1. Introduction
BNS and NSBH mergers generate not only GWs but also electromagnetic (EM) radiation and neutrino bursts (cosmic rays are also generated by these events, but they are impossible to detect due to their low speed — they take hundreds to thousands of years to reach Earth after event (Perkins et al. Reference Perkins2024) and allow a multi-messenger approach to be used for detailed observation of stellar death. Multi-messenger astronomy improves our understanding of compact object mergers by combining gravitational wave data with EM and neutrino observations (Bartos et al. Reference Bartos2017). Global projects such as LIGO, Virgo, and KAGRA, in collaboration with telescopes for EM observations and neutrino detectors, have created a robust infrastructure for obtaining all kinds of observational data on these types of events.
Global networks such as GRANDMA (Global Rapid Advanced Network Devoted to the Multi-messenger Addicts) and powerful telescopes such as ZTF (Zwicky Transient Facility) help monitor gravitational wave alerts and search for transients, coordinating work with telescopes from observatories around the world (Graham et al. Reference Graham2021). The GRANDMA project, which includes the Shamakhi Astrophysical Observatory named after N. Tusi, has enhanced coordination between telescopes in different countries, improving observation strategies through joint monitoring with various telescopes. These efforts allow for immediate dissemination of event alerts, rapid and real-time organization of observations, and study of the remnants and their surroundings. These follow-ups aim to confirm or constrain the presence of kilonovae (Metzger Reference Metzger2019) and relativistic jets, thus expanding the possibilities for detecting and studying these remnants. In these projects, gravitational wave detectors and ground-based telescopes work together to provide faster follow-up observations, thereby building our knowledge in this field of research.
NSBH mergers have also become an important research area, playing a key role in the study of GWs. NSBH mergers can result in a variety of astrophysical events, including kilonovae (KNe), relativistic jets, and hence potential observable EM counterparts (Kasen et al. Reference Kasen2017). What happens after a NSBH merger depends on several factors, such as the total mass, spin, and the equation of state of neutron star matter. In such mergers, a more massive black hole is typically formed, often accompanied by an accretion disk that emits EM radiation (Foucart Reference Foucart2018).
NSBH mergers remain extremely rare among GW sources. All BNS, BBH, and NSBH mergers observed so far are listed in the Fourth Open Gravitational-wave Catalog (4-OGC). The 4-OGC catalog covers all such observations conducted between 2015 and 2020. It also includes data collected the first through third observing runs (O1, O2, O3a, and O3b) of Advanced LIGO and Advanced Virgo. This updated catalog includes seven BBH mergers that were previously not recorded as highly significant during the O3b observing run. In total, 94 observation events have been recorded: 90 of which are BBHs, 2 are NSBHs, and 2 are BNS (Nitz et al. Reference Nitz2023). Therefore, despite having very limited resources, the study of NSBH mergers is currently a highly significant area of research.
The coalescence of a neutron star with a black hole represents another remarkable class of compact object mergers in astrophysics. These systems, though rarer than their BNS counterparts, can also produce detectable gravitational waves, offering a complementary probe into the behavior of dense matter, stellar evolution, and strong-field gravity.
A NSBH merger occurs when a neutron star spirals inward toward a black hole due to the emission of gravitational radiation, eventually merging with it. The outcome and observational signatures of such an event largely depend on several critical parameters, including the mass ratio of the two objects and the orbital properties of the system. In particular, whether an electromagnetic (EM) counterpart is produced hinges on these properties. If the mass ratio is sufficiently low and the black hole’s spin is high or favorably aligned, the neutron star can be tidally disrupted outside the black hole’s event horizon, leading to the formation of an accretion disk and potential emission of short GRBs and other EM signals.
GW200105 and GW200115 are notable observational examples of NSBH mergers confirmed by the LIGO-Virgo Collaboration, however, no optical or high-energy electromagnetic counterparts were detected in either case. Despite this, theoretical models suggest that under favorable conditions, these systems could generate observable EM signals, especially if the neutron star is disrupted before being consumed by the black hole.
The detection (or absence) of EM counterparts from NSBH mergers provides valuable constraints on merger dynamics and the physical properties of neutron stars, such as their radii and tidal deformability. Furthermore, these events play a crucial role in enriching multi-messenger astrophysics by offering insights into the diverse outcomes of compact binary coalescences (Broekgaarden et al. Reference Broekgaarden2022).
2. Properties of GW 200115 and GW 200105 events
The events GW 200115 and GW 200105, are confirmed to be NSBH mergers based on their mass and spin characteristics (LIGO & Virgo Collaboration 2020). Unlike binary neutron star mergers, NSBH mergers are less likely to produce an observable KN because the neutron star may be completely consumed by the black hole without forming a significant accretion disk (LIGO & Virgo Collaboration 2020). However, if the neutron star is disrupted before being fully swallowed, a KN might still be detected.
The detection of GW 200115 and GW 200105 has prompted extensive follow-up observations to find EM counterparts. Collaborations like GRANDMA have played a significant role in coordinating optical follow-ups of these GW events (LIGO & Virgo Collaboration 2020, Noysena et al. Reference Noysena2020). By analyzing data from various telescopes around the world, researchers were looking for KN signatures or afterglows associated with these mergers.
