Plausible Indication of Gamma-Ray Absorption by Dark Matter in NGC 1068

TL;DR

The paper addresses the mismatch between IceCube's high-energy neutrinos from NGC 1068 and the weak gamma-ray flux observed by Fermi-LAT. It proposes that gamma rays are absorbed by a dense dark-matter spike surrounding the central supermassive black hole, deriving a model-independent attenuation measure $\Delta \mu$ and inferring a target cross section $\sigma_{\rm DM-\gamma}/m_{\rm DM}$ that can reconcile LAT and IceCube data. To realize this attenuation, the authors explore simple inelastic DM–photon scatterings producing a dark photon or an ALP final state, and demonstrate viable regions in $(m_{\rm DM}, \epsilon)$ that fit the gamma-ray spectrum while satisfying collider and cosmological constraints. The approach highlights a testable link between AGN environments, dark matter properties, and high-energy astrophysical observations, offering a less-tuned alternative to extreme astrophysical scenarios. A key takeaway is that a cross section of order $\sigma_{\rm DM-\gamma}/m_{\rm DM} \sim 10^{-28}-10^{-30}$ cm$^2$/GeV could provide the needed absorption, though its realization requires careful model-building to satisfy multi-mavelength and cosmological bounds.

Abstract

NGC 1068 is the brightest extragalactic source in high-energy neutrinos as seen by IceCube, yet the accompanying gamma-ray flux is orders of magnitude weaker. It has been argued that this indicates that the bulk of neutrinos and gamma rays are emitted in the innermost vicinity of the central supermassive black hole, which is transparent to neutrinos, but opaque to gamma rays. Even in such extreme scenarios for the acceleration of cosmic rays, astrophysical models typically overestimate the low-energy gamma-ray flux and/or require some fine-tuning in the physical parameters. Here we suggest instead that the dark matter surrounding the supermassive black hole may absorb the gamma rays, inducing the observed deficit. We show that for a dark matter-photon scattering cross section in the range $σ_{\rm DM-γ}/m_{\rm DM} \simeq 10^{-28}-10^{-30}$ cm$^2$/GeV, Fermi-LAT measurements can be well reconciled with IceCube data. We also present some simple particle physics examples that achieve the correct spectral energy dependence while respecting complementary constraints.

Figures (6)

• Figure 1: Absorption coefficient needed to attenuate the expected gamma-ray flux from NGC 1068 Inoue_2020Murase:2022dog to the observed values by Fermi-LAT (including error bars) 2012ApJ...755..164A2020ApJS..247...33A, for both $pp$ (left panel) and $p\gamma$ (right panel) scenarios and different locations of the emitting region of neutrinos and gamma rays. Absorption coefficients larger than these values would be compatible with Fermi-LAT data as well, if including an additional component of gamma rays arising from the starburst activity with high supernova rates Eichmann_2022.
• Figure 2: Family of dark matter distributions around the supermassive black hole of NGC 1068 yielding a column density of $\Sigma_{\rm DM}=10^{30}$ GeV/cm$^2$, assuming emission from $R_{\rm em}=10^4 R_S$, and for different values of the initial NFW-like profile index $\gamma$.
• Figure 3: High-energy neutrino and gamma-ray spectral distribution from NGC 1068, including IceCube IceCube:2022der, Fermi-LAT 2012ApJ...755..164A2020ApJS..247...33A, and MAGIC MAGIC:2019fvw data. The solid blue lines reflect the predicted high-energy neutrino flux via $pp$ (left panel) and $p\gamma$ (right panel) interactions at $R_{\rm em} \simeq 10^{4}R_S$Murase:2022dog. The solid green lines show the corresponding gamma-ray flux, which overshoots Fermi-LAT measurements at most energy bins. The dashed dotted lines show the attenuated photon flux due to absorption by dark matter particles, for different values of the cross section, and the parameters of the dark matter spike as normalized in Eq. \ref{['eq:absorption_coeff']} with $\gamma=1$.
• Figure 4: Absorption coefficient needed to reconcile the photon flux produced by $pp$(red) and $p\gamma$(blue) interactions at $R_{\rm em}=10^{4}R_S$ with Fermi-LAT data, due to DM-photon inelastic scatterings inducing a dark photon (left panel) or an ALP (right panel) in the final state, as a function of the photon energy, for different values of the fermion dark matter mass, dark photon/ALP mass, and mixing.
• Figure 5: Spectral energy distribution from NGC 1068 for $pp$ processes (left panels) and $p\gamma$ processes (right panels) with emitting region $R_{\rm em} =10^{4}R_S$. We show the expected gamma-ray fluxes in astrophysical models and the corresponding attenuated fluxes induced by dark photon production (upper panels) and ALP production (lower panels), for different parameter choices inducing a sizable depletion of the gamma-ray flux.