Abstract
Abstract Searches for electromagnetic counterparts of gravitational-wave signals have redoubled since the first detection in 2017 of a binary neutron star merger with a gamma-ray burst, optical/infrared kilonova, and panchromatic afterglow. Yet, one LIGO/Virgo observing run later, there has not yet been a second, secure identification of an electromagnetic counterpart. This is not surprising given that the localization uncertainties of events in LIGO and Virgo’s third observing run, O3, were much larger than predicted. We explain this by showing that improvements in data analysis that now allow LIGO/Virgo to detect weaker and hence more poorly localized events have increased the overall number of detections, of which well-localized, gold-plated events make up a smaller proportion overall. We present simulations of the next two LIGO/Virgo/KAGRA observing runs, O4 and O5, that are grounded in the statistics of O3 public alerts. To illustrate the significant impact that the updated predictions can have, we study the follow-up strategy for the Zwicky Transient Facility. Realistic and timely forecasting of gravitational-wave localization accuracy is paramount given the large commitments of telescope time and the need to prioritize which events are followed up. We include a data release of our simulated localizations as a public proposal planning resource for astronomers.
Highlights
We show that the discrepancy between the predicted and as-built localization performance in O3 was largely due to differences between the signal-to-noise ratio (S/N) threshold for detection that was assumed in LRR versus what was used in practice
We have used the same source distribution and detector networks as LRR to make an apples-to-apples comparison with our simulations, quantifying the large impact that the S/N threshold assumption has on figures of merit that are important to observers such as detection rate, distance, and sky localization precision
The updated S/N thresholds result in a modest increase in the number of detections, the large distances and large sky areas explain a significant reduction in the fraction of gravitational wave (GW) events for which KNe are detectable
Summary
The detection of the first binary neutron star (BNS) merger GW170187 (Abbott et al 2017a) by the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO; LIGO Scientific Collaboration et al 2015) and Virgo (Acernese et al 2015), its short gamma-ray burst (GRB) 170817A (Goldstein et al 2017), its afterglow (e.g., Hallinan et al 2017; Troja et al 2017), and its kilonova (KN) AT2017gfo (e.g., Evans et al 2017; Kasliwal et al 2017; Kilpatrick et al 2017; Pian et al 2017; Shappee et al 2017; Smartt et al 2017) has shown significant promise for multimessenger constraints on many areas of physics, including the neutron star (NS) equation of state (e.g., Bauswein et al 2017; Abbott et al 2018; Radice et al 2018; Coughlin et al 2019a, 2018a, 2020; Dietrich et al 2020), cosmology (e.g., Abbott et al 2017b; Hotokezaka et al 2019; Dietrich et al 2020), and nucleosynthesis (e.g., Chornock et al 2017; Coulter et al 2017; Cowperthwaite et al 2017; Pian et al 2017). Improvements in flagging and excision of bad data and the estimation of false alarm rates (FARs) made it possible in O3 to detect CBC signals in just a single detector (Callister et al 2017; Sachdev et al 2019; Godwin et al 2020; Nitz et al 2020), or with a reduced network S/N threshold when multiple detectors were online. These advances had the positive impact of increasing LIGO and Virgo’s astrophysical reach in O3 and increasing the number of candidates detected.
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