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Limits on the Ejecta Mass During the Search for Kilonovae Associated with Neutron Star-Black Hole Mergers: A case study of S230518h, GW230529, S230627c and the Low-Significance Candidate S240422ed

M. Pillas, S. Antier, K. Ackley, T. Ahumada, D. Akl, L. de Almeida, S. Anand, C. Andrade, I. Andreoni, K. A. Bostroem, M. Bulla, E. Burns, T. Cabrera, S. Chang, H. Choi, B. O'Connor, M. W. Coughlin, W. Corradi, A. R. Gibbs, T. Dietrich, D. Dornic, J. -G. Ducoin, P. -A. Duverne, M. Dyer, H. -B. Eggenstein, M. Freeberg, M. Fausnaugh, W. Fong, F. Foucart, D. Frostig, N. Guessoum, V. Gupta, P. Hello, G. Hosseinzadeh, L. Hu, T. Hussenot-Desenonges, M. Im, R. Jayaraman, M. Jeong, V. Karambelkar, S. Karpov, M. Kasliwal, C. D. Kilpatrick, S. Kim, N. Kochiashvili, K. Kunnumkai, M. Lamoureux, C. U. Lee, N. Lourie, J. Lyman, F. Magnani, M. Masek, G. Mo, M. Molham, F. Navarete, D. O'Neill, M. Nicholl, A. H. Nitz, K. Noysena, G. S. H. Paek, A. Palmese, R. Poggiani, T. Pradier, O. Pyshna, Y. Rajabov, J. C. Rastinejad, D. J. Sand, P. Shawhan, M. Shrestha, R. Simcoe, S. J. Smartt, D. Steeghs, R. Stein, H. F. Stevance, M. Sun, A. Takey, A. Toivonen, D. Turpin, K. Ulaczyk, A. Wold, T. Wouters

TL;DR

The study tackles the challenge of constraining kilonova ejecta from neutron star–black hole mergers detected by gravitational waves, by combining public LVK alerts with extensive optical follow-up for four NSBH candidates from the O4 run. It uses POSSIS-based kilonova light curves and fits ejecta masses via NSBH parameter grids (varying BH spin and EOS) to derive upper limits on dynamical and disk winds ejecta and to assess viewing-angle constraints, accounting for observation depth and sky-area coverage. Although three of the four candidates (S230518h, GW230529, S230627c) yield limited ejecta constraints due to localization and depth, S230518h provides $m_{dyn},m_{wind} < 0.03\,M_\odot$ and a viewing angle $\theta>25^\circ$, while S240422ed is likely non-astrophysical as KN would have been detectable if present. The work highlights that early, multi-band, wide-area follow-up near the predicted KN peak (around ~1 day post-merger in optical bands) is crucial for constraining KN scenarios, and it outlines a framework for end-to-end KN detectability assessments that can be refined with better GW localization and richer KN models. This approach advances our ability to translate non-detections into meaningful astrophysical constraints on NSBH ejecta and KN emission.

Abstract

Neutron star-black hole (NSBH) mergers, detectable via their gravitational-wave (GW) emission, are expected to produce kilonovae (KNe). Four NSBH candidates have been identified and followed-up by more than fifty instruments since the start of the fourth GW Observing Run (O4), in May 2023, up to July 2024; however, no confirmed associated KN has been detected. This study evaluates ejecta properties from multi-messenger observations to understand the absence of detectable KN: we use GW public information and joint observations taken from 05.2023 to 07.2024 (LVK, ATLAS, DECam, GECKO, GOTO, GRANDMA, SAGUARO, TESS, WINTER, ZTF). First, our analysis on follow-up observation strategies shows that, on average, more than 50% of the simulated KNe associated with NSBH mergers reach their peak luminosity around one day after merger in the $g,r,i$- bands, which is not necessarily covered for each NSBH GW candidate. We also analyze the trade-off between observation efficiency and the intrinsic properties of the KN emission, to understand the impact on how these constraints affect our ability to detect the KN, and underlying ejecta properties for each GW candidate. In particular, we can only confirm the kilonova was not missed for 1% of the GW230529 and S230627c sky localization region, given the large sky localization error of GW230529 and the large distance for S230627c and, their respective KN faint luminosities. More constraining, for S230518h, we infer the dynamical ejecta and post-merger disk wind ejecta $m_{dyn}, m_{wind}$ $<$ $0.03$ $M_\odot$ and the viewing angle $θ>25^\circ$. Similarly, the non-astrophysical origin of S240422ed is likely further confirmed by the fact that we would have detected even a faint KN at the time and presumed distance of the S240422ed event candidate, within a minimum 45% credible region of the sky area, that can be larger depending on the KN scenario.

Limits on the Ejecta Mass During the Search for Kilonovae Associated with Neutron Star-Black Hole Mergers: A case study of S230518h, GW230529, S230627c and the Low-Significance Candidate S240422ed

TL;DR

The study tackles the challenge of constraining kilonova ejecta from neutron star–black hole mergers detected by gravitational waves, by combining public LVK alerts with extensive optical follow-up for four NSBH candidates from the O4 run. It uses POSSIS-based kilonova light curves and fits ejecta masses via NSBH parameter grids (varying BH spin and EOS) to derive upper limits on dynamical and disk winds ejecta and to assess viewing-angle constraints, accounting for observation depth and sky-area coverage. Although three of the four candidates (S230518h, GW230529, S230627c) yield limited ejecta constraints due to localization and depth, S230518h provides and a viewing angle , while S240422ed is likely non-astrophysical as KN would have been detectable if present. The work highlights that early, multi-band, wide-area follow-up near the predicted KN peak (around ~1 day post-merger in optical bands) is crucial for constraining KN scenarios, and it outlines a framework for end-to-end KN detectability assessments that can be refined with better GW localization and richer KN models. This approach advances our ability to translate non-detections into meaningful astrophysical constraints on NSBH ejecta and KN emission.

