Possible $ν$ Source Class: 3-sigma Detection of High-Energy Neutrinos from Supermassive Black Hole Binary Candidates

TL;DR

This work searches for high-energy neutrino emission from a new source class, supermassive black hole binaries (SMBHBs), by combining IceCube's 10-year public data with a catalog of 693 SMBHB candidates. Using an unbinned maximum-likelihood stacking approach across multiple EM-based SMBHB selection channels and three weighting schemes, the authors report a maximum significance around $3\sigma$ in the most physically motivated (neutrino-flux) weighting, with weaker signals under other schemes. They also explore potential connections to nano-Hertz gravitational waves and quantify the energy fraction of SMBHB mergers carried by neutrinos relative to gravitational waves. The results provide the first evidence that SMBHBs can be high-energy neutrino emitters, highlighting them as multimessenger sources and guiding future observations with next-generation detectors to conclusively test this hypothesis.

Abstract

Identifying the sources of high-energy (TeV-PeV) astrophysical neutrinos is crucial for studies in both astrophysics and particle physics. Despite extensive searches for more than a decade, which revealed several individual potential sources and only one potential source class, the origins of these neutrinos remain largely unresolved; thus, more source classes should be investigated. In this work, we conduct the first search for high-energy neutrino emission from a new source class, supermassive black hole binaries (SMBHBs), which are also theoretically motivated. We perform an unbinned maximum-likelihood-ratio analysis on our constructed catalog of 693 SMBHB candidates and 10 years of IceCube public data. Our results show positive correlations, with higher significance in more physically motivated scenarios and the highest significance at 3.0$σ$. In addition, we also study potential connections between SMBHBs' high-energy neutrino and nano-Hz gravitational-wave emissions, the latter being the main target of pulsar timing arrays. Our results provide the first evidence of SMBHBs being high-energy neutrino emitters.

Figures (8)

• Figure 1: Sky map in the equatorial coordinate system with the neutrino events and sources from our catalog. The blue gradient shows the distribution of events from the 10-year IceCube public data (April 2008--July 2018). The sky is divided into $1^{\circ} \times 1^{\circ}$ bins, with the color indicating the number of detected neutrino events in each bin. The red dots indicate the locations of the SMBHB candidates from the catalog we construct, described in Sec. \ref{['sec_catalog']}, while the encircled pink markers represent the three notable sources that contributed most to the 3$\sigma$ found in our work (see Sec. \ref{['sec_results']}).
• Figure 2: Our stacking-analysis results using the neutrino-flux weighting scheme for the three nested catalogs. The red curves denote the per-flavor neutrino fluxes that maximize the full spatial-energy likelihood, while the blue curves correspond to the spatial-focused likelihood. Left: log-likelihood values as a function of $E_\nu^2 dF/dE_\nu$ at 100 TeV. The gray band ($\blacksquare$) represents the measurements by the IceCube Collaboration from muon track analysis IceCube:2021uhz. Right: our inferred neutrino spectra or the UL from the likelihood analysis. For the spatial-energy analysis results, the spectral shapes correspond to the best-fit $\Gamma$ from the analysis, whereas for the spatial-focused analysis results, which are energy-independent, we use the canonical $\Gamma=-2$. For comparison, the grey bands and points with error bars represent the total diffuse HE astrophysical per-flavor neutrino fluxes measured by the IceCube Collaboration using different datasets. The bands in dark gray ($\blacksquare$) and light gray ($\blacksquare$) were measured using muon-neutrino and starting track events, respectively IceCube:2021uhzAbbasi:2024jro. The dark gray data points with error bars ($+$) were measured using combined electron and tau neutrino events Aartsen:2020aqd. The light gray points with errorbars ($+$) were measured using the PeV energy partially contained events IceCube:2021rpz.
• Figure 3: Same as Fig. \ref{['fig:neutrino_flux_plot']} but (1) use the bolometric-flux weighting scheme and (2) red and blue lines in the right panel are flux ULs instead of measurements. The spatial-energy analysis results coincide for the whole and association-free catalogs in both panels.
• Figure 4: Same as Fig. \ref{['fig:neutrino_flux_plot']} but (1) use the uniform weighting scheme, (2) all the curves are flux ULs instead of measurements, except for the red solid, (3) numbers of sources are different, see table \ref{['tab_results']}.
• Figure 5: Heatmap of $-2\Delta\ln\mathcal{L}$ as a function of the spectral index $\Gamma$ and the best-fit number of signal events $n_s$ for the case of neutrino-flux weighting scheme and our "whole catalog." A custom color scale is adopted to improve visual contrast across the parameter space. The teal cross indicates the parameter values that maximize the TS.