Kilonova Emission From Black Hole-Neutron Star Mergers. II. Luminosity Function and Implications for Target-of-opportunity Observations of Gravitational-wave Triggers and Blind Searches

Abstract

We present detailed simulations of kilonova and GRB afterglow emission originating from black holeneutron star (BH-NS) mergers. We present kilonova luminosity function and discuss the detectability of kilonova and GRB afterglow in connection with gravitational wave (GW) detections, GW-triggered target-of-opportunity follow-up observations, and blind searches in time-domain survey observations. The predicted absolute magnitude of the radioactive-powered BH-NS merger kilonovae at 0.5 day after the merger falls in the range of [−10, −15.5]. The simulated luminosity function contains the potential viewing angle distribution information of the anisotropic kilonova emission. We simulate the GW detection rates, detectable distances and signal duration, for the future networks of 2nd, 2.5th, and 3rd generation GW detectors. BH-NS mergers tend to produce brighter kilonovae and afterglows if the primary BH has a high aligned-spin, and the NS companion is less-massive with a stiff equation of state. The detectability of kilonova emission is especially sensitive to the spin of the primary BH. If primary BHs typically have a low spin, the electromagnetic (EM) counterpart of BH-NS mergers are hard to discover. For the 2nd generation GW detector networks, a limiting magnitude of m limit ∼ 23 − 24 mag is required to detect the kilonovae from triggered BH-NS mergers even if BH high spin is assumed. This could provide a plausible explanation for the lack of kilonova detection from BH-NS candidates during LIGO/Virgo O3 (early 2nd generation detectors): either there is no EM counterpart (plunging events), or the current follow-up observations are too shallow to make a detection. On the other hand, these observations may still have the chance to detect the on-axis jet afterglow associated with a shortduration gamma-ray burst or an orphan afterglow, if the line of sight is not too far away from the jet axis (presumably the axis of binary merger). In future GW detection eras, more remote GW signals can be detected, but their associated kilonovae are more difficult to detect. Follow up observations can in any case detect possible associated sGRB afterglows, from which kilonova signatures may be studied. For time-domain blind survey observations, a high-cadence search in redder filters is recommended to detect more BH-NS merger kilonovae and afterglows in the current and future GW detection eras.

No Detectable Kilonova Counterpart is Expected for O3 Neutron Star–Black Hole Candidates

“…However, the predicted brightness for most of kilonovae still be too faint for the follow-up observations of LSST. This result is basically consistent with other recent studies (e.g., Zappa et al 2019;Zhu et al 2020b;Drozda et al 2020). For future GW-triggered multi-messenger observations, one may search for potential sGRB and afterglow as ideal EM counterparts of NSBH GW events.…”

“…However, in spite of many efforts for follow-up observations, no confirmed EM counterpart candidate was identified 1 (e.g., Anand et al 2020;Andreoni et al 2020;Coughlin et al 2020a;Gompertz et al 2020;Page et al 2020;Kasliwal et al 2020;Sagués Carracedo et al 2020). One plausible explanation for the lack of detection of an EM counterpart is that present EM searches are too shallow to achieve distance and volumetric coverage for the probability maps of LVC events (Coughlin et al 2020b;Sagués Carracedo et al 2020;Zhu et al 2020b). However, it is also possible and even likely that the EM counterparts are intrinsically missing, e.g.…”

“…for NSBH mergers there might not be material left outside of the merged BH remnant since the NS is not tidally disrupted but is swallowed as a whole into the BH (i.e. the so-called "plunging" event) (Zhu et al 2020b).…”

Population Synthesis of Black Hole Binaries with Normal-star Companions. I. Detached Systems

“…The detection of BH-NS mergers is of great interest as they are expected to emit across a broad electromagnetic spectrum and have been suggested to produce radio flares (Nakar & Piran 2011;Hotokezaka et al 2016), kilonovae (Li & Paczyński 1998;Zhu et al 2020), short gamma-ray bursts (Paczynski 1986;Gompertz et al 2020), and so on. A recent quite comprehensive study of BH-NS mergers can be found in Broekgaarden et al (2021).…”

An Infrared Search for Kilonovae with the WINTER Telescope. I. Binary Neutron Star Mergers

“…In contrast, the near-infrared emission is expected to be isotropic, longlived (>1 week), and ubiquitous in models regardless of mass ratio, viewing angle, or remnant lifetime (Kasen et al 2017). Models predict the detection rates of kilonovae in the near-infrared could be up to ∼ 8 − 10 times higher than in optical wavebands (Zhu et al 2020b).…”