-triggered kilonovae can be a numerous class of sources in future surveys and briefly discuss associations with short bright gamma-ray bursts. We finally discuss the detection of the jetted outflow afterglow in addition to the kilonova.
Highlights
The first detection of electromagnetic counterparts to a gravitational-wave (GW) event was a truly historic event (Abbott et al 2017b)
The coalescence of two neutron stars detected by the LIGO-Virgo instruments on August 17, 2017 (GW170817; Abbott et al 2017d) was followed 1.7 s later by a weak short gamma-ray burst (GRB) observed by Fermi and Integral (Goldstein et al 2017; Savchenko et al 2017; Abbott et al 2017c)
After respectively 9 and 16 days the afterglow was detected in X-rays with Chandra (Troja et al 2017) and in radio with the Very Large Array (VLA; Hallinan et al 2017)
Summary
The first detection of electromagnetic counterparts to a gravitational-wave (GW) event was a truly historic event (Abbott et al 2017b). The afterglow light curve was atypical, with a steady rise to a maximum at about 170 days post-merger (Ruan et al 2018; D’Avanzo et al 2018; Nynka et al 2018; Resmi et al 2018; Mooley et al 2018b) While such a behavior could result from either a radial or an angular structure of the ejecta (e.g., Gill & Granot 2018), very long baseline interferometry observations showing a displacement of the unresolved source by about 2.5 mas in 5 months (Mooley et al 2018a; Ghirlanda et al 2019) provided firm evidence for the latter. Joint fits to the afterglow photometry and imagery show that GW170817 was observed under an inclination angle of 15+−21..57 deg (Mooley et al 2018b; Ghirlanda et al 2019)
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