Improving low-latency multi-messenger follow-up of neutron star-black hole mergers with mode-by-mode filtering

Author(s)

Iacovelli, Francesco, Wadekar, Digvijay, Roulet, Javier, Berti, Emanuele, Corsi, Alessandra

Abstract

Rapid parameter estimation for neutron star-black hole (NSBH) mergers is essential for deciding whether, where, and how electromagnetic facilities should follow up gravitational-wave alerts. Current low-latency analyses typically use only the dominant quadrupole harmonic, leaving strong degeneracies among luminosity distance, inclination, and intrinsic binary parameters. We show that mode-by-mode filtering of the $(2,2)$, $(3,3)$, and $(4,4)$ signal-to-noise-ratio (SNR) time series enables low-latency marginalization over higher-order-mode information at a computational cost comparable to quadrupole-only analyses. Applied to simulated NSBH detections in a LIGO-Virgo network at design sensitivity, our method improves constraints on luminosity distance, viewing angle, localization volume, and source-frame secondary mass, thereby sharpening crucial estimates of electromagnetic detectability and host-galaxy association. We also validate the approach on public data for previously detected NSBH events, finding the largest improvement for the asymmetric, higher-SNR event GW190814.

Figures

Low-latency PE for a GW190814-like injected signal in a LIGO--Virgo network at O5 design sensitivity. We compare our method including higher harmonics with a quadrupole-only analysis similar to those currently used in low-latency LVK pipelines. We also show full PE results from \href{https://github.com/bilby-dev/bilby}{\texttt{bilby}}. Including HMs recovers the full-PE luminosity-distance and inclination posteriors much more faithfully, while also improving polarization and mildly improving $m_2^{\rm src}$.
Caption Low-latency PE for a GW190814-like injected signal in a LIGO--Virgo network at O5 design sensitivity. We compare our method including higher harmonics with a quadrupole-only analysis similar to those currently used in low-latency LVK pipelines. We also show full PE results from \href{https://github.com/bilby-dev/bilby}{\texttt{bilby}}. Including HMs recovers the full-PE luminosity-distance and inclination posteriors much more faithfully, while also improving polarization and mildly improving $m_2^{\rm src}$.
Accurate distance and volume estimates determine how efficiently observers can search for EM counterparts or host galaxies. We show the HM-to-$(2,2)$ ratio of the 90\% c.l. width of the $d_L$ posterior and the 90\% c.l. localization comoving volume for all analyzed NSBH systems; color gives the corresponding ratio of candidate galaxies followed up before reaching the true host (smaller is better), and point size scales with viewing angle. Including HMs usually shrinks the distance--volume search space and reduces the host-galaxy follow-up burden.
Caption Accurate distance and volume estimates determine how efficiently observers can search for EM counterparts or host galaxies. We show the HM-to-$(2,2)$ ratio of the 90\% c.l. width of the $d_L$ posterior and the 90\% c.l. localization comoving volume for all analyzed NSBH systems; color gives the corresponding ratio of candidate galaxies followed up before reaching the true host (smaller is better), and point size scales with viewing angle. Including HMs usually shrinks the distance--volume search space and reduces the host-galaxy follow-up burden.
The viewing/inclination angle $\Theta$ controls whether a relativistic jet is likely to be visible, so rapid constraints on $\Theta$ directly affect GRB follow-up decisions. We show the injected $\Theta$ and the HM-to-$(2,2)$ ratio of the posterior width on $\Theta$; color gives the ratio of the {\it Swift}-BAT GRB-detection probability from Eq.~\eqref{eq:f_grb}, and black crosses mark events with vanishing probability in both analyses. We see that HMs sharpen viewing-angle inference and can correctly strengthen or weaken the case for rapid GRB follow-up, depending on the reconstructed orientation.
Caption The viewing/inclination angle $\Theta$ controls whether a relativistic jet is likely to be visible, so rapid constraints on $\Theta$ directly affect GRB follow-up decisions. We show the injected $\Theta$ and the HM-to-$(2,2)$ ratio of the posterior width on $\Theta$; color gives the ratio of the {\it Swift}-BAT GRB-detection probability from Eq.~\eqref{eq:f_grb}, and black crosses mark events with vanishing probability in both analyses. We see that HMs sharpen viewing-angle inference and can correctly strengthen or weaken the case for rapid GRB follow-up, depending on the reconstructed orientation.