S200105ae was detected on January 5, 2020, at 16:24:26.057 UTC. Although this event was observed by both LIGO detectors, it was recorded only by the Livingston detector (Hanford detector was temporarily offline). Initial analyses indicated that it was an NSBH merger. S200105ae was initially classified as a 97% probability terrestrial signal, but later, its significance was reassessed as being high. However, further analysis showed that the significance of the event had been underestimated, since it was detected by a single instrument, and its chirp-like structure in the spectrograms indicated a real astrophysical event. Finally, it was classified as a NSBH merger with
$\gt98\%$
probability that one component had a mass
$\lt3\,M_\odot$
, making it a neutron star. The initial analysis estimated that the probability of remnant material being left after the merger is
$\lt1\%$
. Its 90% confidence localization covered 7373 square degrees, and its estimated median distance was
$283 \pm 74$
Mpc. Optical follow-up observations by ZTF covered 48% of the event’s localization region. Over three nights, a total of 3300 square degrees were observed. GRANDMA performed tiled observations of the event with the FRAM-Auger, FRAM-CTA-N, TAROT-Calern (TCA), TAROT-Chili (TCH) telescopes. These observations cover about 8.5% of the GW skymap (Noysena et al. Reference Noysena2020). Performed observations were sensitive enough to detect kilonovae brighter than -17.5 magnitude, fading slower than 0.5 mag/day. However, no electromagnetic counterpart associated with S200105ae was detected. Observations were compared with kilonova model simulations (e.g., Figure 1). The data suggested that a kilonova with ejecta mass
$M_{\mathrm{ej,dyn}}\leq 0.02\,M_\odot \quad \text{and} \quad M_{\mathrm{ej,pm}} \leq 0.04\,M_\odot$
would have been detectable at 283 Mpc in a polar direction. The observational limits from this event helped exclude scenarios with low mass ratios, high black hole spins, and large neutron star radii (Anand et al. Reference Anand and Coughlin2021, Shawhan et al. Reference Shawhan2020).
Simulation of a black-hole-neutron-star merger (GW200115) (© T. Dietrich, Potsdam University and Max Planck Institute for Gravitational Physics), N. Fischer, S. Ossokine, H. Pfeiffer (Max Planck Institute for Gravitational Physics), S.V. Chaurasia (Stockholm University)
Figure 1 Long description
A cross-sectional view of a black-hole-neutron-star merger simulation. The image shows a detailed internal structure with various layers and features. The outer layer appears as a swirling, translucent material, likely representing the accretion disk. Inside, a dense, central region is visible, which could be the neutron star being pulled into the black hole. An inset in the top left corner provides a zoomed-out view, showing the relative positions and sizes of the black hole and neutron star. Arrows point from the inset to specific features within the main image, highlighting key areas of interest.
The S200115j event was detected on January 15, 2020, at 04:23:09.742 UTC by both the LIGO and Virgo detectors. It was initially classified as a “MassGap” event, meaning one of the objects had a mass in the 3–5 solar mass range. But since one of the components had a mass of
$\lt3\,M_\odot$
, it was considered a neutron star with
$\gt99\%$
probability. The false alarm rate (FAR) was estimated to be 1 per 1513 years, indicating that it was a highly significant event. Initially, the probability of remnant material remaining after the merger was
$8.7\%$
, but further detailed analyses later updated this probability to
$\gt99\%$
.
This made S200115j an important candidate for electromagnetic follow-up observations. The 90% confidence localization of S200115j was initially 908 square degrees, consisting of two distinct lobes. A revised LALInference skymap refined this area to 765 square degrees, concentrating most of the probability in the southern lobe. The estimated median distance was
$340 \pm 79$
Mpc, slightly different from the initial estimate of
$331 \pm 97$
Mpc (Shawhan et al. Reference Shawhan2020).
Optical follow-up observations with ZTF began within minutes of the skymap (sky map) release. On the first night, 36% of the skymap was covered, but poor weather conditions significantly limited observations on the following nights. In total, ZTF observed 22% of the skymap during the observation period. GRANDMA performed tiled observations of LIGO/Virgo S200115j event with the FRAM-Auger, FRAM-CTA-N, TAROT-Calern (TCA), TAROT-Chili (TCH) and TAROT-Reunion (TRE) telescopes. These observations cover about 13.2% of the cumulative probability of the event’s skymap (Noysena et al. Reference Noysena2020). No kilonova optical counterpart was detected. Sensitivity of observations allowed the detection of kilonovae brighter than -17 magnitude. Compared to S200105ae, S200115j was located at a greater distance, which meant that only kilonova models at closer distances could be constrained. Models with high ejecta mass (
$\gt0.05\,M_\odot$
) were ruled out only for polar viewing angles (Shawhan et al. Reference Shawhan2020).
3. Conclusion and Discussion
It has been determined that NSBH mergers generally produce more massive ejecta compared to binary neutron star mergers. As a result, this leads to a slower evolution in KN brightness after reaching its peak. In future observations, should goal for consistent and deep observations across all wavelengths, particularly in the i- and z-bands, is crucial, as these bands are considered more suitable for detecting NSBH kilonovae.
Due to the large distance of the S200115j event and the limited sky coverage, the constraints on kilonova properties have been weaker. Deeper observations in the i- and z-bands could improve the study of the optical counterparts of NSBH kilonovae.
Although observations of GW 200115 and GW 200105 have not revealed confirmed electromagnetic counterparts, ongoing and future studies, including the GRANDMA contribution, will enhance our ability to detect and study such phenomena. The coming generation of gravitational-wave observatories, such as the Einstein telescope and Cosmic Explorer, will give us even deeper insight into the late stages of stellar evolution, using observations of mergers of compact objects and other sources of gravitational waves (Punturo et al. Reference Punturo2010).
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