Which active galaxies might be neutrino emitters?

TL;DR

This work investigates which AGN types are most likely to emit IceCube neutrinos by comparing high-confidence neutrino-emitting AGNs (blazars and Seyferts) against a large hard X-ray selected control sample from BASS DR2. The authors combine intrinsic hard X-ray flux measurements with MIR variability from WISE/NEOWISE to identify long-term central engine fluctuations as a possible prerequisite for neutrino production, defining MIR variability as $\\delta f=(f_{\\max}-f_{\\min})/f_{\\max}$ and establishing a tight relation between MIR and X-ray luminosities for Seyferts with quasar-like MIR colors: $\\log L_{\\mathrm{W1}} = 0.74 \,\\log L_{14-150\ \\mathrm{keV}} + 43.94$. The key finding is that neutrino emitters tend to exhibit either high hard X-ray fluxes or large MIR variations, with notable examples like NGC 1068 and NGC 4151 illustrating these traits; this suggests long-timescale central-engine fluctuations may be critical for neutrino production and can guide IceCube counterpart searches. The paper also provides a list of Seyferts meeting criteria similar to known emitters to facilitate stacking analyses, offering a path to test models of coronal, wind/outflow, and jet-related neutrino production in AGNs and to refine multi-messenger association strategies.

Abstract

The IceCube Neutrino Observatory has identified several individual neutrino emitters associated with supermassive black hole accretion phenomena, including blazars, tidal disruption events, and, unexpectedly, Seyfert galaxies. A key open question is which types of active galactic nuclei (AGNs) are most likely to be neutrino emitters. Here we show that high-confidence extragalactic neutrino emitters tend not only to have higher hard X-ray fluxes but also to be more variable in mid-infrared (MIR) than other AGNs in the \textit{Swift} BAT AGN Spectroscopic Survey. MIR variations effectively trace long-term fluctuations in AGN accretion disks and/or jets. In addition to the role of X-ray flux emphasized in previous studies, we speculate that long-term central engine fluctuations may also be critical for neutrino production. This hypothesis may inform IceCube neutrino-electromagnetic counterpart association studies and provide new insights into cosmic ray acceleration sites. First, the observed neutrinos are unlikely to originate from AGN host galaxies or from interactions between large-scale (dozens of parsecs) winds/outflows and the surrounding interstellar medium. Second, if neutrinos are produced in the X-ray corona, the corona should exhibit strong magnetic turbulence dissipation or magnetic reconnection whose rate changes substantially on timescales of years. Third, the relativistic jets of blazar neutrino emitters may be intrinsically unstable over years. Finally, if neutrinos are related to interactions between small-scale winds/outflows and torus clouds, such winds/outflows must be highly episodic.

Figures (3)

• Figure 1: The distributions of the $W1$ peak luminosity versus the 14–150 keV luminosity for Seyfert galaxies (including NGC 1068 and NGC 4151). Top panel: Green circles denote Seyferts with quasar-like MIR colors ($W1-W2\geq 0.8\ \mathrm{[mag]}$) in their brightest states, while purple triangles represent Seyferts with galaxy-like colors ($W1-W2< 0.8\ \mathrm{[mag]}$). Error bars show $1\sigma$ uncertainties (too small to be visible). The green dash-dotted line represents the best-fit relation for Seyferts with quasar-like MIR colors (excluding NGC 1068, indicated by the gray dot, which shows a significant MIR excess). Bottom panel: Residuals of the $W1$-band luminosity ($L_\mathrm{W1}$) relative to the expected values from the best-fit relation for Seyferts with quasar-like MIR colors($L_\mathrm{W1,exp}$; the green dash-dotted line). Both subsamples show small residuals, suggesting that the $W1$-band peak luminosity is a reliable tracer of the $14$–$150\ \mathrm{keV}$ luminosity.
• Figure 2: Hard X-ray fluxes ($f_{14-150\ \mathrm{keV}}$) and MIR variations ($\delta f$) for the neutrino emitters and the no-$\nu$ sample. Gray circles and triangles represent Seyferts with quasar-like MIR colors ($W1-W2\geq 0.8\ \mathrm{[mag]}$) and Seyferts with galaxy-like MIR colors ($W1-W2< 0.8\ \mathrm{[mag]}$) in the no-$\nu$ sample, respectively. Gray squares with blue borders denote jetted AGNs in the no-$\nu$ sample. Gray dashed curves show the number density of the no-$\nu$ sample. Yellow and blue filled symbols denote Seyfert and blazar neutrino emitters, respectively. The solid black bars in the upper-left corner show systematic uncertainties for $14$--$150\ \mathrm{keV}$ flux and MIR variation. Seyfert neutrino emitters exhibit either high hard X-ray fluxes or significant MIR variations.
• Figure 3: The Doppler factors for blazars measured by Homan2021. Note that only $24$ out of $45$ blazars in the control sample have Doppler factor measurements. The color indicates the Doppler factor values. Blazars with higher $\delta f$ tend to have larger Doppler factors, suggesting that the significant apparent MIR variations in blazars without neutrino detection may be attributed to the relativistic beaming effect.