Very-high-energy observations of the Seyfert galaxy NGC 4151 with MAGIC -- Indication of another gamma-ray obscured candidate neutrino source

TL;DR

This study targets the Seyfert galaxy NGC 4151 as a potential high-energy neutrino source by searching for very-high-energy (VHE) gamma rays with the MAGIC telescopes. No gamma-ray excess was detected after ~29 hours of observation, leading to an integral flux upper limit above 200 GeV of $f = 2.3 \times 10^{-12}\ \mathrm{cm^{-2}\ s^{-1}}$ for a $\Gamma = 2.83$ spectrum, and accompanying differential limits up to ~10 TeV. By linking the neutrino flux from IceCube to the gamma-ray production via hadronic processes and accounting for internal gamma-ray absorption, the authors derive a constraint on the size of the neutrino production region, finding $R \lesssim 2 \times 10^{4} R_g$ and, more strictly, $R \approx 3 \times 10^{2} R_g$ as the largest radius consistent with the VHE limits; this implies substantial gamma-ray opacity within a compact region near the SMBH. The results support a scenario in which NGC 4151 is gamma-ray-obscured for neutrino production, akin to NGC 1068, highlighting the importance of simultaneous gamma-ray and neutrino observations and motivating future studies with CTA and expanded IceCube data.

Abstract

Seyfert galaxies are emerging as a promising source class of high-energy neutrinos. The Seyfert galaxies NGC 4151 and NGC 1068 have come up respectively as the most promising counterparts of a 3$σ$ and of a 4.2$σ$ neutrino excesses detected by IceCube in the TeV energy range. Constraining the very-high-energy (VHE) emission associated with the neutrino signal is crucial to unveil the mechanism and site of neutrino production. In this work, we present the first results of the VHE observations ($\sim$29 hours) of NGC 4151 with the MAGIC telescopes. We detect no gamma-ray excess in the direction of NGC 4151, and we derive constraining upper limits on the VHE gamma-ray flux. The integral flux upper limit (at the 95% confidence level) above 200 GeV is $f = 2.3 \times 10^{-12}$ cm$^{-2}$ s$^{-1}$. The comparison of the MAGIC and IceCube measurements suggests the presence of a gamma-ray obscured accelerator, and it allows us to constrain the gamma-ray optical depth and the size of the neutrino production site.

Figures (6)

• Figure 1: The squared angular distance ($\theta^{2}$) to the NGC 4151 position distribution shows the number of gamma-ray candidate events as a function of the squared angular separation between the reconstructed gamma-ray direction and the NGC 4151 position. The distribution of $\theta^{2}$, defined relative to the position of NGC 4151, is indicated with the black points, while the gray histogram corresponds to the $\theta^{2}$ distribution defined relative to one of the background estimation points Fomin94.The vertical dashed line defines the signal region within which the detection significance is computed.
• Figure 2: Multi-messenger SED of NGC 4151 in the VHE band. Black points indicate the MAGIC ULs derived in this work assuming a power-law spectral index $\Gamma$ = 2.83. The orange solid line and band refer to the best-fit and 1-$\sigma$ contour of the single-flavor neutrino spectrum derived from IceCube data collected from muon tracks over 12 years Abbasi24, while the dotted orange corresponds to its extrapolation in energy. The green band refers to the single-flavor neutrino spectrum derived from the publicly available 10-year IceCube dataset Neronov24.
• Figure 3: Multi-messenger SED of NGC 4151 in the HE and VHE bands. Black points indicate the MAGIC ULs obtained in this work assuming a power-law spectral index $\Gamma$ = 2.0. Blue data points indicate the Fermi-LAT spectrum of 4FGL J1210.3+3928 (spatially coincident with NGC 4151) from FermiDR3. Neutrino spectra are the same as in Fig. \ref{['NGC4151_MM_SED']}.
• Figure 4: Phenomenological constraints on the size of the neutrino source obtained comparing the different predictions with MAGIC ULs (black points). The gray dotted curve represents the unabsorbed gamma-ray flux corresponding to the best-fit neutrino spectrum, considering only the EBL absorption, while the gray band accounts for the uncertainty in both $\phi_{\nu_{\mu}}^{\rm 1 TeV}$ and $\Gamma$. Similarly, the dotted light blue and blue curves, along with their respective shaded bands, represent the gamma-ray emission with different level of internal gamma-ray absorption, depending on the assumed source radius: $2 \times 10^4$ (light blue) and $3 \times10^2$ (blue) gravitational radii. The dark gray area below 300 GeV is not taken into account in this analysis.
• Figure 5: SED of an AGN with an X-ray luminosity $L_X = 8 \times 10^{42} \, \rm erg \, s^{-1}$.