Kilonova emission from GW230529 and mass gap neutron star-black hole mergers

TL;DR

The paper assesses kilonova emission from GW230529 under NSBH and BNS scenarios, then forecasts KN production for a LVK O5 mass-gap NSBH population. Using NMMA to map GW-derived parameters to KN ejecta ($M_{\rm dyn}$, $M_{\rm wind}$) and POSSIS-based radiative transfer, it quantifies KN probabilities, ejecta masses, and detectability across EOSs. The results show a $2$–$28\%$ KN probability for GW230529 if it was NSBH (up to $0$–$10\%$ for BNS), and $2$–$3\%$ KN per mgNSBH event during O5, with DECam-like surveys potentially detecting up to about $70\%$ of mgNSBH KNe and yielding roughly $1-2$ multimessenger mgNSBH KN detections per year. These findings position mgNSBH kilonovae as a promising multimessenger channel for near-future gravitational-wave observations and follow-up campaigns, especially in the redder optical/near-infrared bands.

Abstract

The detection of the gravitational wave event GW230529, presumably a neutron star-black hole (NSBH) merger, by the LIGO-Virgo-KAGRA (LVK) Collaboration marks an exciting discovery for multimessenger astronomy. The black hole (BH) has a high probability of falling within the "mass gap" (mg) between the neutron star (NS) and the BH mass distributions. Because of the relatively low primary mass, this system has a higher likelihood of producing an electromagnetic counterpart than previously detected NSBH mergers. We analyze the potential kilonova (KN) emission from GW230529 and find that, if the source was an NSBH merger, there is a $\sim $2-$28\%$ probability (depending on the assumed equation of state) that it produced a KN peaking at $\sim 1$ day post-merger with $g \lesssim 23.5$ and $i < 23$. Hence, it could have been detected by ground-based telescopes. If instead the event was a binary neutron star (BNS) merger, the probability of KN production drops to $\sim $0-$10\%$. Motivated by these results, we simulate a broader population of mgNSBH mergers expected during the fifth LIGO/Virgo/KAGRA observing run (O5) and find a $2$-$3\%$ chance of KN production per event. Such KNe would typically be fainter than GW230529, with $g \lesssim 26$ and $i \lesssim 25$. Based on these findings, DECam-like instruments may be able to detect up to $\sim 70\%$ of future mgNSBH KNe, corresponding to $1-2$ multimessenger mgNSBH per year in O5.

Figures (6)

• Figure 1: Posterior distributions of GW230529 binary parameters under the NSBH assumption and adopting the BBH waveform (NSBHs–BBHw; left) and the NSBH waveform (NSBHs–NSBHw; right) models. Blue shows the full posterior; red shows the subset of samples that produce a kilonova assuming the fiducial EOS. Yellow indicates the simulated LVK O5 mass-gap NSBH population that produces a KN at distances 0-600 Mpc, shown for comparison with GW230529. The contours indicate the 1,2,3$$ of the distributions as well as the entire distribution including any outliers, from darker to lighter color. Note that the KN-producing samples of GW230529 (red) are shifted relative to the peak of the simulated O5 KN producing population (yellow). This is expected as the chirp mass driven $m_1$–$m_2$ degeneracy in GW230529 pushes KN production into the high-$m_1$, low-$m_2$ region, whereas the simulated O5 population, lacking this degeneracy and favoring more equal mass systems, peaks at smaller $m_1$ and larger $m_2$.
• Figure 2: Posterior distributions of GW230529 binary parameters under the BNS assumption (BNSs–BNSw). Blue shows the full posterior; red shows the subset of samples that produce a KN for the softer EOS (left) and the fiducial EOS (right). Yellow indicates the simulated LVK O5 BNS population that produces a KN at luminosity distances of 0-600 Mpc, shown for comparison. The values above the histograms refer to the GW230529 binary posteriors. The contours indicate the 1,2,3$$ of the distributions as well as the entire distribution including any outliers, from darker to lighter color. Only the lowest mass, fastest spinning primaries in the GW230529 posterior are classified as neutron stars; due to degeneracies, these correspond to high mass ($\sim 2M\odot$) secondaries. By contrast, in the generic O5 BNS population, KN production is favored by lower-mass secondaries.
• Figure 3: GW230529 cornerplots of dynamical and wind ejecta mass, in red, for the case of NSBHs-BBHw KN production assuming the GW230529 BBH posterior (top panels), and BNS KN from the BNS posterior (bottom panels), a softer (left panels), fiducial (middle panels), and stiffer (right panels) EOS. The yellow contours show the expected population from mgNSBH (top) BNS mergers (bottom) expected to be detected at O5 sensitivity. The contours indicate the 1,2,3$$ of the distributions as well as the entire distribution including any outliers, from darker to lighter color.
• Figure 4: Simulated KN magnitudes in $g$ (top) and $i$ (bottom) at 1 day post-merger. Solid lines show GW230529 predictions for different posterior models; dashed lines show LVK O5 simulations for mgNSBH and BNS mergers. Left: softer EOS; middle: fiducial EOS; right: stiffer EOS. The black dotted line marks a reference DECam $5$ depth for a 90 s exposure in each filter.
• Figure 5: Simulated KN light curves from 0.5–10 days post–merger in $ugrizy$ for GW230529 assuming NSBHs–BBHw (top six panels) and BNSs–BNSw (bottom six panels). Black curves show 500 randomly sampled KNe lightcurves for the fiducial EOS; shaded regions (green: softer; violet: stiffer) span the 10th–90th percentiles. The black dotted line marks a representative DECam $5$ depth for 90 s exposures in each filter.