Abstract

Neutron star-black hole (NSBH) mergers, detectable via their gravitational-wave (GW) emission, are expected to produce kilonovae (KNe). Four NSBH candidates have been identified and followed-up by more than fifty instruments since the start of the fourth GW Observing Run (O4), in May 2023, up to July 2024; however, no confirmed associated KN has been detected. This study evaluates ejecta properties from multi-messenger observations to understand the absence of detectable KN: we use GW public information and joint observations taken from 05.2023 to 07.2024 (LVK, ATLAS, DECam, GECKO, GOTO, GRANDMA, SAGUARO, TESS, WINTER, ZTF). First, our analysis on follow-up observation strategies shows that, on average, more than 50% of the simulated KNe associated with NSBH mergers reach their peak luminosity around one day after merger in the - bands, which is not necessarily covered for each NSBH GW candidate. We also analyze the trade-off between observation efficiency and the intrinsic properties of the KN emission, to understand the impact on how these constraints affect our ability to detect the KN, and underlying ejecta properties for each GW candidate. In particular, we can only confirm the kilonova was not missed for 1% of the GW230529 and S230627c sky localization region, given the large sky localization error of GW230529 and the large distance for S230627c and, their respective KN faint luminosities. More constraining, for S230518h, we infer the dynamical ejecta and post-merger disk wind ejecta and the viewing angle . Similarly, the non-astrophysical origin of S240422ed is likely further confirmed by the fact that we would have detected even a faint KN at the time and presumed distance of the S240422ed event candidate, within a minimum 45% credible region of the sky area, that can be larger depending on the KN scenario.

Paper Structure

This paper contains 86 sections, 3 equations, 17 figures, 6 tables.

Table of Contents

  1. Introduction
  2. Gravitational-wave alerts during the O4 Observing Run
  3. Sky localization and distance of the O4 alerts
  4. O4 NSBH GW candidate alerts and follow-up campaigns
  5. Summary
  6. S230518h
  7. GW230529
  8. S230627c
  9. S240422ed
  10. Details of optical observations used in this study
  11. Optimization of Kilonova Detection
  12. S230518h
  13. GW230529
  14. S230627c
  15. S240422ed
  16. ...and 71 more sections

Figures (17)

  • Figure 1: The most recently updated 90% credible region area versus the most recently updated luminosity distance (posterior mean distance and posterior standard deviation of distance) for all LIGO/Virgo GW events/candidates of runs O1, O2, O3, and O4 a/b (up to 24/07/2024). O1 to O3 events belong to GWTC catalogs Abbott_20192024PhRvD.109b2001APhysRevX.13.041039.
  • Figure 2: Comparison between the peak time luminosity of our simulated KN dataset in $g$ (first column), $r$ (second column), $i$ (third column), and $J$ (last column)-bands and the time of optical observations (using a mixed of private and public observations reported Table \ref{['followupcoverage']} and Table \ref{['tab:coverage-public']}) for S230518h (first row), GW230529 (second row), S230627c (third row), S240422ed (fourth row). The dashed gray line represents the distribution of peak time considering all m$_{dyn}$-m$_{wind}$-$\theta$ scenarios in the simulated dataset. The solid vertical black line represents the median of the peak time distribution considering only bins containing more than 5% of the distribution (grey dashed line). Observations of the community are shown in color squares.
  • Figure 3: Comparison between the peak time luminosity of our kilonova population in $u$ (left column), $o$ (second column), $z$ (third column), and $R$ (fourth column)-bands and the time of optical observations (using a mixed of private and public observations reported Table \ref{['followupcoverage']} and Table \ref{['tab:coverage-public']}) for S230518h (first row), S230529ay (second row), S230627c (third row), S240422ed (fourth row). The dashed gray line represents the distribution of peak time considering all $m_{dyn}$-$m_{wind}$-$\theta$ scenarios. The solid black line represents the median of the peak time distribution considering only bins containing more than 5% of the distribution. Observations of the community are shown in color squares. All observations in $o$-band are done by ATLAS.
  • Figure 4: Dynamical (top), wind (middle), and total (bottom) ejecta masses given a certain spin component of the BH aligned with the orbital angular momentum. We consider no spin for the NS. The fraction $\xi$ of the disk that is eventually unbound is calculated as a function of the mass ratio, as described in the text. We show results a) using the $SLy$ EOS (for more compact NS) and b) using the $H4$ EOS (for less compact NS).
  • Figure 5: Cumulative histograms showing the fraction of KN scenario that are incompatible with optical observations between 0 and 1 day (left), between 1 and 2 days (middle), between 2 and 6 days (right), as a function of the GW skymap coverage that rules them out, for each NSBH candidates (each colored line). For instance, for S240422ed between 2 and 6 days, $\sim$100% of KN scenarios are ruled out by observations covering more than 45% of S240422ed skymap. The insets show the results for S230627c between 0 and 1 day and GW230529 between 2 and 6 days as the fraction of coverage is too small to be visible in the main figures.
  • ...and 12 more figures