The source-frame secondary mass helps determine whether the lighter object is plausibly a NS and whether tidal disruption could power a kilonova. We compare relative uncertainties on $m_2^{\rm src}$ with HMs ($y$-axis) and without HMs ($x$-axis); color gives the ratio of $P(m_2^{\rm src}<2.7\,{\rm M}_\odot)$ (larger is more accurate), and point size scales with the injected $m_2^{\rm src}$. We see that HMs give modest but useful gains in secondary-mass classification, especially for heavier $m_2^{\rm src}$.
Caption The source-frame secondary mass helps determine whether the lighter object is plausibly a NS and whether tidal disruption could power a kilonova. We compare relative uncertainties on $m_2^{\rm src}$ with HMs ($y$-axis) and without HMs ($x$-axis); color gives the ratio of $P(m_2^{\rm src}<2.7\,{\rm M}_\odot)$ (larger is more accurate), and point size scales with the injected $m_2^{\rm src}$. We see that HMs give modest but useful gains in secondary-mass classification, especially for heavier $m_2^{\rm src}$.
We use our method on the noisy LIGO--Virgo strain data of GW190814 as a real-data test of our method. The black vertical lines show the full-PE posterior sample with the highest matched-filter SNR. We see that HMs move the low-latency posterior toward the full-PE preference for smaller distance and better constrained inclination.
Caption We use our method on the noisy LIGO--Virgo strain data of GW190814 as a real-data test of our method. The black vertical lines show the full-PE posterior sample with the highest matched-filter SNR. We see that HMs move the low-latency posterior toward the full-PE preference for smaller distance and better constrained inclination.
In the main text, we tested our low-latency HM pipeline on mergers detected in 100 simulated one-year NSBH catalogs with O5 LIGO--Virgo design-sensitivity network. Here, we show the intrinsic parameters, redshift, and SNR of all the events (detected+undetected) in the combined set of our catalogs; dashed lines mark the furthest detected-event redshift and the SNR threshold.
Caption In the main text, we tested our low-latency HM pipeline on mergers detected in 100 simulated one-year NSBH catalogs with O5 LIGO--Virgo design-sensitivity network. Here, we show the intrinsic parameters, redshift, and SNR of all the events (detected+undetected) in the combined set of our catalogs; dashed lines mark the furthest detected-event redshift and the SNR threshold.
Population-level one-dimensional summaries complement the event-by-event ratios shown in the main text. We compare the HM pipeline (blue) with the $(2,2)$-only pipeline (orange dashed) for sky area, relative $d_L$ and $m_2^{\rm src}$ widths, viewing-angle width, and SNR. We see that HMs mainly improve distance and viewing-angle information, while sky area and SNR remain largely unchanged.
Caption Population-level one-dimensional summaries complement the event-by-event ratios shown in the main text. We compare the HM pipeline (blue) with the $(2,2)$-only pipeline (orange dashed) for sky area, relative $d_L$ and $m_2^{\rm src}$ widths, viewing-angle width, and SNR. We see that HMs mainly improve distance and viewing-angle information, while sky area and SNR remain largely unchanged.
A small fraction of events in the main text figure about the $d_L$--$\iota$ constraints showed a larger 90\% c.l. localization comoving volume when HMs are included. Here, we show one example of such events. We see that the HM posterior is closer to the injected luminosity distance. Therefore, a larger quoted volume is not necessarily a failure of the HM analysis, because the posterior can become less biased while shifting to distances with larger comoving volume.
Caption A small fraction of events in the main text figure about the $d_L$--$\iota$ constraints showed a larger 90\% c.l. localization comoving volume when HMs are included. Here, we show one example of such events. We see that the HM posterior is closer to the injected luminosity distance. Therefore, a larger quoted volume is not necessarily a failure of the HM analysis, because the posterior can become less biased while shifting to distances with larger comoving volume.
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